J Supercond Nov Magn (2016) 29:361–366DOI 10.1007/s10948-015-3227-y

ORIGINAL PAPER

Temperature Dependence of Coercivity and Magnetizationof Sr1/3Mn1/3Co1/3Fe2O4 Ferrite Nanoparticles

Nadir S. E. Osman1 ·Thomas Moyo1

Received: 3 March 2015 / Accepted: 24 September 2015 / Published online: 2 November 2015© Springer Science+Business Media New York 2015

Abstract Sr1/3Mn1/3Co1/3Fe2O4 ferrite nanoparticles wereobtained by glycol thermal technique. The phase formationwas confirmed by X-ray diffraction. The particle distri-bution and crystallite quality of the nanoparticles wereobserved using transmission electron microscopy Scan-ning electron microscopy was used to monitor the particleshapes and surface morphology. The sample was classi-fied by Barrett–Joyner–Halenda (BJH) measurement as amesoporous material and its surface area was determinedby Brunauer–Emmett–Teller (BET)-surface area measure-ments to be 109.3 m2 g−1. The magnetic properties asa function of temperature were investigated using minicryogen free magnetization measurement system in the tem-perature range of 4 to 300 K and external magnetic field upto 5 T. The magnetization measurements revealed a signifi-cant increase in the coercivity from 0.02 to 1.12 T at 300 and4 K respectively. The coercive fields have been observed tofollow Kneller’s law in the temperature below 200 K. Thesaturation magnetization as a function of the temperaturecan be fitted by the modified Bloch’s law.

Keywords Glycol thermal · Nanoferrite · Mesoporous ·Spin freezing · Remanent magnetization ·Blocking temperature

� Nadir S. E. Osmannadir-sham@hotmail.com

1 School of Chemistry and Physics, Westville Campus,University of KwaZulu-Natal, P/Bag X54001,Durban 4000, South Africa

1 Introduction

Spinel ferrite nanoparticles can be considered to beimportant materials for possible applications for record-ing media, telecommunications, magneto-optic devices,microwave components, high-frequency devices, catalysts,and biomedical applications [1]. The studies of these materi-als tend to concentrate on the optimization of the propertiesby looking for the best substitutions or doping with suit-able elements into either the tetrahedral (A) or octahedral(B) sites of the spinel structure. In normal spinels, diva-lent elements prefer A sites while trivalent elements preferB sites. The resulting properties are known to dependstrongly on elemental compositions, synthesis method androute, particle size distribution, and particle morphologies[2]. The measuring temperature also plays an importantrole in revealing the intrinsic magnetic properties of thecompounds.

A typical spinel ferrite can be specified by the formulaMFe2O4 where M may represent one or more differentmetal ions M1, M2, M3, . . .. Several studies have beenreported in the literature that involve double substitutions oftwo different metal ions (M1 and M2) at equal atomic pro-portions, namely (M1)x(M2)1−xFe2O4 where x = 0.5 [3–5]. These compounds are considered to be at an interestingcomposition midway between (M1)Fe2O4 and (M2)Fe2O4

which can exhibit new and unique properties. In addition,properties for x < 0.5 and x > 0.5 can differ signifi-cantly [3]. Hence, the focused attention on the compositionsat x = 0.5. The common feature of these mixed com-pounds (M1)0.5(M2)0.5Fe2O4 is symmetry with regard toequality in the number of atoms (M1 and M2) that can com-pete for occupation of the tetrahedral and octahedral sites in

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the spinel structure. We have therefore extended these stud-ies to simultaneous substitutions involving three differentmetal ions (M1 = Sr, M2 = Mn and M3 = Co) at theunique composition (M1)1/3(M2)1/3(M3)1/3Fe2O4 whichbelongs to a class of materials that have not yet been widelystudied.

CoFe2O4 ferrites have been widely investigated andfound to exhibit interesting magnetic properties such as highcoercive field and moderate magnetization [6]. Substitutionsby Mn atoms cause the properties to change. Materials withlarger magneto-mechanical quality and higher sensitivity tostress are obtained [7]. In a previous study, simultaneoussubstitution by Mn and Sr was undertaken to produce suc-cessfully Mn0.1 Sr0.2Co0.7Fe2O4 ferrite nanoparticles whichshowed enhanced properties [8]. The aim was to investi-gate the effect of the larger Sr on the Mn-Co ferrite. In thepresent work, we have produced a novel ferrite nanoparticlecompound, Sr1/3Mn1/3Co1/3Fe2O4, which possesses equalstoichiometry with respect to Sr, Mn, and Co substitutionsin the spinel structure. The primary focus is on the magneticproperties induced by the simultaneous substitutions.

