JOURNAL OF APPLIED PHYSICS 102, 013906(2007) Magnetostructural transformation, microstructure, and magnetocaloric effect in Ni-Mn-Ga heusler alloys Babita Ingale Defence Metallurgical Research Laboratory, Hyderabad-500 058, India and Materials Science Centre Indian Institute of Technology, Kharagpur -721 302, India R. Gopalan, a)M. Manivel Raja, and V Chandrasekaran Defence Metallurgical Research laboratory, Hyderabad-500 058, India S Ram Materials Science Centre, Indian Institute of Technology, Kharagpur/ 302, India (Received 9 May 2007; accepted 21 May 2007: published online 5 July 2007 Magnetostructural transformation and the associated magnetic entropy change were investigated in Ni-rich ferromagnetic Heusler alloys. a direct transformation from the ferromagnetic martensite phase to the paramagnetic austenite phase was observed in selected Nis4.8 Mn2o3 Ga249 and Niss Mn,.1 two-alloy compositions. The magnetic and martensitic transformations were incurred at nearly the same temperature(351 K)in the Nis4.8Mn203Ga249 alloy. Such a typical composition involves a change of the magnetic entropy ASM as large as.0 J/kg K at 332 K in an applied magnetic field of 1.2 TO 2007 American Institute of Physics [DO:10.1063/1.2751489] . INTRODUCTION ported in detail by Albertini et al. The origin of the MC effect in the vicinity of the martensitic transition temperature Recently, interest in magnetocaloric (MC)technology in Ni-Mn-Ga alloys has been studied as a function of com- has grown significantly due to the development of new mag- position, which is expressed through the average number of etic materials such as Gd-Si-Ge, Mn-Fe-P-As, Ni-Mn-Ga, valence electron per atom (ela) and La-Fe-Si that exhibit large MC effect near room It is useful to tune the Ni-Mn-Ga alloy composition and temperature.-yIn the above series of MC materials, the fer- microstructure such that the two transitions coappear at tem- romagnetic Ni2 Ga Heusler alloys have attracted consider- perature as high as possible. Altering the stoichiometry of able interest for the application of magnetic Ni, Ga Heusler alloys toward the Ni-rich values uic entro py change ASy as Niso+ Mn 2s-xGa2s, x=0, 2, 3, and 5, the modified Ty and Tc large as-20.7 J/kg K at 333 K under 1. 8 T magnetic field transitions can shift at nearly the same temperature has been reported by Chernechukin et al. in Ni-Mn-Ga al-(2300 K). A typical Niss Mn2oGays composition met loys. It is ascribed to the coincidence of the martensite trans- such TM and Tc concurrence, with a small value ASM formation temperature TM and Curie temperature Tc in a =-0.68 J/kg K. In this work, an attempt is made to coupled magnetostructural transformation. A similar mecha- improve the properties further by tuning the Mn and Ga val nism is reported in Gds Si, Ge, alloys. -3The ASy value ues in this base alloy. The evolution of microstructural fea- (20.7 J/kg K) in Ni-Mn-Ga alloys is larger than that tures, magnetostructural transformation, and magnetic prop. (18.5 J/kg K) in Gds Si2 Gez alloys under common condi- erties is presented in this article tions near room temperature. The Ni-Mn-Ga alloys have other advantages over GdsSi, Ge, and other MC materials such as MnAs,_Sb, 13, I5 perovskite manganese oxides,,or unds. They are biocompatible, EXPERIMENTAL DETAILS easier to synthesize or fabricate in specific shape(ductile comparison to the oxides), and less expensive, especially in A Synthesis of the alloys comparison to the rare-earth or As(highly toxic) containing Three alloys, Nis4. Mn203 Ga24.9, Niss Mn18 Ga26 1, and compound Niss.2Mn18 1 Ga26., were prepared by vacuum arc melting of In a broad composition range, the Ni-Mn-Ga alloys un- ergo a martensitic transformation from a cubic(austenite)to