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Magnetocaloric effect in Ni2+xMn1−xGa Heusler alloys
Physics Letters A 326 (2004) 146–151 www.elsevier.com/locate/pla
Magnetocaloric effect in Ni2+x Mn1−xGa Heusler alloys A.A. Cherechukin a,∗ , T. Takagi a , M. Matsumoto b , V.D. Buchel’nikov c a Institute of Fluid Science, Tohoku University, Sendai 980-8577, Japan b Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan c Chelyabinsk State University, 454021 Chelyabinsk, Russia
Received 7 March 2004; accepted 20 March 2004 Available online 20 April 2004 Communicated by V.M. Agranovich
Abstract Compositional dependence of magnetic entropy change in Ni2+x Mn1−x Ga Heusler alloys was investigated experimentally. The largest entropy change S = 20.7 ± 1.5 J/K kg in the magnetic field H = 1.8 T was revealed in the Ni2.18 Mn0.82 Ga alloy at a magneto-structural phase transition temperature of Ts = 333.2 K. Magnetic-field-induced structural phase transition was observed. 2004 Elsevier B.V. All rights reserved. PACS: 75.30.Sg; 64.70.Kb; 75.50.Cc; 75.20.En Keywords: Magnetic and structural phase transitions; Magnetocaloric effect; Entropy change; Ni2 MnGa
1. Introduction In recent years the materials with high magnetocaloric effect (MCE) have attracted considerable attention as a refrigerant at magnetic refrigeration [1–3]. Magnetic refrigeration is an environmentally friendly cooling technology and more energy efficiently as compared with vapour-cycle ones. This makes MCE interesting for basic research and investigation meaning possible technological application. Many different ferromagnets have been investigated in an attempt to achieve large MCE. Among them only a rare-earth metal Gd and Gd-containing * Corresponding author.
E-mail address: [email protected] (A.A. Cherechukin). 0375-9601/$ – see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2004.03.072
materials Gd5 (Si1−x Gex ) [1–3] have been since long considered as useful materials for room-temperature magnetic refrigerants owing to their large MCE at close to room temperatures with a magnetic entropy change S of about 12–45 J/K kg in comparatively low magnetic fields H up to 2 T. Recently large MCE with S up to 30 J/K kg at 1 T has been established in new materials such as polycrystalline perovskite manganese oxides [4], rare-earth-based compounds [1–3], NaZn13-type [5] and manganese compounds [6]. Some of them exhibit simultaneously a magnetic and a structural phase transition (SPT) [1–3,6]. Ferromagnetic shape memory alloys are intensively investigated in recent years. There are such Ni–Mn– Ga-based Heusler alloys undergone SPT with a shape
A.A. Cherechukin et al. / Physics Letters A 326 (2004) 146–151
memory effect at ferromagnetic state [7–15]. These alloys also seem to be promising materials for investigation of MCE. The up to 5 J/K kg revealed value of S in Ni–Mn–Ga [1,7–9] was reached at by only changing magnetization without SPT. Entropy change equals 18 J/K kg [8] during SPT at H = 5 T. The best results obtained for MCE materials for coupled magneto-SPT is S = 20 J/K kg [9] at a comparatively small magnetic field change of H = 1.6 T. The possibility to control both direct and reverse SPT in Ni–Mn–Ga Heusler alloys through a magnetic field [10] can provide the addition latent heat up to 11 J/g [11]. The SPT can occur at a wide temperature region (4–626 K) and can coincide with the magnetic transition in principle due to changing of composition [10,11]. Last may also be a reason to change the saturated magnetization and probably the value of MCE. Thus the goal of the present Letter is the investigation of MCE in Ni2+x Mn1−x Ga alloys with different compositions of x in order to determine the ones that provide the largest MCE.
