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

利用超低温磁共振成像（ULT-MRI）技术对反铁磁固体氦3中的磁畴结构进行了可视化研究。为了避免原子核有序反铁磁相的非线性动力学，提出了一种从单个小倾角射频脉冲激发后的自由感应衰减（FID）信号中恢复NMR谱的特殊技术。利用这种技术，我们可以在可接受的测量时间内获得核有序反铁磁固体氦3（U_2D_2相）单晶中磁畴的空间分布。需要强调的是，该图像是世界上第一个从微开尔文冷却的物理系统中获得的MRI图像。(1)U_2D_2相的畴壁取向在体心立方晶格中被确定为（110）面，而面对该畴壁的磁畴取向在体心立方晶格中被确定为（110）面。 ...更多信息 使它们的各向异性轴（100）相对于畴壁法线倾斜45度。(2)当外部磁场超过Hc时，平衡相转变为伪铁磁相，其不具有如U2 D2相的情况下的自旋各向异性。然而，通过在经历比Hc强得多的外部磁场之前和之后拍摄U2 D2阶段的MRI图像，我们可以确认磁畴分布被再现。由于U2 D2相（其为单轴的）和伪铁磁相（其为各向同性的）中的自旋对称性不同，因此没有理由在变为不同相之后恢复畴结构。我们的MRI实验清楚地证实了这种记忆效应。这种记忆的物理原因尚未得到证实，但我们怀疑其原因是由U2 D2相的单轴晶格畸变产生的晶格应变。(3)研究了超流^3 He-B中的晶体生长。与众所周知的^4 He超流中不受任何输运问题控制的情况相反，固体^3 He的这种增长是由穿过固液界面的自旋输运控制的。少

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