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Intermetallics 19 (2011) 1839e1848

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Intermetallics

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Effect of Co addition on martensitic phase transformation and magneticproperties of Mn50Ni40-xIn10Cox polycrystalline alloys

Zhigang Wu a, Zhuhong Liu b, Hong Yang a, Yinong Liu a,*, Guangheng Wu c

a School of Mechanical and Chemical Engineering, The University of Western Australia, Crawley, WA 6009, AustraliabDepartment of Physics, University of Science and Technology Beijing, Beijing 100083, ChinacBeijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Science, Beijing 100080, China

a r t i c l e i n f o

Article history:Received 19 May 2011Received in revised form22 July 2011Accepted 5 August 2011Available online 28 August 2011

Keywords:A. Magnetic intermetallicsB. Alloy designB. Shape-memory effectsB. Martensitic transformationsB. Magnetic properties

* Corresponding author. Tel.: þ61 8 6488 3132; faxE-mail address: liu@mech.uwa.edu.au (Y. Liu).

0966-9795/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.intermet.2011.08.001

a b s t r a c t

This study investigated the use of Co to enhance the magnetic driving force for inducing the martensitictransformation of Mn50Ni40-xIn10Cox alloys. These alloys present a martensitic transformation froma Hg2CuTi-type austenite to a body centered tetragonal martensite, with a large lattice distortion of 15.7%elongation along the c direction and 8.2% contraction along a and b directions. The martensitic trans-formation temperatures, transformation enthalpy and entropy changes decreased with increasing the Cocontent in these alloys. The maximum magnetization of the austenite increased significantly, whereasthat of the martensite changed much less prominently with increasing the Co substitution for Ni, leadingto increase of the magnetic driving force for the transformation. The magnetization increase of theaustenite is found to be due to (i) formation of ferromagnetically coupled MneMn due to new atomicconfiguration in off-stoichiometric composition, (ii) magnetic moment contribution of Co and (iii)widening of the temperature window for magnetization of the austenite by lowering the temperature ofthe martensitic transformation. These findings clarify the effect of Co addition on martensitic trans-formation and magnetic properties in Mn-rich ferromagnetic shape memory alloys, and provide usefulunderstanding for alloy design for magnetoactuation applications.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Ternary NieMneZ(Z ¼ In,Sn,Sb) alloys have attracted muchattention in the past few years as a new type of ferromagnetic shapememory alloys (FSMAs) since their discovery in 2004 [1]. UnlikeNi2MnGa alloy [2], which relies on magnetic crystallographicanisotropy of the martensite, these NieMneZ(Z ¼ In,Sn,Sb) alloysexhibit a martensitic transformation between a ferromagneticaustenite and a paramagnetic martensite. The different magneticstates between the two phases provide a much greater magneticdriving force, thus the possibility for a magnetic-field-inducedmartensitic transformation. Such transformations are referred toasmetamagnetic transformations in recognition of their concurrentmetallurgical and magnetic changes. The magnetic driving force fora metamagnetic transformation is provided by the Zeeman EnergyEZeeman¼m0DM,H, where m0 is the permeability of a vacuum, DM isthe saturation magnetization difference between the austenite andmartensite and H corresponds to the strength of the applied field.

: þ61 8 6488 1024.

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TheDM between the ferromagnetic austenite and the paramagneticmartensite, as in the case of NieMneZ(Z ¼ In,Sn,Sb), is muchgreater than the DM between the easy and hard magnetizationdirections of the same crystal structure, as in the case of NieMneGaalloys, thus giving possibility for much more powerful magnetic-field-induced martensitic phase transformation and mechanicalactuation.

In Ni2MnZ(Z ¼ In,Sn,Sb) alloys, the net magnetic momentmainly comes from the contribution of Mn [3]. By substituting Mnfor X, DM has been found to increase in Ni2Mn1þxIn1-x alloys but todecrease in Ni2Mn1þxSn1-x alloys [4,5]. At the meantime,increasingMn content also causes rapid increase of themartensitictransformation temperatures, to above the Curie temperature ofthe austenite [6,7]. This results in the transformation beingbetween a paramagnetic austenite to a paramagnetic martensite,thus losing the advantage of large magnetic driving force fortransformation and jeopardizing the possibility for magneticactuation. This limits the range of Mn content feasible in Ni2Mn-X(In,Sn,Sb) alloys.

A new approach is to develop Mn2NiZ(Z ¼ Ga, In,Sn,Sb) alloys.These alloys have the obvious advantage by having more Mn in thematrix, which has the highest magnetization contribution among

Fig. 1. X-ray diffraction spectra of Mn50Ni40-xIn10Cox alloys.

