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Effect of Al doping on structural and magnetic properties of Ni50Mn37AlxSb13−x alloy
Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Effect of Al doping on structural and magnetic properties of Ni50Mn37AlxSb13 x alloy Mayukh K. Ray a, K. Bagani a, R.K. Singh b, B. Majumdar b, S. Banerjee a,n a b
Surface Physics Division, Saha Institute of Nuclear Physics, Kolkata 700064, India Defense Metrological Research Laboratory, Hyderabad 500058, India
art ic l e i nf o
a b s t r a c t
Keywords: Magnetism Exchange bias Magnetocaloric Martensite Ferromagnetism Heusler alloy
The Ni50Mn37AlxSb13 x (x ¼0, 1, 3 and 5) alloys were prepared by tri-arc melting technique. The replacement of Sb by Al increases the martensitic transformation temperature (TM) as well as ferromagnetic to paramagnetic transformation temperature (T C A ) within the austenite phase. The increase in TM is found to due to the enhancement of hybridization between 3d states of Ni and Mn atoms. We also observed a large exchange bias field (HEB) of 470 Oe for x ¼ 0 and it decreases with the Al concentration for field cooled (FC) magnetic hysteresis loop. A large magnetic entropy change (ΔSM) of 10 J/kg-K is found for x ¼ 1 alloy under a field change (ΔH) of 50 kOe and it decreased for further higher concentration of Al doping. The possible reasons for observed behaviors are discussed. & 2014 Published by Elsevier B.V.
1. Introduction There have been renewed interest in the transition metal based Heusler alloys particularly Ni–Mn–Z (Z ¼Sn,Sb,In) type, primarily due to their multifunctional properties that includes ferromagnetic shape memory effect (FMSME), magnetocaloric effect (MCE), magnetoresistance (MR) and exchange bias (EB) effect [1–5]. Most of these above mentioned applicable properties are arises due to the coupling between the structure and its magnetic nature. Heusler alloy is a typical system to study the ordered atomic configuration and magnetic coupling for intermetallic compounds. Therefore, synthesizing and investigating new systems becomes attractive in this field. In structural transformation, the alloys undergo transition from its higher symmetric austenite phase to lower symmetric martensite phase upon cooling. At high temperature austenite phase crystallize into the cubic L21 structure comprising four interpenetrating FCC sublattices, while at lower temperature martensite phase may have tetragonal, orthogonal or monoclinic structures [6]. Basically, the magnetism of Heusler alloy is considered to be a local moment system and the moment is confined to the Mn atoms of Ni–Mn based alloys [4]. The Mn–Mn indirect exchange interaction (Rudermann–Kittel– Kasuya–Yoshida type) plays the key role to determine the magnetic nature of Heusler alloys. Recently off-stoichoimetric Ni–Mn–Sb alloy is drawing much attention because of their
n
Corresponding author. E-mail address: [email protected] (S. Banerjee).
multifunctional properties. In this study we tried to find out a material with the above mentioned properties near to room temperature. With this motivation we prepared a series of Ni50Mn37AlxSb13 x (x ¼0, 1, 3, 5) alloys, where we replace Sb by nonmagnetic Al and studied their structure and magnetic properties.
2. Experimental details The polycrystalline Ni50Mn37Sb13 xAlx (where x¼ 0, 1, 3, 5) was prepared by arc melting technique in argon atmosphere using Ni, Mn, Sb and Al of 99.98% purity (Alfa Aesar). To make sure about the compositional homogeneity the sample was flipped and re-melted four times. The structural characterization was done by powder X-Ray diffraction (XRD) using CuKα radiation. The compositional analysis was done by Energy Dispersive X-ray analysis (EDAX) attached in scanning electron microscopy (SEM). The magnetic measurements were carried out in physical property measurement system (PPMS, Quantum design).
3. Results and discussion The room temperature X-Ray diffraction pattern (see Fig. 1.), using CuKα confirms that x¼ 0 and 1 alloys are crystallized into highly ordered cubic L21 phase (only for x ¼1 alloy is shown in Fig. 1). For x ¼3 there are martensite peaks near to the main (220) austenite peak and x ¼5 is in martensite phase (see inset of Fig. 1).
http://dx.doi.org/10.1016/j.physb.2014.04.054 0921-4526/& 2014 Published by Elsevier B.V.
