Effect of Fe-substitution on microstructure, hysteresis behaviour and magnetocaloric effect in...

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Journal of Magnetism and Magnetic Materials 321 (2009) 1300–1305

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Journal of Magnetism and Magnetic Materials

0304-88

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/jmmm

Effect of Fe-substitution on microstructure, hysteresis behaviour andmagnetocaloric effect in Gd5Si2Ge2 Alloys

D.M. Raj Kumar, M. Manivel Raja, R. Gopalan �, A. Sambasiva Rao, V. Chandrasekaran

Defence Metallurgical Research Laboratory, Hyderabad 500 058, India

a r t i c l e i n f o

Article history:

Received 25 June 2008

Received in revised form

5 November 2008Available online 3 December 2008

Keywords:

Gd–Si–Ge–Fe alloys

Microstructure

Structural and magnetic transitions

Magnetocaloric effect

53/$ - see front matter & 2008 Elsevier B.V. A

016/j.jmmm.2008.11.094

esponding author. Tel./fax: 9140 2434 0884.

ail address: [email protected] (R. Gopalan)

a b s t r a c t

Effect of Fe-substitution on the phase formation, partitioning behaviour of Fe in the co-existing phases,

magneto-structural transition, magnetic entropy change and associated hysteresis losses has been

investigated in Gd5Si2Ge2 alloy. The virgin alloy crystallizes in monoclinic Gd5Si2Ge2-type phase, while

Fe-substituted alloys form mixed monoclinic Gd5Si2Ge2-type and orthorhombic Gd5Si4-type phases.

Electron probe microanalysis reveals that Fe does not dissolve in the matrix, but influences the

magneto-structural transitions. Magneto-structural characterization of the Fe-containing alloys reveals

that the Fe-substitution suppresses the structural transition observed at 273 K in virgin alloy. A

maximum magnetic entropy change, DSM of 6.5 J/kg-K at 273 K was observed for a field change of 2 T in

Gd5Si2Ge2 alloy. The Fe-substituted alloys exhibit lower value of DSM but with reduced hysteresis losses.

& 2008 Elsevier B.V. All rights reserved.

1. Introduction

Recently a giant magnetocaloric effect (GMCE) has beenreported in intermetallic Gd5Si2Ge2 compound near-room tem-perature [1]. This has been attributed to a magnetic field-inducedfirst-order structural transition from the high-temperature mono-

clinic paramagnetic phase (P1121/a) to the low-temperatureorthorhombic ferromagnetic phase (Pnma) upon cooling [2–6].These compounds offer great technological promise in providingan eco-friendly and efficient magnetic refrigeration [7–9].Although the Gd5(SixGe1�x)4 alloys with xp0.5 provide aremarkably wide temperature range where GMCE can be preciselytuned from �30 K (x ¼ 0) to�276 K (x ¼ 0.5) just by changing Si toGe ratio, for some applications it is desirable that the GMCE occursclose to room temperature and even above. Pecharsky et al.studied the effect of alloying additions in Gd–Si–Ge alloys in orderto enhance the transition temperature at which GMCE occurs [10].In addition to the GMCE, large thermal and magnetic hysteresishave also been reported in Gd5(SixGe1�x)4 system [11,12]. Theselosses are attributed to the first-order structural/magnetic transi-tions during increasing and decreasing temperature/magneticfield. It is important to consider the hysteresis effect whileassessing the usefulness of a material as a refrigerant, since itaffects the overall efficiency of the magnetic refrigeration. Effortsto reduce the hysteresis losses in Gd–Si–Ge alloys have alsoresulted in minor alloying additions which can profoundly affectthe hysteresis behaviour and also modify the associated MCE

ll rights reserved.

.

properties. It has been reported that with the addition of about1 at% Fe, the magnetic hysteresis losses of Gd5Si2Ge2 alloy can besignificantly reduced (�90%) without decreasing its refrigerantcapacity [13].