147
(a)
2. Experimental procedure Polycrystalline ingots of Ni2+x Mn1−x Ga with composition x = 0.16; 0.18; 0.19; 0.20; 0.21 were prepared by arc melting of high-purity (99.99%) initial elements in argon gas atmosphere on a cold bottom. Then they were annealed at 1100 K for 9 days and quenched in ice water. A vibrating sample magnetometer was used for magnetic measurements and determination the regions of start Ms and finish Mf temperatures of the direct SPT, start As and finish Af temperatures of the reverse SPT, and the Curie point TC . The cooling at the magnetization-curves determination was very slow to approach a stable temperature. The temperature was controlled by a thermocouple within the accuracy of 0.1 K. For the different temperature regions the temperature step varies from 1 to 2.5 K. 3. Results and their discussion Figs. 1–5(a) show the M(H )-curves of the Ni2+x Mn1−x Ga samples with composition x = 0.16; 0.18; 0.19; 0.20; 0.21 for selected values of the temperature. The curves are determined during cooling at the temperature regions in which magnetic transition
(b) Fig. 1. (a) Magnetization and (b) magnetic entropy change, the insert displays magnetization at H = 0.05 T with TC and SPT temperatures of Ni2.18 Mn0.82 Ga at 326–351 K.
and SPT occur. It is also clearly seen from the M(H ) graphs (Figs. 1–5) that in the alloys at the same temperature, saturated magnetization MSat decreases with the decrease in the Mn-contents (Fig. 6). We can obtain the magnetic entropy change by using the numerical experimental M(H ) dependences in the following way. From the Maxwell relation [16] the entropy change in a magnetic field H from start Hs to finish Hf is given by the formula: Hf S = Hs
∂M ∂T
dH. H
(1)
148
A.A. Cherechukin et al. / Physics Letters A 326 (2004) 146–151
(a)
(a)
(b)
(b)
Fig. 2. (a) Magnetization and (b) magnetic entropy change, the insert displays magnetization at H = 0.05 T with TC and SPT temperatures of Ni2.19 Mn0.81 Ga at 298–351 K.
Fig. 3. (a) Magnetization and (b) magnetic entropy change, the insert displays magnetization at H = 0.05 T with TC and SPT temperatures of Ni2.20 Mn0.80 Ga at 313–369 K.
Numerically integrating Eq. (1) by using a trapezoidal rule we obtain (as it is shown in [3]) an entropy change at an average temperature Tav = (Tu + Tl )/2: n−1 H M1 + 2 S(Tav ) = Mk + Mn , (2) 2T
The calculated magnetic entropy change dependencies on temperature using (2) in a magnetic field H = 1.8 T for the studied alloys are presented in Figs. 1–5(b). The largest entropy change S = 20.7±1.5 J/K kg in magnetic field H = 1.8 T is revealed in Ni2.18 Mn0.82Ga alloy at Ts = 333.2 K (Fig. 1(b)). The temperature Ts is approximately equal to the temperature of the coupled magneto-SPT in the alloy. The magneto-SPT induced by changes of the magnetic field observed in the alloy is shown in Fig. 1(a). It is indicated by different magnetic curves at the same con-
k=2
where temperature difference T = Tu − Tl , Mk is the difference in the values of neighbouring magnetization curves, measured at up Tu and low Tl temperatures at the appointed magnetic field Hk = (k − 1) × H, H = const.
A.A. Cherechukin et al. / Physics Letters A 326 (2004) 146–151
149
(a)
(a)
(b)
(b)
Fig. 4. (a) Magnetization and (b) magnetic entropy change, the insert displays magnetization at H = 0.05 T with TC and SPT temperatures of Ni2.21 Mn0.79 Ga at 309–355 K.
Fig. 5. (a) Magnetization and (b) magnetic entropy change at magnetic field H = 0.25 T (2), 0.7 T ("), 1.8 T (Q), the insert displays magnetization at H = 0.05 T with TC and SPT temperatures of Ni2.16 Mn0.84 Ga at 306–360 K.
stant temperatures. Increasing of magnetic field causes an increase in the concentration of the martensite [10], which possesses ferromagnetic properties (in contrast to austenite in this case) and has a larger magnetization at the same magnetic field and temperature. So the magnetization increases due to both the magnetic field and the magnetic-field-induced direct SPT. When the magnetic field decreases the reverse SPT is not so much pronounced at the same temperature because of the hysteresis of the first-order transition (see the insert with M(T ) dependence in the Fig. 1(b)).
