Magnetocaloric Effect of Ni56Mn18.8Ga24.5Gd0.7 Alloy

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JOURNAL OF RARE EARTHS 24 (2006) 579 - 581

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Magnetocaloric Effect of Ni56Mnls.8GaUs5 G&., Alloy Huang Peng (-& @)', Zhang Zeyu ($k%1.)'", Long Yi ( A $&)2", Duan Jingfang ($%##)', Wu Guangheng (XX@)',Chang Yongqin ( 1 . Institute .f Scientific and Technical Information of China , Beijing 100038, China ; 2 . School of Materials

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Science and Engineering, University of Science and Technology Beying , Beijing 100083, China ; 3 . State Key Laboratory for Magnetism , Institute of Physics , Chinese Academy of Sciences , Beijing 100080, China) Received 13 February 2006; revised 26 April 2006

Abstract: With the addition of Gd , the Ni56Mnl*.8Ga2,5Gdo,7alloy exhibits non-modulated martensite phase at room temperature . From the illustration of Gd microstructure, it can be found that Gd exists along the subgrain boundaries. Hence, the crystalline size decreases and the mechanical properties improve. Ac-susceptibility results show that Ni56Mnls.8Ga24.5 Gd,,7 alloy still undergoes simultaneous structural and magnetic transitions and transforms from ferromagnetic martensitic phase to paramagnetic austenitic phase with increasing temperature. The maximum magnetic entropy change is 13. 4 J. (kg*K)- I under 1.9 T field at 338 K . The giant magnetocaloric effect found in Ni56Mnl8.8Ga24.5Gdo.7 alloy is attributed to the concurrently occurring first-order structural- and magnetic-phase transitions. Key words: NiMnGa; magnetocaloric effect (MCE); magnetic transition; structural transition; magnetic entropy; rare earths Article ID: 1002 - 0721 (2006)05 - 0579 - 03 CLC number: TM273 Document code: A

In recent years materials with high magnetocaloric effect (MCE) have attracted considerable attention owing to its potential application as a magnetic refrigerant. Many material systems that underwent the first-order magnetic transition have been found to exhibit a giant MCE. Their typical representatives are Gd5(Si,Ge1-.)4[1321and La ( Fe,Si,-, )13[3'41 alloys. NiMnGa is a ferromagnetic shaped memory alloy which undergoes a reversible first-order structural phase transition (SPT) with the variation in temperature. It has been reported that in the vicinity of martensitic and magnetic transitions application of magnetic field in~ ] . example, duces a large change of e n t r ~ p y [ ~ - For the structural- and magnetic-phase transitions of Ni54 MnZlGa occur at 314 and 338 K , and its entropy change are 10.4 and 3 . 9 J . ( k g . K ) - ' under a mag-

netic field of 1 . 8 T , respectively. Depending on the modification in the composition of NiMnGa, the SFT can coincide with magnetic tsansition, which may be responsible for the induction of the giant MCE. The best results obtained for coupled SPT and magnetic transition in Niz. 18Mn,,82Ga is 20.7 f 1. 5 J * (kg K ) at a magnetic field change of 1 . 8 TI6]. Gd is known to be an excellent magnetic refrigerant owing to its high MCE and suitable Curie temperature (293 K ) . Otherwise, rare earth elements are also known to improve the mechanical properties of various alloys and intermetallic compounds. NiMnGa samples are very brittle and badly machinable because of the presence of i n t e r g a n h fracture. The significant disadvantage limits the possible application in magnetic refrigeration. Thus the goal of the present study is to

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* Corresponding author (E-mail : zeyu - h a n g @ 126. com) Foundation item: Pmject supported by the National Natural Science Foundation of China (50571008) ; National Basic Research Program of China (2006CB601101)

Biography: Zhang Zeyu (1976 - 1, Male, Doctor Copyright @2M)6, by Editorial Committee of Journal of the Chinese Rare Earths Society. Published by Elsevier B .V .

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investigate the SET with simultaneous magnetic transition in Ni56MnlE,BGa24.5G&.7 with the addition of Gd.

1 Experimental Polycrystalline ingots of Ni,Mnla.aGa24.5W.7were prepared by arc melting of high-purity (99.95%) Ni, Mn , Ga and (99.9%) Gd in argon gas atmosphere on a water-cooling copper bottom. Each arc-melted ingot was flipped over and remelted thrice. The ingot was annealed at 1173 K for 3 d in an evacuated quartz tube and quenched in ice water. The crystal structure at room temperature was confirmed by powder X-ray diffraction with Cu Ka radiation. Samples with suitable sizes were cut from the middle of the ingots with a sparkcutting machine. The chemical compositions were determined by energy-dispersive spectrometer. Transformation temperatures M s, M F, A s , A F were measured using differential scanning calorimeter Netzsch DSC 204. During DSC experiment, the heating and cooling rates were 10 “c amin-’, respectively. The ac-susceptibility was measured under a magnetic field of 397.885 A-m-’ and a frequency of 77 Hz. The magnetic properties were measured using a vibrating sample magnetometer (Lakeshore-7300) . The temperature was controlled within the accuracy limit of 0.1 K.