Recently, the phase formation and thermal hysteresis inGd5Si2Ge2 alloy with substitution of transition elements havebeen reported [14–16]. The addition of Sn and Pb in Gd5Si2Ge2

alloys has been found to enhance the MCE [17,18]. However thetemperatures at which the magnetic entropy change peaks up inSn- and Pb-doped alloys are 277 and 274 K, respectively. In thecase of Fe/Ga substitution, the temperature could be enhancedfurther for room temperature MCE applications [10]. Although,the magnetocaloric properties of these alloys have been reported,not much work has been directed towards metallurgical under-standing and its correlation to MCE. We have earlier reported sucha metallurgical study in Ga substituted Gd5Si2Ge2 alloys [19]. Inthe present study we have processed series of Gd5Si2Ge2�xFex

alloys with varying Fe concentration and investigated thestructure-magnetic property correlation. The partitioning beha-viour of Fe in the co-existing phases, the microstructure of thealloys and the associated magnetic properties along with thethermal hysteresis behaviour are discussed in detail.

2. Experimental details

Elemental raw materials, Gd (99.9 wt%), Si (99.999 wt%), Ge(99.999 wt%) and Fe (99.9 wt%), were used for arc-melting ofGd5Si2Ge2�xFex alloys with x ¼ 0, 0.05, 0.10 and 0.15 under argonatmosphere. Each alloy was melted and re-melted four times in

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Table 1Compositional analysis of the phases in Gd5Si2Ge2�x Fex annealed alloys using

EPMA.

Phase ID Composition (at%) Phase identified Phase type

x Gd Si Ge Fe

0 Matrix 55.35 22.84 21.81 0 Gd5Si2.03Ge1.97 5: 4-type

0.05 Matrix 55.47 23.58 20.87 0.08a Gd5Si2.13Ge1.88 5: 4-type

Bright 62.00 17.96 19.93 0.11a Gd5Si1.38Ge1.62 5: 3-type

Dark 44.37 29.44 12.93 13.26 Gd5Si3.32(Ge,Fe)2.95 Fe–Si rich

0.10 Matrix 55.63 22.63 21.53 0.21a Gd5Si2.05Ge1.94 5: 4-type



0.15 Matrix 56.10 22.08 21.72 0.10a Gd5Si1.97 Ge1.95 5: 4-type



a Values are below the detection limit.

Fig. 1. X-ray powder diffraction patterns of Gd5Si2Ge2�xFex alloys; (a) x ¼ 0, (b)

x ¼ 0.05, (c) x ¼ 0.10 and (d) x ¼ 0.15.

D.M. Raj Kumar et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 1300–1305 1301

order to ensure the homogeneity. The as-cast alloys weresubsequently annealed at 1573 K for 1 h in argon atmospherefollowed by gas quenching to room temperature. The chemicalcomposition of the alloys was determined by the wet-chemicalmethod (complexo-metric titration) and instrumental techniquesusing characteristic emission/absorption spectrum. The inductivecoupled plasma-optical emission spectroscopy (ICP-OES) and theatomic absorption spectroscopy (AAS) techniques were used foranalyzing Ge and Fe, while wet-chemical analysis was used for Gdand Si (Table 1). Crystal structure/phase identification was carriedout by X-ray diffraction using a PHILIPS X-ray diffractometer withgraphite monochromator and Cu–Ka (l ¼ 1.54058 A) radiation.The microstructure of the alloys was investigated using Leo440iscanning electron microscope (SEM). The composition of theconstituent phases was determined by electron probe microana-lysis (EPMA) using a CAMECA-SX100 instrument.

Thermo-magnetic characterizations were carried out usingan ac magnetic susceptometer (CryoBIND) in the temperaturerange 77–350 K. The ac susceptibility (wac) measurements weredone with a driving field of 170 mOe at a frequency of 430 Hz.The magnetic and structural transitions were investigated usingmodulated differential scanning calorimeter (MDSC, TA Instru-ment’s Q100 Model). The magnetization isotherms used forevaluation of the magnetic entropy change DSM, were measuredusing vibrating sample magnetometer (VSM, ADE make EV9Model). The measurements were carried out for both increasingand decreasing fields in the vicinity of the magnetic phasetransition temperatures in magnetic field varying from 0–2 T. Thetemperature dependence of the magnetic entropy change DSM

was calculated for the set of isotherms with increasing magneticfield.

