Cobalt Oxides (From Crystal Chemistry to Physics) || Electronic and Magnetic Properties of Ruddlesden-Poepper-Type Cobaltites

4Electronic and Magnetic Properties of Ruddlesden–Poepper-Type Cobaltites

The Ruddlesden–Poepper (RP) cobaltites have been explored these past few decadesfor the influence of the dimensionality of their structure upon their magnetic andtransport properties. The low dimensionality of their structure not only induces astrong anisotropy of their properties but is also at the origin of particular chargeordering (CO) phenomenon with respect to the 3D perovskite frameworks.

4.1Cobalt Valence and Spin State Transitions

In the layered cobaltites (A, Ln)nþ 1ConO3nþ 1, with A¼ Sr and Ba, the cobalt valencecan be varied over a very large range, so that themixed valence of cobalt is observed. Itis the case of the single-layered RP cobaltites (n¼ 1), Ln2�xSrxCoO4, for which thecobalt valence ranges from Co2þ for Ln2CoO4 to Co

4þ for Sr2CoO4. While the limitmember Ln2CoO4 is easily obtained, it is more difficult to obtain the exclusivepresence of Co4þ in Sr2CoO4, as shown from theO1sXAS spectra of thinfilms of thelatter phase, which reveal that the maximum cobalt valence obtained in Sr2CoO4 isgreater than or equal to Co3.6þ [1]. For the intermediate compositions, inLn2�xSrxCoO4, the mixed valency Co2þ /Co3þ is quite stable, especially in thecompositional range, 0< x� 1. It is this mixed valence that is at the origin ofmagnetic disorder and spin glass behavior often observed for these oxides below100K. Charge orderingmay also be obtained, as shown for the half-doped compound(x¼ 0.50) La1.5Sr0.5CoO4, which exhibits a checkerboard ordering of the Co2þ andCo3þ species. In the double-layered RP cobaltites (n¼ 2), large oxygen deficiency isoften observed as pointed out in Section 1. This is the case of the cobaltiteSr3Co2O7�d, which shows d-values ranging from 0.94 to 1.62 [2, 3], that is,corresponding mainly to the mixed Co2þ /Co3þ valence (d¼ 1.62; VCo¼ þ 2.38)or to a slight amount of Co4þ in the Co3þ matrix (d¼ 0.94; VCo¼ þ 3.06). Anaverage cobalt oxidation state, VCo¼ þ 3.5, is obtained in the oxidizedSr2.75Ce

þ 40.25Co2O6.7 phase, which shows a clear ferromagnetic (FM) signature at

TC¼ 175K [4]. The cobalt valence þ 2.6 in Sr2Y0.8Ca0.2Co2O6 is expected to be theaverage of those of theCo2þ andCo3þ present in the ratio 0.4 : 0.6 in this compound.

Cobalt Oxides: From Crystal Chemistry to Physics, First Edition. Bernard Raveau and Md. Motin Seikh.� 2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA.

j179

The formal valence of cobalt ions is Co3.75þ in Sr2Y0.5Ca0.5Co2O7, but the actualvalence is close to þ 3 because of charge transfer between oxygen and cobalt causedby strong covalence bonding between the two [5].

Like single perovskite cobaltites, the RP-type cobaltites exhibit a very complexbehavior of the spin state of cobalt, depending on their structure, on the temperature,and of course on the cobalt valency. In undoped La2CoO4, which is an antiferro-magnetic insulator, Co2þ is in the high-spin state [6]. No spin state transition hasbeen detected in the temperature range 77–290K from susceptibility measure-ments [7]. In Sr2CoO4 (Co4þ , d5), there are three possible spin states, namely,low-spin (LS) (t52g), intermediate-spin (IS) (t42ge

1g), and high-spin (HS) (t32ge

2g) states.

The observed saturated moment of 1.8mB/Co site is close to what would be expectedfor the intermediate-spin configuration akin to the case of SrCoO3 (Co

4þ ) [8, 9]. First-principles calculations predict a magnetic moment of 1.97mB, which also supportsthe intermediate-spin state as well [10].

The spin state of the Co3þ ions for LaSrCoO4 is similar to that of LaCoO3, that is, itexhibits a paramagnetic ground state with the IS state of the Co3þ ion. The IS groundstate for LaSrCoO4 is also supported by optical conductivity spectra. Two broad bandsare observed at around 2 and 3.5 eV [11, 12]. The optical conductivity for LaSrCoO4 issimilar to that of LaCoO3 [13], where the spin configuration is IS for the Co3þ ion.The observed meff values for LnSrCoO4 (Ln¼ La, Ce, Pr, Nd, Eu, Gd, and Tb) samplesdecrease regularly from2.82 to 1.44 mBwith the decrease in the rare-earth ionic radiusrLn

3þ . For these compounds, it was suggested that the IS configuration appears to befavored rather than the HS or the LS state due to the ligand field splitting of the egstates and to the distortion of the CoO6 octahedron that appears as the Ln3þ sizedecreases. The TbSrCoO4 sample hasmore Jahn–Teller distortions compared to thatof other samples. The gain of kinetic energy of eg electron also contributes to thestabilization of the IS ground state with respect to the LS one. The distortion isreflected by the bond length ratio dCo–O(ap)/dCo–O(eq), which increases with thedecrease in A-site rare-earth ionic radius rLn

3þ . The ratio changes from 1.069 forLaSrCoO4 to 1.074 for TbSrCoO4 [14]. It is pertinent to mention that the ratio dM–O

(ap)/dM–O(eq) is approximately 1.20 (M¼Mn, e1g) for the eg-orbital-driven Jahn–Tellerdistortion in LaSrMnO4 [15] and 1.05 (M¼Ru, t42g) for the t2g-orbital-driven one inCa2RuO4 [16]. Thus, the observed bond length ratio in the LnSrCoO4 series indicatesthat the eg states of the IS configuration do not only correspond to fully occupied3dz2orbitals but also to partially occupied 3dx2�y2 states. It was observed that inLa1�xSr1þ xCoO4, the doped holes are mainly accommodated in the t2g orbital stateswith less Jahn–Teller distortion while keeping the IS configuration [17]. Similarobservation was reported for the hole-doped system Sr1.05Ln0.95CoO4 (Ln¼ La, Ce,and Nd) [18].

However, usingneutron diffraction studies [19] onLaSrCoO4, itwas suggested thatthe most probable ground state is the HS–LS ordered state rather than the IS state,where LS andHS cobalt ions are in 1 : 1 ratio in the low-temperature phase. In view ofthe measured effective magnetic moment meff �2.6 mB of LaSrCoO4, [20, 21], theunrestricted Hartree–Fock approximation calculations also support the HS–LSscenario in LaSrCoO4. For a fixed Hund�s coupling j, it was shown that the ground

180j 4 Electronic and Magnetic Properties of Ruddlesden–Poepper-Type Cobaltites

state of LaSrCoO4 transforms first from the antiferromagnetic high-spin state to theferromagnetically ordered high-spin–low-spin state and then to the nonmagneticlow-spin state as the crystal field splitting Dq increases. The intermediate-spin statenever becomes the ground state of LaSrCoO4 [22].

The Co3þ spin state in the mixed valence state in La2�xSrxCoO4 has been studiedusing various techniques [21, 23–27]. However, the results are conflicting and allthree possible scenarios HS, IS, and LS have been proposed. The effective magneticmoment meff in La2�xSrxCoO4 (0.4� x� 1.0) takes amaximum value around x¼ 0.5,it decreases rapidly around x� 0.7, and then becomes almost constant after-wards [21]. The meff value decreases from �4.0mB for x¼ 0.4–0.5 to 2.6mB forx¼ 0.8–1.0. The steep reduction in the resistivity in the a–b plane along with themeff change beyond x� 0.7 was brought into a coherent picture by the spin statetransition of Co3þ fromHS to IS, which occurs at x� 0.7. The values ofH andmeff forLa2�xSrxCoO4, derived from the linear regions of inverse susceptibility, are shown inFigure 4.1. TheWeiss temperature increases nearlymonotonously fromH¼� 200Kfor x¼ 0.4 up toH¼ 190K for x¼ 1.4 and is symmetrically centered around x¼ 1.0,where the change from antiferromagnetic interactions to ferromagnetic ones occurs.The effective moment decreases from meff¼ 4 mB for the Co

2þ /Co3þ sample x� 0.5to meff¼ 2.7 mB for the pure Co3þ sample x¼ 1.0. This is essentially in accordancewith the expected spin states for HS-Co2þ (S� 1.5, meff� 3.87 mB) and IS-Co3þ

(S� 1, meff� 2.83mB). For Co4þ -doped samples with x> 1.0, the effective moment

first levels at meff� 2.7–2.8mB, but finally increases to meff¼ 3.1mB for x¼ 1.4,pointing to a stabilization of the IS-Co4þ state. At elevated temperatures, themagnetic susceptibility of La2�xSrxCoO4 deviates from the common Curie–Weissbehavior, which is ascribed to a gradual change in the Co3þ /4þ spin states [28].Similar results were obtained in La1�xSr1þ xCoO4 (0� x� 0.4) and it was presumed

Figure 4.1 Composition dependence of Weiss temperature H data and effective spin number Smeff¼ 2 H{S(S þ 1)}, derived from linear regions of inverse susceptibility for La2�xSrxCoO4.Adapted from Refs [21, 28].

