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Magnetocaloric effect and magnetoresistance correlation in Ge-doped Mn2Sb

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2014 Mater. Res. Express 1 046101

(http://iopscience.iop.org/2053-1591/1/4/046101)

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Magnetocaloric effect and magnetoresistancecorrelation in Ge-doped Mn2Sb

Vikram Singh1, Rohit Kumar1, Pallab Bag1, R Rawat1 andPallavi Kushwaha2,31 UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road,Indore-452001, India2Department of Condensed Matter Physics and Materials Science, Tata Institute of FundamentalResearch, Mumbai, 400 005 IndiaE-mail: rrawat@csr.res.in

Received 6 June 2014, revised 28 July 2014Accepted for publication 27 August 2014Published 3 October 2014

Materials Research Express 1 (2014) 046101

doi:10.1088/2053-1591/1/4/046101

AbstractMagnetocaloric effect (MCE) and magnetoresistance (MR) in Ge-doped Mn2Sbsystems with near-room-temperature, first-order antiferromagnetic (AFM) toferrimagnetic (FRI) transitions have been studied. They show an inverse MCEwith a 3.2 J/kg-K isothermal change in entropy (ΔS), and a refrigeration capacitythat varies linearly up to 130 J kg−1 for a 13 Tesla magnetic field change. MR(dominated by change in electronic structure) and ΔS (dominated by change inmagnetic entropy) are shown to have similar temperature dependence but withopposite signs due to coupled electronic and magnetic changes across thetransition. The ratio of the peak values of MR (%) and ΔS is found to be −5.6 (J/kg-K)−1, which remains nearly constant for the studied range of magnetic fieldchange. Existing data of MR and MCE in other Mn2Sb systems (with sub-stitution elements other than Ge) with nearby transition temperatures also shownearly same value for this ratio. Therefore, this ratio can be related to couplingbetween magnetic and electronic changes and will be an useful parameter forsystems with such transitions.

Keywords: first order magnetic transition, magnetocaloric effect, magnetoresis-tance, Mn compound, magnetization, heat capacity

3 Max Planck Institute for Chemical Physics of Solids, Nthnitzer Strasse 40, 01187 Dresden, Germany.

Materials Research Express 1 (2014) 0461012053-1591/14/046101+09$33.00 © 2014 IOP Publishing Ltd

1. Introduction

The magnetocaloric effect (MCE) is defined as a reversible isothermal change in magneticentropy, or an adiabatic change in temperature with magnetic field change. Brownʼs [1]demonstration of near-room-temperature magnetic refrigeration in 1976 and the observation ofgiant MCE in FeRh compound above room temperature in 1990 [2] actuated the interest innear-room-temperature magnetic refrigeration [3], which is believed to be more environmen-tally friendly than conventional gas-compression-based refrigeration [3, 4]. The success ofmagnetic refrigeration depends on the availability of materials with large MCE in thetemperature range of interest. Since MCE depends on the rate of change of magnetization withtemperature, materials with first-order transitions are a natural choice [2, 5–11]. In addition, ifmagnetic transition is coupled with lattice and electronic changes, then these contributions mayenhance the MCE near transition temperature (e.g., Gd5Ge2Si2 [5] and FeRh [2, 8]), and cangive rise to added functionality. The temperature span of magnetic refrigeration can be extendedby using composites made of materials with gradually varying transition temperatures. DopedMn2Sb, which shows a ferrimagnetic (FRI) to antiferromagnetic (AFM) transition accompaniedby a discontinuous change in unit cell parameters and electrical resistivity [12–14], meets manyof these criteria. The AFM to FRI transition temperature (TN) in these systems varies frombelow 100K to above 300 K with substitution element concentration, and at a rate of 4–7K/Tesla with an applied magnetic field [11, 15, 16]. Below TN, field-induced AFM to FRItransitions result in giant magnetoresistance (MR) [16–19]. The reports on the correlationbetween MR and MCE [20–22] suggest that materials with large MR can serve as potentialmaterials for large MCE.

In the case of doped Mn2Sb, Cr substitution at the Mn site is widely studied, as thetransition temperature can be tuned systematically to near room temperature by just ≈5 atomic% substitutions [12]. The heat capacity study in Mn2-xCrxSb by Engelhardt et al showed thatlatent heat associated with the transition is of the order of a few hundred Joule/mol [15].Recently, Caron et al [11] reported MCE with varying x in these systems, which showed largenear-room-temperature MCE around x = 0.1 composition. Although, TN variation up to roomtemperature for a variety of substitutions exists in the literature, the results by different groupsvaried (e.g., Co-doped Mn2Sb [16, 19, 23, 24]). Ge is the only element other than Cr for whichthe transition temperature up to room temperature has been found to be reproducible by variousgroups [25–27]. Zhang et al [25] reported near-room-temperature giant MR for a 12% Gesubstitution for Sb. Recent high-field studies on Ge-doped Mn2Sb also suggest the presence oflarge MR in these systems [26, 27]. However, MCE in Ge-substituted Mn2Sb systems remainsunexplored.