3. Results and discussion

3.1. Structure and microstructure analysis

The X-ray powder diffraction analysis (Fig. 1) was carried outfor all the alloys and it revealed that the virgin Gd5Si2Ge2 alloyis mainly composed of monoclinic Gd5Si2Ge2-type phase. Sincethe monoclinic Gd5Si2Ge2-type and the orthorhombic Gd5Si4-typephases have overlapping peaks, it is important to identify the

Fig. 2. SEM back scattered electron images of Gd5Si2Ge2�xFex alloys; (a) x ¼ 0, (b)

x ¼ 0.05,(c) x ¼ 0.1 and (d) x ¼ 0.15. [Matrix phase is 5:4-type, Bright phase is 5:3-

type and Dark phase is FeSi-rich].

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D.M. Raj Kumar et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 1300–13051302

phases clearly by analyzing with the standard PDF profiles of thephases given in Fig. 1. The XRD peaks at �30.51 and �31.21 whichcorrespond to (2 3 0) and (2 31) reflections of the monoclinicGd5Si2Ge2-type phase and the absence of peak at 301 correspond-ing to the orthorhombic Gd5Si4-type phase confirm the formationof monoclinic phase. It is interesting to observe from Fig. 1 thatthe Fe-substituted alloys have signature peaks corresponding tothe orthorhombic Gd5Si4-type phase, along with the monoclinicphase peaks indicating the presence of both the Gd5Si2Ge2-typeand Gd5Si4-type mixed phases. From XRD analysis, it appears thatthe presence of Fe results in the formation of the mixed matrixphases. These results are very much similar to that reported inGd5Si2Ge1.9Fe0.1 alloy [13]. The presence of two signature peaks at�35.21 and 36.61 corresponding to a hexagonal Gd5Ge3-typephase observed in the Fe-containing alloys reveal that the alloyshave some detectable fraction of the hexagonal Gd5Ge3-type (5:3)minor phase. These peaks overlap with those of the monoclinicGd5Si2Ge2-type phase. However, from the intensity of these peaksobserved in the x ¼ 0 alloy it can be said that the virgin alloyforms monoclinic Gd5Si2Ge2-type matrix phase along with sometraces of 5:3 minor phase. In order to determine microchemistry

Fig. 3. Elemental mapping of Gd5Si2Ge1.85Fe0.15 alloy using EPMA.

of these phases, we have carried out detailed microstructuralstudies.

The backscattered scanning electron micrographs (BSE) of theGd5Si2Ge2�xFex alloys are shown in Fig. 2. The microstructures ofthe Fe-substituted alloys display the presence of 5:4-type matrix(monoclinic Gd5Si2Ge2+orthorhombic Gd5(Si,Ge)4 mixed phases)along with traces of 5:3-type and Fe-rich minor phases whereasthe virgin Gd5Si2Ge2 alloy, displays the presence of 5:4-type(monoclinic Gd5Si2Ge2) matrix. The presence of 5:3-type minorphase is not clearly seen in this alloy. The composition of theconstituent phases was determined by EPMA and the results aregiven in Table 1. The analysis reveals that in Fe-containing alloys,Fe is absent in the matrix as well as in 5:3 secondary phases; butis present as a FeSi-rich phase. The fraction of the minor phaseswas found to increase with increase in Fe concentration.