2 Experimental Details

The Sr1/3Mn1/3Co1/3Fe2O4 ferrite nanoparticles were pro-duced by glycol thermal technique using a Watlow seriesmodel PARR 4843 stirred pressure reactor The start-ing materials were highpurity SrCl26H2O, MnCl24H2O,CoCl24H2O, and FeCl26H2O. Stoichiometric metal chlo-rides were dissolved in deionized water using a magneticstirrer for 30 min. Drops of NH4OH were added slowly

Fig. 1 X-ray diffraction pattern for the as-preparedSr1/3Mn1/3Co1/3Fe2O4 ferrite nanoparticles

to the mixture until a pH of about 9. The precipitatewas washed using deionized water over Whatman glassmicrofiber filter and AgNO3 standard solution was used toconfirm that all the chloride ions had been removed. Thewet precipitate was reacted in ethylene glycol at 200 ◦Cin a pressure reactor under continuous stirring for 6 h at apressure of about 100 Psi. After the reaction, the mixturewas washed with deionized water and finally by ethanol.The final product dried using a 200 W infra-red lamp.The powder was homogenized using an agate mortar andpestle. The phase and structural characterizations of thesample was obtained by a Phillips X-ray diffractometer typeModel: PANalytical, EMPYREAN using CoKα radiation.The morphology and microstructure of the nanoparticleswere investigated by high-resolution transmission electronmicroscope (HRTEM) (type Jeol JEM-1010) and high-resolution scanning electron microscope (HRSEM) (UltraPlus ZEISS-FEG HRSEM instrument). The synthesized ele-mental compositions were identified by energy-dispersiveX-ray spectroscopy (EDX). The textural and porosity char-acteristics of the nanoparticles were investigated usingmicrometrics TriStar II 3020 instrument using liquid N2 asthe analysis gas at liquid N2 temperature. A mini cryogenfree measurement system was used to perform lowtem-perature magnetization measurements from 4 to 300 K inmagnetic fields of up to 5 Tesla.

3 Results and Discussion

The X-ray diffraction (XRD) peaks are shown in Fig. 1. Thepeaks are indexed and identified as a corresponding to sin-glephase spinel structure. No impurities peaks are observedin the XRD pattern. The highest peak intensity in Fig. 1

Fig. 2 Plot of βcosθVs 4Sinθ for Sr1/3Mn1/3Co1/3Fe2O4 ferritenanoparticles

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Fig. 3 HRSEM images (a) andHRTEM (b) for as-preparedSr1/3Mn1/3Co1/3Fe2O4 ferritenanoparticle

is the (311) peak which was used to calculate the latticeparameter (a) using the Bragg’s equation and a = d(h2 +k2 + l2)1/2 where d is the inter-planar spacing and hkl arethe Miller indices [9]. The average crystalline size was cal-culated using the Scherrer equation D = 0.9λ/(β cos θ),where β is the full width at half maximum of the (311)XRD peak and θ is the Bragg’s angle [10]. The micros-train was determined using the Williamson–Hall plot [11] asshown in Fig. 2. The scattering of points around the linearfitting line in Fig. 2 suggests that the sample has homoge-nous microstrain. The obtained values of lattice param-eter, crystallite size and microstrain for the as-preparedSr1/3Mn1/3Co01/3Fe2O4 are found to be 0.841 ± 0.03 nm,± 9.26 ± 0.02 nm and 0.0014 ± 0.0002 respectively.

The morphology of the Sr1/3Mn1/3Co1/3Fe2O4 fer-rite nanoparticles was investigated using HRSEM andHRTEM. Figure 3a shows that the particles have sphericalshape with some evidence of agglomeration. Typical trans-mission electron microscopy images for the as-preparedSr1/3Mn1/3Co1/3Fe2O4 are shown in Fig. 3b. The particlessize was estimated also from HRTEM image to be about 8 ±

2 nm. The HRTEM image also shows a well-resolved crys-talline structure for the sample under investigation since thelattice fringes appear clearly in Fig. 3b. The HRTEM resultsare in agreement with the XRD results in terms of particlesize and structure. The results of the elemental compositionsobtained by a typical EDX scan are shown in Fig. 4. Thisqualitatively confirms the presence of the expected elementsin the sample.