4.1 Cobalt Valence and Spin State Transitions j181

that the IS configuration could be stabilized by the tetragonal distortion of theoctahedral site, as revealed by theX-ray diffraction data [29]. The transition of the spinstate from theHS (x� 0.6) to the IS state (x� 0.8) is associatedwith the transfer of theeg electronbetween theCo

2þ and theCo3þ sites,which is expected to stabilize the IS-Co3þ state [21]. The orbital filling-induced spin state transition from the HS state tothe IS state is supported also by the optical conductivity spectrum for La2�xSrx-CoO4 [12, 30]. Such a spin state transition was also supported by 59Co NMRmeasurement [26]. Zero-field NMR investigations suggest an anomalous spin statetransition for La2�xSrxCoO4, where the antiferromagnetic state is suddenly replacedby a ferromagnetic state when the doping concentration becomes x� 0.6 [20].However, the observed magnetic anisotropy in La2�xSrxCoO4 (0.3� x� 0.8) singlecrystals was correlated with the low-spin ground state (S¼ 0) of Co3þ for x> 0.4 anda high-spin ground state (S¼ 3/2) of Co2þ . The spin state of Co3þ for x� 0.4 shouldbe the LS state [25].

For the highly doped system of La2�xSrxCoO4 (x> 1), with a mixed Co3þ /Co4þ

valence, the Co3þ and Co4þ ions are both in IS states, at least in the highertemperature range [28]. On the basis of susceptibility data, in Sr2�xYxCoO4 (x¼ 0,0.1, 0.3, 0.5, 0.67, 0.83, and 1), the Co3þ and Co4þ ions were suggested to be presentin the intermediate-spin states when x� 0.67, at least for the higher temperaturerange above TC [31]. Figure 4.2 shows the variation in the effective moments with xand spin state of the cobalt ions in Sr2�xYxCoO4.

Another interesting composition of the series is La1.5Sr0.5CoO4, which exhibitscharge ordering and spin ordering (SO) much below TC¼ 850K, at quite differenttemperatures, namely, TCO� 750K and TSO� 30K, respectively. Moreover, thesystem shows an incommensurate magnetism [27, 32]. Neutron scattering experi-ments have revealed the presence of nonmagnetic Co3þ in the spin ordered state ofLa1.5Sr0.5CoO4. However, it was claimed that these Co3þ ions are in the IS state andthat they become nonmagnetic as a result of the quenching of the spin angular

Figure 4.2 Variation in meff with x in Sr2�xYxCoO4. Two meff values are given for x¼ 0.87 (and x¼ 1),in which the larger value was obtained from fitting the higher temperature susceptibility data whilethe smaller one comes from lower temperature data. Adapted from Ref. [31].


momentum due to the strong planar anisotropy in the spin ordered state. At lowtemperatures, the Co3þ ions in La1.5Sr0.5CoO4 are in the IS state, which favors the JTdistorted CO phase. At higher temperature, a spin entropy-driven transition to theHS state occurs, with consequent disappearance of the JTmodulation andmelting ofthe charge ordering. Local effects, like the spin state transition and JTdistortion, wereproposed to drive the CO phase in La1.5Sr0.5CoO4 [32].

However, there is ample evidence, including experimental and theoretical studies,which suggests the low-spin ground state of Co3þ in La1.5Sr0.5CoO4. Figure 4.3shows the charge and spin ordering of the half-doped La1.5Sr0.5CoO4. Unlike thedimer Mott insulator, where the localization of electron takes place per everydimerized two sites, the charge ordering concerns the charge population on everyother site. In the checkerboard charge ordering, the alternate site in the a–b plane ofthe high-temperature tetragonal structure accommodates a hole Figure 4.3a. Elasticneutron scattering experiments revealed that the spin order is usually short rangedand is presumed to be the reflection of the short-range nature of charge and/or stripesuperlattice. Consequently, the short-range nanoscale spin correlations could beassociated with stripe stacking faults (Figure 4.3b). Charge ordering in this materialoccurs independently of magnetic ordering, which is driven by lattice electrostaticsand local spin entropy competing with the crystal field splitting of the energy levels ofcobalt [33]. Using LSDA þ U calculations, including the spin–orbit coupling andmultiplet effect, it was shown [34] that the checkerboard charge order in La1.5Sr0.5-CoO4 consists of HS-Co2þ and LS-Co3þ . Due to a small Co2þ t2g crystal fieldsplitting, the spin–orbit interaction produces an orbital moment of 0.26 mB andaccounts for the observed easy in-plane magnetism.

Co-L2.3 and O-K edge X-ray absorption spectroscopy studies revealed that chargeordering in La1.5Sr0.5CoO4 involves high-spin (S¼ 3/2) Co2þ and low-spin (S¼ 0)Co3þ ions [35].

In the calcium-phase La2�xCaxCoO4 (0.5� x� 0.8), the effective moment meffdecreases with increasing x, from 4.0 mB (x¼ 0.5) to 3.0 mB (x¼ 0.7–0.8), and a steepreduction in the Weiss temperature H is observed. These drastic changes in themagnetic properties suggest a spin state transition of the Co3þ ions from the

Figure 4.3 La1.5Sr0.5CoO4: (a) checkerboard charge and spin order at half doping. (b) Stacking faultgiving rise to short-range correlation and magnetic incommensurability in the stripe picture.Adapted from Ref. [33].

4.1 Cobalt Valence and Spin State Transitions j183

high-spin state to the intermediate-spin state. On the other hand, the effectivemoment of La2�xBaxCoO4 (0.5� x� 0.9) is almost 4.0 mB and there is no changein the Weiss temperature H. These facts suggest that the intermediate-spin state isrelated to the Co�O bond length, that is, to crystal field [36].

Unquenched orbital moments and spin-orbit interaction in cobalt ions complicatethe assignment of their electron configuration based on the observedmoment alone.Consequently, it is difficult to uniquely assign spin states based solely on the effectivemoment in the mixed valence cobaltites. This is the case of the Sr3Co2O6þd (n¼ 2)RP phases (d> 0), whose effective moment of 5.259mB/f.u. is consistent either withboth Co3þ and Co4þ adopting IS configurations or with Co3þ in a high-spin stateand Co4þ in a low-spin state. However, whatever the assumption of the spin stateeither HS-Co3þ and LS-Co4þ or IS for Co3þ and Co4þ , the effective moment issomewhat less than that derived from the Curie–Weiss fit. This discrepancy wasattributed to spin–orbit coupling, enhancing the Land�e factor (g� 2.1) for both cobaltions [37]. Gd2SrCo2O7 exhibits a structural transition at 575K from the F42/mnm tothe Bbmm space group, with a marked axial elongation of the CoO6 octahedra.The transition to the orthorhombic Bbmm symmetry allows an increase in the lengthof the bridging Co�O bond. This transition was attributed to the spin crossover ofoctahedrally coordinated Co3þ from IS to HS [38, 39].

In Sr2Y0.8Ca0.2Co2O6, the Co2þ and Co3þ ions are present in the ratio 0.4 : 0.6 and

the observed magnetic moment is 2.93 mB for VCo¼ þ 2.6 [40]. In this case, only thecombination of LS-Co2þ (t62ge

1g, S¼ 1/2) and HS-Co3þ (t42ge

2g, S¼ 2) gives a value of

2.80mB close to the observed value. The 2.40mB moment results from the combi-nation of the HS state and IS states of Co2þ (t52ge

2g, S¼ 3/2) and Co3þ (t52ge

1g, S¼ 1),

respectively. The consideration of LS-Co2þ is unusual and it is difficult to assign aspin state to cobalt ions. The meff value in Sr2.75Ce0.25Co2O6þd increases from 3.3 to4.2 mB/Co as the oxygen content, d, increases from�0.10 to þ 0.7. To account for themeff value of the d��0.10-phase with the cobalt valence VCo� þ 2.65, one canconsider a mixture of high spin Co2þ (S¼ 3/2, meff¼ 3.9mB) with various possiblespin states of Co3þ like HS, IS, or LS. On the other hand, for the d¼ 0.7 phase(VCo¼ þ 3.5), the experimental value of meff¼ 4.2 mB/Co cannot be explained by amixture of IS-Co3þ (S¼ 1)/Co4þ (S¼ 3/2) yielding to meff¼ 3.3mB/Co. This leads tothe consideration of higher spin states for the Co3þ and Co4þ species [4].

The meff value of Sr3�dCo1.9Nb0.1O6.65�d, 3.75 mB could be explained byconsidering high-spin Co3þ and low-spin Co4þ , leading to an expected valuemeff¼ 3.47mB. However, the presence of large amounts of oxygen vacancies createsdifferent kinds of oxygen coordination for cobalt, making it difficult to ascertain thespin state of cobalt. For the oxyhydroxide hydrate derivative, Sr3�dCo1.9Nb0.1O4.86�d(OH)3.04.0.4H2O, the topotactic reaction with water most probablycreates CoO5(OH) polyhedra in which the Co�OH distance is larger than the Co�Oones. This means that the majority of Co3þ cations may be considered as fivefoldcoordinated rather than sixfold. This tends to stabilize the high-spin state Co3þ as inthe Sr2CoO3Cl oxychloride. Within 300K, the oxyhydroxide hydrate derivativedoes not show a linear behavior in the x�1 plot and above that temperature it startslosing water [41].