Z. Wu et al. / Intermetallics 19 (2011) 1839e18481840

the three constituents [3]. A magnetic-field-assisted shape changeof w4% has been achieved in single crystalline Mn2NiGa [8].However, the magnetic driving force in this alloy is small due to thelimited magnetization difference (w9 emu/g) between theaustenite and martensite [8,9]. A progress has been made recentlywith off-stoichiometric Mn50Ni40In10 [10,11] and Mn48CoxNi32-xGa20 [12] alloys, which showed a relative large DM of about 40emu/g and 30 emu/g respectively, making these alloys validcandidates for ferromagnetic shape memory actuation. To furtherimprove DM, we have recently reported our study ona Mn50Ni37In10Co3 polycrystalline alloy. This alloy exhibited a largeDM of w89 emu/g and a complete reversible metamagnetictransformation [13]. These limited early findings indicate a possiblesolution to challenge of enhancing magnetic driving force forinducing metamagnetic transformation, a prerequisite formagnetically actuated shape memory alloys.

The saturation magnetization of the alloys depends greatly onthe magnetic moment distribution from Mn atoms. However, thestudy on the magnetic moment distribution of Mn in the off-stoichiometric alloys is much lacking. Very recently, Lazpita et al.proposed a model of magnetic interaction between Mn atoms inthe off-stoichiometric NieMneGa alloys [14]. In their model, theexcess of Mn atoms at Ga sites couple antiferromagnetically withthe Mn at Mn sites when Ni atoms are at their proper sties, whiletheMn at Ga sites couple ferromagnetically with theMn atMn siteswhenMn excess occupies Ni sites. However, the systematic analysison atomic configuration in Mn-rich off-stoichiometricMneNieZ(Z ¼ In,Sn,Sb) alloys is still missing. Moreover, themagnetic interactions between the constituents may rise up toanother level of complexity after Co doping in these ternary alloys,since Co doping in NieMneZ(Ga,Al,In,Sn,Sb) has been found to beeffective in inducing its metamagnetic transformation [15e19].These findings all indicate that Co doping in NieMneZ alloysgreatly enhances correlation of ferromagnetic interaction of theaustenite, resulting in the significantly increased the magneticdriving for metamagnetic transformation. A popular argument isthat when Co enters the NieMneZ Heusler lattice, it has the effectof turning the antiferromagnetically coupled MneMn atoms intoferromagnetically couples ones [12,19]. However, the actualdetailed explanation of the effect of Co doping on the magneticmoments changes of the constituents of the austenite andmartensite is yet established in MneNieZ(Z ¼ In,Sn,Sb) alloys. Inthis study, we further expand our investigation on a series ofMn50Ni40-xIn10Cox alloys, with an emphasis on analyzing themagnetic moment interactions between MneMn and MneCoatoms in our proposed model.

2. Experimental Procedures

Polycrystalline Mn50Ni40�xIn10Cox (x ¼ 0, 1, 2 and 3) alloy ingots(compositions are in atomic percentage) were prepared by meansof arc melting in argon atmosphere using high purity (99.99%)elemental metals. The samples are referred to as Co0, Co1, Co2, andCo3, respectively. The button shaped ingots were heat treated at1173 K for 24 h in vacuum followed by quenching into water forhomogenization. Transformation behaviour of the alloys wasstudied by means of differential scanning calorimetry (DSC) usinga TA Q10 DSC instrument with a cooling/heating rate of 10 K/min.Phase identification and crystal structures were determined bymeans of X-ray powder diffraction using Cu-Ka radiation. Thecompositions were determined by means of X-ray energy disper-sive spectrometry (EDS), which is in a Zeiss 1555 field-emissionscanning electron microscope (FESEM). The magnetic propertieswere studied using a super conducting quantum interferencedevice magnetometer (SQUID).

3. Experimental results

3.1. Crystal structure

Fig. 1 shows XRD spectra of powder samples of Mn50Ni40-xIn10Cox alloys measured at room temperature. The diffractionpeaks of Co0 and Co1 alloys are indexed to body-centered tetrag-onal non-modulated martensite structure, which is also observedin Mn2NiGa alloys [8]. The Co2 alloy has a mixed structure of body-centered cubic austenite and tetragonal martensite. This indicatesthat the addition of 2 at.% of Co lowers the martensitic trans-formation temperatures to below the room temperature. The Co3alloy shows a pure austenite structure with b.c.c. Fundamentallattice reflections of (220), (400), (422) and (440) and superlatticereflections of (111), (200), (311) and (222). The superlattice struc-ture can be determined by comparing the relative intensities of(111) and (200). It is evident that I111/I200 > 1, as shown in the insetof Fig. 1, implying that the superlattice is of the Hg2CuTi-type,consistent with other Mn2NiZ(Z ¼ Ga,Sn,Sb) alloys [20e22].

Fig. 2 shows the effect of Co addition on the lattice parametersand unit cell volumes for the austenite and martensite at roomtemperature. It is seen that the transformation from the cubicaustenite to tetragonal martensite is realized by an expansion in thec direction and equal contractions in the a and b directions (a ¼ b),which is consistent with Mn2NiGa alloy [8]. The lattice distortioncan be estimated to be (cM-aA)/aA ¼ 15.7% along the c direction and(aMeaA)/aA¼�8.2% for the a and b directions for alloy Co2. Both theexpansion in c direction and contractions in a and b directions arelarger than those in Ni2MnGa alloy, which are 8.4% and �6.6%respectively [2]. This large lattice deformation implies higher fric-tional resistance to the propagation of transformation interfaces,leading to large transformation hysteresis. The unit cell volumes ofboth the austenite and martensite are found to slightly increasewith increasing substitution of Co for Ni, obviously related to theslightly larger size of Co atom relative to Ni. It is also evident thatthe transformation from the austenite to martensite is accompa-nied by a volume contraction, of �2.4%. The large volume changemay induce cracking in the material during transformation cycles.

