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Influence of Ni/Mn concentration on the structural, magnetic and magnetocaloric properties in Ni50−x Mn37+x Sn13 Heusler alloys

This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2010 J. Phys. D: Appl. Phys. 43 425002 (http://iopscience.iop.org/0022-3727/43/42/425002) View the table of contents for this issue, or go to the journal homepage for more

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JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 43 (2010) 425002 (6pp)

doi:10.1088/0022-3727/43/42/425002

Influence of Ni/Mn concentration on the structural, magnetic and magnetocaloric properties in Ni50−xMn37+xSn13 Heusler alloys S Esakki Muthu1 , N V Rama Rao2,3 , M Manivel Raja2 , D M Raj Kumar2 , D Mohan Radheep1 and S Arumugam1 1 Centre for High Pressure Research, School of Physics, Bharathidasan University, Tiruchirappalli-620024, India 2 Defence Metallurgical Research Laboratory, Hyderabad-500 058, India

E-mail: [email protected] and [email protected]

Received 12 July 2010, in final form 9 September 2010 Published 7 October 2010 Online at stacks.iop.org/JPhysD/43/425002 Abstract We report the structure, magnetism and magnetic entropy change in a Mn-rich Ni50−x Mn37+x Sn13 Heusler alloy system in the composition range 0 x 4. An excess Mn content stabilizes the cubic austenite phase at room temperature. Martensitic transition decreases from 305 to 100 K with increasing Mn concentration (x: 0 → 4) and also it was found to shift to a lower temperature with the application of a higher magnetic field. The exchange bias blocking temperature was found to decrease drastically from 149 to 9 K with increasing Mn concentration. A large magnetic entropy change (SM ) of 32 J kg−1 K−1 has been achieved for a field change of 5 T in the x = 3 alloy. (Some figures in this article are in colour only in the electronic version)

Recently, Ye et al have theoretically shown that excess Mn at the Sn site stabilizes the martensite phase in the Ni–Mn–Sn alloy system [11]. Generally, the magnetic moment decreases in the martensite phase for the composition with excess Mn which couples antiferromagnetically with Mn atoms. The effect of variation of Mn/Sn concentration on the structural and magnetic properties in Ni–Mn–Sn alloys has been studied extensively [10, 12, 13], however; there are very few reports on the effect of variation of Ni/Mn concentration on the magnetostructural properties in Ni–Mn–Sn alloys [14]. Hence, we have taken up this study to investigate in detail the influence of Ni/Mn content on the structural and magnetic properties in Ni50−x Mn37+x Sn13 (x = 0, 1, 2, 3, 4) alloys. In addition to this, we have also studied the IMCE and the effect of magnetic field on the structural transition in these alloys.

1. Introduction Recently, Ni–Mn–X (X = Ga, Sb, In, Sn), Heusler based alloy systems have gained attention due to their vast applications in magnetic refrigeration, magnetic actuated devices and spintronic devices [1, 2]. On selectively tuning the composition, this alloy system exhibits a wide range of physical properties such as magnetic-field-induced transition, inverse magnetocaloric effect (IMCE), giant magnetoresistance, giant Hall effect, giant magnetothermal conductivity, magnetic superelasticity effects, exchange bias and shape memory effect [3–8]. The complex behaviour exhibited by the Ni–Mn based Heusler alloy is due to the strong coupling between magnetism and structure, and also the magnetic behaviour of these alloys strongly depends on the distance between Mn atoms [9]. The Ni–Mn–Sn alloy system is, in particular, an interesting class of materials due to the large reported IMCE [10]. The structural and magnetic properties of these alloys are very sensitive to the composition. 3

2. Experimental The ingots of Ni50−x Mn37+x Sn13 (x = 0, 1, 2, 3, 4) alloys were prepared by melting the high purity starting

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J. Phys. D: Appl. Phys. 43 (2010) 425002