4.2Magnetic Properties of RP Phases

The influence of the structure dimensionality upon the magnetism and electricalconductivity of perovskite and Ruddlesden–Poepper-type mixed oxides has beenstudied in the past few decades [42]. Figure 4.4 shows the DC magnetization of theRuddlesden–Poepper series compound Srnþ 1ConO3nþ 1 with n¼ 1, 2, 3, 4, and 1.The n¼ 1-phase Sr2CoO4 shows amaximumTC of 250K.TCdecreases as n increasesand goes down to �170K for n¼1 SrCoO3. Nevertheless, this type of compoundshave been relatively less studied, probably due to the difficulty to synthesize themandto stabilize the Co3þ , Co4þ formal states in the K2NiF4 structure. Importantly, theseoxides exhibit pronounced anisotropic physical properties as expected from thereduced dimensionality of their structure. Moreover, charge ordering phenomenonis observed, as shown for La1.5Sr0.5CoO4.

4.2.1The n¼ 1 – RP Cobaltites Ln2�xAxCoO4

The parent compound of the Ruddlesden–Poepper series with n¼ 1, La2CoO4,exhibits commensurate long-range magnetic ordering at TN¼ 228K, similar toLa2CuO4 and La2NiO4. Interestingly, it shows an unusual magnetic transition to adifferent, equally antiferromagnetic state at T2¼ 103K [43]. These transition tem-peratures that are different from those previously reported for this phase, that is,TN¼ 275K and T2¼ 135K [44], were ascribed to a variation in the oxygen

Figure 4.4 The DC magnetization as a function of temperature for Srnþ 1ConO3nþ 1 withn¼ 1,2,3,4, and 1. (Inset) Curie–Weiss fitting for n¼ 1 sample. Adapted from Ref. [8].

4.2 Magnetic Properties of RP Phases j185

stoichiometry. In any case, the second magnetic transition at T2 is connected with afirst-order structural phase transition, from the high-temperature orthorhombicphase to the low-temperature tetragonal phase. This transition is accompanied by aCo2þ (S¼ 3/2) spin rotation or flipping of spin in the CoO2 plane, leading to a newspin structure. Susceptibility measurements on single crystal revealed that the in-plane value is about 50% higher than that of the out-of-plane value. This magneticanisotropy suggests that the spins predominantly lie in the plane of the octahedrallayers [25, 44].

The substitution of Sr for La in La2CoO4 has been extensively studied by severalauthors, both theoretically and experimentally, leading for the cobaltites La2�xSrx-CoO4 to a wide homogeneity range (0� x� 2) [12, 14, 17, 21, 25, 28, 34, 35, 45–47].The mixed Co2þ /Co3þ valency in La2�xSrxCoO4, for the 0< x< 1 region, bringsabout magnetic disorder and spin glass behavior is found below 100K for thesesystems, for 0.40� x� 1.0, as it was observed for La1�xSrxCoO3 with low dopinglevels [21]. The effective magnetic moment in the high-temperature paramagneticphase decreases with the increase in Co3þ content and it falls from �4.0mBfor x¼ 0.4–0.5 to �2.6mB for x¼ 0.8–1.0. The Weiss temperatures also follow thesame trend and the results were interpreted in terms of cobalt spin state transition.From the meff value�3.5–4.2mB for 0.4< x< 0.6, it was suggest that for x� 0.7, Co3þ

species are in the HS state and a transition to an IS ground state occurs for x> 0.7.However, the results of unrestricted Hartree–Fock calculations differ little by threedifferent magnetic phases, namely, an antiferromagnetic HS phase (0< x< 0.39),a ferromagnetic HS phase (0.39� x< 0.52), and an antiferromagnetic LS–HSferromagnetic ordered phase (0.52� x< 1.1) [48].

Almost similar features were observed in Pr2�xSrxCoO4 (0.39� x� 0.73) [47].Neutron scattering studies showed that unlike the cuprates or nickelates, the cobaltiteLa2�xSrxCoO4 retains the commensurate antiferromagnetic order of La2CoO4 in avery short range up to a Sr content of x¼ 0.3 [49]. The commensurate magnetic andcommensurate charge ordering around the half-doped La2�xSrxCoO4 (x¼ 0.4) wasascribed to a structural distortion [49]. Again, a zero-field NMR investigation [20]suggested that the magnetic state of La2�xSrxCoO4 suddenly transforms from anantiferromagnetic state to a ferromagnetic state when the doping concentration x islarge (x� 0.6). The ferromagnetic contribution starts to develop for x� 1.1 atT� 175K in La2�xSrxCoO4, but to an extent much less than that observed in thecorresponding Ln1�xSrxCoO3 materials.

The consideration of the La2�xSrxCoO4 system shows that four compositionalranges are of particular interest: the half-doped phase La1.5Sr0.5CoO4 that exhibitscharge ordering, the compositions around LaSrCoO4 (x¼ 1) that correspond to thetransition of the AFM to the FM state, the unique ferromagnet Sr2CoO4 (x� 2), andthe Sr-rich Sr2�yLnyCoO4 oxides that exhibit a cluster glass behavior.

4.2.1.1 The Half-Doped RP Phase La1.5Sr0.5CoO4

Interestingly, an elastic neutron scattering study revealed nanoscale incommensu-rate magnetic and charge superstructures, which can be described as the quasi-regular stacking of charge lines separating antiferromagnetically ordered stripe


domains, in the hole-doped cobaltite La1.5Sr0.5CoO4, below�30K [33]. However, theauthors suggested that the stripe-type superstructure is not at the origin of incom-mensurate short-rangemagnetism in this half-doped cobaltite La1.5Sr0.5CoO4, ratherdriven by lattice electrostatics and local spin entropy competing with the crystal fieldsplitting of Co ions energy levels. Consequently, the magnetic incommensurabilityresults from an inhomogeneous exchange modulation induced by charge ordering.The independence of the charge and the spin ordering was shown by neutronscattering [27]. At the spin ordering temperature, no anomaly takes place in thecharge ordered state. The charge and spin ordered La1.5Sr0.5CoO4 was treated as astrongly frustrated square lattice antiferromagnet [27]. The Co3þ ion is effectivelynonmagnetic (1.4mB/Co

3þ ) and the effectivemoment (2.9mBperCo2þ ) is alsomuch

lower than the spin value. This was attributed to the planar anisotropy that quenchesthe spin angular momentum on the Co3þ site, similar to the orbital momentumquenching in the crystal field. Co-L2,3 and O-K X-ray absorption spectroscopymeasurements also suggest that Co2þ is in the high-spin state (S¼ 3/2), whereasCo3þ exhibits the low-spin state (S¼ 0) [35]. Nevertheless, it was also reported thatthe Co3þ ions are in IS state and that the spin entropy-driven charge melting takesplace at 825K, where the Co3þ ion transforms from the intermediate- to the high-spin state [32].

Similar near-ideal checkerboard-type ordering of Co2þ /Co3þ charges at tempera-tures below�800K was also reported in La1.5Sr0.5CoO4 [50]. A magnetic correlationbelow 60K was also suggested, but with a magnetic ordering only below 30K. Thestability of the magnetic ordering was attributed to the dominant antiferromagneticCo2þ–Co2þ interactions acting in a straight line through LS-Co3þ ions. Local spindensity approximation plus Hubbard U calculations revealed that the checkerboardcharge order in La1.5Sr0.5CoO4 can be explained by considering HS-Co2þ andLS-Co3þ only. The consideration of IS or HS states of Co3þ does not agree withthe low-temperature spin ordering since it would imply strong antiferromagneticinteraction with HS-Co2þ [34].

4.2.1.2 The Magnetic Transition Region Around LaSrCoO4

Owing to the presence of only trivalent cobalt, LaSrCoO4 represents the crossovercomposition from the Co2þ /Co3þ region to the Co3þ /Co4þ region in the La2�xSrx-CoO4 system. It was reported [17] that LaSrCoO4 behaves like a paramagnet at higherapplied field, but that at lower field it significantly deviates from the Curie–Weiss fitbelow the characteristic temperature TG (see Figure 4.5). The effective moment percobalt ion varies in the range 2.6 mB� 3.0 mB. Similar features were observed for otherhigher x compositions, suggesting the existence of competing ferromagnetic andantiferromagnetic interactions. The results were interpreted in terms of Griffiths-type cluster state, that is, the presence of critical fluctuations of the ferromagneticinteractions under random potential in a wide temperature range between localmaxima and much lower global TC. In other words, in a system with randomlydistributed spins, a finite probability of formation of ferromagnetic clusters alwaysexists in the paramagnetic background for the temperature range of TC<T< TG,whereTG is called theGriffiths temperature. The domain betweenTG andTC range is


called the Griffiths phase. Accordingly, the Griffith temperature TG� 200K forLa2�xSrxCoO4 (1.0� x� 1.5) lies between the observed TC 142 and 250K forLa0.5Sr1.5CoO4 and Sr2CoO4, respectively. Chichev et al. [28] corrected the ferromag-netic contribution arising from impurity phase in LaSrCoO4 and showed that it isparamagnetic even at 10 K. On the other hand, at higher temperatures (�650K) itdeviates from the Curie–Weiss behavior and it is associated with themetal–insulatortransition and lattice expansion, in conjunction with the spin state change similar toLaCoO3. Single-crystal magnetic measurements of La2�xSrxCoO4 (0.3� x� 0.8)were performed [25], which illustrate a high anisotropic behavior (Figure 4.6). Theyshow that xab> xc, that is, the c-axis is the hard axis of magnetization. Similar resultswith higher in-plane magnetization were obtained for La2�xSrxCoO4 (0.3� x� 1.0) [21, 25]. This result was explained by considering the low-spin state of Co3þ

for x� 0.4 and it was supported by a full-multiplet calculation. An effective crystalfield may take care of the band structure and covalency effects of Mott and charge–transfer insulators with well-localized moments. The crystal anisotropy leads toanisotropy of the orbital moment. This anisotropic orbital moment drives the spinmoment anisotropy via spin–orbit coupling since the spin is aligned in the direction

Figure 4.5 Temperature dependence ofinversemagnetization x�1(H/M) atH¼ 100Oe(open circles) and H¼ 5000 Oe (solid circles)for La2�xSrxCoO4. Typical spin glass behavior

observed in AC susceptibility at lowtemperatures below 10 K for LaSrCoO4

(x¼ 1) as displayed in the inset. Adaptedfrom Ref. [17].


of the maximum orbital momentum [25, 51]. It is important to mention here that inthin-film ferromagnetic Sr2CoO4 (TC� 250K), the magnetic easy axis is the c-axis [10].

