Magnetic Domain Structure and Anisotropic Thermal Conductivity

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We have completed a construction of new refrigerator for ultra low temperature MRI and variable frequency MRI system with improved resolution, stability. Measurement speed was greatly improved so that we could measure the true 3 dimensional MRI image in 30 minutes. With this improvement we could determine the direction of magnetic domain wall without any assumption. It is [110] in the bcc lattice, which confirmed our previous determination under some assumption. We also investigated how the stable phase evolve during the field induced phase transition between U2D2 phase and CNAF phase. It showed two stage time evolution. During the first stage, a seed of stable phase appeared at many places in a crystal. This seed grew rapidly until it stoped growing at some certain size, due to a loss of chemical potential by the latent heat release. During this stage, time evolution is controlled by the nucleation rate. Nucleation of other seeds kept occuring until the entire crystal reached to a temperature of coexistence for two phases. Then the slow second stage, which is controlled by the thermal conduction and latent heat release throgh liquid-solid interface, and thus phase transition proceeded from liquid-solid interface side of a crystal. Through the analysis of the first stage and memory effect, we proposed a possibility that this nucleation occured on the magnetic planar defect associated with crystalline linear defect.