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The observation of martensite peaks for x¼ 3 might be due to the presence of martensitic transition (MT) near room temperature, where both the phases coexists. The XRD pattern of x ¼3 is indexed with mixed structure of cubic austenite and 4 layered orthorhombic (4O) structure while x¼ 5 alloy is indexed with 4 layered orthorhombic (4O) martensite structure. The calculated lattice parameters correspond to cubic austenite phase of x¼ 0, 1, and 3 alloys are 5.967, 5.965, 5.964 Å, respectively. The decrease in cell parameter with the increase of Al concentration is an indication of contraction of cell volume in the austenite phase. The cell volume contraction is attributed due to Al (1.43 Å) has lower atomic radius than Sb (1.82 Å). The zero field cooled warming (ZFCW), field cooled cooling (FCC) and field cooled warming (FCW) thermo-magnetization (M (T)) for all the alloys are shown in Fig. 2. Now if we look at the ZFCW M (T) curves (for x ¼0, 1, and 3), it can be seen that with
θ Fig. 1. The room temperature XRD pattern of x¼ 0 alloy and inset is showing splitting of (220) austenite peak for other compositions.
the increase of temperature, magnetization is nearly constant upto a temperature (Tn). Tn is believed to be exchange bias blocking temperature and has been confirmed from FC hysteresis loop measurements [7]. Further increase of temperature gives rise a step like anomaly continued upto a temperature Ts (spin freezing temperature), followed by an abrupt rise in magnetization correspond to structural transformation (at TM) thereafter the increase of temperature lead to ferromagnetic to paramagnetic (FM–PM) type magnetic transition (at T C A ). The characteristic transition temperatures for start and finish of forward (Tms and Tmf) and reverse (Tas and Taf) martensite transformation is indicated in Fig. 2. All the characteristic temperatures were determined from the intersections of straight lines extrapolated from both sides of the bended portion of M (T) curves. The martensite or structural transformation temperature TM was determined using TM ¼ (Tms þTaf )/2 equation. It can be clearly seen that the austenite phase is FM in nature but the magnetic nature of martensite phase is quite complex (FM and AFM coexists in this phase). A thermal hysteresis between FCC and FCW curves is an indication of first order nature of structural transition [6]. If one compare the M (T) curves of different alloys, it can be seen that the structural transition for x ¼3 and x ¼5 alloy occurred in a wide range of temperature which indicate that the magneto-structural coupling diminishes for x ¼3 and 5 alloy. From the M (T) one can also find that the TM increases with the Al concentration. It is well known that the e/a and volume of the unit cell are the two important parameters to determine the structural stability of this type of alloy. There have been numerous studies in transition metal based heusler alloys, where a systematic variation of the physical properties are observed with the e/a and unit cell volume. In the case of Sb, the total numbers of s þp electrons are 5, while for Al it is 3. So, replacement of Sb by Al will therefore decrease in e/a in this alloy compositions. In accordance with Hume–Rothery rule, lattice instability would expect to increase with e/a [6]. Therefore, increasing Al concentration would lead to decrease in TM. But we have seen that with the Al doping, TM start to increase and this phenomenon cannot be explained on the basis of e/a ratio. So, we
Fig. 2. The ZFCW, FCC and FCW M (T) curves for (a) x¼ 0, (b) x¼ 1, (c) x¼ 3 and (d) x¼ 5 alloys.
Please cite this article as: M.K. Ray, et al., Physica B (2014), http://dx.doi.org/10.1016/j.physb.2014.04.054i
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need to consider the unit cell volume to explain our observation. Replacing Sb (atomic radius 1.82 Å) by Al (atomic radius 1.43 Å) decreases the cell volume in austenite phase. The decreased cell volume increase the hybridization between 3d states of Ni and Mn atoms which contribute to stabilize the martensite phase and hence increase the TM [8]. There is splitting between FCC and ZFCW M (T) just below the T C A and this splitting increase after the system undergoes martensitic transformation. A similar tendency has been reported in many other Ni–Mn based bulk polycrystalline systems due to presence of some antiferromagnetic components embedded in ferromagnetic matrix [6]. In the martensite phase, contraction of lattice parameter leads to decrease in Mn–Mn distance. Now due to decrease in Mn–Mn distance, antiferromagnetic (AFM) interaction gets strengthen and this enhanced AFM interaction takes a toll on FM and hence the drop in magnetization around TM arises. The variation of characteristic temperatures (TAC, TM and Tn) with the Al concentrations and corresponding
Fig. 3. Variation in of TM and T C A with the Al concentration and corresponding magnetic phase in this temperature range.