One should notice the low-temperature magnetic state of Co3þ ions (IS state) inLaSrCoO4, in contrast to their nonmagnetic ground state in LaCoO3. Figure 4.7shows the magnetization loop for La2�xSrxCoO4 (1.0� x� 1.4). This figure clearlyshows the appearance of the ferromagnetic state at x¼ 1.1 in the mixed Co3þ /Co4þ

state. However, the saturation magnetic moment, 0.8mB/Co for x¼ 1.4, is muchlower than the expected one for IS-Co3þ and LS-Co4þ mixture as observed in

Figure 4.6 Inverse susceptibility of La2�xSrxCoO4 for two different directions of themagnetic field.The form of the curves and the magnetic anisotropy strongly deviate from Curie–Weiss behavior.Adapted from Ref. [25].

Figure 4.7 Magnetization loops for La2�xSrxCoO4 at 10 K. Adapted from Ref. [28].


ferromagnetic cubic perovskites La1�xSrxCoO3. This suggests that the magneticstructure of this layered cobaltite is complex. It was shown that for x¼ 1.1 theitinerant carrier-mediated ferromagnetic interactions give rise to a short range or two-dimensional ferromagnetic phase. For higher x-values, for example, x¼ 1.4, thisshort-range or two-dimensional FM ordered phase is transformed to 3Dferromagnetism [28].

The magnetic properties of the SrLnCoO4 (Ln¼ La, Ce, Pr, Nd, Eu, Gd, and Tb)were also studied [14, 18]. All the samples are paramagnetic and themeff values 2 of thesamples decrease regularly from 2.82 mB for Ln¼ La to 1.78mB for Ln¼Tb with theA-site rare-earth ionic radius rLn

3þ .

4.2.1.3 The 2D Ferromagnet Sr2CoO4

The single-layered Sr2CoO4 oxide is a unique two-dimensional ferromagnet, withfairly highTC� 250K. Slight variations inTC are noticed, depending on the synthesisconditions. However, the saturationmagnetization is rather controversial. A value ofthe saturation moment of 1.8mB/Co for epitaxial films of Sr2CoO4 was reported [10],whereas high-temperature and high-pressure synthesized polycrystalline sampleswere shown to exhibit a saturation moment of 1.0mB/Co [31]. LDA calculationsshowed that the saturation moment should be �2.0mB/Co, which is close to that ofthe thin-film value.However, the consideration of on-site Coulomb interactions givesthe value 1.0mB/Co, which corresponds to values observed for the bulk samples [52].In the paramagnetic phase, the effective moment is found to be 3.72 mB/Co,corresponding to the S¼ 3/2 spin configuration [31].

The reduced dimensionality of Sr2CoO4 leads to a large anisotropy in both themagnetic and the electrical properties [10, 31].Magneticmeasurements on thinfilmsshow that the c-axis is the easy axis of magnetization (upper panel of Figure 4.8). Incontrast to the three-dimensional SrCoO3 perovskite, Sr2CoO4 behaves as a hardmagnet with a coercive field (HC) as high as 2.5 T. Such a highHCwas supposed to becaused by anisotropy [8, 31].

4.2.1.4 The Sr-Rich Sr2�xLnxCoO4 Spin Glass-Like CobaltitesThe partial substitution of Sr by rare-earth elements in Sr2CoO4 leads to theformation of interesting magnetic phases [17, 53, 54]. The Sr-rich Sr2�xLaxCoO4

0.6� x� 1 compositions have been reported to show an anomalous cluster glass-typebehavior as x decreases [45]. The ferromagnetic ordering takes place at �150K,followed by a blocking process around 125K. SrLaCoO4 transforms to the spin glassstate and this has also been reported for Sr1.25Nd0.75CoO4 [54]. The appearance of aspin glass state in the doped compounds is a generic phenomenon, arising fromcompeting ferromagnetic and antiferromagnetic interactions. Note that theSr1.5La0.5CoO4 compound retains the FM ground state of Sr2CoO4, in contrast tothe charge ordered La1.5Sr0.5CoO4 [17]. However, the ground state of theSr1.5Pr0.5CoO4 compound is reported to be spin glass with a Curie temperature�200K [53, 55, 56]. But PrSrCoO4 is paramagnetic, like LaSrCoO4, having onlyIS-Co3þ ion. The substitution of Sr by Y or Gd in Sr2�x(Y/Gd)xCoO4 drastically


decreases the TC [31, 57]. For Sr2�xGdxCoO4, the ferromagnetic TC decreases to 230and 200K, for x¼ 0.1 and 0.3, respectively [57]. In Sr2�xYxCoO4,TC reaches 150K forx¼ 0.5 and the ferromagnetic state ceases to exist for x� 0.67 for Y-doped samples, asshown in Figure 4.9.

Sr1.25Pr0.75CoO4 becomes ferromagnetic at TC¼ 230K and TC drops down asthe Sr content decreases and becomes paramagnetic at x¼ 0.5 [58]. However, aglassy behavior has been reported for Sr1.5Pr0.5CoO4 [59] and an anisotropicexchange bias is observed at the interface of the phase-segregated FM clusters andthe spin glass regions in Sr1.5Pr0.5CoO4. Sr2�xNdxCoO4 (0.40� x� 0.75) shows aparamagnetic to ferromagnetic transition. A prominent cusp in the magnetizationcurve, related to the spin glass state, was noticed at 18 K forNd0.75Sr1.25CoO4 [54]. Thecusp intensity decreases with x. A Griffiths singularity was reported inSr2�xNdxCoO4 (0.40� x� 0.75) at TG� 210K, as it is observed in Sr2�xLaxCoO4

(0.50� x� 1) [28].

2

1

0

100

50

0

1.0

0.5

0.0

Sr2CoO4

9 T (H//c)

0 T

H = 0.05 T

H // c

H // b

0 100

ρ c (

mΩ

cm

)

ρc

ρb

M (

μ B/C

o)

ρb (m

Ω cm

)

200 300

T (K)

Figure 4.8 Temperature dependence of the magnetization M (upper panel) and in-planeresistivity b and interplane resistivity c (lower panel) for Sr2CoO4 thin films. Adaptedfrom Ref. [10].


4.2.2The n¼ 2 RP Cobaltites

The magnetic behavior of the Ruddlesden–Poepper cobaltites with n¼ 2 is verycomplicated [4, 37, 41, 60–62]. The high-temperaturemagnetic susceptibility study ofLn2MCo2O7 (Ln¼ Sm and Gd; M¼Sr and Ba) [60] showed a paramagnetic stateabove 300K and higher temperature plateaus were suggested to be connected with aspin state transition of Coþ 3.

Several studies of the Sr3Co2O6þd cobaltites, concerning themagnetic behavior atlow temperature, suggest that the d< 0 samples are dominant antiferromagnetic innature and that the oxygen-rich phase shows pronounced short-range ferromagneticordering. The magnetic properties of Sr3Co2O6þd for �0.62�d��0.09 are com-plex [61]. Themagnetization curve of Sr3Co2O5.38 shows two successive transitions at185 and 120K followed by a small cusp around 90K (Figure 4.10). This temperaturevariation reflects the complexity of themagnetic behavior in this phase. Interestingly,not only the high temperature data but also the data in between the two peaks can befitted into the Curie–Weiss law with meff¼ 1.13 mB. These transitions are associatedwith the antiferromagnetic ordering. Sr3Co2O5.91 also shows two transitions at 225and 155K and the latter transition is associated with the ferromagnetic component.Very similar types of transitions are reported for compositions with �0.36�d��0.20 [37]. However, the magnitude of the peaks is very sensitive to the oxygencontent andweakly depends on the temperature, which suggests that such a behavioris generic for this system and does not arise from inhomogeneity. However, onlyone peak at 188K was reported for Sr3Co2O5.64 [61], which is antiferromagneticin nature with a small FM component, as indicated by the opening of the hysteresisloop at 170K.

Figure 4.9 Temperature dependence of the field cooled DC magnetization of Sr2�xYxCoO4

measured at a magnetic field of 20 Oe. Adapted from Ref. [31].


