013906-3 Ingale et al. J.Appl.Phys.102.013906(2007 Cooling Tc 2 Ing FIG. 4. DSC thermograms in(a)Nis4s Mn2o3 Ga 4g:(b) Niss Mni& Ga26 and(c)Niss., MnI& Ga26.7 alloys showing the forward (on cooling)and nermomagnetic curves in (a) Nis.sMn203Gaz4g:(b) Niss MnI8gGa26: and(c)Niss.2 Mn. Ga267 alloys Niss.2Mn18 1 Ga26.7, presents an altogether different magneto- B. Microstructure structural transformation process. The ferromagnetic marten site has first transformed into the ferromagnetic austenite and The SEM images of the elongated thin plates or strips in then into the paramagnetic austenite. These two transforma- the Nis4. Mn203Ga249 [ Fig. 2(a)] and Niss,8. Ga26. 1 [Fig. tions can be clearly seen in Fig 3. A decrease in the Tc value 2(b)] infer the martensite twins of the alloys. There is an with an increase in the Ga in these alloys can be attributed to bvious difference in the NM and 7M martensites in the twin weakening the Mn-Mn magnetic exchange coupling in the widths in the two samples. Relatively fine twins constitute Mn-Ga pairing 7M modulated Niss MnI8. Ga26. alloy in Fig. 2(b). Also, In Fig 4, the DSC experiments corroborate the simulta- Jiang et al. observed such fine twins in a Niss Mn18gGa26 neous structural and magnetic transformations. The values of alloy. No such distinct twins are observed in the the transformation temperatures obtained from the DSC Nis 2 Mn18. Ga26.7 alloy [Fig. 2(c)]consisting of the austenite curves are given in Table I. The symbols A, and A refer to phase. Thus, a common implication of the two SEM and the austenite start and the austenite finish temperatures, re- XRD independent studies is the presence of NM, 7M, and spectively, in the heating cycle, while those of M, and M, austenite phases in the three alloys, respectively refer to the martensite start and the martensite finish tem- In the 14/mmm tetragonal crystal unit cell, Ga occupies peratures, respectively, in the cooling cycle. The TM value is the corners and body center, with Mn between any one pair estimated as(M, +A)/2 from the DSC data. It can be seen of Ga along the c-axis Ni occupies the face centers such that from Table I that the TM value is improved to 300 K or even two of them lie at 4 and 4 heights. This specific system has more in Nis4sMn203 Ga249 or Niss Mn,.1 alloys from a a twin plane (112) and, according to the Murray's model 267 K value in the Niss.2 Mn181 Ga26.7 alloy lattice, a partial substitution among Ni/Mn/Ga must reflect The enthalpy changes AH were estimated to be 7.5, 6.0, in a concomitant shrinkage in the final lattice of the cubic and 4.7 J/g from the Dsc thermograms in the system. Nevertheless, it is quite possible that the presence of Nis4&Mn2o3 Ga249, NissMn8o Ga261, and Niss.2 Mn8, Ga26 the twin plane tunes a direction change in the lattice at the alloys, respectively. The Nis48Mn203 9 alloy, which has twin boundaries. It adds a shear of the lattice about a plane the martensite transformation from NM to austenite, involves normal to the [110] direction, and that reflects in the con- a larger AH value than the Niss Mn189Ga261 alloy of the comitant lattice expansion. As mentioned above, the accom- modulated structure (7M). The volume free-energy change panying strain from a shear lattice in the parent austenite on forming martensite from austenite at the equilibrium phase results in formation of a modulated structure of the transformation temperature To is martensite phase. Hence, a twinned microstructure of alloy can also be taken as a fingerprint in analyzing XRD of for mation of a modulated Ni-Mn-Ga structure TABLE L The transformation temperatures and the ela ratio in the three C Magnetic and thermal properties TA (K (K) ela As demonstrated from the thermomagnetic curves in Fig. NieMn Ga 340 364 338 318 351 7.65 3, the Nis4. 8Mn20..9 and NissMn18. Ga26.1 alloys exhibi Niss MnIggGa2. 3143283102963197.61 a single transformation of Tc and Ty values very close to Niss.3MnIg.Ga266253272262 each other. The mixed ferromagnetic martensite and the fer- A, and Ae-austenite start and austenite finish temperatures, respectively. romagnetic austenite phases have transformed into the para- M, and M -martensite start and martensite finish temperatures, respec- magnetic austenite phase at Tc. The third alloy,B. Microstructure The SEM images of the elongated thin plates or strips in the Ni54.8Mn20.3Ga24.9 Fig. 2
