AI 中文总结
该研究利用偏振分辨二次谐波产生（SHG）显微镜，实现了对薄膜赤铁矿中磁对称性、奈尔矢量取向及磁畴演化的局部成像，揭示了莫林转变下的磁畴变化规律，为相关研究提供了新的探测手段。
AI 中文摘要
赤铁矿是一种反铁磁氧化物，也是候选的交替磁绝缘体，其奈尔序在莫林转变时从易轴相重新取向为易平面相。解释交替磁输运和对称敏感的光学响应，需要了解奈尔矢量相对于晶体轴的取向，以及所探测器件内的畴结构。在此，我们表明偏振分辨二次谐波产生（SHG）显微镜可分辨外延（0001）取向赤铁矿薄膜中的磁对称性和畴。在莫林温度范围内，SHG偏振各向异性从与易轴取向一致的近似六重图案，演变为与易平面取向一致的明显二重图案。基于磁偶极和电四极贡献的对称性分析重现了这种演变。重要的是，在磁序反转下奇偶的SHG振幅之间的干涉，使相反的奈尔矢量取向在光学上可区分。相应地，在我们的实验中，施加的面内磁场的相反方向产生了不同的SHG响应。利用这种磁衬度，我们对微米级畴、它们在莫林转变过程中的重组，以及它们在磁和热循环下的重构进行了成像，包括在穿过自旋翻转转变后的剩磁变化。这些结果确立了SHG显微镜作为赤铁矿薄膜中磁对称性、奈尔矢量取向和畴演化的局部探针的地位。
英文摘要
Hematite is an antiferromagnetic oxide and candidate altermagnetic insulator whose Néel order reorients from an easy-axis to an easy-plane phase at the Morin transition. Interpreting altermagnetic transport and symmetry sensitive optical responses requires knowledge of the Néel vector orientation relative to the crystal axes and of the domain structure within the probed device. Here, we show that polarization-resolved second-harmonic generation (SHG) microscopy resolves magnetic symmetry and domains in epitaxial (0001)-oriented hematite films. Across the Morin temperature, the SHG polarization anisotropy evolves from an approximately sixfold pattern consistent with the easy-axis orientation to pronounced twofold patterns consistent with the easy-plane orientation. A symmetry analysis based on magnetic-dipole and electric-quadrupole contributions reproduces this evolution. Importantly, the interference between SHG amplitudes that are odd and even under reversal of the magnetic order renders opposite Néel-vector orientations optically distinguishable. Consistently, opposite directions of an applied in-plane magnetic field produce distinct SHG responses in our experiments. Using this magnetic contrast, we image micrometer-scale domains, their reorganization across the Morin transition, and their reconfiguration under magnetic and thermal cycling, including a remanent change after cycling through the spin-flop transition. These results establish SHG microscopy as a local probe of magnetic symmetry, Néel-vector orientation, and domain evolution in hematite films.
Comments26 pages, 13 figures; First two authors contributed equally to this work