With the increase in the oxygen content (d> 0), the high-temperature peakdisappears, indicating a gradual transformation of the antiferromagnetic phase toferromagnetic phases [37]. The Sr3Co2O6.60 composition shows only one sharp peakat 118K and a clear humparound 60K (Figure 4.11). Thenature of the peak at 118K isof the antiferromagnetic type and shifts to higher temperature at higher applied field,as the magnetization increases, suggesting the presence of short-range ferromag-netic ordering. The effective moment found for Sr3Co2O6.60 is 5.25 mB/f.u.

The calcium-substituted phase Sr2Y0.8Ca0.2Co2O6 shows a broad antiferromag-netic ordering between 270 and 300K [40]. The onset AFM ordering is associatedwith the buckling of the CoO2 layers in conjunction with the structural change fromtetragonal to orthorhombic. Such a buckling strongly affects the physical propertiesand leads to anisotropic antiferromagnetic ordering of the moments, 2.93mB/Co siteof average valence þ 2.6 [40]. The magnetic and electrical properties of a largenumber ofRP-type cobaltites, (Sr,Ca,Ln)3Co2O6�d (Ln¼Sm,Eu,Gd, Tb,Dy,Ho, andY), were investigated [63]. All the calcium-doped Sr2Y1�xCaxCo2O6�d are antiferro-magnetically ordered below 300K. However, the long-range antiferromagneticordering of Sr2Y1�xCaxCo2O6�d (0.2� x� 0.5; 0�d� 0.2) switches to a magneti-cally glassy state at higher oxygen content or at a cobalt valence of þ 3.4. Sr2Y0.5-

Ca0.5Co2O7 was shown to be a ferromagnetic insulator with a TC¼ 169K [64].The magnetic moment in the high-temperature paramagnetic phase is 2.89mB.The ferromagnetic feature appears at a Coulomb gap of 57 K, in contrast to theMott insulator, where the Coulomb correlations drive the antiferromagneticinsulating state.

Figure 4.10 Temperature dependence of magnetic susceptibility for Sr3Co2O5.38. The inset showsthe magnetization versus applied field at 5, 100, and 170 K. Adapted from Ref. [61].


The cerium-doped Sr2.75Ce0.25Co2O6.7 is ferromagnetic with a TC¼ 175K anda saturation magnetization of 0.8 mB/Co, despite a cobalt valency of þ 3.5 [4].Figure 4.12 shows the temperature-dependent susceptibility of cerium-doped sam-ples. Sr2.75Ce0.25Co2O5.9 exhibits weak magnetism, that is, dominating antiferro-magnetic interactions in the paramagnetic phase, with meff¼ 3.3mB/Co with formalcobalt valence � þ 2.65. In contrast, the oxygen-treated sample Sr2.75Ce0.25Co2O6.7

is ferromagnetic with a large thermomagnetic hysteresis loop at low temperature(Figure 4.12). TheZn-doped samples Sr3Co2–xZnxO6þd are reported to exhibit a spinglass behavior with the freezing temperature, Tg, in the range 25–30K [65].

Figure 4.12 The temperature dependence of the magnetic susceptibility (x) measured at 0.3 T foras-prepared Sr2.75Ce0.25Co2O5.9 and oxygen-treated Sr2.75Ce0.25Co2O6.7. Adapted from Ref. [4].

Figure 4.11 Inverse magnetic susceptibility (x�1) versus temperature T taken on cooling forSr3Co2O6.60. (Inset) x(T) FC data taken in a magnetic field H¼ 100 Oe (open circles) and 4000 Oe(open squares) for Sr3Co2O6.60. Adapted from Ref. [37].


The niobium-doped n¼ 2RP cobaltite Sr3�dCo1.9Nb0.1O6.65�d exhibits interestingmagnetic features. Figure 4.13 shows the temperature-dependent magnetization ofboth the pristine and the hydrated derivative samples. The oxyhydroxide hydratederivative, Sr3�dCo1.9Nb0.1O4.86�d(OH)3.04 . 0.4H2O, obtained by topotactic reactionof the pristine samples with atmospheric water, shows Brillouin-like temperaturedependence of magnetization, indicating weak ferromagnetic ordering with a highordering temperature (TC¼ 200K) (Figure 4.13a). The TC value of this layered oxideis higher than that of the ferromagnetic perovskite cobaltite, SrCo1�xNbxO3�d

(TC� 150K) [66]. In contrast, the pristine sample exhibits a spin glass state witha freezing temperature Tg� 50K [41]. TheM(H) curve (Figure 4.13b) of the hydratedderivative shows an abrupt increase in magnetization at low field and remainsunsaturated with 0.1mB/Co in 5 T, which is much lower than that of the pristinesample. Based on these observations, the authors suggested an antiferromagneticstate for the hydrated derivative. From AC susceptibility measurements, the devel-opment of spin canting at 200K was proposed. This impressive change in themagnetic nature of the layered cobaltite by water reaction was interpreted by thepartial reduction ofCo4þ intoCo3þ . It results in an increase in the antiferromagneticcoupling between high-spin Co3þ species, in association with the change in cobaltcoordination due to water intercalation [41]. The Ti-substituted hydrated phasesSr3Co1.7Ti0.3O5(OH)2,xH2O and Sr4Co1.6Ti1.4O8(OH)2,xH2O of the RP series withn¼ 2 and 3, respectively, also show a cluster or spin glass-type behavior and aferromagnetic phase appears below 125K [62].

The RP phase Sm2BaCo2O7�d was also reported [67]. This phase exhibits a strongantiferromagnetic character and a small ferromagnetic component appears only

Figure 4.13 (a) Temperature dependenceof DC magnetization (M) for the pristineRP oxide, Sr3�dCo1.9Nb0.1O6.65�d, andits oxyhydroxide hydrate derivative,Sr3�dCo1.9Nb0.1O4.86�d(OH)3.04�0.4H2O.

The data were recorded in a magneticfield of 0.3 T. (b) The M versus H loopsat 5 K for the pristine RP oxide and itsoxyhydroxide hydrate derivative. Adaptedfrom Ref. [41].


around 100K as shown in Figure 4.14. The inverse susceptibility follows linearityonly for a small region between 470 and 500K, but the authors did not report theeffective moment.

4.3Electrical Properties of RP Phases

4.3.1The n¼ 1 RP Phases Ln2�xSrxCoO4

The parent compound of the n¼ 1 series, La2CoO4, was suggested to be a Mottinsulator [6]. However, it has also been assigned a charge transfer-type insulator withthe optical gap of Eg¼ 5–6 eV [30]. The substitution of La by Sr introduces holes intothe CoO2 layers and consequently modifies its transport properties. Unlike thecuprates, nickelates, or manganites, the La2�xSrxCoO4 compounds show quitepeculiar transport properties and are insulating for a very wide range of x-values[21, 24, 25, 68]. The electrical conduction for the low doped range in La2�xSrxCoO4

remains of an activation character with the mixed Co2þ /3þ valency. The magnitudeof the resistivity in La2�xSrxCoO4 significantly decreases with the increase in Co3þ

content. Unlike the cubic perovskite La1�xSrxCoO3 cobaltite, which shows a metal–insulator transition at x¼ 0.2, La2�xSrxCoO4 fails to show a metal–insulator tran-sition even at x¼ 0.5, probably due to its low dimensionality. The in-plane resistivityof La2�xSrxCoO4 for 0.4� x� 0.7 follows the thermal activation behavior with anactivation energy of �0.5 eV, but for x� 0.8, the thermal activation law is notobeyed [21].

Figure 4.14 Susceptibility and inversesusceptibility curves of Sm2BaCo2O7�d, left andright y-axes, respectively measured in 100 G for4–400 K (SQUID) and in 3000 G for 300–800K

(Faraday balance). Inset showsmagnetic field dependence of themagnetization at 10 K. Adaptedfrom Ref. [67].


For 1� x� 1.40, the Ln2�xSrxCoO4 RP phases show a semiconducting behavior,and their resistivity decreases as x increases. Within the temperature range 250K<T< 300K, the resistivity data follow the nonadiabatic small polaronmodel, r/T 3/

2exp[Ea/kBT], whereas for 80 K<T< 220K the data give a better fit with the Mottvariable range hopping (VRH) expression, r/ exp[T0/T]

1/(dþ 1), where d is thedimensionality of the system and T0 is related to the density of states at the Fermienergy and the localization length [45].

It should be pointed out that the almost localized eg electrons for the intermediatedoping range may affect the spin state of the Co3þ ions. Measurements of theresistivity of La2�xSrxCoO4 (x¼ 1.0–1.4) up to 900K show amonotonous variation inagreement with a thermal activation law (Figure 4.15) [28]. In the hole-dopedsamples with x > 1, the resistivity decreases significantly and becomes �10mV cmat room temperature for x¼ 1.4, along with a drop in activation energy defined asEA¼ d(lnr)/d(1/T) and itinerant polaronic carriers are also suggested (see inset inFigure 4.15).

As stated above for the magnetic properties of these oxides, the half-dopedLa1.50Sr0.50CoO4 phase, LnSrCoO4 and Sr2CoO4 cobaltites have been the object ofextensive studies.

4.3.1.1 The Half-Doped Ln1.5Sr0.5CoO4 CobaltiteThe stable charge ordered insulating state of the half-doped transitionmetal oxides isgenerally assisted by cooperative Jahn–Teller distortions. In the layered oxides,

Figure 4.15 Electrical resistivity and local activation energy defined as EA¼ d(lnr)/d(1/T) forLa2�xSrxCoO4. Adapted from Ref. [28].

