IEEE TRANSACTIONS ON MAGNETICS, VOL. 38, NO. 5, SEPTEMBER 2002 2841 Magnetic Properties of Ni–Mn–Ga Ribbon Prepared by Rapid Solidification Oleg Heczko, Peter ˇ Svec, Duˇ san Janiˇ ckoviˇ c, and Kari Ullakko Abstract—Magnetic shape memory Ni Mn Ga alloy was prepared in the form of the thin ribbon by planar-flow casting method. Optical microscopy and X-ray diffraction were used to check the microstructure of as-quenched and annealed ribbons (800C/24 h and 72 h). The as-quenched ribbon was microcrys- talline with the grain size about 1.5–3 m which after annealing increased to about 40 m. Annealing also increased the marten- sitic transformation and Curie temperatures. Annealed ribbon transformed to martensite at 299 K with reverse transformation to austenite at about 308 K. Room-temperature martensite had five-layered modulated structure similar to that observed in bulk material. Magnetization at room temperature, 63 Am /kg and Curie temperature, 367 K, were close to that of the master alloy. Magnetization loop of as-quenched ribbon was very flat due to high level of quenched-in stresses. The loops of annealed ribbon were round indicating random distribution of easy magnetization axes. Magnetocrystalline anisotropy constant of the annealed ribbon was J/m . Thermoelastic strain due to martensitic transformation was about 0.3%. Index Terms—Magnetic shape memory alloy, martensitic trans- formation, Ni–Mn–Ga ribbon, rapid quenching. I. INTRODUCTION M AGNETIC shape memory (MSM) materials are of a great interest for their giant field-induced strain up to 6% and fast response in magnetic field [1]–[3]. To date the giant field-induced strain has been observed only in bulk single-crystalline material. Due to induction of eddy currents the cycling is limited to low frequencies. To increase the frequency range the material can be utilized in the form of a thin layer or ribbon and powder in suitable medium as for giant magnetostrictive materials. Rapid quenching may offer a possibility to prepare a ribbon with a specific texture which may be favorable to MSM effect. The process can also serve to produce precursor for powder production. The structure of stoichiometric Ni MnGa ribbon was studied by Chernenko and Vitenko [4] who reported improved ther- moelastic properties compared with the bulk at 220 K. Struc- tural and some magnetic properties of the Ni MnGa ribbon with Ni excess and martensitic structure at room temperature were Manuscript received February 2, 2002; revised April 27, 2002. This work was supported in part by The National Technology Agency (Tekes), Finland, and in part by the consortium of Finnish companies (Outokumpu Research Oy, Metso Paper Oyj, Nokia Research Oyj, and AdaptaMat Oy). O. Heczko and K. Ullakko are with the Department of Engineering Physics and Mathematics, Laboratory of Biomedical Engineering, Helsinki University of Technology, 02015 HUT Espoo, Finland (e-mail: oleg.heczko@hut.fi; kari.ullakko@hut.fi). P. ˇ Svec, D. Janiˇ ckoviˇ c are with the Department of Metal Physics, Institute of Physics, Slovak Academy of Science, SK-842 28 Bratislava, Slovakia. Digital Object Identifier 10.1109/TMAG.2002.802471. studied by Albertini et al. [5]. They reported that the ribbon pos- sessed in plane anisotropy due to strong texture with the tetrag- onal -axis in the plane of ribbon. In this paper, we present the investigation of structural and magnetic properties of the ribbon having martensitic transfor- mation above room temperature. Additionally, the comparison with the bulk material is made. The composition of the studied alloys was selected to be close to the alloy(s) exhibiting giant MSM effect [2], [3], [5], [6]. II. EXPERIMENT The ingot of Ni Mn Ga was cast in AdaptaMat OY, Finland from pure elements. The ribbon was prepared by planar-flow casting method in the Institute of Physics, Slovak Academy of Science, Bratislava, Slovakia. Small pieces from the ingot (master alloy) were melted in a quartz tube and rapidly solidified on rotating copper wheel. The ribbon was about 5 mm wide and about 80 m thick. As a result of casting in the air the ribbon surface was oxidized. The as-quenched ribbons were sealed in evacuated quartz ampoules and annealed in 1073 K for 24 and 72 hours. As-prepared ribbons were brittle and got even more brittle after annealing. Optical microscopy shows that the ribbon is breaking along grain boundaries. The composition of the ribbon was the same as the master alloy within error margin of energy dispersive spectroscopy method, i.e., 1 at%, thus, no significant loss of Mn during preparation process occurred. The structure of the ribbon was studied by X-ray diffraction (XRD) (MDR X’pert Philips diffractometer) and optical microscopy (DMRX Leica). The temperature dependencies of the low field ac magnetic susceptibility and resistivity were measured to determine the interval of the martensitic trans- formations and possible another structural changes. Magnetic properties were measured by vibrating sample magnetometer. Thermoelastic strain or shape recovery effect was detected by dilatometer in which strain as a function of temperature was measured under constant tensile stress of 2 MPa. III. RESULTS AND DISCUSSION The optical microscopy showed that the as-quenched ribbon was finely polycrystalline with average grain sizes about 1.5 m on wheel side and 3.8 m on air side. After annealing for 72 hours, the grain size increased to 40 m. XRD showed that at room temperature the annealed ribbon had five-layer modulated tetragonal martensitic structure with lattice pa- rameters nm and nm and . This structure is identical with the structure observed in the 0018-9464/02$17.00 © 2002 IEEE