3
magnetic phase diagram is shown in Fig. 3. The total scenario of M (T) during cooling is PM austenite transformed to FM austenite and then on further lowering the temperature the system goes to a mixed FM–AFM martensite and finally a martensite phase with strong AFM anisotropy is observed below Tn. It is well known that off-stoichiometric Ni–Mn–Z (Z¼ Sn, In, Sb) alloys shows exchange bias at lower temperature for the composition where the ferromagnetic and antiferromagnetic exchange interaction compete. As it is identified that cell parameters gets modified during the martensite transformation and this change in interatomic distance modifies the exchange interaction. Because of this modification in exchange interaction, AFM region can pin the FM region in different configurations. Now due to different spin configurations, an extra field is required to overcome the microscopic torque arising from different spin configurations. As a result hysteresis loop shifted towards the negative field axis for FC magnetic hysteresis loop. In order to quantify the exchange bias field, we measured magnetic hysteresis (M (H)) loop in the field range 40 kOerH r þ 40 kOe at 5 K after cooling the sample from 350 K under different cooling field (Hcool). It can be seen that the exchange bias field (HEB) goes through a maxima (nearly 0.3 kOe for x ¼0, 1 and 3 alloys) on the increase of Hcool. The cooling field dependence of HEB is shown in the inset of Fig.4(a)–(d). Initially cooling field helps to increase the HEB by reducing the FM/AFM interface width. A similar variation of HEB with the Hcool observed for Ni–Mn–Sb ribbon [9]. Now we consider Hcool ¼0.3 kOe as at this field HEB found to be maximum and measured FC M (H) loop at different temperatures to understand the dependence of AFM anisotropy on temperature for all these alloys (see Fig. 5) [9]. The values of HEB and coercive field (HC) was calculated by using HEB ¼ (H1 þ H2)/2 and HC ¼|H1 H2/2 formula respectively, where H1 and H2 denote the negative and positive field at where the magnetization become zero. From the Fig.5, one can easily see that the HEB decreases continuously with the increasing temperature and disappear near to a temperature (Tn) and hence Tn is called exchange bias blocking temperature [7]. One can assume that the anisotropy of AFM weakens as the
Fig. 4. FC 300 Oe M (H) loop at 5 K for (a) x ¼ 0 (b) x¼ 1 (c) x¼ 3 and (d) x ¼5 alloys and inset of every panel is showing variation of HEB with Hcool.
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Fig. 5. Temperature variation of HEB and HC for all the alloys calculated from FC 300 Oe M (H) loop.