4.3 Electrical Properties of RP Phases j197

such an ordering is of checkerboard type as observed in manganites or nickelates.The possible existence of stripe phases in La2�xSrxCoO4 was suggested for x¼ 0.5 atlower temperatures [21], like in cuprates and nickelates.

As it was pointed out, La2�xSrxCoO4 is a Mott insulator in the doping range0� x� 1.0. InMott–Hubbard insulators, the doped charges get self-localized, givingrise to the charge ordered state. Charge ordering could be related to the cooperativeordering of the lattice polarons to minimize the lattice strains [69, 70]. Neutronscattering investigations revealed this polaron glass state in La2�xSrxCoO4, withx¼ 0.5 below TCO� 750K. However, it was correlated with the checkerboardarrangement of empty and occupied 3dz2�r2 orbitals of Co

3þ and Co2þ ions (seeFigure 4.16) and viewed as a limiting case of charge stripe order with shortestpossible stripe spacing [27]. Figure 4.17 shows the charge and spin orderingconfiguration in La1.5Sr0.5CoO4. Note that there is an amazing difference in COand SO transition temperatures, TC/TS� 25, and the absence of any CO anomaly atTsin La1.5Sr0.5CoO4. It was argued that the relativistic spin–orbit coupling-mediatedsingle-ion anisotropy effectively decouples charge ordering from low-energy spinfluctuations. The charge and spin orderings in La1.5Sr0.5CoO4 are independentphenomena [27]. The extreme insulating character of the charge ordered La1.5Sr0.5-CoO4 was attributed [35] to the active spin blockade phenomenon [71]. It wassuggested that the Co3þ ions are in IS state and transform to the HS state abovethe charge ordering temperature. Thus, below TCO, local effects, like spin statetransition and JT distortion, are efficient to drive the charge ordered phase inLa1.5Sr0.5CoO4 [32]. Similar type of spin and charge ordering is also observed in

Figure 4.16 Schematic drawing of Co�Obonding orbitals and checkerboard order of Co2þ /Co3þ

valence in the ab plane of La1.50Sr0.50CoO4. Adapted from Ref. [27].


La2�xCaxCoO4 system [72, 73]. A checkerboard charge order on Co2þ /Co3þ ispresent at room temperature in La2�xCaxCoO4 [72, 74].

The possible existence of stripe phases in La2�xSrxCoO4 around x� 0.5 wasspeculated [21]. It is well known that the checkerboard charge ordering occurs at hightemperature, which coexists with a magnetic ordering below 50K and the chargeordering is totally independent of spins. Elastic neutron scattering studies [33] ofLa1.5Sr0.5CoO4 showed short-range incommensurate magnetic ordering, but noevidence of incipient 1D charge stripes was found. However, neutron scatteringstudies on La2�xSrxCoO4 [49] revealed the possible evidence for stripe phases. It wasshown that the commensurate AFM magnetic ordering of La2CoO4 persists for Srcontents up to 0.3 due to efficient trapping of the charge carriers. However, anincommensurate magnetism manifests at the intermediate doping range which isakin to the stripe phases observed in cuprates [75] and nickelates [76].

4.3.1.2 The LnSrCoO4 CobaltitesLaSrCoO4 seems to be a paramagnetic semiconductor. Like the parent La2CoO4,LaSrCoO4 is also a charge transfer-type insulator with an optical gap of0.95 eV [12, 17, 21, 28]. LaSrCoO4 undergoes a broad resistivity transition at 400–900K, associated with diffusive changes of the magnetic susceptibility and volumeexpansion. The magnitude of the resistivity of LaSrCoO4 is close to that ofLaCoO3 [21]. In the optical conductivity spectra two broad bands are observed forLaSrCoO4 at 2 and 3.5 eV [12]. The resemblance in optical conductivity betweenLaSrCoO4 and LaCoO3, with IS Co3þ , has led some authors to consider LaSrCoO4

also as a 3d6-based Mott insulator [12, 17]. However, one should remember thatLaCoO3 is nonmagnetic at lower temperature and thus, it basically undergoes atransition from a spin-gapped insulator to a Mott insulator [77].

Figure 4.17 Schematic spin and charge configuration of the CoO2 plane in La1.5Ca0.5CoO4.Conventional and magnetic unit cells represent the broken and solid lines, respectively. Spinstructure of Co3þ is realized only in x¼ 0.5. Adapted from Ref. [72].


The effect of A-site rare-earth ionic radius rLn3þ on the physical properties of

transitionmetal oxides is quite usual. This is the case of the LnSrCoO4 series that hasbeen investigated for a large number of lanthanides (Ln¼ La, Ce, Pr, Nd, Eu, Gd, andTb) [14]. Aprogressive narrowing of the bandwidth due to the increasing distortion ofthe octahedral framework is observed. This is correlated with the successivelyreduced tolerance factor t, as rLn

3þ decreases. The lanthanide-based cobaltitesSrLnCoO4 (Ln¼ La, Ce, Pr, Nd, Eu, Gd, and Tb) show a semiconducting behaviorand their high-temperature resistivity behavior follows the thermal activation law,r(T)¼ r0exp(Ea/kBT) [14]. Figure 4.18 shows the ln r against T�1 plot of thehomologous series SrLnCoO4 (Ln¼ La, Ce, Pr, Nd, Eu, Gd, and Tb). The activationenergy Ea increases from 144meV for SrLaCoO4 to 312meV for SrTbCoO4, as rLn

3þ

decreases (inset in Figure 4.18). This is similar to what was observed for perovskitesand attributed to the change in tolerance factor [78].

4.3.1.3 Sr2CoO4 and Some Sr-Rich Phases Sr2�xLnxCoO4

Sr2CoO4 shows both ferromagnetic and metallic properties, with anisotropy of itsresistivity. These properties are strongly linked to the tetravalent nature of cobalt.Nevertheless, this square lattice does not show any superconductivity, contrary toNaxCoO2 phases, which exhibit a triangular lattice. Sr2CoO4 shows a temperature-independent resistivity below TC, on the order of 10

�4–10�3V cm, which is relativelyhigher than that of a typical metal [8, 31]. Its resistivity shows a kink at theferromagnetic TC¼ 255K. Band structure calculations indicate that Sr2CoO4 shouldbe a ferromagnetic metal or half metal in the thin-film or polycrystalline form,respectively [52].

The Sr-rich cobaltites Sr2�xYxCoO4 exhibit also an interesting evolution of theirresistivity (Figure 4.19). In Sr2�xYxCoO4, the resistivity changes significantly as x

Figure 4.18 Plot of ln(r) against T�1 for SrLnCoO4 (Ln¼ La, Ce, Pr, Nd, Eu, Gd, and Tb). The solidlines stand for the thermally activated conduction fitting. The inset shows the variation in activationenergy Ea. Adapted from Ref. [14].


increases. The resistivity of SrYCoO4 (x¼ 1) becomes 3 103V cm at 300K.The larger x content samples do not follow the thermal activation law at lowertemperatures. The thermal activation energy for the x< 0.5 samples remains almostconstant, 10–20meV, and it increases steeply near x¼ 0.5, where the ferromagneticstate ceases to exist and becomes �130meV for x¼ 1.0 (Figure 4.19) [31].Very similar features are observed for the Pr-doped Sr2�xPrxCoO4 samples and theactivation energy becomes maximum at x¼ 1.0 [56]. The Nd-doped samples,Sr2�xNdxCoO4 (x¼ 0.40, 0.67, and 0.75) are also semiconductors, and both thermalactivation and small polaron model can explain the transport mechanism of thesystem. The resistivity decreases as x increases [46, 54]. The Eu-doped Eu2�xSrxCoO4

samples with x < 1 also show a very low resistivity that is comparable to that of theundoped Sr2CoO4 [58].

4.3.2The n¼ 2 RP Phases

Rather few investigations have been carried out on n¼ 2 RP-type cobaltites. Theoxides Ln2MCo2O7 (Ln¼Sm and Gd; M¼ Sr and Ba) exhibit a semiconductor tometal transition at high temperature [60]. The TIM values for Sm2SrCo2O7,Gd2SrCo2O7, and Sm2BaCo2O7 are 1053, 1053, and 593K, respectively. Figure 4.20shows the temperature-dependent semiconducting-like resistivity of Sr3Co2O5.91,which follows the simple thermal activation behavior r¼r0exp (Ea/kBT) with anactivation energy of 0.23 eV. The lower oxygen content phases Sr3Co2O5.38 andSr3Co2O5.64 are highly resistive [61]

Sr2Y0.8Ca0.2Co2O6 is electrically insulating [40]. All the samples of the seriesSr2Y1�xCaxCo2O6�d (0.2� x� 0.5; 0�d� 0.2) are insulating. Though, on the

10000.12

0.08

0.04

0.000.0 0.2

0.5

0.3

0.1

0.67

0.83

1

X=0

0.4 0.6 0.8 1.0

X

Ea

(eV

)100

10

1

0.1

0 50 100 150 200 250 300 350

Temperature [K]

ρ [Ω

cm

]

Figure 4.19 Temperature dependence of the electrical resistivity (r) for the Sr2�xYxCoO4 system.Inset shows the activation energy Ea. Adapted from Ref. [31].


basis of partially filled mixed valence band of Sr2Y0.8Ca0.2Co2O5.92 and Sr2Y0.5-

Ca0.5Co2O5.76, one would expect a metallic behavior, it was found to be insulatorprobably due to the 2D localization of the electron [63]. However, on furtheroxidation of Sr2Y0.5Ca0.5Co2O5.76 to Sr2Y0.5Ca0.5Co2O7, it becomes semicon-ducting and follows two types of VRH conductivity [64]. Below 30 K, it followsEfros–Shklovskii-type conduction and above 30 K it transforms to the Mott-typeVRH conductivity.