Fig. 3. DSC curves of the martensitic transformation of Mn50Ni40-xIn10Cox alloys.

Fig. 2. Effect of Co addition on lattice parameters and unit cell volume ofMn50Ni40-xIn10Cox alloys.

Z. Wu et al. / Intermetallics 19 (2011) 1839e1848 1841

3.2. Alloy composition

All these alloys show single phase microstructure, confirmed bythe SEM observation. The compositions of these alloys weredetermined by quantitative EDS analysis, as summarized in Table 1.The Mn contents for all four alloys are approximately 49 at.%,indicating a volatilization loss of w1 at.% of Mn during the arcmelting process. The continuous reduction of Ni is compensatedwell by the addition of Co, as the designed nominal compositions.The content of In remained nearly unchanged for all four alloys, atbetween 9.9 at.% and 10.5 at.%. The valence electron concentrationsof the alloys (e/a ratio) are calculated using the compositionsobtained from the EDS analysis based on the sum of s, p andd electrons for Mn (7), Ni (10), Co (9) and In (3). It is seen that the e/a ratio of the alloys decreased from 7.837 to 7.756 with increasingCo substitution for Ni from 0 to 3 at.%, obviously due to the smallernumber of valence electrons of Co (9) relative to that of Ni (10).

Table 2The martensitic and austenitic transformation starting, finishing and peak temper-

3.3. Martensitic transformation

Fig. 3 shows DSC curves of the Mn50Ni40-xIn10Cox alloys. It is seenthat the martensitic transformation behaviour evolves progres-sively, to lower temperatures, with increasing the Co content of thealloys. The transformation thermal parameters, including starting,finishing and peak temperatures (Ms, Mf, Mp, As, Af and Ap) for theforward and reverse transformation, transformation hysteresis(DT¼ApeMp), enthalpy change (DH) and entropy change (DS), of thealloys are summarized in Table 2. DH is obtained directly from the

DSC measurement, and DS is calculated as DS ¼ DHT0

, where

T0 ¼ 12ðMp þ ApÞ.

Fig. 4 shows the effect of Co substitution for Ni on phasetransformation temperatures (Mp and Ap) and transformationhysteresis (DT) of the alloys. It is seen that the transformationtemperatures decreased progressively with increasing the Cocontent. This is in good agreement with the effect of Co doping inNieMneGa [23] and NieMneSb [24] alloys. This is obviously

Table 1Composition and e/a ratio of Mn50Ni40-xIn10Cox alloys.

Mn at.% Co at.% Ni at.% In at.% e/a ratio

x ¼ 0 49.0 e 41.1 9.9 7.837x ¼ 1 48.8 0.9 40.0 10.3 7.806x ¼ 2 49.3 1.9 38.5 10.3 7.781x ¼ 3 49.3 3.0 37.2 10.5 7.756

related to the e/a ratio decrease with the increase of Co substitutionfor Ni. The DT remained practically unchanged, between 20 and26 K for the alloys of different Co content. It is known that thetransformation hysteresis generally corresponds to the frictionalresistance to the martensitic transformation, stemming largelyfrom the lattice mismatch, distortion and volume change of thetransformation. Generally, a larger lattice distortion means themartensitic transformation requires higher energy to overcome thefriction during the motion of the phase boundaries, thus leading tolarger transformation hysteresis. It is seen in Fig. 2 that the latticedistortions and the volume change are practically the same for allthe four alloys, thus resulting in nearly constant transformationhysteresis for the transformation.

Fig. 5 shows the effects of Co addition on the transformationenthalpy (DH) and entropy (DS) changes of the alloys, as functionsof transformation temperature To in (a) and e/a ratio in (b). It is to benoted that for the martensitic transformation both DH and DS arenegative values and the plot customarily neglects this. It is seen thatboth the enthalpy and entropy changes increased continuouslywith increasing To and with e/a ratio, caused by Co addition. Theinfluence of e/a ratio on the entropy change of martensitic trans-formation has been reported for Ni50þxMn25-xGa25 [25], Ni50Mn50-

xInx [7], and Ni50Mn50-xSnx [6] alloys. In these alloy systems, DSincreases with increasing transformation temperatures and e/a ratio, which is in good agreement with the findings of this study.Similar phenomenon has also been observed in Ni50Mn40-xSn10Fexand Ni50Mn37(In,Sb)13 alloys in our previous studies [26,27].

3.4. Thermomagnetization behaviour

Fig. 6 shows the thermomagnetization behaviour of the fouralloys. The sample was first cooled down to 10 K in a zero magneticfield prior to themeasurement. Amagnetic field was applied at 10 K

atures (Ms, Mf, Mp, As, Af, Ap), transformation hysteresis (DT ¼ ApeMp), enthalpychange (DH) and entropy change (DS) of Mn50Ni40-xIn10Cox alloys.