temperature progress and thus AFM spins cannot pin the FM spins, resulting no exchange bias above their respective Tn. One more interesting fact is that the decrease of HEB with temperature is different for different alloy compositions. If we look into the HEB curve for x¼ 3 alloy, one can see that the HEB value at 5 K and the dHEB/dT is less than that for x ¼0 alloy. This may be due to the fact that the strength of exchange interaction and the anisotropy of AFM have different temperature dependency for different alloy composition. The HC initially increases with the temperature and shows a maximum near to Tn and then decreases with the temperature. As the temperature progress i.e., anisotropy decreases, the FM spins are able to drag more AFM spins giving rise to increase in HC [7,10]. The magnetic entropy change due to the application of magnetic field (H) was calculated using the Maxwell's equation RH ΔSM ¼ 0 ð∂M=∂TÞdH [11]. The usual numerical approximation of the integral is to use the isothermal magnetization measurement at small discrete temperature intervals by equation. ΔSM ¼ ∑ i
Mi þ 1 Mi ΔH i Tiþ 1 Ti
The Mi þ 1 and Mi denote the experimental values of magnetization at Ti þ 1 and Ti under the application of field Hi. The positive sign of observed ΔSM near to the respective TM is an indication of the inverse magnetocaloric effect (IMCE). Such positive ΔSM can be understood by considering the magnetic ordering of austenite and martensite phase. If we consider the forward martensite transformation i.e., transition of the system from its austenite to martensite phase during cooling, one can readily find that the relative magnetic ordering of austenite phase is higher than the martensite phase. Now in such condition the magnetic entropy of austenite phase (SM A ) has lower value compared to the magnetic entropy of martensite phase (SM M ) leading to the net magnetic entropy change ΔSM ¼SM M SM A 40 and hence the positive ΔSM near to their respective TM is observed. The maximum observed ΔSM near around the TM for x ¼1 alloy was found to be 10 J/kg-K for 50 kOe field change (ΔH). The variation of ΔSM as function of Al concentration is shown in the inset of Fig.6. Such a variation of ΔSM with the Al concentration can be explained as follows: when the Al concentration is low (xr 1) the magnetic ordering of austenite phase is increased quite reasonably compared to x ¼0 alloy (i.e., SM A is reduced for x r1) on the other hand the magnetic ordering of martensite phase (just below Tmf) is also reduced (i.e., SM M is increased for xr 1) (see Fig. 2). Now as ΔSM depend on the
Fig. 6. The temperature variation of ΔSM for x ¼1 alloy at ΔH¼ 50 kOe and inset is showing the same for all compositions.
difference in degree of magnetic ordering between austenite and martensite phase, thus ΔSM is enhanced for x¼ 1 alloy. For x ¼3 and 5 alloy, the magnetic ordering of austenite phase is reduced (i.e., SM A is increased) but in the martensite phase magnetic ordering is increased substantially (i.e., SM M is decreased) thus as a net effect a decrease in ΔSM around the TM is observed. On the similar note if we see the M (T) curves for all the alloys, it is clear that the magnetization difference between austenite and martensite phase is less for x¼ 3 and 5 alloy and also the martensite transition width is larger. Now, since ΔSM depends on ð∂M=∂TÞ and its value is small for x ¼3 and 5 alloy and hence we observe the lower value of ΔSM.
4. Conclusions In conclusion, we have seen that after Al doping in Ni50Mn37Sb13 alloy the TM increases and this can be explain by considering the enhanced hybridization between Ni and Mn atom due to the contraction in lattice parameter. The observation of exchange bias is mainly due to co-existence of FM and AFM in lower temperature. This AFM interaction induced in the system during martensite transformation due to the modification in interatomic distance, more specifically is due to the decreased in Mn–Mn distance. The decrease in ΔSM for x¼ 3 and 5 alloy due to the increase of relative magnetic ordering of martensite phase of these alloy compositions. The decreased in ΔSM and decrease in HEB for x¼ 3 and 5 is due the enhancement of ferromagnetic magnetic ordering in the martensite phase.
Acknowledgments The author likes to thank material science division of VECC, Kolkata for XRD facility and UGC for financial assistance. References [1] R. Knainuma, Y. Imano, W. Ito, Y. Sutou, H. Morito, S. Okamoto, O. Kitakami, K. Oikawa, A. Fujita, T. Kanomata, K. Isida, Nature 439 (2006) 957. [2] M. Khan, I. Dubenko, S. Stadler, N. Ali, Appl. Phys. Lett. 91 (2007) 072510. [3] B.M. Wang, Y. Liu, L. Wang, S.L. Huang, Y. Zhao, H. Zhang, Appl. Phys. Lett. 104 (2008) 043916. [4] A. Planes, L. Manosa, M. Acet, J. Phys.: Condens. Matter 21 (2009) 233201.