In other alloys with coinciding magnetic and SPT the entropy changes decrease with decreasing Mnconcentration, decreasing in this way saturated magnetization at the same temperature Figs. 2–4. They have a positive sign in the interval of magnetic field H = 0–1.8 T and equal 15.8 ± 1.3, 6.0 ± 0.5, 5.5 ± 0.5 J/K kg for Ni2+x Mn1−x Ga with x = 0.19; 0.20; 0.21, nearby magneto-SPT temperature Ts ≈ 341.8, 350, 320.6 K, respectively (Figs. 2–4(b)). The magnetic-field-induced SPT is less pronounced and
150
A.A. Cherechukin et al. / Physics Letters A 326 (2004) 146–151
Fig. 6. Saturated magnetization MSat and magnetic entropy change S versus composition x in Ni2+x Mn1−x Ga alloys at 306 K.
clearly observed only in the alloy with a concentration of x = 0.19 (Fig. 2(a)). In the alloy Ni2.16 Mn0.84Ga, that undergo SPT at ferromagnetic state when TC > Af , we observed quite different behavior of S because of the next reasons. Magnetization of parent phase is more easily saturated than that of the martensitic phase, because the martensitic phase has higher magnetocrystalline anisotropy energy than the parent phase (see Fig. 5(a)) [13–15]. In the low magnetic field magnetization of the parent phase is higher than the martensitic phase. The magnetizations become equal at approximately 0.25–0.3 T, and subsequently at the higher magnetic fields the magnetization of the martensite becomes higher than that of the parent phase. This behavior in magnetization of the alloy leads to the following results in magnetic entropy change. In magnetic field H = 0.25 T, at SPT, entropy change is negative and equals −0.7 ± 0.1 J/K kg at martensitic transition temperature TM = 312.4 K (Fig. 5(b)). S changes its sign at an approximate magnetic field of H = 0.7 T starting from the high-temperature side, and rapidly increases in the 0.7–1.8 T field interval. Thus at H = 1.8 T S = 10.4 ± 1.1 J/K kg and has the narrow temperature span (up to 5 K). At the same time magnetic entropy change at the Curie point TC = 337.3 K equals 3.6 ± 0.4 J/K kg and spreads over 10 K, demonstrating the same tendency like one described in Refs. [1,9]. It is very surprising that the magnetic entropy change in this alloy at SPT is several
times bigger than that at the Curie point. That can be useful for magnetically refrigeration, because of the possibility of utilizing the additional latent heat of the first-order SPT in these alloys. Now we can compare the results. In the Fig. 6, the saturated magnetization MSat at approximately 306 K when all of the alloys are in ferromagnetic martensite state, and magnetic entropy change S depending on Mn-content are presented. It is clearly seen that MSat increase with the Mn-content increasing, because of the relatively large Mn magnetic moments ∼ 4µB [13,15]. The exchange interactions between the magnetic moments are strongly dependent on the Mn– Mn distance and therefore can be control by chemical composition. The entropy change is also correlated with the Mn-content and increases with increasing MSat in the alloys with coupled magneto-SPT. It gives the opportunity to increase the MCE by this way. However, Ni2.16 Mn0.84Ga alloy with separated magnetic and SPT is an exception from the rule.
4. Conclusion The dependence of magnetic entropy change on compositional x = 0.16; 0.18; 0.19; 0.20; 0.21 in Ni2+x Mn1−x Ga Heusler alloys was investigated. The largest entropy change S = 20.7 ± 1.5 J/K kg in magnetic field H = 1.8 T is revealed in Ni2.18 Mn0.82Ga alloy at magneto-SPT temperature Ts = 333.2 K. This alloy displays magneto-SPT, induced by changes in the magnetic field. In the alloy with TC > Af , the magnetic entropy change at SPT changes its sign with the increasing of the magnetic field and in magnetic field H = 1.8 T is several times bigger than the one at the Curie point. A correlation between saturated magnetization and magnetic entropy change is established. It is found that increasing of saturated magnetization leads to an increase of magnetic entropy change in the alloys with coupled magneto-SPT.
Acknowledgement The work was partly supported by the Russian Ministry of Education, Grant-in-Aid No. E02-3.4-35.