S Esakki Muthu et al

elements (99.9% pure) using an arc melting furnace in argon atmosphere. The samples were remelted four times to ensure homogeneity. The alloys were annealed under high vacuum at 1175 K for 6 h and then quenched with Ar gas. Elemental compositions of the alloys were determined using scanning electron microscopy (SEM, Leo 440i) attached with an x-ray energy dispersive spectroscopy (EDS) setup and were found to be close to the nominal composition. The structural analysis was carried out using a Philips 3121 X-ray diffractometer with Cu Kα radiation. The magnetization measurements were performed by means of a physical property measurement system (PPMS-9T) using a vibrating sample magnetometer (VSM) module (Quantum Design, USA). The sample was initially cooled in zero magnetic field and the data were collected on warming by applying a magnetic field of 5 mT in the temperature range 4–330 K (referred to as zero-fieldcooled (ZFC)); subsequently, the data were collected upon cooling without removing the applied field in the temperature range between 330 and 4 K (referred to as field-cooled (FC)). Magnetization as a function of magnetic field was recorded up to a field of 5 T in the vicinity of structural transitions. The isothermal magnetization curves were recorded for both increasing and decreasing field.

3. Results and discussion 3.1. Structural studies Figure 1 shows the x-ray diffraction patterns of Ni50−x Mn37+x Sn13 (x = 0, 1, 2, 3, 4) alloys at room temperature (RT). It can be noted that the first two alloys (x = 0, 1) display one kind of pattern while the other three alloys show another kind of pattern. As seen in figures 1(a) and (b), the alloys with x = 0, 1 show the presence of mixed martensite and austenite phases at RT. The martensite phase is indexed as a four-layered orthorhombic structure (4O) while the austenite phase is indexed as a cubic L21 structure. The notation C (h k l) and O (h k l) in figure 1 refers to the Miller indices for the L21 and 4O structure, respectively. Also, the fraction of martensite phase present in the x = 0 alloy is more than that of the x = 1 alloy as reflected by the higher relative intensities observed for the 4O phase. The lattice parameter ac is calculated to be 0.5983 nm and 0.5984 nm for the x = 0, 1 alloys, respectively, in the austenite phase. On the other hand, the x-ray analysis of alloys with x = 2, 3, 4 reveals the presence of only cubic L21 austenite phase. The appearance of super lattice reflections (1 1 1) and (3 1 1) suggests that the alloys are crystallized in the highly ordered cubic L21 Heusler structure. It can be seen that increasing the Mn content in this Ni50−x Mn37+x Sn13 alloy series stabilizes the austenite phase at RT, or in other words, a decrease in Ni/Mn ratio suppresses the martensite phase and favours the formation of the austenite phase at RT. The lattice parameters were determined to be ac = 0.5981 nm, 0.5992 nm and 0.600 nm for alloys with x = 2, 3 and 4 respectively. The value of ac increases linearly with increasing Mn content in the concentration range x = 2–4. This could be due to the replacement of smaller atomic radius Ni (0.162 nm) by the larger atomic radius Mn (0.179 nm).

Figure 1. X-ray diffraction patterns for Ni50−x Mn37+x Sn13 (x = 0, 1, 2, 3, 4) alloys.

3.2. Magnetic studies The ZFC and FC magnetization curves for Ni50−x Mn37+x Sn13 (x = 0, 1, 2, 3, 4) alloys are shown in figure 2. All the samples show complex magnetic behaviour with distinct transitions which correspond to first- and second-order transformations. At the high-temperature end, the ZFC and FC curves for the x = 0 alloy (figure 2(a)) show a second-order transition at 320 K that corresponds to the Curie temperature of the austenite phase TCA , where the sample transforms from paramagnetic to ferromagnetic austenite. With decreasing temperature, the magnetization increases and reaches a peak value at 305 K in the FC curve. In the temperature range 255–313 K, the FC and ZFC curves display hysteresis indicating the first-order structural transition from austenite to martensite phase. The characteristic transformation temperatures, martensite start and finish (Ms , Mf ), and austenite start and finish (As , Af ), where the magnetization changes its path, are indicated on the ZFC and FC curves. As the temperature decreases below Ms 2

