Magnetic anisotropy and structures of magnetic domain walls in antiferromagnetic solid Helium 3

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A visualization of magnetic domain structure in antiferromagnetic solid helium 3 has performed by the Ultra Low Temperature Magnetic Resonance Imaging (ULT-MRI) technique. To avoind a nonlinear dynamics in the nuclear ordered antiferromagnetic phase, a special technique of recoverring NMR specrum out of a Free Induction Decay (FID) signal after a single small tipping angle RF excitation pulse was developed. By using this technique we could obtain spatial distribution of magnetic domains in a single crystal of nuclear ordered antiferromagnetic solid helium 3 (U2D2 phase) in the acceptable amount of measuring time. It shuld be stressed that this image was the world first MRI image obtained from the phyical system cooled in the micro Kelvin regime.As a result we could obtain the following information. (1) The domain wall orientation in the U2D2 phase, which was not known for a long time, was identified as (110) plane in the bcc lattice and the magnetic domains facing to this domain wall h … More ave their anisotropy axis (100) tilted to 45 degrees from the domain wall normal. (2) When an external magnetic field exceeds Hc, equilibrium phase changes into a pseudo-ferromagnetic phase which has no spin anisotropy as in the case of U2D2 phase. However by taking MRI images in the U2D2 phase before and after experiencing an external magnetic field much stronger than Hc, we could confirm that the magnetic domain distribution was reproduced. Since spin symmetry in the U2D2 phase, which is uni-axial and pseudo-ferromagnetic phase, which is isotropic, are different, there are no reason to recover domain structure after changing into a different phase. This memory effect was clearly confirmed by our MRI experiment. The physical reason of this memory is not confirmed yet, however we suspect that the cause is a lattice strain produced by the uni-axial lattice distortion in the U2D2 phase. (3) Crystal growth in superfluid ^3He-B was investigated. On the contrary to the well-known situation in superfluid ^4He, which is not governed by any transport problem, this growth of solid ^3He was controlled by the spin transport across the solid-liquid interface. Less