In order to identify the distribution/partitioning characteristicsof the elements among various phases, we also carried out theelemental mapping using EPMA. Elemental mapping of theGd5Si2Ge1.85Fe0.15 alloy (Fig. 3) reveals that Gd is almost uniformlypresent except that the fraction is slightly more in the 5:3 phaseand significantly less in the Fe-rich phase with respect to thematrix phase. The Si content is rich in the Fe-rich phase while it islow in the matrix phase (5:4). The distribution of Ge is found to beuniform in the matrix phase but considerably low in the Fe-rich

Inte

nsity

(a.u

)

0 5 10 15 20 25 30Length (µm)

Gd

Si

Ge

Fe

Fig. 4. Line profile compositional analysis of Gd5Si2Ge1.85Fe0.15 alloy using EPMA.

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phase. It is interesting to note that Fe is absent in the matrix andthe 5:3 secondary phase and segregates to the grain boundaries ofthe matrix phase. Earlier report predicted that the major effect ofthe iron addition is to combine primarily with the silicon resultingin a matrix phase that is depleted in silicon compared to the singlephase of the alloy without iron [13]. The higher concentration

Fig. 5. DSC thermograms of Gd5Si2Ge2�xFex alloys with x ¼ 0, 0.05, 0.10 and 0.15;

Heat flow is upward direction.

Fig. 6. Ac susceptibility curves of Gd5Si2Ge2�xFex alloys with x ¼ 0, 0.05, 0.10 and 0.15. T

respectively, upon heating and cooling.

of Gd, together with the deficiency of Si in the matrix phase,could play a role in the different magnetic behaviour of theFe-containing alloys and hence the MCE characteristics.

The composition of the FeSi-rich phase that exists at the grainboundary was estimated to be Gd5Si3.2Ge1.5Fe1.8. The composi-tional line profile of the elements across different phases of thex ¼ 0.15 alloy is in good agreement with the elemental mapping.One can clearly observe the presence of Fe in the phase havingdark contrast in the BSE micrograph (Fig. 4) and its absence in thematrix and 5:3 phases. Si is present in all the phases, but thefraction is significantly higher in the Fe-rich phase and lower inthe 5:3 phase than that in the matrix phase. The partitioning of Gdand Ge among various phases is much similar except that thefraction of Gd is slightly more and that of Ge is slightly less in the5:3 phase compared to that in the matrix phase. We furtherinvestigated the magneto-structural transformation behaviour ofthese alloys using MDSC and VSM.

3.2. Structural and magnetic transitions

Fig. 5 shows the DSC thermograms upon heating and coolingrevealing the characteristics of structural/magnetic transitionsof the Gd5Si2Ge2�xFex alloys. The thermogram of the x ¼ 0 alloyupon heating, displays a peak (endothermic) at 276 K associatedwith a simultaneous structural and magnetic transition. Thechange of slope at 298 K is the signature of a magnetic transition.The alloy shows a thermal hysteresis of �10 K for the structuraltransition upon cooling. However, the Fe-substituted alloys display

he inset shown for x ¼ 0 and x ¼ 0.1 are the regions from 260–325 and 275–345 K,

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D.M. Raj Kumar et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 1300–13051304

only changes in slope which reveals the presence of only magnetictransition. The change in slope observed in the DSC thermogramsupon heating and cooling of the Gd5Si2Ge2�xFex alloy sampleswith negligible thermal hysteresis of 1–2 K can be attributed to thesecond-order ferromagnetic to paramagnetic transition (Tc) in thetemperature range 298–302 K. The absence of a structural transi-tion in the Fe-substituted alloys suggests that the presence of Fechanges the distributions of Si and Ge in the matrix due to theformation of the hexagonal 5:3 and FeSi-rich phases.

