101 Thermomagnetic analysis and domain structure in the phase transition region of Ni-Mn-Ga and... © 2009 Advanced Study Center Co. Ltd. Rev.Adv.Mater.Sci. 20(2009) 101-106 Corresponding author: Teodor Breczko, e-mail: tbreczko@uwb.edu.pl THERMOMAGNETIC ANALYSIS AND DOMAIN STRUCTURE IN THE PHASE TRANSITION REGION OF Ni-Mn-Ga AND Co-Ni-Ga SHAPE MEMORY ALLOYS Teodor Breczko 1 , Svetlana Ilyashenko 2 , Dmitry Bykov 2 , Oleg Korpusov 2 , Miroslav Bramowicz 3 and Rostislav Grechishkin 2 1 Department of Mathematics and Computer Science, University in Bialystok, 64 Sosnowa str., 15-887 Bialystok, Poland 2 Laboratory of Magnetoelectronics, Tver State University, 170000 Tver, Russia 3 Laboratory of Metal Physics and Nanotechnology, Faculty of Technical Sciences University of Warmia and Mazury in Olsztyn, 11 Oczapowski str. 10-719 Olsztyn, Poland Received: February 03, 2008 Abstract. The recently developed ferromagnetic shape memory alloys (FSMA) exhibit large field induced strains thereby rendering new potentials for application in transducers, actuators and other novel devices. Magnetically controlled strain in FSMA is based on the reorientation of the twin structure of martensite under applied magnetic field. A detailed study of both martensitic and magnetic domain structure is presented for oriented single-crystalline bulk and texturized pow- dered samples embedded in polymer matrix, and polycrystalline bulk and rapidly quenched ribbon alloys. Optical microscopy including magneto-optical indicator film technique was employed along- side with X-ray and AFM/MFM studies for the characterization of the coexisting structural and magnetic domains. It is shown that only 180° magnetic domains exist in twin plates because mar- tensite possesses uniaxial magnetic anisotropy having magnetization vector M oriented along easy c-axes at angles of ±45° with respect to the twin boundaries. Due to magnetostatic coupling the 180° magnetic domains of neighbouring twins cooperate with each other forming continuous macrodomains running through the whole crystallite or single crystal sample and changing the direction of M by ±90° in a zigzag fashion at each intersection of the twin boundary. 1. INTRODUCTION Combination of ferromagnetism and structural phase transitions in Heusler alloys is perspective for the production of new devices based on the magnetic field control of the size and shape of the actuator active elements. The structural phase tran- sitions in these elements proceed by the transfor- mation of high-temperature austenite cubic phase into a tetragonal low-temperature martensite phase [1]. Extremely high (up to 10%) magnetically in- duced deformations were shown to exist in the fam- ily of Ni 2+x Mn 1-x Ga alloys [2]. Also important, though studied in less detail, are some other groups of Heusler alloys, in particular, those on the basis of Co–Ni–Ga [3–5]. Detailed experimental studies of the regulari- ties of formation and realignment of both marten- sitic and magnetic domain structure (DS) are nec- essary for modelling and simulation of magnetically induced phenomena in these alloys. These ques- tions are studied intensively by a number of re- search groups [6-12]. However the available data are still fragmentary and need to be extended and generalized. In the present work we focus our at- tention on the study of martensite and magnetic DS of mono-, poly-, and nanocrystalline Ni 2+x Mn 1-x Ga and Co 2+x Ni 1-x Ga alloys.