The texture and porosity characteristic of the as-preparednanoparticle Sr1/3Mn1/3Co1/3Fe2O4 ferrite was determinedby N2 adsorption–desorption isotherm study. The resultsobtained in Fig. 5 indicate that the material can be classi-fied as a mesoporous sample, as shown by the type (IV)hysteresis loop [12]. Figure 5 also shows that the adsorp-tion of nitrogen gas by the sample increased from 30 to180.4 cm3/g when the relative pressure (P/P0) changesfrom 0.03 to 1.0. The pore size range is wide, since thegap between the adsorption and desorption is rather wide.Furthermore, the pores of the sample tend to be empty asshown by the low P /P0 onset point. The surface area ofthe material was determined by Brunauer–Emmett–Teller

Fig. 4 EDX measurement ofthe as-preparedSr1/3Mn1/3Co1/3Fe2O4 sample

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(BET) surface area measurements to be 109.3 m2/g. Thisis expected, since related materials (in terms of structure)are reported to have similar surface area [13]. The Barrett–Joyner–Halenda (BJH) pore size distribution measurementsshow that most of the pore diameters are located below50 nm (see inset in Fig. 5). This is additional evidence ofmesoporous structure for our sample [14].

Figure 6 displays M–H curves as a function of mea-suring temperature in the range of 4 to 300 K. Saturationmagnetizations MS were calculated from the empiricalexpression given in reference [15]. MS increases as the tem-perature decreases. Figure 6 also revealed distortion in theloops (near zero field) which appear to become more promi-nent in the temperature range below 100 K. This distortionin the loops is related to the spin freezing phenomena [8]due to significant time dependence of the remnant magneti-zation. In order to see the development of the coercivity withdecreasing the measuring temperature, we magnified thearea around the origin of the M–H loops as it is seen fromthe inset in Fig. 6. Systematic increases in the coercivityhave been observed as the temperature decrease indicat-ing that the material becomes magnetically harder at lowertemperature. The values of the coercive fields change from0.0200 T at 300 K to 1.12 T at 4 K. The coercive field isa sensitive property to the temperature of a sample. Moremagnetic moments are frozen into anisotropic directions atlower temperature. As the temperature increases the valueof the coercive field decreases [16]. The temperature depen-dency of the coercivity can be explained based on the effectsof thermal fluctuations of the blocked moments across the

Fig. 5 N2 gas adsorption isotherms for as-preparedSr1/3Mn1/3Co1/3Fe2O4 ferrite nanoparticle. The inset shows thepores’ size distribution

Fig. 6 Hysteresis loops of Sr1/3Mn1/3Co1/3Fe2O4 ferrite nanoparti-cles as a function of measuring at various temperatures in the range 4to 300 K in the external magnetic field up to ± 5 T. The inset showsthe magnification around the origin

anisotropy barrier [17] Fig. 7 shows the variation of coer-civity with measuring temperature. The spin freezing effectis dominant at low temperature particularly below blockingtemperature (TB). This can arise due to exchange coupling

Fig. 7 Coercivity temperature dependence for as-preparedSr1/3Mn1/3Co1/3Fe2O4 ferrite nanoparticles. The red line shows thefit curve according to the modified Kneller’s law. The straight red linein the inset shows the T 1/2 dependence

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Fig. 8 Variation of the remanence magnetization (Mr ) with the mea-suring temperature of Sr1/3Mn1/3Co1/3Fe2O4 ferrite nanoparticles.The inset shows the variation of the reduced remanence magnetization(Mr/Ms ) with the measuring temperature

between the core and surface spins. The temperature depen-dence of the coercive field in this case can be expressed interms of the Kneller’s law HC(T ) = HC(0)[1 − (T /TB)α]where HC (0) is the coercivity at T = 0 K, TB is the blockingtemperature, and α is the constant that usually takes thevalue of 0.5 for non-interacting mono-domains in the tem-perature range of T <TB [16]. In the present case, Kneller’slaw appears to fit the experimental data in Fig. 6 well belowabout 200 K. The fit parameters HC(0) and TB to Kneller’slaw in Fig. 7 are 1.40 ± 0.03 T and 223 ± 9 K, respectively.

