We combine magnetometry and magnetic small-angle neutron scattering to study the influence of the microstructure on the macroscopic magnetic properties of a nanocrystalline Ni bulk sample, which was prepared by straining via high-pressure torsion (HPT). As seen by magnetometry, the mechanical deformation leads to a significant increase of the coercivity compared with nondeformed polycrystalline Ni. The neutron data reveal a significant spin-misalignment scattering caused by the high density of crystal defects inside the sample, which were created by the severe plastic deformation during the sample preparation. The corresponding magnetic correlation length, which characterizes the spatial magnetization fluctuations in real space, indicates an average defect size of 11 nm, which is smaller than the average crystallite size of 60 nm. In the remanent state, the strain fields around the defects cause spin disorder in the surrounding ferromagnetic bulk, with a penetration depth of ∼22 nm. The range and amplitude of the disorder is systematically suppressed by an increasing external magnetic field. Our findings are supported and illustrated by micromagnetic simulations, which for the case of nonmagnetic defects (holes) embedded in a ferromagnetic Ni phase, further highlight the role of localized spin perturbations for the magnetic microstructure of defect-rich magnets such as HPT materials.

中文翻译：

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揭示纳米晶镍中缺陷诱发的自旋失调

我们结合磁力分析法和磁性小角中子散射技术，研究了微观结构对纳米晶镍块状样品宏观磁性能的影响，该样品是通过高压扭力（HPT）应变制备的。如通过磁力分析所见，与未变形的多晶Ni相比，机械变形导致矫顽力显着增加。中子数据显示，由于样品内部高密度的晶体缺陷引起的自旋未对准散射，这是由样品制备过程中的严重塑性变形造成的。表征实际空间中空间磁化强度波动的相应磁相关长度表示平均缺陷尺寸为11 nm，小于60 nm的平均微晶尺寸。在剩余状态下，缺陷周围的应变场在周围的铁磁体中引起自旋失调，穿透深度约为22 nm。不断增加的外部磁场有系统地抑制了无序的范围和幅度。我们的发现得到微磁模拟的支持和说明，对于嵌入铁磁Ni相中的非磁缺陷（孔），它进一步突出了局部自旋扰动对于诸如HPT材料之类的富含缺陷的磁体的磁微结构的作用。