Small Angle Neutron Scattering Studies of Magnetic Textures: Competing Anisotropies in Vortex Lattices and Development of the Sample and Scanning-Aperture Stage for Spatially Resolved Skyrmion Lattices

The two magnetic textures of superconducting vortex lattices (VLs) and magnetic skyrmion lattices (SkLs) are very different in their formation mechanisms and host materials, yet they have many similar attributes that allow them to both be studied using the same experimental technique, small angle neutron scattering (SANS). SANS provides statistical information of an entire VL or SkL in bulk crystals and conveys information pertaining to the magnetic texture’s uniformity, orientation, separation, and phase. The VL is highly sensitive to the host superconductor’s characteristics, like anisotropies present in the system. The effects of the anisotropies in relationship with adjustable external parameters can be seen in VL orientations and domain formations. Magnetic field-angle rotations combined with adjusting the applied magnetic field magnitude creates a delicate balance between the anisotropies in MgB2. As a result, there are three distinct field-angle regimes in which the anisotropies are cooperative, balanced, and competitive, where there are unusual existences of single-domain L phases and discontinuous phase transitions into novel coexistences of the L and I phases. The previously unexplored skyrmion mesoscale encompasses how and where domains form and their relationships with other coexisting magnetic textures. The Sample and Scanning-Aperture Stage (SSAS), a bespoke apparatus designed and built by the author of this work, was specifically designed to bridge the gap between the skyrmion micro- and macroscales and perform spatially resolved measurements of SkLs. The SSAS was recently utilized to observe spatial variations in the helical phase and equilibrium and metastable SkLs in a polycrystalline sample of Co8Zn8Mn4. The spatial variations observed across a small region of the sample are remarkably consistent for the metastable skyrmions upon cooling and warming. These measurements show that the reversible transition between triangular and square SkLs upon cooling and warming are spatially reversible, reoccurring in the same sample locations.

超导涡旋晶格（superconducting vortex lattices, VLs）与磁性斯格明子晶格（magnetic skyrmion lattices, SkLs）这两种磁结构，其形成机制与宿主材料均存在显著差异，但二者拥有诸多共性属性，可通过同一实验技术——小角中子散射（small angle neutron scattering, SANS）开展研究。小角中子散射（SANS）可获取体晶体中完整VL或SkL的统计信息，传递有关磁结构均匀性、取向、间距与物相的相关数据。VL对宿主超导体的特性高度敏感，如体系中存在的各向异性；外加可调外部参数与各向异性的耦合效应，可通过VL取向与畴结构体现。对于二硼化镁（MgB₂）而言，磁场角度旋转结合外加磁场强度的调控，可实现各向异性间的微妙平衡。据此可划分出三种截然不同的磁场角度区间，分别对应各向异性协同、平衡与竞争状态，其中存在单畴L相的反常存在形式，以及向L相与I相共存的新型态转变的不连续相变。此前尚未被充分探索的斯格明子介观尺度范畴，涵盖了畴的形成方式与位置，及其与其他共存磁结构的关联。本研究作者自主设计搭建的样品与扫描孔径台（Sample and Scanning-Aperture Stage, SSAS），正是为填补斯格明子微观与宏观尺度间的研究空白而开发，可实现SkL的空间分辨测量。近期，研究团队利用SSAS在Co₈Zn₈Mn₄多晶样品中观测到螺旋相、平衡态与亚稳态SkL的空间变化。在升降温过程中，于样品小区域内观测到的空间变化，对于亚稳态斯格明子而言具有高度一致性。上述测量结果表明，升降温过程中三角与四方SkL间的可逆转变具备空间可逆性，即在样品的同一位置重复出现。