Rearrangement of crystallographic domains driven by magnetic field in ferromagnetic Ni2MnGa and antiferromagnetic CoO

Abstract

We have investigated the rearrangement of crystallographic domains (martensite variants) in Ni2MnGa ferromagnetic shape memory alloy and CoO antiferromagnetic oxide by applying magnetic field up to 8.0 MA/m. From the result of optical microscope observation of Ni2MnGa single crystal, when a magnetic field is applied along [001]p (p represents a parent phase), the rearrangement of crystallographic domains occurs and the single domain state is obtained below TMs 202 K. The same rearrangement occurs but partially when a magnetic field is applied along [110]p. On the other hand, when a magnetic field is applied along [111]p, the rearrangement does not occur. In case of the CoO single crystal, when a magnetic field is applied along [001]p below TMs 293 K, the rearrangement occurs at 170 K ≤ T ≤ 293 K, but does not occur at T < 170 K. When a magnetic field is applied along [110]p and [111]p, the rearrangement does not occur below TMs. In order to explain the rearrangement in the alloy and the oxide, we have evaluated the magnetic shear stress, τmag, which is derived from the difference in magnetic energy among crystallographic domains and have compared it with the shear stress required for the twinning plane movement, τreq. As a result, we have found that the rearrangement occurs when the value of τmag is larger than or equal to the value of τreq for the present alloy and oxide. © 2009 IOP Publishing Ltd.