4.4Phase Separation in RP Phases

The substitution of strontium for La in the insulating antiferromagnet La2CoO4

induces hole doping. The doped holes in La1�xSr1þ xCoO4 (0� x� 0.5) mostly enterthe t2g orbital states while keeping an intermediate-spin configuration of the Co3þ

ions [17]. In this doping range, there is a competition between the ferromagneticand the antiferromagnetic interactions leading to phase separation, that is, to thepresence of ferromagnetic clusters. It was suggested [28] that in La2�xSrxCoO4 up tox¼ 1.2, the long-range ferromagnetic ordering remains 2D in nature, but that forfurther increase in x the 3D ferromagnetic order starts below 150K, which thensaturates at about 70K. At this point, phase separation takes place where ferromag-netic regions coexist with the residual 2D paramagnetic phase. For a certaindoping range in the Pr-doped Sr2�xPrxCoO4, magnetic phase separation hasbeen suggested, which could be arising out from the large lattice mismatch betweenPr and Sr [59].

Figure 4.20 Temperature-dependent resistivity for Sr3Co2O5.91. The inset shows the linear fit of theln r versus temperature data. Adapted from Ref. [61].


Very few studies claim the existence of phase separation in the n¼ 2 RP phases.Nevertheless, these systems exhibit spin glass and cluster glass behavior and showthe appearance of ferromagnetic phases within antiferromagnetic matrix, whereaslarge thermomagnetic hysteresis loops are observed. All these features support thepresence of coexisting phases. This is the case of Sr2Y0.8Ca0.2CoO6.79 and Sr2Y0.5-

Ca0.5Co2O5.62, where the coexistence of ferromagnetism and antiferromagnetismwas suggested to originate from spin frustration [63]. The Ti-substituted hydratedSr3Co1.7Ti0.3O5(OH)2,xH2O and Sr4Co1.6Ti1.4O8(OH)2,xH2O also reveal the coexis-tence of spin glass and ferromagnetic phase [62].

4.5Magnetoresistance of RP Phases

The increase in the 2D character of the structure in Ruddlesden–Poepper systems,which enhances electronic correlations, can lead to higher magnetoresistance withrespect to 3D perovskites, even at lower temperatures [79–81]. This dimensionaleffect is evident from the large values of MR observed in the layered 2D Sr2CoO4

(MR� 7.5%) compared to 3D SrCoO3 (MR� 3%) [8].Reasonably large magnetoresistance was observed for Sr2CoO4 near the ferro-

magnetic transition temperature and in the lower temperature region. Figure 4.21shows the temperature variation inMR for Sr2CoO4. Apeak in the negativeMR value

00

–1

–2

–3

–4

–5

0.30 0.062

0.060

0.058

0.056

0.054

0.052

0.050220 240 260

T = 255 K

280

0.25

0.20

0.15

0.10

0.05

50 50100 150 200

Temperature [K]

(a) (b)

250 300 0

0T

5T

100 150 200

Temperature [K]

250 300

[ρ5T

–ρ0T

]/ρ 0

T%

ρ[Ω

cm

]

Figure 4.21 Temperature dependences of the resistivity under 0 and 50 kOe (b) andmagnetoresistance (a) for Sr2CoO4. Inset shows the resistivity within the temperature range aroundTc. Adapted from Ref. [31].

4.5 Magnetoresistance of RP Phases j203

is observed in the vicinity of TC (�4%), which is an intrinsic feature of magneto-resistance, that is, the ferromagnetic order is improved by the applied externalmagnetic field, causing a decrease in the electrical resistivity. The observed kink at TC

in the resistivity curve disappears in an applied field of 5T (inset of right panel inFigure 4.21). Corresponding to the magnetic hysteresis, the field hysteretic magne-toresistance is also observed at lower temperature. TheMR value reaches�6% at 5 Tand 5K for Sr2CoO4 [31]. About �7.5 % MR value was observed at 5 K and 7 T inSr2CoO4 [8].

The Sr-rich La1�xSr1þ xCoO4 (0� x� 0.40), which shows a cluster glass behavior,exhibits a large magnetoresistance [45]. Figure 4.22 shows the MR effect ofLa1�xSr1þ xCoO4. The composition La0.8Sr1.2CoO4 shows a maximum MR of�24% at T¼ 15K under a field of 5 T. The quasi-metallic/semiconducting ferro-magnetic La0.5Sr1.5CoO4 phase has also been reported to exhibit a large MR value atlower temperature, which was attributed to spin-dependent scattering at the grainboundaries and domain walls in polycrystalline samples [17].

Figure 4.22 Field dependence of the electrical resistivity of the La1�xSr1þ xCoO4 samples:(a) x¼ 0.20, measured at 15 and 25 K. (b) x¼ 0.30, measured at 5 and 10 K. Adapted from Ref. [45].


The doping effect on MR is significant in Sr2�xLnxCoO4. For Ln¼Y and Gd,though TC decreases as x increases, the MR values increase notably [57]. At 5 K and7 T, the MR values increase up to 17 and 14% for Gd and Y, respectively, at x¼ 0.3in Sr2�xLnxCoO4 and the magnetoresistance decreases with the increase intemperature. The MR value of Sr1.7Gd0.3CoO4 is about two times smaller thanthat of the pure a-axis-oriented Sr2CoO4 single-crystal thin films and it is two timesgreater than that in Sr2CoO4 polycrystalline bulks. This effect is related to the spin-dependent tunneling MR at the grain boundaries [10, 31, 57]. The EuSrCoO4

sample shows about 10% MR at 8 T at 5 K with negligible field-hystereticmagnetoresistance. This effect was suggested to be due to spin-dependentscattering at the grain boundaries [58]. At 150 K, Sr2�xLaxCoO4 was reported toshowMR values of 1, 2, and 3% for x¼ 1, 0.75, and 1.25, respectively. However, theMR is enhanced to 7% at 5 K for La1.25Sr0.75CoO4. First-principles band structurecalculations indicate a highly spin polarization metallic state for Sr1.5La0.5CoO4

and the associated MR in Sr2�xLaxCoO4 system was suggested to be spin-polarized magnetoresistance [82].

Very few magnetoresistance properties have been investigated for n¼ 2RP phases. One example is given here for Sr2.75Ce0.25Co2O6.7. The ferromagneticstate (TC¼ 175K) of this phase is much more conducting compared to the high-temperature paramagnetic phase and the conductivity is sensitive to the appliedfield. Figure 4.23 shows the isothermal magnetic field-dependent resistivity ofSr2.75Ce0.25Co2O6.7. The figure shows that the sensitivity of the resistivity to themagnetic field increases at lower temperatures and it exhibits amagnetoresistance ofabout 4% at 15K, which is comparable to that of the cubic ferromagnetic metallicperovskites SrCoO3�d [4].

Figure 4.23 Isothermal magnetic field dependence of the resistivity for Sr2.75Ce0.25Co2O6.7.Adapted from Ref. [4].

4.5 Magnetoresistance of RP Phases j205

4.6Thermoelectric Properties of RP Phases

In La2�xSrxCoO4 (x¼ 1.0–1.4), for doping x> 1, the thermopower rapidly decreasesas x increases (see Figure 4.24). Thus, although nonmetallic conduction is observed inthese cobaltites, the thermopower measurements reflect an itinerant nature of thepolarons in this Co4þ -doped cobaltite. Interestingly, in all the samples, themaximumthermopower coefficient is reached at around room temperature (Figure 4.24). Thehigher value�280mV/K is obtained for the x¼ 1 composition. It is important to pointout that the magnitude of S(T) in this doped layered phase is significantly lower thanthat of the cubic perovskite [28]. A similar behavior was described for the same oxides,in the same compositional range, showing a positive Seebeck coefficient, suggestingcharge carriers to be holes [45]. Different from the previous results, the S(T) evolution(Figure 4.25) shows a plateau indicating a small polaron transport regime, whereas inthe low-temperature regime it follows a VRH model, S(T)¼AT1/(nþ 1), where n¼ 2(2D behavior) for x� 0.20 and n¼ 1 (3D behavior) for x> 0.20.

Nevertheless, recent measurements on a series of samples of compositionSrLnCoO4 [14] showed that a larger value of S(T)� 291 mV/K is obtained at 61 K(Figure 4.26). This observation is in contradiction with that previously reported [28].Such a higher S-value at lower temperature was attributed to degeneracies in Co3þ

sites, as observed in the layered NaxCoO2 cobaltite [83]. At room temperature, largerS-values are observed for smaller lanthanides, for example, S300K¼ 320 mV/K forSrTbCoO4. The S(T) value increases as the rare-earth size, rLn

3þ , decreases(Figure 4.26). This result was related to the enhanced spin entropy of magnetically

Figure 4.24 Thermoelectric power in La2�xSrxCoO4. Adapted from Ref. [28].


weak interacted system of narrow bandwidth, which arises from the poor orbitaloverlap as the size of lanthanides decreases [14, 18].