The real part of the ac susceptibility (wac) of the Gd5Si2Ge2�xFex

alloys upon heating and cooling are shown in Fig. 6. The Gd5Si2Ge2

alloy shows two magnetic transitions, a first-order transition (Ts)at 273 K upon heating which reverses at 268 K upon cooling(shown in inset) and a second-order transition (Tc) at 303 K withno hysteresis. The x ¼ 0.05 alloy shows only second order at 303 Kand first-order transition could not be traced which is due to thedeviation in the composition of the alloy to the Si-rich side asrevealed from EPMA (Table 1). The x ¼ 0.1 and 0.15 alloys show aless pronounced first-order transition (Ts) at 284 and 290 K,respectively, with negligible hysteresis upon heating and cooling

Fig. 7. Magnetization vs field curves of (a) Gd5Si2Ge2 compound in the

temperature range 258–308 K and (b) Gd5Si2Ge1.9Fe0.1 alloy in the temperature

range 278–328 K. The curves qualitatively illustrate the large hysteresis losses of

the Gd5Si2Ge2 compound and relatively smaller values of the Gd5Si2Ge1.9Fe0.1 alloy.

(inset of x ¼ 0.1). This can be attributed to the low-volumefraction of the monoclinic phase present in these alloys asevidenced from the XRD results. The increase in Ts is due to thepresence of Fe which is in agreement with the earlier reports [10].These alloys show a second-order magnetic transition around305 K with no hysteresis upon heating and cooling. These resultsare in good agreement with that of the MDSC thermograms.An additional feature which is apparent in the wac data of theFe-doped alloys at low temperature reflects an antiferromagnetictransition of the 5:3-type Gd5(Si,Ge)3 secondary phase present inthese alloys whose Neel temperature (TN) appears to be less than100 K. The transition is well pronounced in the alloys with x ¼ 0.1and 0.15 due to more volume fraction of the 5:3 phase presentin these alloys as revealed from the microstructural studies. Thedifference in the Neel temperature (TN) of the 5:3 phase observedin the Fe-substituted alloys is due to the change in the ratio of Sito Ge content. From the ac susceptibility measurements we canconclude that Fe enhances the Ts and reduces the thermalhysteresis observed in the pure Gd5Si2Ge2 alloy.

Fig. 8. Comparison of hysteresis losses of Gd5Si2Ge2�xFex alloys with x ¼ 0, 0.05,

0.10 and 0.15 plotted as a function of temperature.

Fig. 9. Temperature dependence of magnetic entropy change (DS)M of Gd5Si2-

Ge2�xFex alloys (x ¼ 0, 0.05, 0.10 and 0.15).

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3.3. Isothermal magnetization curves and hysteresis losses

The typical isothermal magnetization curves obtained uponincreasing and decreasing fields (Fig. 7) illustrate the hysteresislosses in virgin Gd5Si2Ge2 and Gd5Si2Ge1.9Fe0.1 alloys for a tempe-rature range in the vicinity of their respective magnetic transi-tions. In the temperature interval of 258–308 K, the Gd5Si2Ge2

compound exhibits large hysteretic losses (Fig. 7a), coincidingwith the same temperature interval in which the correspondingmagnetic entropy change, DSM, attains peak value for a maximumfield of 2 T. In the case of Gd5Si2Ge1.9Fe0.1 alloy smaller hysteresisloss in the temperature range 278–328 K was observed. Asummary of the hysteretic loss values over the temperature rangeof interest is presented in Fig. 8. These hysteretic losses weredetermined from Fig. 7 by computing the area under eachmagnetization (M) vs field (H) loop as discussed by Pecharskyet al [13]. It can be seen clearly that the addition of about 0.05 at%of iron to the alloy resulted in the reduction of the hysteresislosses by nearly 80% as compared to the Fe-free alloy. Thecomparison of the hysteresis loss of the other Fe-substitutedalloys with the Gd5Si2Ge2 compound would not be very appro-priate as their temperature ranges were different. However, thepeak values of the hysteresis loss of these alloys were very lowcompared to the peak value of the Fe-free alloy.