J. Phys. D: Appl. Phys. 43 (2010) 425002


Figure 2. (a)–(e) ZFC and FC thermomagnetic curves of Ni50−x Mn37+x Sn13 (x = 0, 1, 2, 3, 4) alloys at a field of 5 mT. (f ) Thermomagnetic curve measured upon cooling/heating for Ni50−x Mn37+x Sn13 (x = 3) alloys at a field of 5 T.

the magnetization decreases up to Mf in the FC curve as the fraction of the austenite phase progressively decreases. Since the saturation magnetization of the martensite phase is lower than the austenite phase, the total magnetization of the mixed

austenite and martensite phases decreases with temperature in the temperature range Ms –Mf . On further lowering the temperatures, i.e. below Mf , there is a splitting between the ZFC and FC curves, and also the ZFC curve exhibits a step-like 3

J. Phys. D: Appl. Phys. 43 (2010) 425002


Table 1. The values for the structural and magnetic transition temperatures in Ni50−x Mn37+x Sn13 alloys determined from the thermomagnetic curves. Transformation temperatures (K) x value

Ms (K)

Mf (K)

As (K)

Af (K)

TCA (K)

e/a

0 1 2 3 4

305 294 201 162 100

255 185 151 132 48

267 218 193 153 54

313 304 252 196 122

320 309 303 297 291

8.11 8.08 8.05 8.02 7.99

behaviour in magnetization. This transition is denoted as TEB which corresponds to the exchange bias blocking temperature of the sample. The splitting between the ZFC and FC curves indicates that the sample is magnetically inhomogeneous. This behaviour is also observed in Ni–Mn–X (X = Sn, Sb, In) alloys [15–18]. The observation of magnetic inhomogeneity and exchange bias blocking temperature can be attributed to the presence of antiferromagnetic (AF) interactions arising from the AF coupling between the Mn atoms in the Mn sites and Mn atoms in the Ni/Sn sites. The presence of AF interaction in this material system was confirmed through the Neutron diffraction and ferromagnetic resonance studies elsewhere [19, 20]. This AF interaction also leads to the pinning of ferromagnetic domains in different configurations resulting in the irreversible magnetic behaviour in FC and ZFC curves. A similar thermomagnetic behaviour is observed for the other alloys with x = 1, 2, 3, 4 as shown in figures 2(b)–(e). For the x = 4 alloy, the TEB is observed at a very low temperature which is evident from the inset shown in figure 2(e). The characteristic temperatures As , Af , Ms and Mf are given in table 1. With the increase in Mn content (x = 0–4) in this alloy series TCA shifts from 320 to 291 K as observed from table 1. It is generally known that TCA is determined mainly by the ferromagnetic Mn–Mn interaction strength in the Ni–Mn–Sn alloys and the excess Mn content leads to AF coupling with Mn at the Ni/Sn sites which in turn reduces the strength of ferromagnetic interactions. As a result the TCA decreases with increasing Mn content in these alloys. Further, the value of Ms decreases from 305 to 100 K upon increasing the Mn concentration (x = 0 to 4) in the Ni50−x Mn37+x Sn13 alloy series which is plotted in figure 3. The Ni–Mn–Sn alloys with a different composition range (Ni50−x Mn39+x Sn11 ) also show a similar trend [21]. The observation of compositional dependence on the structural transformation in these alloys can be correlated with their valence electron concentration ratio (e/a). It can be seen from figure 3 that the decrease in e/a ratio (or increase in Mn content) decreases the structural transformation temperature (Ms ) in Ni50−x Mn37+x Sn13 alloys. Similar results have been reported in other Ni–Mn–X (X = In, Ga) Heusler alloys [22, 23]. Krenke et al have studied the effect of variation of Mn/Sn on the structural and magnetic properties in Ni0.50 Mn0.50−x Snx alloys [13]. The authors found that the structural transition temperature decreases with decreasing Mn content or e/a ratio (keeping Ni content constant). A comparison of these results with

Figure 3. Variation of Ms and TCA as a function of e/a ratio (or Mn content) for Ni50−x Mn37+x Sn13 (x = 0, 1, 2, 3, 4) alloys.