Here we present our magnetization, MR, and MCE studies for Ge-substituted Mn2Sb withnear-room-temperature TN. In addition to moderate inverse MCE around TN, it shows an inversecorrelation between MR and MCE. The ratio of MCE and MR is found to be nearly constant notonly for the system studied here, but also for other Mn2Sb systems reported earlier.

2. Experimental details

The polycrystalline Ge-doped Mn2Sb with nominal composition, Mn2Sb0.9Ge0.1, is prepared byarc melting the constituent elements of purity better than 99.9% under a high-purity argon gas

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atmosphere. To insure better homogeneity, melting is repeated three times. Powder x-raydiffraction (XRD) measurements are performed in an Advanced D-8 Bruker diffractometerusing Cu-Kα radiation to determine the phase purity and lattice parameters. Rietveld refinement[28] of the obtained XRD pattern is carried out considering a Cu2Sb-type tetragonal structure(space group = P4/nmm). Figure 1 shows that the sample is single phase with lattice parametersa = 4.0540(1)A and c = 6.5164(3)A. Temperature-dependent resistivity (ρ) in the presence ofvarious constant applied magnetic fields is measured by a standard four-probe method using anin-house-built resistivity setup along with an 8-Tesla superconducting magnet system from M/s.Oxford Instruments, UK. The in-field measurements are performed in longitudinal geometry.Specific heat is measured using an in-house-built semi-adiabatic heat pulse calorimeter duringwarming [29]. The magnetization measurements as a function of magnetic field and temperatureM(H,T) were performed in a commercial 14 T MPMS from Quantum Design, Inc.

3. Results and discussions

The magnetization measured in the presence of a 0.1 Tesla magnetic field during cooling andsubsequent warming is shown in figure 2(a). It shows a sharp drop in magnetization near roomtemperature with a small hysteresis for the cooling and warming curves, indicating the first-order nature of the FRI to AFM transition. The average of the temperatures at which thetemperature derivative of magnetization shows the maxima for the cooling and warming curvesis found to be 273K (taken as TN) with ≈ 2.6K hysteresis. The inset shows typical isothermalmagnetization measured at 255, 265, and 280K. At 255K, a sharp rise in magnetization forfields above ≈ 4 Tesla indicates a field-induced AFM to FRI transition. With a reduction in themagnetic field, the field-induced FRI state is transformed back to an AFM state at a lower fieldthan the field-increasing cycle. At 265K, the field required for the AFM-FRI transition shiftedclose to zero Tesla; however at 280K, the low field M-H curve appears almost reversible butshows the presence of a hysteretic region at a higher field. The low field rise in magnetizationcan be attributed predominantly to domain dynamics, as it seems to merge with the high fieldmagnetization curve at 255K and 265K. These values of magnetization, 0.6 μ f u/ . .B are inagreement with earlier reports on doped Mn2Sb systems [25]. The high field region suggests thepossibility of another field-induced transition, as discussed in the following sections.

The specific heat (CP) measured during zero field warming is shown in figure 2(b), Itshows a peak around the first-order AFM-FRI transition, which is observed in the magnetization

Figure 1. Room temperature powder XRD pattern of Mn2Sb0.9Ge0.1 (open circle) along,with a fitted curve (line curve). The bottom line curve shows the difference between themeasured and fitted curves, and the vertical line indicates the Bragg position.

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measurement. To account for nonmagnetic contributions, we used a Debye function for latticecontribution and a linear term for electronic and anharmonic contributions. The bestrepresentative curve is shown in figure 2(b) as a line curve with a Debye temperature of298K and a coefficient of linear term to specific heat of 14mJ/mol-K2. The entropy changeassociated with the transition (Sdiff) obtained from the difference of measured and simulatedcurves is shown in the inset of figure 2(b). It shows an entropy change of 2.4 J/molK, which ismuch higher than the 1.31 ± 0.07 J/molK observed by Engelhardt et al [15] for Mn1.9Cr0.1Sb.This could be due to limitations in modeling the nonmagnetic contribution. The inset shows thatthere is a limited temperature range (marked by vertical lines in the main panel) over which Sdiff

Figure 2. (a) Magnetization (M) versus temperature measured during cooling andsubsequent warming in the presence of a 0.1 Tesla magnetic field. Inset showsisothermal magnetization curves highlighting the field-induced AFM-FRI transition. (b)Measured heat capacity (Cexp) along with simulated nonmagnetic contribution (C

−non mag) as a function of temperature measured during warming. Inset shows theentropy difference, Sdiff, between the two curves around TN, and the vertical line in themain figure indicates the temperature region of sharp entropy change (see text fordetails). (c) Temperature dependence of resistivity (ρ) measured during cooling andsubsequent warming in the presence of 0 and 8 T magnetic fields. Inset shows the MRobtained from these curves.