Gd relative to the target composition of 56% N i , 18.8% Mn, 24.5% Ga, and 0.7% Gd. On the other hand, the grain boundary phase was rich in Gd and Mn. These results strongly suggest that most of the Gd that was added primarily combined with Mn to form the boundary phase. The addition of rare earth elements such as Nd, Sm, and Tb were found to exist along the subgrain boundaries and signzificantly improved the mechanical p r ~ p e r t i e a “ ~ ’ ~ ~ ] . Fig.2 shows the x-ray diffraction pattern of Ni56 MnlE.8Ga24.sGdo.7.It is obvious that the major peaks can be indexed as those from the nonmodulated martensite phase“*’. The crystalline lattice parameters of the alloy are a = b =0.7716 nm, c =0.6553 nm. The DSC trace and ac-susceptibility of Ni5aMnlE.s GaX.,G&.7 are shown in Fig. 3. Peaks corresponding to the forward and reverse martensitic transformations can be seen from the DSC line, and steps comsponding to the Curie temperature can be seen from the acsusceptibility line. Transformation temperature were determinedas M , = 5 5 . 1 “c, M f = 5 0 . 1 “c, A , = 5 7 . 3 “c , and A [ = 63.5 “c . Peak temperatures for the forward and reverse transformations are 60.4 and 52.6 “c , respectively The Curie temperature for the for -

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2 Results and Discussion Fig. 1 shows the metallograph of Ni56Mnl~.8Ga24.~ Gd,., that is characterized by a dominant matrix phase and by a minor (darker) phase located along the grain boundaries of the matrix phase. Energy-dispersive spectroscopy (EDS) analysis conducted on the two different phases yielded the following results. The composition of the phases are: ( 1 ) for the dominant matrix phase - 54.48% Ni, 19.14% Mn, 26.06% Ga, and 0 . 3 2 % Gd; ( 2 ) for the darker 5ain boundary phase - 35.05% Ni, 23.89% Mn, 14.18% Ga, and 26.88% Gd. It is noted that the dominant matrix phase had a larger concentration of both Mn and Ga and a smaller concentration of Ni and

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Fig.2 X-ray diffraction spectra of NiJh18.8Ga~.SGd0.7 I

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Metallograph of NisMn1,.8Ga~,&&,,

DSC and ac-susceptibility results of Ni, Mn18.8Ga,.,

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Zhang Z Y et a l . Effectof Ni5&fn18.8Ga24.5Gdo.7

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ward and reverse transformations are 62 and 54 "c, respectively, and are attributed to the hysteresis of the first-order transition. It can be seen from Fig. 3 that the SFT and magnetic transition of NiS6Mn18.8Ga,,, are concurrent with increasing temperature. During heating, the alloy transformed from ferromagnetic to paramagnetic state, represented by ac-susceptibility line, and from martensite to austenite, shown by DSC line, at the same time. The simultaneous beginning and ending of the SFT and magnetic transition probably play a key role in the abnormal magnetic behavior of Ni56Mn18.8Ga24.5Gd,,7 alloy as shown in Fig. 4. The magnetization isotherms of Niss Mn18.8Gaza,sGc&,7 for selected temperature during heating are presented in Fig. 4. The curve measured at 64 "c overlaps with the other two curves, which may be the result of the process of SFT and magnetic transition. The isothermal magnetic entropy change can be obtained using the numerical experimental M-H curves from the Maxwell relation. The calculated magnetic entropy change dependence on temperature under a magnetic field for Ni56Mn18.8Gax.5G&.7 is presented in Fig. 5 . The maximum peaks are 6. 3 J kg-' * K - I ( H = l T ) and 13.4 J * k g - l . K - ' ( H = 1.93 T ) at 65 T . The peaks are dissymmetric. This may be due

to the addition of Gd to NiMnGa alloy. The giant MCE found in Ni56Mn18.8Ga,,sG&,7alloy is due to the concurrent occurrence of magnetic-phase transition and SPT, which is sensitive to the component.

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Magnetization isotherms of Nis6Mn18,8Ga,.sGd, on field increase

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Magnetic entropy change of Ni&hl8.~.Ga~.5G&.7

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Conclusion With the addition of Gd, the NiS6Mn18.8GaN.5

Gd,,, alloy exhibits nonmodulated martensite phase at room temperature. Most of Gd that is added primarily combines with Mn along the subgrain boundaries. The solubility of Gd into the nonmodulated martensite matrix is found to be very low. During heating, the alloy transforms from ferromagnetic to paramagnetic state and from martensite to austenite at the same time. The maximum magnetic entropy change is 1 3 . 4 J * ( k g * K - ' ) under 1 . 9 T field at 65 "c. The giant MCE found in Ni56 Mn18,8Ga,,s No,, alloy is due to concurrent occurrence of magnetic phase transition and SPT, which is sensitive to the component.

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