Please cite this article as: M.K. Ray, et al., Physica B (2014), http://dx.doi.org/10.1016/j.physb.2014.04.054i
M.K. Ray et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎ [5] D.Y. Cong, S. Roth, M. Potschke, C. Hurrich, L. Schultz, Appl. Phys. Lett. 97 (2010) 021908. [6] T. Krenke, M. Acet, E.F. Wassermann, X. Moya, L. Manosa, A. Planes, Phys. Rev. B 72 (2005) 014412. [7] Z. li, C. Jing, J. Chen, S. Yuan, S. Cao, J. Zhang, Appl. Phys. Lett. 91 (2007) 112505. [8] V.V. Kokorin, I.A. Osipenko, T.V. Shirina, Phys. Metals Metallogr. 67 (1989) 173.
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[9] M.K. Ray, K. Bagani, R.K. Singh, B. Majumdar, S. Banerjee, J. Appl. Phys. 114 (2013) 123094. [10] A.K. Nayek, M. Nicklas, S. Chadov, Y. Skourski, J. Winterlik, C. Felser, Phys. Rev. Lett. 110 (2013) 127204. [11] M. Khan, I. Dubenko, S. Stadker, N. Ali, J. Appl. Phys. 102 (2007) 113914.
Please cite this article as: M.K. Ray, et al., Physica B (2014), http://dx.doi.org/10.1016/j.physb.2014.04.054i
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Effect of Al doping on structural and magnetic properties of Ni50Mn37AlxSb13 x alloy Mayukh K. Ray a, K. Bagani a, R.K. Singh b, B. Majumdar b, S. Banerjee a,n a b
Surface Physics Division, Saha Institute of Nuclear Physics, Kolkata 700064, India Defense Metrological Research Laboratory, Hyderabad 500058, India
art ic l e i nf o
a b s t r a c t
Keywords: Magnetism Exchange bias Magnetocaloric Martensite Ferromagnetism Heusler alloy
The Ni50Mn37AlxSb13 x (x ¼0, 1, 3 and 5) alloys were prepared by tri-arc melting technique. The replacement of Sb by Al increases the martensitic transformation temperature (TM) as well as ferromagnetic to paramagnetic transformation temperature (T C A ) within the austenite phase. The increase in TM is found to due to the enhancement of hybridization between 3d states of Ni and Mn atoms. We also observed a large exchange bias field (HEB) of 470 Oe for x ¼ 0 and it decreases with the Al concentration for field cooled (FC) magnetic hysteresis loop. A large magnetic entropy change (ΔSM) of 10 J/kg-K is found for x ¼ 1 alloy under a field change (ΔH) of 50 kOe and it decreased for further higher concentration of Al doping. The possible reasons for observed behaviors are discussed. & 2014 Published by Elsevier B.V.
1. Introduction There have been renewed interest in the transition metal based Heusler alloys particularly Ni–Mn–Z (Z ¼Sn,Sb,In) type, primarily due to their multifunctional properties that includes ferromagnetic shape memory effect (FMSME), magnetocaloric effect (MCE), magnetoresistance (MR) and exchange bias (EB) effect [1–5]. Most of these above mentioned applicable properties are arises due to the coupling between the structure and its magnetic nature. Heusler alloy is a typical system to study the ordered atomic configuration and magnetic coupling for intermetallic compounds. Therefore, synthesizing and investigating new systems becomes attractive in this field. In structural transformation, the alloys undergo transition from its higher symmetric austenite phase to lower symmetric martensite phase upon cooling. At high temperature austenite phase crystallize into the cubic L21 structure comprising four interpenetrating FCC sublattices, while at lower temperature martensite phase may have tetragonal, orthogonal or monoclinic structures [6]. Basically, the magnetism of Heusler alloy is considered to be a local moment system and the moment is confined to the Mn atoms of Ni–Mn based alloys [4]. The Mn–Mn indirect exchange interaction (Rudermann–Kittel– Kasuya–Yoshida type) plays the key role to determine the magnetic nature of Heusler alloys. Recently off-stoichoimetric Ni–Mn–Sb alloy is drawing much attention because of their
n
Corresponding author. E-mail address: [email protected] (S. Banerjee).
multifunctional properties. In this study we tried to find out a material with the above mentioned properties near to room temperature. With this motivation we prepared a series of Ni50Mn37AlxSb13 x (x ¼0, 1, 3, 5) alloys, where we replace Sb by nonmagnetic Al and studied their structure and magnetic properties.