Fig. 9 Magnetization temperature dependence for the as-preparedSr1/3Mn1/3Co1/3Fe2O4 ferrite nanoparticles. The red line shows thefit curve according to Bloch’s model

Table 1 The fitting parameters based on the modified Bloch’slaw model for the as-prepared Sr1/3Mn1/3Co1/3Fe2O4 ferritenanoparticles

Parameter MS (0) TO β A Tf χ2

±0.1 ±13 ±0.1 ±0.06 ±1

(emu/g) (K) (emu/g) (K)

Value 82.7 672 2 0.96 17 0.9996

The inset in Fig. 7 depicts a linear variation of HC(0)

with T 1/2 as expected with a high correlation coefficient of0.99398.

The variation of the remanent magnetization (Mr ) andthe reduced remanent magnetization (Mr/MS) with temper-ature is shown in Fig. 8. Both Mr and Mr/MS decreaseas the temperature increases indicating that the magneticanisotropy strength reduces at a higher temperature [18].For non-interacting single-domain particles with randomlyoriented easy axes, Mr/MS takes a value of 0.5 for uniax-ial anisotropy and 0.832 for cubic anisotropy according toStoner–Wohlfarth theory [19]. The inset in Fig. 8 shows thatthe values of Mr/MS for our sample exceed 0.5 but is lessthan 0.832. This implies cubic and uniaxial anisotropy.

Figure 9 shows the temperature dependence of the sat-uration magnetization MS(T ) which we attribute to spinwaves excitations. Below the Curie temperature, the mag-netization is usually described in terms of the Bloch’s law:Ms(T ) = Ms(0)[1 − (T /T0)

β ]. The parameter MS(0) isthe saturation magnetization at 0 K, T0 is the temperature atwhich the saturation magnetization would disappear, and β

is the Bloch’s exponent [20]. It is well known for bulk mate-rials that the exponent β takes the value of 3/2. However,due to the finite size effect of the nanoparticles, the magne-tization tends to deviate from pure Bloch’s law [21]. In Fig.9, there is a slight increase in the magnetization below about40 K. This increase is attributed to frozen surface spins [22].The best fit to the data in Fig. 8 is the so-called modifiedBloch’s law expressed as MS(T ) = MS(0)[1 − (T /T0)

β +A exp[−T/Tf ], where A and Tf are experimental param-eters which depend on the nanoparticle size. The resultsextracted from the fit in Fig. 9 are presented in Table 1.

4 Conclusions

The Sr1/3Mn1/3Co1/3Fe2O4 ferrite nanoparticles were suc-cessfully produced by glycol thermal method. The single-phase formation was identified by XRD. The simulta-neous substitution of Sr, Mn, and Co in the form ofSr1/3Mn1/3Co1/3Fe2O4 leads to a slight increase in thelattice parameter from 0.836 ± 0.001 nm (for Mn0.1

Sr0.2Co0.7Fe2O4 [8]) to 0.841 ± 0.003 nm. The BET

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and BJH measurements show a high surface area valueassociated with mesoporous property. Evidence of surfacespin freezing was deduced based on the modified Bloch’slaw for the temperature dependence of the saturation mag-netization. The effect of spin freezing was observed asindicated by some distortion in the magnetic hysteresisloops and increased coercivity at lower temperature. Thetemperature dependence of the coercive fields has beenshown to follow Kneller’s law below about 200 K. We havealso found significant increases in the coercive field at roomtemperature from 0.0045 T (for Mn0.1 Sr0.2Co0.7Fe2O4 [8])to 0.0200 T for the current compound. At 4 K, the coercivefield increases from 0.30 T [8] to 1.12 T. The correspond-ing crystallite sizes from XRD increased from 8.06 ± 0.01nm to 9.26 ± 0.02 nm. Hence, the increase in crystallite sizecan also contribute to the increase in coercive field.

Acknowledgments The authors would like to thank the NationalResearch Foundation (NRF) of South Africa for the equipment grantfor the cryogen free measurement system and Sudan University ofScience and Technology for the study leave (NSEO).

Conflict of interests The authors declare that they have no compet-ing interests.

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