Ms (K) Mf (K) Mp (K) As (K) Af (K) Ap (K) DT (K) DH (J/g) DS(J/K,kg)

x ¼ 0 381 350 373 373 403 396 23 9.7 25.0x ¼ 1 318 302 316 334 348 341 25 6.7 23.4x ¼ 2 262 239 249 257 282 269 20 4.6 17.8x ¼ 3 175 161 169 185 199 195 26 2.9 15.9

Fig. 4. Effect of Co addition on phase transformation peak temperatures (Mp and Ap)and transformation hysteresis (DT).

Z. Wu et al. / Intermetallics 19 (2011) 1839e18481842

and then the measurement was taken upon heating to 395 K ata rate of 10 K/min and cooling back again to 10 K in the same field.Fig. 6(a) shows the M(T) curves of the alloys between 10 and 395 Kin a field of 50 Oe. It is seen that the Co0 alloy showed a milddecrease of magnetization at between 320 and 340 K upon heating,owing to the Curie transition of the alloy. Based on the DSCmeasurement (Fig. 3), the Ms temperature of this alloy is 381 K.However, the M(T) data shows that the hysteresis between theheating and cooling curves prevailed at between 320 and 340 K,

Fig. 5. Effect of Co addition on enthalpy and entropy changes, (a) as function oftransformation temperature To ¼ (MpþAp)/2, and (b) as function of e/a ratio.

Fig. 6. Thermomagnetization behaviour of Mn50Ni40-xIn10Cox alloys under a field of (a)H ¼ 50 Oe and (b) H ¼ 70 kOe.

and continued to present down to 100 K, shown in the inset ofFig. 6(a). This implies that the martensitic transformation is notcomplete and the austenite remains in this alloy at very lowtemperature. Therefore, the Curie temperature corresponds to thatof the remaining austenite at below Ms temperature, denoted TA

C¼ 322 K of Co0.

In the Co1 alloy, the martensite is antiferromagnetic-like at lowtemperatures, as evidenced by the nil magnetization. It is alsoworth noting the cooling curve retraced the heating curve at theentire low temperature regime below the martensitic trans-formation. Normally a splitting phenomenon between the zerofield cooled and field cooled M(T) curves is observed at lowtemperatures in NieMn based alloys [6,7], which indicates thecoexistence of ferromagnetic (FM) and antiferromagnetic (AFM)ordering at the martensitic state. However, the M(T) data of Co1suggests that the FM structure is vanished and the AFM exchange isdominant at the martensitic phase. The AFM martensite startedtransforming to FM austenite at 320 K upon heating, followedimmediately by the Curie transition of the austenite at TA

C ¼ 345 K.Upon cooling, the magnetization of the austenite increased rapidlythrough its Curie transition, followed by a short magnetizationplateau before demagnetization rapidly at 325 K upon furthercooling via the transformation from the FM austenite to AFMmartensite.

The martensitic transformation in Co2 alloy can be clearlyobserved, shown as the magnetization change upon both heating

Fig. 7. The maximum magnetization of the austenite and martensite (MA and MM) andmagnetization difference between the phases (DM) as a function of Co addition.

Z. Wu et al. / Intermetallics 19 (2011) 1839e1848 1843

and cooling, as evidenced by the obvious transformation hysteresis.The Curie transition for the austenite occurred at 378 K. Similar toCo1 alloy, the nil magnetization and superposition ofM(T) curves atbelow themartensitic transformation suggest that themartensite ismainly antiferromagnetic ordered.

Co3 showed clear martensitic transformation as the abruptmagnetization change upon heating and cooling in the temperaturerange between 170 and 195 K. The TA

C of Co3 is determined to be393 K. Unlike Co1 and Co2, the separation between the heating andcooling curves appeared at below 50 K in Co3, suggesting thecoexistence of FM ordering together with AFM exchange at itsmartensitic state. It is seen that the martensitic transformationshifted to lower temperatures whereas the Curie transition shiftedto higher temperatures with increasing Co content in these alloys.The increased TA

C is attributed to the fact that the exchange inter-action between CoeMn is stronger than that between NieMn [12].

Fig. 6(b) shows the M(T) curves of the four alloys between 10and 395 K in a field of 70 kOe. The magnetization of the Co0 alloydid not change much during the heating and cooling cycle, atbetween 12 and 17 emu/g. The minor increase of the magnetizationfrom 12.5 to 15.5 emu/g at around 380 K upon heating correspondsto the partial occurrence of the martensitic transformation. Withincreasing the Co content, it is clear that the martensitic trans-formation shifted to lower temperatures. More notably, for alloysthrough Co1 to Co3, the magnetization of the austenite increasedsteadily with increasing Co content at given temperatures. Forexample, the magnetization increased from 52 to 70 emu/g withthe increase of Co content from 1 to 3 at.% at 350 K, giving rise to anaverage increase of 9 emu/g per 1 at.% Co.