copper crucible under high-purity argon atmosphere.The al a tetragonal/orthorhombic (martensite) phase upon cooling. loys recovered after each batch melting were reverted and The magnetostructural transformation is sensitive to the remelted(four times)to ensure homogeneous chemical dis composition.The compositional dependence of magnetic persion and alloying. The arc melted buttons were annealed and structural transformations in the martensitic at 1123 K for 2 days in vacuum followed by water quench Ni2+ Mn Gai+ (r+y+z=0) Heusler alloys has been re ing. Their chemical compositions were determined using an inductively coupled plasma-atomic emission spectroscopy Correspondingauthorelectronicmailrg_gopy(@yahoo.com (ICP-AES). Electron probe microanalysis(EPMA)was used 0021-8979/2007/102(1/013906/5/s23 102,013906-1 e 2007 American Institute of Physic
Magnetostructural transformation, microstructure, and magnetocaloric effect in Ni-Mn-Ga Heusler alloys Babita Ingale Defence Metallurgical Research Laboratory, Hyderabad – 500 058, India and Materials Science Centre, Indian Institute of Technology, Kharagpur – 721 302, India R. Gopalan,a M. Manivel Raja, and V. Chandrasekaran Defence Metallurgical Research Laboratory, Hyderabad – 500 058, India S. Ram Materials Science Centre, Indian Institute of Technology, Kharagpur – 721 302, India Received 9 May 2007; accepted 21 May 2007; published online 5 July 2007 Magnetostructural transformation and the associated magnetic entropy change were investigated in Ni-rich ferromagnetic Heusler alloys. A direct transformation from the ferromagnetic martensite phase to the paramagnetic austenite phase was observed in selected Ni54.8Mn20.3Ga24.9 and Ni55Mn18.9Ga26.1 two-alloy compositions. The magnetic and martensitic transformations were incurred at nearly the same temperature 351 K in the Ni54.8Mn20.3Ga24.9 alloy. Such a typical composition involves a change of the magnetic entropy SM as large as −7.0 J/kg K at 332 K in an applied magnetic field of 1.2 T. © 2007 American Institute of Physics. DOI: 10.1063/1.2751489 I. INTRODUCTION Recently, interest in magnetocaloric MC technology has grown significantly due to the development of new magnetic materials such as Gd-Si-Ge, Mn-Fe-P-As, Ni-Mn-Ga, and La-Fe-Si that exhibit large MC effect near room temperature.1–9 In the above series of MC materials, the ferromagnetic Ni2MnGa Heusler alloys have attracted considerable interest for the application of magnetic refrigeration.6,7,10–14 A magnetic entropy change SM as large as −20.7 J/kg K at 333 K under 1.8 T magnetic field has been reported by Chernechukin et al.9 in Ni-Mn-Ga alloys. It is ascribed to the coincidence of the martensite transformation temperature TM and Curie temperature TC in a coupled magnetostructural transformation.9 A similar mechanism is reported in Gd5Si2Ge2 alloys.1–3 The SM value −20.7 J/kg K in Ni-Mn-Ga alloys is larger than that −18.5 J/kg K in Gd5Si2Ge2 alloys under common conditions near room temperature.1,2 The Ni-Mn-Ga alloys have other advantages over Gd5Si2Ge2 and other MC materials such as MnAs1−xSbx, 13,15 perovskite manganese oxides,9,16 or rare-earth-based compounds.10 They are biocompatible, easier to synthesize or fabricate in specific shape ductile in comparison to the oxides, and less expensive, especially in comparison to the rare-earth or As highly toxic containing compounds. In a broad composition range, the Ni-Mn-Ga alloys undergo a martensitic transformation from a cubic austenite to a tetragonal/orthorhombic martensite phase upon cooling. The magnetostructural transformation is sensitive to the composition.9–13 The compositional dependence of magnetic and structural transformations in the martensitic Ni2+xMn1+yGa1+z