A.A. Cherechukin et al. / Physics Letters A 326 (2004) 146–151
References [1] O. Tegus, E. Bruck, L. Zhang, Dagula, K.H.J. Buschow, F.R. Boer, Physica B 319 (2002) 174. [2] V.K. Pecharsky, K.A. Gshneidner Jr., Phys. Rev. Lett. 78 (1997) 4494. [3] V.K. Pecharsky, K.A. Gshneidner Jr., J. Appl. Phys. 86 (1999) 565. [4] X. Bohigas, J. Tejada, E. Barco, X.X. Zhang, M. Sales, Appl. Phys. Lett. 73 (1998) 390. [5] S. Fujieda, A. Fujita, K. Fukamichi, Appl. Phys. Lett. 81 (2002) 1276. [6] H. Wada, Y. Tanabe, Appl. Phys. Lett. 79 (2001) 3302. [7] J. Marcos, A. Planes, L. Manosa, F. Casanova, X. Battle, A. Labarta, B. Martinez, Phys. Rev. B 66 (2002) 224413. [8] F.X. Hu, B.G. Shen, J.R. Sun, G.H. Wu, Phys. Rev. B 64 (2001) 132412.
151
[9] L. Pareti, M. Solzi, F. Albertini, A. Paoluzi, Eur. Phys. J. B 32 (2003) 303. [10] I.E. Dikshtein, D.I. Ermakov, V.V. Koledov, L.V. Koledov, T. Takagi, A.A. Tulaikova, A.A. Cherechukin, V.G. Shavrov, JETP Lett. 72 (2000) 373. [11] V.A. Chernenko, E. Cesari, V.V. Kokorin, I.N. Vitenko, Scripta Metal. Mater. 33 (1995) 1239. [12] A.A. Cherechukin, T. Takagi, H. Miki, M. Matsumoto, M. Ohtsuka, J. Appl. Phys. 95 (2004) 1740. [13] P.J. Webster, K.R.A. Ziebeck, S.L. Town, M.S. Peak, Philos. Mag. B 49 (1984) 295. [14] K. Ullakko, J.K. Huang, C. Kantner, R.C. O‘Handley, V.V. Kokorin, Appl. Phys. Lett. 69 (1996) 1966. [15] R. Tickle, R.D. James, J. Magn. Magn. Mater. 195 (1999) 627. [16] L.D. Landau, E.M. Lifshitz, L.P. Pitaevskii, Electrodynamics of Continuous Media, in: Course of Theoretical Physics, vol. 8, Pergamon, Elmsford, NY, 1984.
Magnetocaloric effect in Ni2+x Mn1−xGa Heusler alloys A.A. Cherechukin a,∗ , T. Takagi a , M. Matsumoto b , V.D. Buchel’nikov c a Institute of Fluid Science, Tohoku University, Sendai 980-8577, Japan b Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan c Chelyabinsk State University, 454021 Chelyabinsk, Russia
Received 7 March 2004; accepted 20 March 2004 Available online 20 April 2004 Communicated by V.M. Agranovich
Abstract Compositional dependence of magnetic entropy change in Ni2+x Mn1−x Ga Heusler alloys was investigated experimentally. The largest entropy change S = 20.7 ± 1.5 J/K kg in the magnetic field H = 1.8 T was revealed in the Ni2.18 Mn0.82 Ga alloy at a magneto-structural phase transition temperature of Ts = 333.2 K. Magnetic-field-induced structural phase transition was observed. 2004 Elsevier B.V. All rights reserved. PACS: 75.30.Sg; 64.70.Kb; 75.50.Cc; 75.20.En Keywords: Magnetic and structural phase transitions; Magnetocaloric effect; Entropy change; Ni2 MnGa
1. Introduction In recent years the materials with high magnetocaloric effect (MCE) have attracted considerable attention as a refrigerant at magnetic refrigeration [1–3]. Magnetic refrigeration is an environmentally friendly cooling technology and more energy efficiently as compared with vapour-cycle ones. This makes MCE interesting for basic research and investigation meaning possible technological application. Many different ferromagnets have been investigated in an attempt to achieve large MCE. Among them only a rare-earth metal Gd and Gd-containing * Corresponding author.
