螺旋磁体MnSi的无穷物理学：从超声研究推断的磁相变的场演化
Inexhaustible physics of the helical magnet MnSi: field evolution of the magnetic phase transition inferred from ultrasound studies
摘要：
在温度范围为2 - 40 K和磁场至7特斯拉的MnSi单晶中测量了纵向和横向超声速度和衰减。MnSi在外加磁场中的磁相图似乎取决于实验装置，这与由于样品的圆盘形状而产生的退磁因子的差异有关。MnSi在零磁场中的磁相变由c 11弹性常数的准不连续性表示，其随磁场显著变化。值得注意的是，c11不连续性几乎消失的区域紧密对应于沿着磁性到顺磁性转变的skyrmion相的范围。这意味着，c11弹性常数几乎是连续的，通过从斯基尔米恩顺磁相的过渡。在c11中的相变线和极小值线的交叉处发生c11的不连续性的恢复和增强的吸声。在c11的最小值处的强波动使得所提到的交叉点类似于临界端点，在临界端点处，二阶相变与一阶相变相遇。在具有较小退磁因子的垂直设置的情况下，skyrmion畴具有减小的温度范围，这表明磁场不均匀性在skyrmion出现中起着重要作用，因此，开辟了一条操纵skyrmion的途径。在平行设置中发现的（001）平面中的剪切模量的小各向异性也很可能是由磁场不均匀性引起的，其扭曲了Skyrmion晶体的六方对称性。
英文摘要：
The longitudinal and transverse ultrasound speeds and attenuation were measured in a MnSi single crystal in the temperature range of 2 - 40 K and magnetic fields to 7 Tesla. The magnetic phase diagram of MnSi in applied magnetic field appears to depend on the experimental setups, which is related to a difference in demagnetization factors arising due to the disc shape of the sample. The magnetic phase transition in MnSi in zero magnetic field is signified by a quasi discontinuity in the c11 elastic constant, which varies significantly with magnetic field. It is notable that the region where the c11 discontinuity almost vanishes closely corresponds to the extent of skyrmion phase along the magnetic to paramagnetic transition. This implies that the c11 elastic constant is almost continuous through the transition from the skyrmion to paramagnetic phases. A recovery of the discontinuity of c11 and enhanced sound absorption occur at the crossing of the phase transition line and the line of minima in c11. The powerful fluctuations at the minima of c11 make the mentioned crossing point similar to a critical end point, where a second order phase transition meets a first order one. The skyrmion domain in the case of a perpendicular setup with a smaller demagnetization factor has a reduced temperature range, which suggests that the magnetic field inhomogeneity plays an important role in the skyrmion occurrence and, hence, opens a way of skyrmion manipulation. The small anisotropy of the shear moduli in the (001) plane found in the parallel setup is most probably also caused by the magnetic field inhomogeneity, which distorts the hexagonal symmetry of the skyrmion crystal.
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