Magnetic Domain Structure and Anisotropic Thermal Conductivity

我们已经完成了超低温MRI和变频MRI系统的新冰箱的构建，提高了分辨率，稳定性。测量速度大大提高，使我们可以在30分钟内测量出真正的三维MRI图像。利用这一改进，我们可以确定磁畴壁的方向，而无需任何假设。它是bcc晶格中的[110]，这证实了我们在某些假设下的先前确定。我们还研究了场诱导U_2D_2相和CNAF相相变过程中稳定相的演变。它表现出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中的许多地方。这种种子生长迅速，直到它停止生长在一定的大小，由于潜热释放的化学势的损失。在此阶段，时间演化由成核速率控制。其他晶种继续成核，直至整个晶体达到两相共存温度。然后是缓慢的第二阶段，这一阶段受液-固界面热传导和潜热释放的控制，从晶体的液-固界面一侧开始发生相变。通过对第一阶段和记忆效应的分析，提出了这种形核可能发生在与晶体线缺陷相联系的磁性平面缺陷上。

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.