It is also seen that the magnetization behaviour of Co1 and Co2were completely reversible after a complete transformation cycleunder 70 kOe. In contrast, the magnetization loop of alloy Co3 didnot close at 10 K. It is due to the kinetic arrest of the austenite phaseunder the influence of high magnetic field. This effect has also beenobserved in several NieMneIn alloys [28e30]. The magnetizationat the finishing point on the cooling curve comprises of thecontributions of the newly formed martensite and the retainedaustenite. The enhanced magnetization of Co3 at the end of thecooling implies that more austenite has been retained by the highmagnetic field. This is reasonable given the significantly loweredmartensitic transformation temperature, i.e., reduced thermody-namic driving force for the transformation, of this alloy.

3.5. Magnetization

The maximum magnetizations of the austenite and martensite(MA and MM) are taken at Af and As from the heating M(T) curvesunder 70 kOe. The magnetization difference between the austeniteand martensite is obtained from DM ¼ MA�MM. The MA, MM, DM,and TA

C are summarized in Table 3. Fig. 7 shows MA, MM and DM asfunctions of Co content. It is seen thatMA increased greatly with theincrease of Co content, from 15.5 emu/g in Co0 to 118 emu/g in Co3,whileMM

first decreased from 12.5 emu/g in Co0 to 3 emu/g in Co1,and then it increased to 29 emu/g in Co3 alloy. DM shows a steady

Table 3Effect of Co addition on maximum magnetizations of the austenite and martensiteobtained at Af and As (MA and MM), magnetization difference of the transformation(DM), and Curie temperatures of the austenite (TA

C ).

MA MM DM TACCo0 15.5 12.5 3 322Co1 55 3 52 345Co2 78 5 73 378Co3 118 29 89 393

increasing trendwith increasing Co content in the alloys, giving riseto a maximumvalue of 89 emu/g in Co3 alloy. The greatly increasedDM is beneficial for obtaining large magnetic driving force formetamagnetic transformation, i.e. Zeeman Energy, in these alloys.

The giant magnetization difference across the martensitictransformation is obviously due to the distinct magnetic statesbetween the austenite and martensite. To further examine themagnetic configurations in the austenitic and martensitic phases,M(H) curves were carried out at 5 and 350 K for the Co0, Co1, Co2and Co3 alloys, respectively. Fig. 8 shows the magnetization of thealloys at (a) 5 K and (b) 350 K. It is known that all the alloys are atmartensitic state at 5 K based on the thermomagnetizationmeasurements (Fig. 6). Fig. 8(a) shows that Co0 has a relativelyquick magnetizing behaviour at the beginning of M(H) curve,indicating the existence of FM ordering at its martensitic state.However, based on the form of M(H) curve and the low magneti-zation of 15 emu/g at 50 kOe, the AFM exchange is expected tocoexist with FM structure at 5 K Co3 alloy shows a similar M(H)behaviour, but with stronger magnetic correlations than that inCo0, evidenced by the higher magnetization of 26 emu/g at 50 kOe.The M(H) curves of Co1 and Co2 are nearly linear, particularly forCo1, strongly suggesting the existence of long-range AFM orderingat the martensitic state of these alloys.

Fig. 8(b) shows the magnetization behaviour of the austeniticphase at 350 K of Co1, Co2 and Co3 alloys. The absence ofM(H) dataof Co0 is due to its martensitic state at 350 K, which is irrelative forcomparison with other alloys at the austenitic state. The austeniteof Co1 showed a gradual magnetization growth and maximized at42 emu/g upon magnetizing. It is known that the TA

C (345 K) of Co1is very close to the magnetizing temperature (350 K), therefore, theshape of the M(H) data indicates the existence of magnetic short-range correlations in the paramagnetic austenitic state. Co2 andCo3 presented very typical FM behaviour due to the rapid increaseof magnetization at the initial portion of M(H) curves and the highmagnetization magnitude of 59 and 70 emu/g, respectively.

4. Discussion

4.1. Entropy change

It is seen in Fig. 5 that the value of the entropy change of thetransformation decreased significantly with Co doping, by 36%reduction with addition of 3 at.% Co. Entropy change (DS) of

Fig. 9. Illustration of the effect of Co addition on entropy change of the alloys.

Fig. 8. Magnetization curves at (a) 5 K and (b) 350 K of Mn50Ni40-xIn10Cox alloys.

Z. Wu et al. / Intermetallics 19 (2011) 1839e18481844

a martensitic phase transformation is a measure of the difference ofdegree of order between the austenite and martensite. In additionto being a function of temperature itself, DS is generally consideredto have three contributions, including crystal structural ordering(DSlatt), magnetic structure ordering (DSmag) and electronic struc-ture ordering (DSel). In Ni2þxMn1-xGa and X2MnSn (X ¼ Co, Ni, Pd,Cu) alloys, it has been shown that the electronic contribution DSel toDS is small [25,31]. The crystal structural contribution DSlatt to DSdepends on the crystal structures of the transformation. For thepresent four alloys, the structural change is the same and themagnitudes of lattice distortions of the transformation are similaraccording to the XRD measurements. Thus, the crystal structuralcontribution to the entropy change is also expected to beunchanged for these alloys. In this regard, the increase of totalentropy change DS with Co content is attributed to the magneticordering contribution, neglecting the temperature effect.