x+y+z=0 Heusler alloys has been reported in detail by Albertini et al.17 The origin of the MC effect in the vicinity of the martensitic transition temperature in Ni-Mn-Ga alloys has been studied as a function of composition, which is expressed through the average number of valence electron per atom e/a. 12 It is useful to tune the Ni-Mn-Ga alloy composition and microstructure such that the two transitions coappear at temperature as high as possible. Altering the stoichiometry of Ni2MnGa Heusler alloys toward the Ni-rich values Ni50+xMn25−XGa25, x=0, 2, 3, and 5, the modified TM and TC transitions can shift at nearly the same temperature 300 K. 17,18 A typical Ni55Mn20Ga25 composition met such TM and TC concurrence, with a small value SM =−0.68 J/kg K.18 In this work, an attempt is made to improve the properties further by tuning the Mn and Ga values in this base alloy. The evolution of microstructural features, magnetostructural transformation, and magnetic properties is presented in this article. II. EXPERIMENTAL DETAILS A. Synthesis of the alloys Three alloys, Ni54.8Mn20.3Ga24.9, Ni55Mn18.9Ga26.1, and Ni55.2Mn18.1Ga26.7, were prepared by vacuum arc melting of high-purity 99.99% starting elements in a water-cooled copper crucible under high-purity argon atmosphere. The alloys recovered after each batch melting were reverted and remelted four times to ensure homogeneous chemical dispersion and alloying. The arc melted buttons were annealed at 1123 K for 2 days in vacuum followed by water quenching. Their chemical compositions were determined using an inductively coupled plasma-atomic emission spectroscopy ICP-AES. Electron probe microanalysis EPMA was used a Corresponding author; electronic mail: rg_gopy@yahoo.com JOURNAL OF APPLIED PHYSICS 102, 013906 2007 0021-8979/2007/1021/013906/5/$23.00 © 2007 American Institute of Physics 102, 013906-1
013906-2 Ingale et al. J.Appl.Phys.102.013906(2007 balance the contribution from the sample holder. The data were collected during the heating as well as the cooling cycles at a given 20 K/min rate. A hysteresis of the thermo- gram occurs in the two cycles due to thermochemistry of the aPec ample of MC effects of interest in this work s|a=06320mm 2|b=0.553 Orthorhombic(7M) c=0.5369 ca 1 alloys are nearly single phase with no detectable secondary phases. The Nis4. 8Mn203 Ga24.9 alloy has a nonmodulated (NM)tetragonal martensite structure with lattice parameters Diffraction angle 20(degree) a=0.5460 nm and c=0.6471 nm Fig. 1(a)]. The lissMn1&oGa261 alloy [Fig. 1(b)] resumes a seven-layer iGM -ray dittranon patterns of (a) Nis4 Mn 20 Ga24. (b) modulated(7M) structure of orthorhombic symmetry: a =0. 6320 nm. b=0.5573 nm. and c=0.5369 nm was ob served. The NM or 7M structures are analyzed by splitting of by confirming the alloys'chemistry. The as-cast alloy ingots the (202) peak into three peaks:(220), (202), and(022) were cut into small species and used for the proposed studies in this work Similar XRD patterns were reported in NissMn18 Ga26 B Measurements and analyses Mn- ga alloys in terms of xrd can be considered onlv as a The crystalline structure of the alloys was analyzed by preliminary observation rather than the confirmation. Using X-ray diffraction(XRD). The XRD patterns of the samples XRD data, Martynov et ad 2i reported c/a>l(a were measured (after cutting, crushing, and pulverizing the 0.5520 nm and c=0.6440 nm) for the NM martensite alloy ingots as a course powder)using a Philips diffracto- phase. The crystal structures of the five-layer modulated meter with 0.154 058 nm Cu Ka radiation. The microstruc (5M)and seven-layer modulated(7M) martensite phases are ture characterization was done using a Leo model 440i scan- still complex. Such structure can form in the martensite ning electron microscope(SEM) Magnetic proprieties were phase from the parent (p)austenite phase by a periodic shuf- measured with a DMS-1600 model vibrating sample magne- fling of the lattice along(110)[110]p or with a long-period tometer(VSM) with a magnetic field H up to 12 kOe. Ther- stacking of (110)p close-packed planes. 