Figure 4. Shift of Ms in an applied magnetic field of 5 T for different Mn content in Ni50−x Mn37+x Sn13 (x = 0, 1, 2, 3, 4) alloys.

our data suggests that the effect of either varying Ni/Mn or Mn/Sn on structural transformation is the same i.e. the structural transition increases with increasing e/a ratio in this alloy system. It is also observed that the exchange bias blocking temperature (TEB ) decreases drastically from 149 to 9 K upon increasing the x from 0 to 4. It could be due to the weakening of the strength of AF interactions with increasing x value in these alloys. This is supported by the experimental results that the saturation magnetization at 5 K increases with increasing x value, which suggests that an increase in x content would reduce the AF interactions. It is known that exchange bias arises below TEB due to the competing antiferromagnetic and ferromagnetic interactions, and the weakening of the AF interactions destabilizes the exchange bias region which leads to a decrease in TEB values. In order to study the effect of magnetic field on the structural transition, the magnetization was measured as a function of temperature upon heating and cooling in a field of 5 T. The measurements were performed for all the samples and a typical thermomagnetic plot is shown in figure 2(f ) for the Ni50−x Mn37+x Sn13 (x = 3) alloy. These alloys show a shift in 4

J. Phys. D: Appl. Phys. 43 (2010) 425002


Ms [T = Ms (5 T) − Ms (5 mT)] towards lower temperatures as the magnetic field increases. A maximum shift of 30 K is observed for the x = 0 alloy for an applied field of 5 T. The lowering of the martensitic transition in the Ni–Mn–Sn system implies that the external magnetic field favours the formation of the austenitic phase. It is due to the fact that the saturation magnetization of the martensitic phase is smaller than that of the austenite phase. A similar behaviour has also been observed in Ni–Mn–Sb and Ni–Mn–In alloys. This behaviour is in contrast to that observed in Ni–Mn–Ga alloys, wherein the martensitic transition shifts to higher temperatures upon increasing the magnetic field [24]. The observed T values in Ni50−x Mn37+x Sn13 alloys were found to decrease with increasing Mn content as shown in figure 4. In general, a large difference in saturation magnetization between the austenite and martensite phases (M) leads to a large shift in the structural transition temperature (T ) due to an applied magnetic field [25]. T for a magnetic field change (H ) can also be approximately calculated by the Clausius–Clapeyron relation M T ≈ H, (1) S where S is the entropy change between the austenite and martensite phases. The T values for the x = 0–2 alloys were calculated from equation (1) wherein the S values were obtained from DSC measurements. For the x = 2 alloy, the calculated T ≈ 18 K (M = 50 emu g−1 , S = 13.8 J kg−1 K−1 and H = 5 T) is in close agreement with the experimental value. However, for the x = 0 and 1 alloys there is a difference in the calculated and experimental values as it is difficult to calculate the exact S values from the DSC curves because of the overlapping of the peaks corresponding to the structural and magnetic transitions and also the broad nature of the peaks. The observed M values at the transition increase with increasing Mn content. Accordingly, the T value should increase with increasing Mn content, but a decreasing trend is observed experimentally (figure 4) which can be attributed to the strong dependence of T on the S values. The magnetic behaviour of the Ni50−x Mn37+x Sn13 alloys were studied by measuring the isothermal magnetization curves in the vicinity of structural and magnetic transitions and a typical curve for the x = 3 alloy is shown in figure 5. A typical paramagnetic behaviour is observed at 318 K before ferromagnetic transition. In the region of martensitic transition, the isothermal magnetization curves are complex with the signature of a field-induced transition. At temperatures 184 and 187 K, the metamagnetism can be seen clearly, where the magnetization suddenly starts to rise around a field of 3.8 T. It is due to the field-induced reverse transition from the low magnetic martensite to the high magnetic austenite phase. The magnetic entropy change (SM ) is calculated from the isothermal magnetization curves using the Maxwell relation: SM = 0