Fig. 9 shows a schematic of the contributions of DSlatt, DSmag toDS as functions of Co content in the alloys. It is known thatSAlatt > SMlatt , thus the DSlatt ¼ SMlatt � SAlatt < 0 for the forward A/Mtransformation. In the Figure DSlatt remains a constant negativevalue irrespective of Co content. On the other hand, it is known thatSAmag < SMmag , thus DSmag ¼ SMmag � SAmag > 0. This is because of thehigher magnetic ordering in the austenite relative to themartensite. The introduction of Co enhances the FM ordering of theaustenite [13,16,19], thus decreasing the magnetic entropy of the

austenite. Meanwhile, the magnetic interaction between the atomicconstituents in the martensite is not affected too much, evidencedby the minor magnetization change of the martensite relative tothose of the austenite (Fig. 8). This results in positive increase ofmagnetic entropy change of the transformation with increasing Cocontent. Consequently, the total entropy change for the A/Mtransformation becomes less negative with increasing Co content,as observed in Fig. 5.

4.2. Magnetic moment interactions

4.2.1. Structure of stoichiometric Mn2NiInThe crystal structure of the austenite is Hg2CuTi-type super-

lattice cubic structure. This structure is commonly observed in Mn-rich Heusler alloys, such as Mn2NiGa [20] and Mn2CoZ(Z ¼ Al,Ga,Ge,In,Sn,Sb) [32]. In this structure, Mn atoms occupy A (0,0,0)site and B (1/4,1/4,1/4) site, leaving C (1/2,1/2,1/2) site to Ni atomsand D (3/4,3/4,3/4) site to the third element atoms. This structure isillustrated in Fig. 10, showing the unit cell models for both theaustenite in (a) and martensite in (b) of a stoichiometric Mn2NiInalloy. Such structure can be expressed in a stacking order of Mn-Mn-Ni-X (F43m space group) along the diagonal direction of theunit cell.

Based on the calculation by Chakrabarti et al. [9], the spinmagnetic moments of Mn(A), Mn(B) and Ni in Mn2NiIn alloyare �3.08, 3.42 and 0.13 mB, respectively. The magnetic moment ofIn is very small and is neglected in the present discussion. The spinmagnetic moments of Mn atoms are symbolically depicted usingarrows on the atoms shown in Fig. 10. The length of the arrowsroughly represents the magnitude of the magnetic moment of theatom. The exchange interaction between Mn atoms is known todepend strongly on MneMn distance in the lattice. Early studieshave shown that the magnetic interaction between Mn atomschanges fromAFM to FMwhen theMneMn distance is increased toabove a critical value of approximately 0.30 nm [33]. For theaustenite with Mn-Mn-Ni-In stacking order in the unit cell(Fig. 10(a)), the distance between the nearest neighboring Mn at Asite and B site is AB ¼

ffiffiffi

3p

a=4 ¼ 0:2604 nm and the distancebetween the second nearest neighboringMn at two adjacent A sitesis AA ¼

ffiffiffi

2p

a=2 ¼ 0:4251 nm using the lattice constant ofa ¼ 0.6013 nm for the Co3 alloy. This implies that the moments ofMn(A)-Mn(B) form antiparrallel coupling and Mn(A)eMn(A) formparallel coupling. It is seen that the spin directions of Mn magneticmoments at A site are antiparrallel with those of Ni at C site, andthey are also opposed to those of Mn at B site. This forms a ferri-magnetic structure in this atomic configuration, with antiparrallelaligned magnetic moments between (A,C) and (B,D) sub-lattices.

Assuming that the stoichiometric Mn2NiIn alloy also undergoesthe same structural transformation to a tetragonal martensite as for

Fig. 10. Atomic configuration in the unit cell of Mn2NiIn alloy: (a) unit cell of the austenite with Mn-Mn-Ni-In stacking order (Hg2CuTi structure) and (b) unit cell of the martensitewith Mn-Mn-Ni-In tetragonal structure.

Z. Wu et al. / Intermetallics 19 (2011) 1839e1848 1845

the present alloys, the distance between A site and B site changesvery little by the transformation, at AB ¼

ffiffiffi

3p

a=4 ¼ 0:2612 nm inthe martensite, and the distance between two adjacent A sites isshortened to AA ¼

ffiffiffi

2p

a=2 ¼ 0:3899 nm as shown in Fig. 10(b).Therefore, Mn(A)-Mn(B) still forms AFM interaction and Mn(A)eMn(A) forms FM interaction in the martensite. This implies that themagnetic exchange interactions in the martensite are similar withthose in the austenite, which is ferrimagnetic.