19.21 The basic unit momagnetic measurements were carried out at a fixed H cell of the 5M phase is approximated to a tetragonal or =500 Oe value in order to determine Ty and Tc values. monoclinic cell,cla<l, while that of the 7M phase is de Magnetization as a function of temperature was measured termined to be orthorhombic or monoclinic(a=0.6140 nm using a variable temperature cryostat attached to the VSM. b=0.5780 nm, and c=0.5510 nm) The data were collected during a heating cycle at a rate of 10 ence, in such a complex modulated structure of the K/min. The temperature was controlled within an accuracy martensite phase in Ni-Mn-Ga alloy, the c/a ratio is taken as +l K. For different regions, the temperature ste the measure to predict roughly the kind of the modulation; in this study we have adopted a similar approach, taking into A modulated differential scanning calorimeter (TA in- account the (202) splitting as we discussed above. It is evi- struments DSC model Q100)was used to monitor the heat dent from XRD in Fig. 1(c)that in the Niss. 2 Mn8 Ga26 How during magnetostructural transformations. The sample, alloy the austenite phase has tuned at the expense of the sealed in a standard aluminum cup with a lid, was measured martensite phase at room temperature. A small change in the against a similar cup with a lid under identical conditions to Mn/Ga ratio tunes the phase formation sensitively FIG. 2. Typical SEM images NisssMnxoaGaz49; Niss MniggGa26
by confirming the alloys’ chemistry. The as-cast alloy ingots were cut into small species and used for the proposed studies in this work. B. Measurements and analyses The crystalline structure of the alloys was analyzed by x-ray diffraction XRD. The XRD patterns of the samples were measured after cutting, crushing, and pulverizing the alloy ingots as a course powder using a Philips diffractometer with 0.154 058 nm Cu K radiation. The microstructure characterization was done using a Leo model 440i scanning electron microscope SEM Magnetic proprieties were measured with a DMS-1600 model vibrating sample magnetometer VSM with a magnetic field H up to 12 kOe. Thermomagnetic measurements were carried out at a fixed H =500 Oe value in order to determine TM and TC values. Magnetization as a function of temperature was measured using a variable temperature cryostat attached to the VSM. The data were collected during a heating cycle at a rate of 10 K/min. The temperature was controlled within an accuracy ±1 K. For different regions, the temperature steps were varied from 1 to 2.5 K. A modulated differential scanning calorimeter TA instruments DSC model Q100 was used to monitor the heat flow during magnetostructural transformations. The sample, sealed in a standard aluminum cup with a lid, was measured against a similar cup with a lid under identical conditions to balance the contribution from the sample holder. The data were collected during the heating as well as the cooling cycles at a given 20 K/min rate. A hysteresis of the thermogram occurs in the two cycles due to thermochemistry of the sample of MC effects of interest in this work. III. RESULTS AND DISCUSSION A. Phase transformation and crystal structure Analysis of XRD patterns in Fig. 1 reveals that all three alloys are nearly single phase with no detectable secondary phases. The Ni54.8Mn20.3Ga24.9 alloy has a nonmodulated NM tetragonal martensite