The profound difference between the two sets of theisothermal magnetic curves shown in Fig. 7 gives an insight intothe effect of iron substitution on the magnetocaloric responseof the Gd5Si2Ge2 alloy. For the alloy without iron (Fig. 7a), themagnetization measured in the first half loop shows a distinctmagnetic transition with increasing field between 270 and 285 Kand this transition occurs at higher fields with increase intemperature. Earlier reports explained that this type of transitionto be of field-induced first-order structural change from theparamagnetic monoclinic phase to a ferromagnetic orthorhombicphase [8,12,20]. The M–H loops clearly show that this field-induced phase transition reverses upon decreasing the field, butwith some hysteresis in the transition field. Below 265 K the alloyexhibits ferromagnetic behaviour, while it is paramagnetic above295 K. On the contrary, the M–H loops of the Gd5Si2Ge1.9Fe0.1 alloy(Fig. 7b) do not show any field-induced magnetic transition in thetemperature range 275–330 K. In fact, the magnetic data show agradual shift from ferromagnetic behaviour to superparamagneticbehaviour as evidenced by the appearance of curvature in theM–H plots with increasing temperature. The structural (Fig. 1)and the magnetization data (Fig. 6) strongly suggest that the maineffect of the iron addition in the Gd5Si2Ge2 alloy is to suppress thefield-induced phase transition in the temperature range275–320 K, resulting in much smaller hysteresis losses.

3.4. Magnetic entropy change (DSM)

Variation of the magnetic entropy change, DSM with tempera-ture, for the Gd5Si2Ge2�xFex alloys, is shown in Fig. 9. These datawere computed from the isothermal M–H data of the alloys for afield change of DH ¼ 2 T [21]. It can be seen that the magneticentropy change is maximum at the first-order magnetic transitionand its value is 6.5 J/kg-K at 276 K for x ¼ 0 alloy. The peak value ofDSM for Fe-substituted alloys is lower in magnitude compared tothe x ¼ 0 alloy and occurs at higher temperatures as they exhibitonly second-order magnetic transitions in the temperature range300–303 K. For Gd5Si2Ge1.9Fe0.1 alloy, the DSM value has decreasedfrom 6.5 to 3.6 J/kg-K, and occurs at a temperature of 300 K. Thereduction in DSM value could be attributed to the absence ofstructural transitions in Fe-substituted alloys. However, the widthof the DSM peak in Fe-containing alloys is considerably broader

and also the peak occurs at higher temperatures close to roomtemperature compared to the iron free alloy. Thus Fe-containingGd5Si2Ge2 alloys appear better candidates for room temperaturemagnetic refrigeration applications.

4. Conclusions

The Gd5Si2Ge2�xFex alloys were investigated for microstructur-al and magnetocaloric properties. The Fe-substituted alloysdisplay three phase microstructure with Gd5(Si,Ge)4 matrix phasealong with Gd5(Si,Ge)3 and dark FeSi-rich minor phases. TheGd5Si2Ge2 alloy shows a first-order magnetic transition withhysteresis of 5–8 K whereas the Fe-substituted alloys show asecond-order magnetic transition at higher temperatures withnegligible hysteresis upon heating and cooing. EPMA reveals thatFe does not dissolve in the matrix but forms a FeSi-richprecipitate. This results in different ratio of Si/Ge in the matrixthat is responsible for the observed magneto-structural transi-tions in the Fe-substituted alloys. A maximum magnetic entropychange (DS)M of 6.5 J/kg-K was obtained for the Gd5Si2Ge2 alloywhile for typical Gd5Si2Ge1.9Fe0.1 alloy it is 3.6 J/kg-K for a fieldchange of 2 T. However, the hysteretic losses calculated for Fealloys are low compared to the Fe-free alloy. The advantages ofhaving enhanced transition temperature (Ts), low hysteresis lossesand broad operating temperature range make the Fe-substitutedalloys potentially better magnetic refrigerants than pureGd5Si2Ge2 alloys.

Acknowledgements

The authors acknowledge the financial support from DefenceResearch and Development Organization (DRDO) of India to carryout this research work. We are thankful to G. Malakondaiah,Director, DMRL for his keen interest on this research work. We arealso thankful to SFAG and EMG in DMRL for their experimentalsupport.

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