H

∂M(H, T ) ∂T

Figure 5. Isothermal magnetization curves for Ni50−x Mn37+x Sn13 (x = 3) alloy; (a) in the vicinity of martensite transition, (b) magnetic transition. The measurements were carried out at an interval of 3 K; however, for the sake of clarity in (b) the curves are displayed for 6 K interval.

The SM values calculated at different temperatures for Ni50−x Mn37+x Sn13 (x = 0, 1, 2, 3, 4) are shown in figure 6. The sign of SM is determined by the sign of ∂M/∂T and the positive sign of SM represents the IMCE. The x = 3 alloy displays the highest SM value of 34 J kg−1 K−1 around 187 K for a field change of 5 T. For the x = 0, 1, 2 and 4 alloys the SM values are 0.2 J kg−1 K−1 , 2.2 J kg−1 K−1 , 12 J kg−1 K−1 and 11 J kg−1 K−1 , respectively. The SM value was also calculated in the range of temperatures inside the region of the first-order phase transition using the magnetization data by the modified Maxwell equation proposed and used in [26]. The calculated SM in the first-order phase transition region by the modified Maxwell equation for a typical sample of x = 3 (which shows the highest SM value) is shown in the inset of figure 6 for comparison between these two methods. It can be seen that SM obtained from the modified Maxwell equation yields a lower value of 32 J kg−1 K−1 . Previously, Krenke et al have reported a highest SM value of 18 J kg−1 K−1 near room temperature for a field of 5 T in Ni0.50 Mn0.50−x Snx (x = 0.13) alloys [10]. For composition Ni50−x Mn39+x Sn11 (x = 7), a maximum SM value of 10.4 J kg−1 K−1 has been reported in a low field of 1 T near 200 K [21]. By comparing these results with our data, we achieved the highest SM value in the Ni–Mn–Sn alloy system and this enhanced

dH .

(2)

H

5

J. Phys. D: Appl. Phys. 43 (2010) 425002


Acknowledgments The authors are grateful to the DST and UGC for financial support. The authors would like to thank the Defence Metallurgical Research Laboratory (DMRL), Hyderabad, for support.