E-mail address: [email protected] (A.A. Cherechukin). 0375-9601/$ – see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2004.03.072
materials Gd5 (Si1−x Gex ) [1–3] have been since long considered as useful materials for room-temperature magnetic refrigerants owing to their large MCE at close to room temperatures with a magnetic entropy change S of about 12–45 J/K kg in comparatively low magnetic fields H up to 2 T. Recently large MCE with S up to 30 J/K kg at 1 T has been established in new materials such as polycrystalline perovskite manganese oxides [4], rare-earth-based compounds [1–3], NaZn13-type [5] and manganese compounds [6]. Some of them exhibit simultaneously a magnetic and a structural phase transition (SPT) [1–3,6]. Ferromagnetic shape memory alloys are intensively investigated in recent years. There are such Ni–Mn– Ga-based Heusler alloys undergone SPT with a shape
A.A. Cherechukin et al. / Physics Letters A 326 (2004) 146–151
memory effect at ferromagnetic state [7–15]. These alloys also seem to be promising materials for investigation of MCE. The up to 5 J/K kg revealed value of S in Ni–Mn–Ga [1,7–9] was reached at by only changing magnetization without SPT. Entropy change equals 18 J/K kg [8] during SPT at H = 5 T. The best results obtained for MCE materials for coupled magneto-SPT is S = 20 J/K kg [9] at a comparatively small magnetic field change of H = 1.6 T. The possibility to control both direct and reverse SPT in Ni–Mn–Ga Heusler alloys through a magnetic field [10] can provide the addition latent heat up to 11 J/g [11]. The SPT can occur at a wide temperature region (4–626 K) and can coincide with the magnetic transition in principle due to changing of composition [10,11]. Last may also be a reason to change the saturated magnetization and probably the value of MCE. Thus the goal of the present Letter is the investigation of MCE in Ni2+x Mn1−x Ga alloys with different compositions of x in order to determine the ones that provide the largest MCE.
147
(a)
2. Experimental procedure Polycrystalline ingots of Ni2+x Mn1−x Ga with composition x = 0.16; 0.18; 0.19; 0.20; 0.21 were prepared by arc melting of high-purity (99.99%) initial elements in argon gas atmosphere on a cold bottom. Then they were annealed at 1100 K for 9 days and quenched in ice water. A vibrating sample magnetometer was used for magnetic measurements and determination the regions of start Ms and finish Mf temperatures of the direct SPT, start As and finish Af temperatures of the reverse SPT, and the Curie point TC . The cooling at the magnetization-curves determination was very slow to approach a stable temperature. The temperature was controlled by a thermocouple within the accuracy of 0.1 K. For the different temperature regions the temperature step varies from 1 to 2.5 K. 3. Results and their discussion Figs. 1–5(a) show the M(H )-curves of the Ni2+x Mn1−x Ga samples with composition x = 0.16; 0.18; 0.19; 0.20; 0.21 for selected values of the temperature. The curves are determined during cooling at the temperature regions in which magnetic transition
(b) Fig. 1. (a) Magnetization and (b) magnetic entropy change, the insert displays magnetization at H = 0.05 T with TC and SPT temperatures of Ni2.18 Mn0.82 Ga at 326–351 K.
and SPT occur. It is also clearly seen from the M(H ) graphs (Figs. 1–5) that in the alloys at the same temperature, saturated magnetization MSat decreases with the decrease in the Mn-contents (Fig. 6). We can obtain the magnetic entropy change by using the numerical experimental M(H ) dependences in the following way. From the Maxwell relation [16] the entropy change in a magnetic field H from start Hs to finish Hf is given by the formula: Hf S = Hs
∂M ∂T
dH. H
(1)
148
A.A. Cherechukin et al. / Physics Letters A 326 (2004) 146–151
(a)
(a)
(b)
(b)
Fig. 2. (a) Magnetization and (b) magnetic entropy change, the insert displays magnetization at H = 0.05 T with TC and SPT temperatures of Ni2.19 Mn0.81 Ga at 298–351 K.
Fig. 3. (a) Magnetization and (b) magnetic entropy change, the insert displays magnetization at H = 0.05 T with TC and SPT temperatures of Ni2.20 Mn0.80 Ga at 313–369 K.