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varies sharply. The change in Sdiff is found to be about 1.5 J/molK within this temperaturewindow. If we extrapolate linearly the high-temperature and low-temperature regions of theSdiff, then the difference between these two curves gives a value of 1 J/mol K, which isassociated with sharp variation in Sdiff. These values, 1.5 J/mol K and 1 J/mol K, provide anupper and lower bound to entropy change across the transition and conform to earlier studies[15]. Corresponding limits for latent heat are 305 Jmol−1 to 392 Jmol−1.

The temperature dependence of resistivity (ρ-T) in zero-field and 8T for a Mn2Sb0.9Ge0.1compound is shown in figure 2(c). The residual resistivity ratio (taken as the ratio of themaximum and minimum values of resistivity in the AFM state, which are ρ(250K) and ρ(5K),respectively) is found to be ≈ 2, indicating a good quality of the sample. AFM-FRI transition isaccompanied by a hysteresis of (≈2.8K) and the average temperature of the minimum in thetemperature derivative of resistivity during cooling and warming is found to be 271.5K. Withthe application of an 8 Tesla magnetic field, the transition is shifted to a lower temperature(≈246K). The shift in transition temperature with the magnetic field is comparable to a Cr-doped Mn2Sb system with a near-room transition temperature [11, 15]. The small hysteresis inresistivity at high temperatures in the presence of 8 Tesla could arise due to the presence ofanother transition at high field. It may be related to the high field rise of M in isothermal M-Hmeasurements for T ⩾ 280K. The inset in figure 2(c) shows MR as a function of temperature,which is obtained as MR(%) = [{ρ(H, T)/ρ(0, T)}-1]*100, where ρ(H,T) and ρ(0,T) areresistivity in the presence of an applied magnetic field (H) and in the absence of an appliedmagnetic field, respectively. The magnetic field shifts the AFM-FRI transition to lowtemperature, which gives rise to large negative MR with a peak value of −16%. The observationof large MR around the AFM-FRI transition is consistent with that reported by Zhang et al [25].

Figure 3(a) shows the isothermal magnetization curve measured near TN. As discussedabove, it shows a field-induced transition from AFM to FRI state with a small hysteresis. Thecritical field required for AFM-FRI transition decreases as TN is approached. Increasing fieldisotherms in the form of an Arrott plot are shown in figure 3(b). Spontaneous magnetizationaround TN is found to be less than μ0.35 B/f.u. These curves also show a distinct negative slopebelow TN, which has been taken as a signature of the first-order nature of the magnetic transition[30]. Above TN an upturn is also observed, and the magnetic field value at which this upturnappears decreases with increasing temperature. It could arise either due to an increase in the FRIphase fraction or an increase in the net moment due to the second field-induced transition.Qualitatively, similar magnetization behavior has been observed by Zhang et al [25] forMnSb0.88Ge0.12, where one can see the presence of two transitions near TN. The evidence ofanother transition is also visible in figure 3 of Zhang et al [25], which shows the temperaturedependence of MR.

The isothermal change in entropy (ΔS) with magnetic field change is estimated using theMaxwell relation, ∫Δ = ∂ ∂S M T dH( ) . The thermal variations of magnetic entropy changes(ΔS) for up to 13 T magnetic field changes around the AFM-FRI transition are shown infigure 4. These are calculated for the field increasing cycle. They show a positive change inentropy, as the magnetic field favors a high temperature state (i.e., FRI). The maximum value of(ΔS) is found to be 3.2 J/kg-K for a 13 T magnetic field change, which is smaller than earlierreports in V- and Cr-modified Mn2Sb [8, 9]. This could be due to broad transitions in thiscompound, which result in a smaller value of ΔS for a smaller field change. For an ideal first-order transition, a tabletop-like behavior is expected for ΔS, where an increase in magnetic fieldchange increases the temperature range of flat maxima with relatively little change in the peak