References [1] Planes A, Mañosa L and Acet M 2009 J. Phys.: Condens. Matter 21 233201 [2] Cong D Y, Roth S, Pötschke M, Hürrich C and Schultz L 2010 Appl. Phys. Lett. 97 021908 [3] Rama Rao N V, Gopalan R, Chandrasekaran V and Suresh K G 2010 Appl. Phys. A: Mater. Sci. Process. 99 265 [4] Yu S Y, Liu Z H, Liu G D, Chen J L, Cao Z X, Wu G H, Zhang B and Zhang X X 2006 Appl. Phys. Lett. 89 162503 [5] Biswas C, Rawat R and Barman S R 2005 Appl. Phys. Lett. 86 202508 [6] Oikawa K, Ito W, Imano Y, Sutou Y, Kainuma R, Ishida K, Okamoto S, Kitakami O and Kanomata T 2006 Appl. Phys. Lett. 88 122507 [7] Sozinov A, Likhachev A A, Lanska N and Ullakko K 2002 Appl. Phys. Lett. 80 1746 [8] Hu F X, Shen B G and Sun J R 2000 Appl. Phys. Lett. 76 3460 [9] Planes A 2010 Physics 3 36 [10] Krenke T, Duman E, Acet M, Wassermann E F, Moya X, Mañosa L and Planes A 2005 Nature Mater. 4 450 [11] Ye M et al 2010 Phys. Rev. Lett. 104 176401 [12] Yuhasz W M, Schlagel D L, Xing Q, McCallum R W and Lograsso T A 2010 J. Alloys Compounds 492 681 [13] Krenke T, Acet M and Wassermann E F 2005 Phys. Rev. B 72 014412 [14] Sharma V K, Chattopadhyay M K, Kumar R, Ganguli T, Tiwari P and Roy S B 2007 J. Phys.: Condens. Matter 19 496207 [15] Khan M, Dubenko I, Stadler S and Ali N 2007 J. Appl. Phys. 102 113914 [16] Li Z, Jing C, Chen J, Yuan S, Cao S and Zhang J 2007 Appl. Phys. Lett. 91 112505 [17] Khan M, Dubenko I, Stadler S and Ali N 2007 Appl. Phys. Lett. 91 072510 [18] Pathak A K, Khan M, Gautam B R, Stadler S, Dubenko I and Ali N 2009 J. Magn. Magn. Mater. 321 963 [19] Aksoy S, Acet M, Deen P P, Mañosa L and Planes A 2009 Phys. Rev. B 79 212401 [20] Aksoy S, Posth O, Acet M, Meckenstock R, Lindner J, Farle M and Wassermann E F 2010 J. Phys.: Conf. Ser. 200 092001 [21] Han Z D, Wang D H, Zhang C L, Xuan H C, Gu B X and Du Y W 2007 Appl. Phys. Lett. 90 042507 [22] Ingale B, Gopalan R, Manivel Raja M, Chandrasekaran V and Ram S 2007 J. Appl. Phys. 102 013906 [23] Krenke T, Acet M, Wassermann E F, Moya X, Mañosa L and Planes A 2006 Phys. Rev. B 73 174413 [24] Gonzalez-Comas A, Obrado E, Mañosa L l, Planes A, Chernenko V A, Hattink B J and Labarta A 1999 Phys. Rev. B 60 7085 [25] Kainuma R et al 2006 Nature 439 957 [26] De Oliveira N A and von Ranke P J 2010 Phys. Rep. 489 89

Figure 6. Temperature dependence of magnetic entropy change for Ni50−x Mn37+x Sn13 (x = 0, 1, 2, 3, 4) alloys calculated using the Maxwell relation. Inset curve displays SM calculated from the Maxwell relation (open circle) and the modified Maxwell relation (solid circle) for the x = 3 alloy.

value of SM is due to the presence of the profound fieldinduced structural transition, particularly for the x = 3 alloy. This IMCE is a consequence of the increase in the magnetic moment of the austenite phase while it transforms from the martensite phase at the magnetostructural transition. As a result the change in ∂M/∂T becomes positive and the sign of SM turns into positive. The observation of a large IMCE and economical concerns make the Ni50−x Mn37+x Sn13 alloys potential candidates for magnetic refrigeration applications.

4. Conclusions We have studied the effect of varying Ni/Mn content on the structural and magnetic transformations in Ni50−x Mn37+x Sn13 (x = 0, 1, 2, 3, 4) alloys. The increase in Mn content stabilizes the austenite phase at RT. The increase in Mn concentration decreases the martensite transition temperature and the exchange bias blocking temperature which arises due to the presence of AF exchange interaction. The effect of varying either Ni/Mn or Mn/Sn on the structural transition temperature was found to be the same. This suggests that the structural transformation is governed mainly by the e/a ratio in this alloy system. We also studied the magnetic entropy change for Ni50−x Mn37+x Sn13 (x = 0, 1, 2, 3, 4) alloys. The x = 3 alloy shows the highest SM value of 32 J kg−1 K−1 for a field change of 5 T.

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