Numerically integrating Eq. (1) by using a trapezoidal rule we obtain (as it is shown in [3]) an entropy change at an average temperature Tav = (Tu + Tl )/2: n−1 H M1 + 2 S(Tav ) = Mk + Mn , (2) 2T
The calculated magnetic entropy change dependencies on temperature using (2) in a magnetic field H = 1.8 T for the studied alloys are presented in Figs. 1–5(b). The largest entropy change S = 20.7±1.5 J/K kg in magnetic field H = 1.8 T is revealed in Ni2.18 Mn0.82Ga alloy at Ts = 333.2 K (Fig. 1(b)). The temperature Ts is approximately equal to the temperature of the coupled magneto-SPT in the alloy. The magneto-SPT induced by changes of the magnetic field observed in the alloy is shown in Fig. 1(a). It is indicated by different magnetic curves at the same con-
k=2
where temperature difference T = Tu − Tl , Mk is the difference in the values of neighbouring magnetization curves, measured at up Tu and low Tl temperatures at the appointed magnetic field Hk = (k − 1) × H, H = const.
A.A. Cherechukin et al. / Physics Letters A 326 (2004) 146–151
149
(a)
(a)
(b)
(b)
Fig. 4. (a) Magnetization and (b) magnetic entropy change, the insert displays magnetization at H = 0.05 T with TC and SPT temperatures of Ni2.21 Mn0.79 Ga at 309–355 K.
Fig. 5. (a) Magnetization and (b) magnetic entropy change at magnetic field H = 0.25 T (2), 0.7 T ("), 1.8 T (Q), the insert displays magnetization at H = 0.05 T with TC and SPT temperatures of Ni2.16 Mn0.84 Ga at 306–360 K.
stant temperatures. Increasing of magnetic field causes an increase in the concentration of the martensite [10], which possesses ferromagnetic properties (in contrast to austenite in this case) and has a larger magnetization at the same magnetic field and temperature. So the magnetization increases due to both the magnetic field and the magnetic-field-induced direct SPT. When the magnetic field decreases the reverse SPT is not so much pronounced at the same temperature because of the hysteresis of the first-order transition (see the insert with M(T ) dependence in the Fig. 1(b)).
In other alloys with coinciding magnetic and SPT the entropy changes decrease with decreasing Mnconcentration, decreasing in this way saturated magnetization at the same temperature Figs. 2–4. They have a positive sign in the interval of magnetic field H = 0–1.8 T and equal 15.8 ± 1.3, 6.0 ± 0.5, 5.5 ± 0.5 J/K kg for Ni2+x Mn1−x Ga with x = 0.19; 0.20; 0.21, nearby magneto-SPT temperature Ts ≈ 341.8, 350, 320.6 K, respectively (Figs. 2–4(b)). The magnetic-field-induced SPT is less pronounced and
150
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Fig. 6. Saturated magnetization MSat and magnetic entropy change S versus composition x in Ni2+x Mn1−x Ga alloys at 306 K.
clearly observed only in the alloy with a concentration of x = 0.19 (Fig. 2(a)). In the alloy Ni2.16 Mn0.84Ga, that undergo SPT at ferromagnetic state when TC > Af , we observed quite different behavior of S because of the next reasons. Magnetization of parent phase is more easily saturated than that of the martensitic phase, because the martensitic phase has higher magnetocrystalline anisotropy energy than the parent phase (see Fig. 5(a)) [13–15]. In the low magnetic field magnetization of the parent phase is higher than the martensitic phase. The magnetizations become equal at approximately 0.25–0.3 T, and subsequently at the higher magnetic fields the magnetization of the martensite becomes higher than that of the parent phase. This behavior in magnetization of the alloy leads to the following results in magnetic entropy change. In magnetic field H = 0.25 T, at SPT, entropy change is negative and equals −0.7 ± 0.1 J/K kg at martensitic transition temperature TM = 312.4 K (Fig. 5(b)). S changes its sign at an approximate magnetic field of H = 0.7 T starting from the high-temperature side, and rapidly increases in the 0.7–1.8 T field interval. Thus at H = 1.8 T S = 10.4 ± 1.1 J/K kg and has the narrow temperature span (up to 5 K). At the same time magnetic entropy change at the Curie point TC = 337.3 K equals 3.6 ± 0.4 J/K kg and spreads over 10 K, demonstrating the same tendency like one described in Refs. [1,9]. It is very surprising that the magnetic entropy change in this alloy at SPT is several
times bigger than that at the Curie point. That can be useful for magnetically refrigeration, because of the possibility of utilizing the additional latent heat of the first-order SPT in these alloys. Now we can compare the results. In the Fig. 6, the saturated magnetization MSat at approximately 306 K when all of the alloys are in ferromagnetic martensite state, and magnetic entropy change S depending on Mn-content are presented. It is clearly seen that MSat increase with the Mn-content increasing, because of the relatively large Mn magnetic moments ∼ 4µB [13,15]. The exchange interactions between the magnetic moments are strongly dependent on the Mn– Mn distance and therefore can be control by chemical composition. The entropy change is also correlated with the Mn-content and increases with increasing MSat in the alloys with coupled magneto-SPT. It gives the opportunity to increase the MCE by this way. However, Ni2.16 Mn0.84Ga alloy with separated magnetic and SPT is an exception from the rule.