4.2.2. Atomic configuration in off-stoichiometric Mn2Ni1þxIn1-xIn Ni-rich off-stoichiometric Mn2Ni1þxIn1�x alloys, the magne-

tization has been found to increase greatly relative to its motheralloyMn2NiIn, as evidenced by themagnetization ofw75 emu/g (at230 K) in Mn50Ni40In10 [10] comparing to 9.27 emu/g in Mn2NiIn[9]. The drastic increase of the magnetization cannot be solelyattributed to the magnetic moment contribution from the “extra”Ni which substitutes for In, as the magnetic moment of Ni is small,typically w0.13 mB. This suggests that the increase may originatefrom the biggest magnetic moment contributor Mn atoms. Thisimplies that the atomic configuration must have changed after theNi substitution for In.

Fig. 11 shows the atomic configuration in the unit cell of theaustenite in (a) and the martensite in (b) of the Co0, with thenominal composition of Mn50Ni40In10. It is seen that some portionof Mn atoms at A site have been replaced by Ni atoms, and thesenew Mn atoms share D site with In atoms. This hypothesis is basedon the rule of preferential site occupation in Mn2YZ (Y: 3delements; Z: IIIeV A group elements) alloys reported by Liu et al.[32]. They observed that Y elements on the right hand side of Mn inthe Periodic Table of Elements prefer to occupy (A,C) sites, whereasY elements to the left of Mn have strong preference for B siteoccupancy. In Mn2YZ (Y ¼ V, Cr, Mn, Fe, Co and Ni; Z ¼ Al, Ga, In, Si,Ge, Sn and Sb) Heusler alloys, this rule of atomic occupancy hasbeen shown to be well obeyed [22,32,34e36]. According to thisprinciple, Ni substitution for In will have the priority to take A sitein preference to Mn. The distance between the new Mn atoms at Dsite andMn atoms at B site is 0.3007 nm, which favors FM exchangeinteraction between the Mn atoms. Therefore, the magnitude of

AFM alignment between Mn(A) and Mn(B) is reduced, and the newMn atoms at D site form FM interactionwith the Mn atoms at B site.

In Fig. 11(a), the spin magnetic moments of the newMn(D) alignparallel with those of Mn(B). At the meantime, the new (A,C) sub-lattice after the replacement of Ni for Mn at A site also forms FMinteraction with Mn(B) and Mn(D), thus creating a local FM struc-ture of Mn(B)-Ni-Mn(D) in the alloy. This explains the increase ofthe magnetization of the austenite in off-stoichiometricMn2Ni1þxIn1-x alloy after Ni substitution for In compared to thestoichiometric Mn2NiIn alloy.

In the martensite unit cell (Fig. 11(b)), the distance changebetween the nearest Mn(A)eMn(B) atoms is very small, from0.2604 to 0.2612 nm, which suggests that the coupling betweenMn(A)eMn(B) does not change after the transformation, stillshowing AFM interaction between them. However, along the a andb directions in the tetragonal unit cell, the distance between A andC sites (same for B and D sites) is shortened to AC¼ BD¼ 0.2758 nm,which strongly favours AFM coupling between Mn(B)eMn(D)[37,38]. This explains the presence of AFM interaction at themartensitic phase obtained from the M(H) data shown in Fig. 8 (a).In this regard, it is reasonable to attribute the disappearance of thelocal FM structure of Mn(B)-Ni-Mn(D) in the martensite to thesignificant decrease of the Mn(B)eMn(D) distance.

4.2.3. Magnetic moment contribution from CoFig. 12 shows the atomic configuration in the unit cells for the

austenite (a) and the martensite (b) after Co doping in off-stoichiometric Mn50Ni40In10 alloys. Followed by the rule of selec-tivity of atomic configuration in Heusler alloys as aforementioned,Co substitution for Ni should just replace Ni at either A site or C site,and no new atomic configuration is formed. By assuming that Coatoms replace the Ni atoms at A site, a stronger local FM structure ofMn(B)-Co-Mn(D) will be formed, shown in Fig. 12(a). Each Co atomat the site of Ni contributes a larger magnetic moment (w1.2 mB)relative to Ni (0.13 mB), based on the calculation of magneticmoments in Mn2NiCoxGa1-x alloys [37]. This results in the furtherincrement of the magnetization of the austenite due to themagnetic moment contribution from Co.

Fig. 11. Atomic configurations in the unit cell of Co0 alloy (Mn50Ni40In10): (a) unit cell of the austenite and (b) unit cell of the martensite. The number of displaced atoms does notrepresent the actual proportion of substitution, which is only for qualitative interpretation.

