013906-4 Ingale et al. J.Appl.Phys.102.013906(2007 TABLE IL. The values for austenite/martensite peak temperatures (A, /Mp), AT, enthalpies(AH)and As observed in the three alloys Room-temperature A △H(J/g) (K)(K)(K) Heating Cooling (/kg K) NisagMn2o 3Ga249 0(at33 NissMnI8gGa26. -6.3-5.2(at307K Niss. Mni8, Ga26.6 4.7 1.3(at264K) △G=△H-ToAS=0 (1) D. Magnetocaloric effect The ASm is an important parameter in quantifying the where AG, and AS are the changes in Gibbs free energy and entropy, respectively. At martensite transformation peak tem- agnetocaloric effect(MCE) in such a specific class of ma- perature MP, transitions. As the Nis4gMn20.3Ga249 and NissMni&9Ga26 alloys involve such features, they are of interest for MCE △G=△H-M2AS<0 (2) and devices. The isothermal magnetization measurements in ig. 5 were explored to estimate the ASM values. As the Combining these two equations isothermal magnetization characteristics are very similar in these two alloys, only the selective plots are included in Fig △G=(△H△n 5 from the representative alloys Nis4. 8Mn20. Ga24.9 and where AT is the undercooling(To-Mp), and approximately the magnetization decreases, with the increase of temperature quals I/2(Ap-Mp; AH is negative for the forward marten- displaying an appreciable discontinuous decrease in the vi- sitic transformation. Since the absolute enthalpy and Ar for cinity of the transition temperatures. The discontinuous nonmodulated are higher than those for 7M, as shown in change of the magnetization is of a desirably large value in Table i. the the Nis4.8Mn20. Ga24.9 alloy. The ASM values were estimated from the isotherms in Fig. 5 in the Maxwell relation △GM<△G2M<0 M(H, T) SEM, and thermomagnetic observations. The Tm value im- alloys assume the values of -7.0 Jlkg K at s85.U We have observed that the ag value for the nm martensii △SM(T,H= is lower than the value in the 7M structure, and that is why its behavior is more stable in performing a reverse martensite where M is the magnetization at field H and temperature transformation, leading to a higher As value consistent with The ASM values estimated at different temperatures the DsC analysis. the three alloys are portrayed in Fig. 6. Th Obviously, the DSC results well corroborate the XRD, Nis4Mn203 Ga249, NissMni8gGa261, and Niss 2 Mn, proved in improving the e/a value(see Table I) in the -5.2 J/kg K at 307 K, and.3 J/kg K at 264 K, respe Nis4. 8Mn203 Ga24.9 and NissMn18. Ga26. 1 alloys. An increase tively, in a magnetic field change 1. 2 T. A large ASm value in the Mn/Ga ratio results in an increase in the ela ratio in achieved in the Nis4 Mn203 Ga24.9 alloy is due to the concur- these specific compositions rent occurrence of the magnetostructural transformations In 3k}(b) 动菇K MAAA 328K 283 FIG. 5. Magnetization isotherms in 335K (b) Niss,2 Mn8. Ga26.7 alloys. 338K ★342K ∴3 Magnetic field(kOe) Magnetic field( kOe)
Gv =
H − T0
S  0,
1 where
Gv and
S are the changes in Gibbs free energy and entropy, respectively. At martensite transformation peak tem￾perature MP,
Gv =
H − Mp
S  0.
2 Combining these two equations,
Gv =

H
T/T0, where
T is the undercooling
T0−Mp, and approximately equals 1/2
Ap−Mp;
H is negative for the forward marten￾sitic transformation.25 Since the absolute enthalpy and
T for nonmodulated are higher than those for 7M, as shown in Table II, then
Gv NM 
Gv 7M  0. We have observed that the
Gv value for the NM martensite is lower than the value in the 7M structure, and that is why its behavior is more stable in performing a reverse martensite transformation, leading to a higher As value consistent with the DSC analysis. Obviously, the DSC results well corroborate the XRD, SEM, and thermomagnetic observations. The TM value im￾proved in improving the e/a value
see Table I in the Ni54.8Mn20.3Ga24.9 and Ni55Mn18.9Ga26.1 alloys. An increase in the Mn/Ga ratio results in an increase in the e/a ratio in these specific compositions. D. Magnetocaloric effect The
SM is an important parameter in quantifying the magnetocaloric effect
MCE in such a specific class of ma￾terials of simultaneously incurring magnetostructural phase transitions. As the Ni54.8Mn20.3Ga24.9 and Ni55Mn18.9Ga26.1 alloys involve such features, they are of interest for MCE and devices. The isothermal magnetization measurements in Fig. 5 were explored to estimate the
SM values. As the isothermal magnetization characteristics are very similar in these two alloys, only the selective plots are included in Fig. 5 from the representative alloys Ni54.8Mn20.3Ga24.9 and Ni55.2Mn18.1Ga26.7. It can be observed that in both examples the magnetization decreases, with the increase of temperature displaying an appreciable discontinuous decrease in the vi￾cinity of the transition temperatures. The discontinuous change of the magnetization is of a desirably large value in the Ni54.8Mn20.3Ga24.9 alloy. The
SM values were estimated from the isotherms in Fig. 5 in the Maxwell relation,
SM
T, H =  H1 H2 M
H, T T H dH,
3 where M is the magnetization at field H and temperature T. The
SM values estimated at different temperatures in the three alloys are portrayed in Fig. 6. The Ni54.8Mn20.3Ga24.9, Ni55Mn18.9Ga26.1, and Ni55.2Mn18.1Ga26.7 alloys assume the values of −7.0 J/kg K at 332 K, −5.2 J/kg K at 307 K, and −1.3 J/kg K at 264 K, respec￾tively, in a magnetic field change 1.2 T. A large
SM value achieved in the Ni54.8Mn20.3Ga24.9 alloy is due to the concur￾rent occurrence of the magnetostructural transformations. In TABLE II. The values for austenite/martensite peak temperatures
Ap /Mp,
T, enthalpies

H and
S observed in the three alloys. Alloy Room-temperature structure Ap
K Mp
K
T
K
H
J/g
SM Heating Cooling
J/kg K Ni54.8Mn20.3Ga24.9 NM 346 326 10 7.5 −7.2 −7.0
at 332 K Ni55Mn18.9Ga26.1 7M 323 303 10 6.0 −6.3 −5.2
at 307 K Ni55.3Mn18.1Ga26.6 Austenite 259 249 5 4.7 −4.5 −1.3
at 264 K FIG. 5. Magnetization isotherms in
a Ni54.8Mn20.3Ga24.9 and
b Ni55.2Mn18.1Ga26.7 alloys. 013906-4 Ingale et al. J. Appl. Phys. 102, 013906 2007