4. Conclusion The dependence of magnetic entropy change on compositional x = 0.16; 0.18; 0.19; 0.20; 0.21 in Ni2+x Mn1−x Ga Heusler alloys was investigated. The largest entropy change S = 20.7 ± 1.5 J/K kg in magnetic field H = 1.8 T is revealed in Ni2.18 Mn0.82Ga alloy at magneto-SPT temperature Ts = 333.2 K. This alloy displays magneto-SPT, induced by changes in the magnetic field. In the alloy with TC > Af , the magnetic entropy change at SPT changes its sign with the increasing of the magnetic field and in magnetic field H = 1.8 T is several times bigger than the one at the Curie point. A correlation between saturated magnetization and magnetic entropy change is established. It is found that increasing of saturated magnetization leads to an increase of magnetic entropy change in the alloys with coupled magneto-SPT.
Acknowledgement The work was partly supported by the Russian Ministry of Education, Grant-in-Aid No. E02-3.4-35.
A.A. Cherechukin et al. / Physics Letters A 326 (2004) 146–151
References [1] O. Tegus, E. Bruck, L. Zhang, Dagula, K.H.J. Buschow, F.R. Boer, Physica B 319 (2002) 174. [2] V.K. Pecharsky, K.A. Gshneidner Jr., Phys. Rev. Lett. 78 (1997) 4494. [3] V.K. Pecharsky, K.A. Gshneidner Jr., J. Appl. Phys. 86 (1999) 565. [4] X. Bohigas, J. Tejada, E. Barco, X.X. Zhang, M. Sales, Appl. Phys. Lett. 73 (1998) 390. [5] S. Fujieda, A. Fujita, K. Fukamichi, Appl. Phys. Lett. 81 (2002) 1276. [6] H. Wada, Y. Tanabe, Appl. Phys. Lett. 79 (2001) 3302. [7] J. Marcos, A. Planes, L. Manosa, F. Casanova, X. Battle, A. Labarta, B. Martinez, Phys. Rev. B 66 (2002) 224413. [8] F.X. Hu, B.G. Shen, J.R. Sun, G.H. Wu, Phys. Rev. B 64 (2001) 132412.
151
[9] L. Pareti, M. Solzi, F. Albertini, A. Paoluzi, Eur. Phys. J. B 32 (2003) 303. [10] I.E. Dikshtein, D.I. Ermakov, V.V. Koledov, L.V. Koledov, T. Takagi, A.A. Tulaikova, A.A. Cherechukin, V.G. Shavrov, JETP Lett. 72 (2000) 373. [11] V.A. Chernenko, E. Cesari, V.V. Kokorin, I.N. Vitenko, Scripta Metal. Mater. 33 (1995) 1239. [12] A.A. Cherechukin, T. Takagi, H. Miki, M. Matsumoto, M. Ohtsuka, J. Appl. Phys. 95 (2004) 1740. [13] P.J. Webster, K.R.A. Ziebeck, S.L. Town, M.S. Peak, Philos. Mag. B 49 (1984) 295. [14] K. Ullakko, J.K. Huang, C. Kantner, R.C. O‘Handley, V.V. Kokorin, Appl. Phys. Lett. 69 (1996) 1966. [15] R. Tickle, R.D. James, J. Magn. Magn. Mater. 195 (1999) 627. [16] L.D. Landau, E.M. Lifshitz, L.P. Pitaevskii, Electrodynamics of Continuous Media, in: Course of Theoretical Physics, vol. 8, Pergamon, Elmsford, NY, 1984.