Z. Wu et al. / Intermetallics 19 (2011) 1839e18481846

4.2.4. Maximum magnetization of ferromagnetic austeniteIt is evident in Fig. 6(b) that the magnetization of the FM

austenite increased progressively with decreasing temperature.The temperature window for FM austenite is limited by twotemperatures, the Curie temperature of the austenite (TA

C ) as theupper boundary and the (reverse) martensitic transformationtemperature (Af) as the lower boundary. The maximum magneti-zation of the austenite (MA) is achieved at Af (upon heating), asseen on the M(T) curves (Fig. 6(b)) of these alloys. It is also seen inFig. 3 that the transformation temperatures decreased significantlywith increasing Co content. This means that the MA values wereactually taken at different temperatures for these alloys and the

Fig. 12. Atomic configurations in the unit cell of Co doped Mn50CoxNi40-xIn10 (x � 1): (a) unitdoes not represent the actual proportion of substitution, which is only for qualitative inter

increase of MA with increasing Co content, shown in Fig. 7 is reallydue to the widening of the temperature window of the FMaustenite, instead of purely due to the effect of alloying. The effectof alloying may be estimated by measuring the magnetization ofthe alloys at a given temperature. As aforementioned, themagnetization at 350 K increased from 52 emu/g for Co1 to 70emu/g for Co3. On the other hand, the maximum magnetization atAf increased from 55 emu/g for Co1 to 118 emu/g for Co3, corre-sponding to the magnetization increase of 31.5 emu/g per at.% of Coaddition. It is evident that widening of the temperature window isthe more prominent factor relative to Co alloying contributing tothe large DM in these alloys.

cell of the austenite and (b) unit cell of the martensite. The number of displaced atomspretation.

Fig. 13. Effect of e/a ratio on Mp, Ap and TAC temperatures of Mn50Ni40-xIn10Cox alloys.

Z. Wu et al. / Intermetallics 19 (2011) 1839e1848 1847

4.3. Transformation diagram

Fig. 13 shows the effect of e/a ratio, as a result of Co doping, on themartensitic transformation temperatures Mp and Ap obtained fromthe DSC measurement and on the Curie transition temperature TACobtained from the magnetization measurements of the Mn50Ni40-xIn10Cox alloys. It is seen that Mp and Ap decreased linearly withdecreasing e/a ratio and TAC increased. This observation is consistentwith the general observation of positive dependence of martensitictransformation temperatures on e/a ratio reported in the literature forNieMneZ(Z ¼ Ga, In,Sn,Sb) alloy systems [6,7,39]. The linear coeffi-cient is estimated to be 25 K per 0.01 change of e/a ratio, which iscomparable to thevaluedeterminedforNi50Mn40-xSn10Fexalloys [26].

The temperature-e/a ratio space shown in Fig. 13 can be tenta-tively divided into three regions representing different crystallo-graphic and magnetic states for the alloys, including austenite(paramagnetic (PM)), austenite (FM) and martensite (AFM). Amongthe three states, two transformation schemes may occur. In theregion to the right of point A, the alloy undergoes a single steptransformation between PM austenite and AFM martensite. Thistransformation is of low interest for magnetic actuation. To the leftof point A, the alloy undergoes the transformation sequence fromaustenite (PM) to austenite (FM) and then to martensite (AFM)upon cooling, expressed as AðPMÞ4AðFMÞ5MðAFMÞ. In thisexpression, the single arrow representsmagnetic transition and thedouble arrow represents the martensitic transformation (in thiscase it is also a concurrent magnetic transition). The phase area ofthe FM austenite opens up with the decrease of e/a ratio, thusproviding a wider temperature window for AðFMÞ5MðAFMÞtransformation.

5. Conclusions

The effects of Co substitution for Ni on the martensitic trans-formation and magnetic behaviour of Mn50Ni40-xIn10Cox alloyswere investigated. The experimental evidences and the discussionslead to the following conclusions:

(1) Co substitution for Ni up to 3 at.% greatly decreases themartensitic transformation temperature from 381 K to 175 K inthese alloys. The martensite has a non-modulated tetragonalstructure, and the crystal structure of the austenite is deter-mined to be Hg2CuTi-type superlattice cubic structure.

(2) The decrease of the phase transformation temperatures isattributed to the decrease of the e/a ratio for the alloys withincreasing Co substitution for Ni. The enthalpy and entropychanges of the transformation are both found to increase withincreasing the e/a ratio of the alloys.

(3) Themaximummagnetization of the austenite (under 70 kOe) issignificantly increased from 15.5 emu/g in the Co0 alloy to 118emu/g in the Co3 alloy, whereas that of the martensite showsmuch less significant change from 12.5 emu/g in the Co0 alloyto 29 emu/g in the Co3 alloy. Consequently, magnetizationdifference between the austenite and the martensite increasessignificantly with increasing Co substitution for Ni. The largestDM for the martensitic transformation obtained is 89 emu/g inalloy Co3.

(4) The increased magnetization of the austenite is attributed tothree reasons: (i) formation of FM structure of Mn(B)-Ni-Mn(D)in off-stoichiometric Mn2Ni1þxIn1-x, due to the readjustment ofatomic configuration in the unit cell caused by Ni substitutionfor In, (ii) higher magnetic moment contribution of Co relativeto Ni, and (iii) widening of the temperature window for FMaustenite.

(5) The lowmagnetization of the martensite, relative to that of theaustenite, is due to the significantly shortened distancebetween Mn(B)eMn(D), which leads to the disappearance ofthe local FM structure in a tetragonal martensitic structure.

Acknowledgments

The authors wish to acknowledge the financial supports by theDepartment of Innovation Industry, Science and Research of theAustralian Government in ISL Grant CH070136, and by NationalNatural Science Foundation of China in Grant No. 51001010.

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