THERMOMAGNETIC ANALYSIS AND DOMAIN STRUCTURE IN THE PHASE TRANSITION REGION OF Ni-Mn-Ga AND Co-Ni-Ga SHAPE MEMORY ALLOYS

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 powdered samples embedded in polymer matrix, and polycrystalline bulk and rapidly quenched ribbon alloys. Optical microscopy including magneto-optical indicator film technique was employed alongside with X-ray and AFM/MFM studies for the characterization of the coexisting structural and magnetic domains. It is shown that only 180 degrees magnetic domains exist in twin plates because martensite possesses uniaxial magnetic anisotropy having magnetization vector M oriented along easy c-axes at angles of +/- 45 degrees with respect to the twin boundaries. Due to magnetostatic coupling the 180 degrees 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 degrees in a zigzag fashion at each intersection of the twin boundary.