structure with lattice parameters a=0.5460 nm and c=0.6471 nm Fig. 1a. The Ni55Mn18.9Ga26.1 alloy Fig. 1b resumes a seven-layer modulated 7M structure of orthorhombic symmetry; a =0.6320 nm, b=0.5573 nm, and c=0.5369 nm was observed. The NM or 7M structures are analyzed by splitting of the 202 peak into three peaks: 220, 202, and 022. Similar XRD patterns were reported in Ni55Mn18Ga26 alloy.19,20 The analysis of the modulated structure in these NiMn-Ga alloys in terms of XRD can be considered only as a preliminary observation rather than the confirmation. Using XRD data, Martynov et al.21 reported c/a1 a =0.5520 nm and c=0.6440 nm for the NM martensite phase. The crystal structures of the five-layer modulated 5M and seven-layer modulated 7M martensite phases are still complex. Such structure can form in the martensite phase from the parent p austenite phase by a periodic shuf- fling of the lattice along 110110P or with a long-period stacking of 110P close-packed planes.19,21 The basic unit cell of the 5M phase is approximated to a tetragonal or monoclinic cell, c/a1, while that of the 7M phase is determined to be orthorhombic or monoclinic a=0.6140 nm, b=0.5780 nm, and c=0.5510 nm. 21–24 Hence, in such a complex modulated structure of the martensite phase in Ni-Mn-Ga alloy, the c/a ratio is taken as the measure to predict roughly the kind of the modulation; in this study we have adopted a similar approach, taking into account the 202 splitting as we discussed above. It is evident from XRD in Fig. 1c that in the Ni55.2Mn18.1Ga26.7 alloy the austenite phase has tuned at the expense of the martensite phase at room temperature. A small change in the Mn/Ga ratio tunes the phase formation sensitively. FIG. 1. X-ray diffraction patterns of a Ni54.8Mn20.3Ga24.9; b Ni55Mn18.9Ga26.1; and c Ni55.2Mn18.1Ga26.7 alloys. FIG. 2. Typical SEM images in a Ni54.8Mn20.3Ga24.9; b Ni55Mn18.9Ga26.1; and c Ni55.2Mn18.1Ga26.7 alloys. 013906-2 Ingale et al. J. Appl. Phys. 102, 013906 2007
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. 2a and Ni55Mn18.9Ga26.1 Fig. 2b 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. 2b. 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. 2c 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 concomitant lattice expansion. As mentioned above, the accompanying 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 formation 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 ferromagnetic austenite phases have transformed into the paramagnetic austenite phase at TC. The third alloy, Ni55.2Mn18.1Ga26.7, presents an altogether different magnetostructural transformation process. The ferromagnetic martensite has first transformed into the ferromagnetic austenite and then into the paramagnetic austenite. These two transformations 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 simultaneous 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, respectively, in the heating cycle, while those of Ms and Mf refer to the martensite start and the martensite finish temperatures, 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 reverse 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, respectively. 013906-3 Ingale et al. J. Appl. Phys. 102, 013906 2007
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 − T0S 0, 1 where Gv and S are the changes in Gibbs free energy and entropy, respectively. At martensite transformation peak temperature MP, Gv = H − MpS 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 martensitic 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 improved 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 materials 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 vicinity 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, SMT, H = H1 H2 MH, 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, respectively, in a magnetic field change 1.2 T. A large SM value achieved in the Ni54.8Mn20.3Ga24.9 alloy is due to the concurrent 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 HJ/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