a and Ni55Mn18.9Ga26.1 Fig. 2
b infer the martensite twins of the alloys. There is an obvious difference in the NM and 7M martensites in the twin widths in the two samples. Relatively fine twins constitute 7M modulated Ni55Mn18.9Ga26.1 alloy in Fig. 2
b. Also, Jiang et al.25 observed such fine twins in a Ni55Mn189Ga26 alloy. No such distinct twins are observed in the Ni55.2Mn18.1Ga26.7 alloy Fig. 2
c consisting of the austenite phase. Thus, a common implication of the two SEM and XRD independent studies is the presence of NM, 7M, and austenite phases in the three alloys, respectively. In the I4 /mmm tetragonal crystal unit cell,3 Ga occupies the corners and body center, with Mn between any one pair of Ga along the c-axis. Ni occupies the face centers such that two of them lie at 1 4 and 3 4 heights. This specific system has a twin plane 112 and, according to the Murray’s model lattice3 , a partial substitution among Ni/Mn/Ga must reflect in a concomitant shrinkage in the final lattice of the cubic system. Nevertheless, it is quite possible that the presence of the twin plane tunes a direction change in the lattice at the twin boundaries. It adds a shear of the lattice about a plane normal to the 110 direction, and that reflects in the con￾comitant lattice expansion. As mentioned above, the accom￾panying strain from a shear lattice in the parent austenite phase results in formation of a modulated structure of the martensite phase. Hence, a twinned microstructure of alloy can also be taken as a fingerprint in analyzing XRD of for￾mation of a modulated Ni-Mn-Ga structure. C. Magnetic and thermal properties As demonstrated from the thermomagnetic curves in Fig. 3, the Ni54.8Mn20.3Ga24.9 and Ni55Mn18.9Ga26.1 alloys exhibit a single transformation of TC and TM values very close to each other. The mixed ferromagnetic martensite and the fer￾romagnetic austenite phases have transformed into the para￾magnetic austenite phase at TC. The third alloy, Ni55.2Mn18.1Ga26.7, presents an altogether different magneto￾structural transformation process. The ferromagnetic marten￾site has first transformed into the ferromagnetic austenite and then into the paramagnetic austenite. These two transforma￾tions can be clearly seen in Fig. 3. A decrease in the TC value with an increase in the Ga in these alloys can be attributed to weakening the Mn-Mn magnetic exchange coupling in the Mn-Ga pairing. In Fig. 4, the DSC experiments corroborate the simulta￾neous structural and magnetic transformations. The values of the transformation temperatures obtained from the DSC curves are given in Table I. The symbols As and Af refer to the austenite start and the austenite finish temperatures, re￾spectively, in the heating cycle, while those of Ms and Mf refer to the martensite start and the martensite finish tem￾peratures, respectively, in the cooling cycle. The TM value is estimated as
Ms+Af/2 from the DSC data. It can be seen from Table I that the TM value is improved to 300 K or even more in Ni54.8Mn20.3Ga24.9 or Ni55Mn18.9Ga26.1 alloys from a 267 K value in the Ni55.2Mn18.1Ga26.7 alloy. The enthalpy changes
H were estimated to be 7.5, 6.0, and 4.7 J/g from the DSC thermograms in the Ni54.8Mn20.3Ga24.9, Ni55Mn18.9Ga26.1, and Ni55.2Mn18.1Ga26.7 alloys, respectively. The Ni54.8Mn20.3Ga24.9 alloy, which has the martensite transformation from NM to austenite, involves a larger
H value than the Ni55Mn18.9Ga26.1 alloy of the modulated structure
7M. The volume free-energy change on forming martensite from austenite at the equilibrium transformation temperature T0 is FIG. 3. Thermomagnetic curves in
a Ni54.8Mn20.3Ga24.9;
b Ni55Mn18.9Ga26.1; and
c Ni55.2Mn18.1Ga26.7 alloys. FIG. 4. DSC thermograms in
a Ni54.8Mn20.3Ga24.9;
b Ni55Mn18.9Ga26.1; and
c Ni55.2Mn18.1Ga26.7 alloys showing the forward
on cooling and re￾verse
on heating transformation temperatures. TABLE I. The transformation temperatures and the e/a ratio in the three alloys. Alloy As a
K Af a
K Ms b
K Mf b
K TM
K e/a Ni54.8Mn20.3Ga24.9 340 364 338 318 351 7.65 Ni55Mn18.9Ga26.1 314 328 310 296 319 7.61 Ni55.3Mn18.1Ga26.6 253 272 262 243 267 7.58 a As and Af:-austenite start and austenite finish temperatures, respectively. b Ms and Mf:-martensite start and martensite finish temperatures, respec￾tively. 013906-3 Ingale et al. J. Appl. Phys. 102, 013906 2007