Magnetic Domain Structure and Anisotropic Thermal Conductivity

完成了新型超低温MRI制冷机和变频MRI系统的建设，提高了分辨率和稳定性。测量速度大大提高，我们可以在30分钟内测量出真实的三维MRI图像。通过这种改进，我们可以在不做任何假设的情况下确定磁畴壁的方向。它是[110]在bcc晶格中，这证实了我们之前在一些假设下的决定。我们还研究了在U2D2相和CNAF相之间的场诱导相变过程中稳定相的演变过程。它表现为两个阶段的时间演化。在第一阶段，稳定相的种子出现在晶体的许多地方。这种种子长得很快，直到长到一定大小时才停止生长，这是由于潜热释放造成的化学势损失。在此阶段，时间演化受成核速率控制。其他种子的成核不断发生，直到整个晶体达到两相共存的温度。然后是缓慢的第二阶段，由热传导和潜热通过液固界面释放控制，从而从晶体的液固界面侧进行相变。通过对第一阶段和记忆效应的分析，我们提出了这种成核发生在与晶体线性缺陷相关的磁性平面缺陷上的可能性。

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.