013906-5 Ingale et al. J.Appl.Phys.102.013906(2007 ACKNOWLEDGMENT The authors thank the Defence Research Development Organization(DRDO), India, for the financial support to carry out this work V. K. Pecharsky and K. A. Gschneidner, Jr, Phys. Rev.Lett.78,4494 V. K Pecharsky and K. A. Gschneidner, Jr, Appl. Phys. Lett. 70, 3299 3S. J. Murray, M. A Marioni, A M. Kukla, J. Robinson, R C O'Handley and S M. Allen, J. Appl. Phys. 87, 5774(2000) F-X.Hu. B.-G. Shen, J.R. Sun, Z.H Cheng, and X.X. Zhang, J Phys. Condens Matter 12, L691(2000) O. Tegus, E. Bruck, K. H.J. Buschow, and F R. de boer, Nature 415, 150 (2002) L. Pareti, M. Solzi, F. Albertini, and A. Paoluzi, Eur. Phys. J. B 32, 303 (2003) E. Bruck, O. Tegus, X. W. Li, F. R. de Boer, and K. H. J. Buschow, FIG.6. Magnetic entropy changes in the (a) Nis4sMn2o3Ga24g:(b)A. Fujita, S Fujieda, Y. Hasegawa, and K Fukamichi, Phys. Rev. B 67. MnIRgGa26.1: and(c) Niss 2 Mn& Ga26.7 alloys in the magnetic field 104416(2003) ge 1.2 T A. Cherechukin, T. Takagi, M. Matsumoto, and V. D. Buchel'nikov, Phys. the Niss. 2Mn181 Ga26.7 alloy, the ASM value distributes slowly F-X. Hu, B.G. Shen, and J -R. Sun, Appl. Phys. Lett. 76, 3460(2000) over a large span of temperature AT-16 K between the TM IJ. Marcos. A Planes. L. Manosa. F. Casanova. X. Battle. A. Labarta, and B. Martinez, Phys. Rev. B 66. 224413(2002) and Tc values. A coupled magnetostructural transformation, J. Marcos, L Manosa, A Planes, F Casanova, XBattle, and A. Labarata AT-0, and a large MCE appear to be interesting for active Phy.Rev.B68,094401(2003) E Bruck, J. Phys. D 38, R381(2005) magnetic refrigerant applications of Nis4&Mn,o Ga,4g alloy F Albertini, A.Paoluzi, LPareti,M.Solzi, L.Righi,E.Villa,S.Besseg- near room temperature hini,and F. Passaretti, J. Appl. Phys. 100, 023908(2006) L CONCLUSIONS H. Wada and Y. Tanabe, Appl. Phys. Lett. 79, 3302(2001) X. Bohigas, J. Tejada, E. Barco, X. X. Zhang, and M. Sales, Appl. Phys The nonmodulated martensite, the seven-layer Lett73,390(1998) E. Albertini, L. Pareti, A. Paoluzi, L. Morellon, P. A. Algarabel, M.R lated martensitic, and the austenite phases were obser Ibarra, and L. Righi, J. AppL. Phys. 81, 4032(2002) room temperature in Nis48Mn203 Ga249, Niss MnI8s 11. Babita, M. Manivel Raja, R Gopalan, V Chandrasekharan,and SRam, and Niss. 2 Mn,8. Ga26.7 alloys, respectively. Among these al- J. Alloys Compd. 432, 23(2007) loys, Nis4.8Mn203Ga24. displayed an enhanced MCE, with as J. Pons, V.AChernenko, R. Santamarta, and ECesari, Acta Mater.48 large ASM value as.0 J/kg K, at 332 K in an applied field 2U.Gaitzsch,SRoth,B.Rellinghaus, and L.Schultz,J. Magn.Magn. .2 T. The origin of the ASy is related to the coincidence of Mater..305,275(2006) structural and magnetic transition temperatures(TM, Tc V. V Martynov and VV Kokorin, J. Phys. m 2, 739(1992) 351 K). Such an appreciable ASy value in such low fields V.V.Martynov, J Phys. IV 5, 91(1995) will enable the material to operate in fields produced by per-YGe, O Soderberg, N Lanska, A Sozinov, K. Ullakko, and V.K. Lin manent magnets. Thus, the Nis4. Mn20.3Gaz49 alloy seems to , droos, J Phys. IV 2, 921(2003) V. A. Chernenko. E. Cesari. V. V. Kokorin. and L. N. vitenko. Scr. Metall. be one of the promising compositions in the Ni-Mn-Ga sys- Mater. 33, 1239(1995) tem for room-temperature magnetic refrigeration and other 25C. Jiang, Y. Muhammad, L. Deng, w. Wu, and H. Xu, Acta Mater. 52 applications. 2779(20