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value of ΔS. MCE studies in Cr-doped Mn2Sb by Caron et al [11] showed higher MCE forsamples with lower TN. They argued that due to tunable TN, as well as consistent MCE with Crconcentration, these could be used as composite materials for applications where largetemperature differences are required, even with low cooling power. Therefore, in spite of itssmaller value compared to other Mn-based giant MCE materials, similar applications can beenvisioned for Ge-substituted Mn2Sb due to the systematic variation of TN with Ge content overa wide temperature range [25–27]. The refrigeration capacity, which is calculated as the areaunder ΔS versus T curve, is found to increase linearly with an increasing magnetic field change,and reaches a value of ≈130 J kg−1 for a 13 Tesla magnetic field change. On the other hand,hysteresis losses are found to be maximum for a 265K isotherm with a value of 6.5 J kg−1 for0–14 Tesla magnetic field cycle.

A comparison of figure 4 with the inset of figure 2(c) suggests a similar trend intemperature dependence of MR and MCE. This is further highlighted in figure 5 for variousfield values. Both ΔS and MR(%) show similar behavior with varying temperatures, but withopposite signs. This is in contrast to the generally expected similar behavior for ΔS and MR

Figure 3. (a) Magnetization versus magnetic field for various constant temperaturesaround the zero field AFM-FRI transition temperature. (b) Isothermal magnetization inthe form of M/H versus H2 for a field-increasing cycle highlighting the negative sloperegion at low temperatures.

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(e.g., in paramagnetic or ferromagnetic systems with increasing magnetic field entropydecreases (or ΔS is negative) due to a decrease in spin disorder). It also leads to a decrease inspin disorder resistivity, which therefore results in a similar trend for MR and ΔS [20–22, 31].In the present case, the AFM-FRI transition is believed to be accompanied by a significantchange in the electronic density of state, and it results in higher resistivity for the AFM state.The similar temperature dependence (but with an opposite sign) is a consequence of couplingbetween magnetic and electronic degrees of freedom. The ratio of MR and MCE peak values isfound to be −5.6 (J/kgK)−1. From the reported MCE [11] and MR(%) [32] for Mn1.9Cr0.1Sb,which also have a TN of around room temperature, this ratio comes out to be −5.3 (J/kgK)−1.Even in vanadium-doped Mn2Sb this ratio appears to be around −5 (J/kgK)−1 with ≈T 240 KN

[33]. The similar values of these ratios for different dopants and samples studied by differentgroups show that they depend on the transition temperature and will be useful parameters to

Figure 4.MCE in the form of isothermal change in entropy (ΔS) for the labeled value ofmagnetic field change, ΔH , calculated from isothermal magnetization data.

Figure 5. Temperature dependence of [a] MR and [b] isothermal change in entropy(ΔS) for the labeled change in magnetic field. It highlights the similarity between thetwo due to strong coupling between magnetic and electronic degrees of freedom.

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characterize magnetic and electronic coupling for such transitions. Similar correlations betweenMCE and MR are expected for systems like Ni-Mn-In, Ni-Mn-Sn, FeRh, etc., which show first-order transitions from a low-temperature, high-resistance AFM state to a high-temperature,lower-resistance FM/PM state. In fact, Chatterjee et al [34] have shown scaling between MRand MCE in Ni2Mn1.36Sn0.64 (T ≈ 120 K)N where the ratio between MR and MCE is reportedto be around −3.36 (J/kg-K)−1. Incidentally, this ratio for another shape memory alloy,Ni45Co5Mn36.5In13.5 with a transition temperature around 240K, appears to be −5.7 (J/kg-K)−1

[35].

4. Conclusions

Systematic magnetotransport and MCE studies in Ge-doped Mn2Sb with a near-room-temperature AFM-FRI transition show about −16% MR and ≈2.8 J/kg-K isothermal change inentropy with an 8 Tesla magnetic field change. The MR and ΔS show an inverse correlation(i.e., both have similar temperature dependence, but with opposite signs). This inversecorrelation between the two appears to result from a of change of density of state at the Fermilevel, which is coupled to the magnetic transition. The ratio of MR(%)) and ΔS is found to be−5.6 (J/kg-K)−1 for the present system. Experimental MR and MCE data available in otherMn2Sb systems by different groups suggest similar values of this ratio. This is an interestingfinding when one considers the sensitivity of the transition temperature and its width on samplepreparation conditions and dopants. Therefore, this parameter could be a useful quantity todefine coupling between magnetic and electronic degrees of freedom. The correlation betweenMR and MCE will also be useful in determining MCE in these materials, where conventionalmethods of its determination are difficult to implement because of pressure, strain, etc.

Acknowledgements

We thank Layant Behera and M. Gupta for powder x-ray diffraction measurements.

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