2. Experimental details The polycrystalline Ni50Mn37Sb13 xAlx (where x¼ 0, 1, 3, 5) was prepared by arc melting technique in argon atmosphere using Ni, Mn, Sb and Al of 99.98% purity (Alfa Aesar). To make sure about the compositional homogeneity the sample was flipped and re-melted four times. The structural characterization was done by powder X-Ray diffraction (XRD) using CuKα radiation. The compositional analysis was done by Energy Dispersive X-ray analysis (EDAX) attached in scanning electron microscopy (SEM). The magnetic measurements were carried out in physical property measurement system (PPMS, Quantum design).
3. Results and discussion The room temperature X-Ray diffraction pattern (see Fig. 1.), using CuKα confirms that x¼ 0 and 1 alloys are crystallized into highly ordered cubic L21 phase (only for x ¼1 alloy is shown in Fig. 1). For x ¼3 there are martensite peaks near to the main (220) austenite peak and x ¼5 is in martensite phase (see inset of Fig. 1).
http://dx.doi.org/10.1016/j.physb.2014.04.054 0921-4526/& 2014 Published by Elsevier B.V.
Please cite this article as: M.K. Ray, et al., Physica B (2014), http://dx.doi.org/10.1016/j.physb.2014.04.054i
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The observation of martensite peaks for x¼ 3 might be due to the presence of martensitic transition (MT) near room temperature, where both the phases coexists. The XRD pattern of x ¼3 is indexed with mixed structure of cubic austenite and 4 layered orthorhombic (4O) structure while x¼ 5 alloy is indexed with 4 layered orthorhombic (4O) martensite structure. The calculated lattice parameters correspond to cubic austenite phase of x¼ 0, 1, and 3 alloys are 5.967, 5.965, 5.964 Å, respectively. The decrease in cell parameter with the increase of Al concentration is an indication of contraction of cell volume in the austenite phase. The cell volume contraction is attributed due to Al (1.43 Å) has lower atomic radius than Sb (1.82 Å). The zero field cooled warming (ZFCW), field cooled cooling (FCC) and field cooled warming (FCW) thermo-magnetization (M (T)) for all the alloys are shown in Fig. 2. Now if we look at the ZFCW M (T) curves (for x ¼0, 1, and 3), it can be seen that with
θ Fig. 1. The room temperature XRD pattern of x¼ 0 alloy and inset is showing splitting of (220) austenite peak for other compositions.
the increase of temperature, magnetization is nearly constant upto a temperature (Tn). Tn is believed to be exchange bias blocking temperature and has been confirmed from FC hysteresis loop measurements [7]. Further increase of temperature gives rise a step like anomaly continued upto a temperature Ts (spin freezing temperature), followed by an abrupt rise in magnetization correspond to structural transformation (at TM) thereafter the increase of temperature lead to ferromagnetic to paramagnetic (FM–PM) type magnetic transition (at T C A ). The characteristic transition temperatures for start and finish of forward (Tms and Tmf) and reverse (Tas and Taf) martensite transformation is indicated in Fig. 2. All the characteristic temperatures were determined from the intersections of straight lines extrapolated from both sides of the bended portion of M (T) curves. The martensite or structural transformation temperature TM was determined using TM ¼ (Tms þTaf )/2 equation. It can be clearly seen that the austenite phase is FM in nature but the magnetic nature of martensite phase is quite complex (FM and AFM coexists in this phase). A thermal hysteresis between FCC and FCW curves is an indication of first order nature of structural transition [6]. If one compare the M (T) curves of different alloys, it can be seen that the structural transition for x ¼3 and x ¼5 alloy occurred in a wide range of temperature which indicate that the magneto-structural coupling diminishes for x ¼3 and 5 alloy. From the M (T) one can also find that the TM increases with the Al concentration. It is well known that the e/a and volume of the unit cell are the two important parameters to determine the structural stability of this type of alloy. There have been numerous studies in transition metal based heusler alloys, where a systematic variation of the physical properties are observed with the e/a and unit cell volume. In the case of Sb, the total numbers of s þp electrons are 5, while for Al it is 3. So, replacement of Sb by Al will therefore decrease in e/a in this alloy compositions. In accordance with Hume–Rothery rule, lattice instability would expect to increase with e/a [6]. Therefore, increasing Al concentration would lead to decrease in TM. But we have seen that with the Al doping, TM start to increase and this phenomenon cannot be explained on the basis of e/a ratio. So, we
Fig. 2. The ZFCW, FCC and FCW M (T) curves for (a) x¼ 0, (b) x¼ 1, (c) x¼ 3 and (d) x¼ 5 alloys.