the Ni55.2Mn18.1Ga26.7 alloy, the SM value distributes slowly over a large span of temperature T 16 K between the TM and TC values. A coupled magnetostructural transformation, T→0, and a large MCE appear to be interesting for active magnetic refrigerant applications of Ni54.8Mn20.3Ga24.9 alloy near room temperature. IV. CONCLUSIONS The nonmodulated martensite, the seven-layer modulated martensitic, and the austenite phases were observed at room temperature in Ni54.8Mn20.3Ga24.9, Ni55Mn18.9Ga26.1, and Ni55.2Mn18.1Ga26.7 alloys, respectively. Among these alloys, Ni54.8Mn20.3Ga24.9 displayed an enhanced MCE, with as large SM value as −7.0 J/kg K, at 332 K in an applied field 1.2 T. The origin of the SM is related to the coincidence of structural and magnetic transition temperatures TM, TC =351 K. Such an appreciable SM value in such low fields will enable the material to operate in fields produced by permanent magnets. Thus, the Ni54.8Mn20.3Ga24.9 alloy seems to be one of the promising compositions in the Ni-Mn-Ga system for room-temperature magnetic refrigeration and other applications. ACKNOWLEDGMENT The authors thank the Defence Research & Development Organization DRDO, India, for the financial support to carry out this work. 1 V. K. Pecharsky and K. A. Gschneidner, Jr., Phys. Rev. Lett. 78, 4494 1997. 2 V. K. Pecharsky and K. A. Gschneidner, Jr., Appl. Phys. Lett. 70, 3299 1997. 3 S. J. Murray, M. A. Marioni, A. M. Kukla, J. Robinson, R. C. O’Handley, and S. M. Allen, J. Appl. Phys. 87, 5774 2000. 4 F.-X. Hu, B.-G. Shen, J.-R. Sun, Z.-H. Cheng, and X.-X. Zhang, J. Phys.: Condens. Matter 12, L691 2000. 5 O. Tegus, E. Brück, K. H. J. Buschow, and F. R. de Boer, Nature 415, 150 2002. 6 L. Pareti, M. Solzi, F. Albertini, and A. Paoluzi, Eur. Phys. J. B 32, 303 2003. 7 E. Brück, O. Tegus, X. W. Li, F. R. de Boer, and K. H. J. Buschow, Physica B Amsterdam 327, 431 2003. 8 A. Fujita, S. Fujieda, Y. Hasegawa, and K. Fukamichi, Phys. Rev. B 67, 104416 2003. 9 A. Cherechukin, T. Takagi, M. Matsumoto, and V. D. Buchel’nikov, Phys. Lett. A 326, 146 2004. 10F.-X. Hu, B.-G. Shen, and J.-R. Sun, Appl. Phys. Lett. 76, 3460 2000. 11J. Marcos, A. Planes, L. Manosa, F. Casanova, X. Battle, A. Labarta, and B. Martinez, Phys. Rev. B 66, 224413 2002. 12J. Marcos, L. Manosa, A. Planes, F. Casanova, X. Battle, and A. Labarata, Phys. Rev. B 68, 094401 2003. 13E. Brück, J. Phys. D 38, R381 2005. 14F. Albertini, A. Paoluzi, L. Pareti, M. Solzi, L. Righi, E. Villa, S. Besseghini, and F. Passaretti, J. Appl. Phys. 100, 023908 2006. 15H. Wada and Y. Tanabe, Appl. Phys. Lett. 79, 3302 2001. 16X. Bohigas, J. Tejada, E. Barco, X. X. Zhang, and M. Sales, Appl. Phys. Lett. 73, 390 1998. 17F. Albertini, L. Pareti, A. Paoluzi, L. Morellon, P. A. Algarabel, M. R. Ibarra, and L. Righi, J. Appl. Phys. 81, 4032 2002. 18I. Babita, M. Manivel Raja, R. Gopalan, V. Chandrasekharan, and S. Ram, J. Alloys Compd. 432, 23 2007. 19J. Pons, V. A. Chernenko, R. Santamarta, and E. Cesari, Acta Mater. 48, 3027 2000. 20U. Gaitzsch, S. Roth, B. Rellinghaus, and L. Schultz, J. Magn. Magn. Mater. 305, 275 2006. 21V. V. Martynov and V. V. Kokorin, J. Phys. III 2, 739 1992. 22V. V. Martynov, J. Phys. IV 5, 91 1995. 23Y. Ge, O. Söderberg, N. Lanska, A. Sozinov, K. Ullakko, and V. K. Lindroos, J. Phys. IV 2, 921 2003. 24V. A. Chernenko, E. Cesari, V. V. Kokorin, and I. N. Vitenko, Scr. Metall. Mater. 33, 1239 1995. 25C. Jiang, Y. Muhammad, L. Deng, W. Wu, and H. Xu, Acta Mater. 52, 2779 2004. FIG. 6. Magnetic entropy changes in the a Ni54.8Mn20.3Ga24.9; b Ni55Mn18.9Ga26.1; and c Ni55.2Mn18.1Ga26.7 alloys in the magnetic field change 1.2 T. 013906-5 Ingale et al. J. Appl. Phys. 102, 013906 2007
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