The thermoelectric power of Nd1�xSr1þ xCoO4 is also rather controversial.Huang et al. [54] observed a decrease in S(T) with temperature for x¼ 0.25 and0.33 compounds, which are p-type (Figure 4.27). The increase in S(T) above TC israther slow. The x¼ 0.6 composition shows relatively weak temperature dependenceandS(T) changes sign around 60K.All the data follow the thermal activation behaviorand a small polaron conduction model. Ang et al. [46] reported highly contradictingresults for x¼ 0.25 and 0.33. They observed an unusual rock bottom in the S(T) curvefor both compositions. The S(T) value increases rapidly below TS¼ 61K for x¼ 0.25and reaches 150mV/K at 26 K. TheTS-value increases with x. The rock bottom of the S

Figure 4.25 (a) Temperature dependence ofthe Seebeck coefficient of La1�xSr1þ xCoO4

(0� x� 0.40) in the temperature range100� T� 450 K. (b) Fitting of the Seebeck

data from the samples with x¼ 0.20, 0.30,and 0.40 to 2D and 3D VRH and to polaronicbehavior in different temperature intervals.Adapted from Ref. [45].

4.6 Thermoelectric Properties of RP Phases j207

(T) curve was related to the LS–IS transition of the highly JT distorted Co3þ

octahedra. The high-temperature and low-temperature data were fitted with theadiabatic small polaronic conduction and 2DMott�s variable range hopping models,respectively [46].

Very few data are available for the thermopower values of n¼ 2 RP phases, whichseem to be rather low, as shown from the thermopower of Sr2Y0.5Ca0.5Co2O7, whichis small (S¼ 12 mV/K) around room temperature and approaches a constant valuebelow 30K and follows the T2/4 dependence [64].

Figure 4.27 Thermoelectric power versus temperature for Nd1�xSr1þ xCoO4. The magnifyinggraphs of S at low temperature are shown in the inset. Adapted from Ref. [54].

Figure 4.26 Temperature-dependent Seebeck coefficient S(T) of SrLnCoO4 (Ln¼ La, Ce, Pr,Nd, Eu, Gd, and Tb). Adapted from Ref. [14].


References

1 Matsuno, J. et al. (2005) Thin Solid Films,486, 113.

2 Viciu, L. et al. (2006) J. Solid State Chem.,179, 5010.

3 Dann, S.E. and Weller, M.T. (1995)J. Solid State Chem., 115, 499.

4 Demont, A. et al. (2008) J. Solid StateChem., 181, 1314.

5 Zaanen, J. and Sawatzky, G.A. (1990)J. Solid State Chem., 88, 8.

6 Yamada, K. et al. (1989) Phys. Rev. B,39, 2336.

7 Le Coustumer, L.R. et al. (1982)Nouv. J. Chem., 6, 7.

8 Wang, X.L. et al. (2005) J. Appl. Phys.,97, 10M519.

9 Potze, R.H. et al. (1995) Phys. Rev. B,51, 11501.

10 Matsuno, J. et al. (2004) Phys. Rev. Lett., 93,167202.

11 Uchida, S. et al. (1993) Physica B,186–188, 975.

12 Moritomo, Y. et al. (1995) J. Phys. Soc. Jpn.,64, 4117.

13 Arima, T. andTokura, Y. (1995) J. Phys. Soc.Jpn., 64, 2488.

14 Ang, R. et al. (2008) J. Phys. D Appl. Phys.,41, 045404.

15 Herrero-Martın, J. et al. (2005)Phys. Rev. B,72, 085106.

16 Friedt, O. et al. (2001) Phys. Rev. B, 63,174432.

17 Shimada, Y. et al. (2006) Phys. Rev. B, 73,134424.


19 Demazeau, G. et al. (1979) Nouv. J. Chim.,3, 171.

20 Furukawa, Y. et al. (1993) J. Phys. Soc. Jpn.,62, 1127.

21 Moritomo, Y. et al. (1997) Phys. Rev. B, 55,R14725.

22 Wang, J. et al. (2000) Phys. Rev. B, 62,14140.

23 Srivastava, K.G. (1963) Phys. Lett., 4, 55.24 Matsuura, T. et al. (1988) J. Phys. Chem.

Solids, 49, 1403.25 Hollamnn, N. et al. (2008)New J. Phys., 10,

023018.26 Itoh, M. et al. (1999) Physica (Amsterdam),

259–261, 997.

27 Zaliznyak, I.A. et al. (2000) Phys. Rev. Lett.,85, 4353.

28 Chichev, A.V. et al. (2006) Phys. Rev. B, 74,134414.

29 Sanchez-Andujar, M. et al. (2004)J. Magn. Magn. Mater., 272–276, 855.

30 Uchida, S. et al. (1993) Physica B,186–188, 875.

31 Wang, X.L. et al. (2005) Phys. Rev. B, 72,064401.

32 Zaliznyak, I.A. et al. (2001)Phys. Rev. B, 64,195117.

33 Savici, A.T. et al. (2007) Phys. Rev. B, 75,184443.

34 Wu, H. and Burnus, T. (2009) Phys. Rev. B,80, 081105(R).

35 Chang, C.F. et al. (2009) Phys. Rev. Lett.,102, 116401.

36 Horigane, K. et al. (2006) Physica B, 378–380, 334.

37 Hill, J.M. et al. (2006) Phys. Rev. B, 74,174417.

38 Siwen, L. and Yufang, R. (1994)Mater. Res.Bull., 29, 993.

39 Hickey, P.J. et al. (2007) Phys. Rev. B, 75,024113.

40 Yamaura, K. et al. (1999) Phys. Rev. B,60, 9623.

41 Motohashi, T. et al. (2005) Chem. Mater.,17, 6256.

42 MohanRam,R.A. et al. (1986) J. Solid StateChem., 63, 138.

43 Gardner, J.S. et al. (1997) Physica B,234–236, 721.

44 Yamada, K. et al. (1989) Phys. Rev. B,39, 2336.

45 S�anchez-And�ujar, M. et al. (2006) SolidState Sci., 8, 901.


47 Sakiyama, N. et al. (2008) Phys. Rev. B, 78,180406(R).

48 Wang, J. et al. (2000) J. Phys. Condens.Matter, 12, 7425.

49 Cwik, M. et al. (2009) Phys. Rev. Lett., 102,057201.

50 Helme, L.M. et al. (2009) Phys. Rev. B, 80,134414.

51 Bruno, P. (1989) Phys. Rev. B, 39, 865.52 Lee, K.-W. and Pickett, W.E. (2006) Phys.

Rev. B, 73, 174428.

Referencesj209

53 Ang, R. et al. (2008) Appl. Phys. Lett., 92,162508.

54 Huang, S. et al. (2006) Phys. Rev. B, 73,094431.

55 Yao, Q.W. et al. (2007) Physica C,460–462, 481.

56 Ang, R. et al. (2008) J. Appl. Phys., 103,103707.

57 Wang,X.L. et al. (2007)Appl. Phys. Lett., 91,062501.

58 Yao, Q.W. et al. (2008) J. Appl. Phys., 103,07B904.

59 Ang, R. et al. (2008) J. Appl. Phys., 104,023914.

60 Siwen, L. and Yufang, R. (1995)J. Solid State Chem., 114, 286.

61 Viciu, L. et al. (2006) J. Solid State Chem.,179, 500.

62 Pelloquin, D. et al. (2005) Chem. Mater.,17, 773.

63 Yamaura, K. et al. (1999) J. Solid StateChem., 146, 277.

64 Yamaura, K. et al. (2001) Phys. Rev. B, 63,064401.

65 Chupakhina, T.I. et al. (2006)Russ. J. Inorg.Chem., 51, 1157.

66 Motohashi, T. et al. (2005) Phys. Rev. B, 71,214424.

67 Gillie, L.J. et al. (2008) Chem. Mater.,20, 6231.

68 Iguchi, E. et al. (1998) J. Solid State Chem.,139, 176.

69 Khomskii,D.I. andKugel, K.I. (2003)Phys.Rev. B, 67, 134401.

70 Khomskii, D.I. andKugel, K.I. (2001) Europhys. Lett.,55, 208.

71 Maignan, A. et al. (2004) Phys. Rev. Lett.,93, 026401.

72 Horigane, K. et al. (2008) Phys. Rev. B, 78,144108.

73 Horigane, K. et al. (2007) J. Phys. Soc. Jpn.,76, 114715.

74 Horigane, K. et al. (2007) J. Magn. Magn.Mater., 310, 774.

75 Tranquada, J. et al. (1995)Nature (London),375, 561.

76 Chen, C.H. et al. (1993) Phys. Rev. Lett.,71, 2461.

77 Tokura, Y. et al. (1998) Phys. Rev. B, 58,R1699.

78 Yamaguchi, S. et al. (1996)Phys. Rev. B, 54,R11022.

79 Mahesh, R. et al. (1996) J. Solid StateChem., 122, 448.

80 Moritomo, Y. et al. (1996)Nature (London),380, 141.

81 Ghosh, S. and Adler, P. (2000) Solid StateCommun., 116, 585.

82 Yao, Q.W. et al. (2007) J. Appl. Phys., 101,09N515.

83 Koshibae, W. et al. (2000) Phys. Rev. B,62, 6869.


Documents

Cobalt Oxides (From Crystal Chemistry to Physics) || Electronic and Magnetic Properties of Ruddlesden-Poepper-Type Cobaltites