Please cite this article as: M.K. Ray, et al., Physica B (2014), http://dx.doi.org/10.1016/j.physb.2014.04.054i
M.K. Ray et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
need to consider the unit cell volume to explain our observation. Replacing Sb (atomic radius 1.82 Å) by Al (atomic radius 1.43 Å) decreases the cell volume in austenite phase. The decreased cell volume increase the hybridization between 3d states of Ni and Mn atoms which contribute to stabilize the martensite phase and hence increase the TM [8]. There is splitting between FCC and ZFCW M (T) just below the T C A and this splitting increase after the system undergoes martensitic transformation. A similar tendency has been reported in many other Ni–Mn based bulk polycrystalline systems due to presence of some antiferromagnetic components embedded in ferromagnetic matrix [6]. In the martensite phase, contraction of lattice parameter leads to decrease in Mn–Mn distance. Now due to decrease in Mn–Mn distance, antiferromagnetic (AFM) interaction gets strengthen and this enhanced AFM interaction takes a toll on FM and hence the drop in magnetization around TM arises. The variation of characteristic temperatures (TAC, TM and Tn) with the Al concentrations and corresponding
Fig. 3. Variation in of TM and T C A with the Al concentration and corresponding magnetic phase in this temperature range.
3
magnetic phase diagram is shown in Fig. 3. The total scenario of M (T) during cooling is PM austenite transformed to FM austenite and then on further lowering the temperature the system goes to a mixed FM–AFM martensite and finally a martensite phase with strong AFM anisotropy is observed below Tn. It is well known that off-stoichiometric Ni–Mn–Z (Z¼ Sn, In, Sb) alloys shows exchange bias at lower temperature for the composition where the ferromagnetic and antiferromagnetic exchange interaction compete. As it is identified that cell parameters gets modified during the martensite transformation and this change in interatomic distance modifies the exchange interaction. Because of this modification in exchange interaction, AFM region can pin the FM region in different configurations. Now due to different spin configurations, an extra field is required to overcome the microscopic torque arising from different spin configurations. As a result hysteresis loop shifted towards the negative field axis for FC magnetic hysteresis loop. In order to quantify the exchange bias field, we measured magnetic hysteresis (M (H)) loop in the field range 40 kOerH r þ 40 kOe at 5 K after cooling the sample from 350 K under different cooling field (Hcool). It can be seen that the exchange bias field (HEB) goes through a maxima (nearly 0.3 kOe for x ¼0, 1 and 3 alloys) on the increase of Hcool. The cooling field dependence of HEB is shown in the inset of Fig.4(a)–(d). Initially cooling field helps to increase the HEB by reducing the FM/AFM interface width. A similar variation of HEB with the Hcool observed for Ni–Mn–Sb ribbon [9]. Now we consider Hcool ¼0.3 kOe as at this field HEB found to be maximum and measured FC M (H) loop at different temperatures to understand the dependence of AFM anisotropy on temperature for all these alloys (see Fig. 5) [9]. The values of HEB and coercive field (HC) was calculated by using HEB ¼ (H1 þ H2)/2 and HC ¼|H1 H2/2 formula respectively, where H1 and H2 denote the negative and positive field at where the magnetization become zero. From the Fig.5, one can easily see that the HEB decreases continuously with the increasing temperature and disappear near to a temperature (Tn) and hence Tn is called exchange bias blocking temperature [7]. One can assume that the anisotropy of AFM weakens as the
Fig. 4. FC 300 Oe M (H) loop at 5 K for (a) x ¼ 0 (b) x¼ 1 (c) x¼ 3 and (d) x ¼5 alloys and inset of every panel is showing variation of HEB with Hcool.
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Fig. 5. Temperature variation of HEB and HC for all the alloys calculated from FC 300 Oe M (H) loop.
temperature progress and thus AFM spins cannot pin the FM spins, resulting no exchange bias above their respective Tn. One more interesting fact is that the decrease of HEB with temperature is different for different alloy compositions. If we look into the HEB curve for x¼ 3 alloy, one can see that the HEB value at 5 K and the dHEB/dT is less than that for x ¼0 alloy. This may be due to the fact that the strength of exchange interaction and the anisotropy of AFM have different temperature dependency for different alloy composition. The HC initially increases with the temperature and shows a maximum near to Tn and then decreases with the temperature. As the temperature progress i.e., anisotropy decreases, the FM spins are able to drag more AFM spins giving rise to increase in HC [7,10]. The magnetic entropy change due to the application of magnetic field (H) was calculated using the Maxwell's equation RH ΔSM ¼ 0 ð∂M=∂TÞdH [11]. The usual numerical approximation of the integral is to use the isothermal magnetization measurement at small discrete temperature intervals by equation. ΔSM ¼ ∑ i
Mi þ 1 Mi ΔH i Tiþ 1 Ti
The Mi þ 1 and Mi denote the experimental values of magnetization at Ti þ 1 and Ti under the application of field Hi. The positive sign of observed ΔSM near to the respective TM is an indication of the inverse magnetocaloric effect (IMCE). Such positive ΔSM can be understood by considering the magnetic ordering of austenite and martensite phase. If we consider the forward martensite transformation i.e., transition of the system from its austenite to martensite phase during cooling, one can readily find that the relative magnetic ordering of austenite phase is higher than the martensite phase. Now in such condition the magnetic entropy of austenite phase (SM A ) has lower value compared to the magnetic entropy of martensite phase (SM M ) leading to the net magnetic entropy change ΔSM ¼SM M SM A 40 and hence the positive ΔSM near to their respective TM is observed. The maximum observed ΔSM near around the TM for x ¼1 alloy was found to be 10 J/kg-K for 50 kOe field change (ΔH). The variation of ΔSM as function of Al concentration is shown in the inset of Fig.6. Such a variation of ΔSM with the Al concentration can be explained as follows: when the Al concentration is low (xr 1) the magnetic ordering of austenite phase is increased quite reasonably compared to x ¼0 alloy (i.e., SM A is reduced for x r1) on the other hand the magnetic ordering of martensite phase (just below Tmf) is also reduced (i.e., SM M is increased for xr 1) (see Fig. 2). Now as ΔSM depend on the
Fig. 6. The temperature variation of ΔSM for x ¼1 alloy at ΔH¼ 50 kOe and inset is showing the same for all compositions.
difference in degree of magnetic ordering between austenite and martensite phase, thus ΔSM is enhanced for x¼ 1 alloy. For x ¼3 and 5 alloy, the magnetic ordering of austenite phase is reduced (i.e., SM A is increased) but in the martensite phase magnetic ordering is increased substantially (i.e., SM M is decreased) thus as a net effect a decrease in ΔSM around the TM is observed. On the similar note if we see the M (T) curves for all the alloys, it is clear that the magnetization difference between austenite and martensite phase is less for x¼ 3 and 5 alloy and also the martensite transition width is larger. Now, since ΔSM depends on ð∂M=∂TÞ and its value is small for x ¼3 and 5 alloy and hence we observe the lower value of ΔSM.
4. Conclusions In conclusion, we have seen that after Al doping in Ni50Mn37Sb13 alloy the TM increases and this can be explain by considering the enhanced hybridization between Ni and Mn atom due to the contraction in lattice parameter. The observation of exchange bias is mainly due to co-existence of FM and AFM in lower temperature. This AFM interaction induced in the system during martensite transformation due to the modification in interatomic distance, more specifically is due to the decreased in Mn–Mn distance. The decrease in ΔSM for x¼ 3 and 5 alloy due to the increase of relative magnetic ordering of martensite phase of these alloy compositions. The decreased in ΔSM and decrease in HEB for x¼ 3 and 5 is due the enhancement of ferromagnetic magnetic ordering in the martensite phase.
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