Revealing defect-induced spin disorder in nanocrystalline Ni

TL;DR

This study combines magnetometry and neutron scattering to reveal how defect-induced microstructural features in nanocrystalline Ni cause spin disorder and influence magnetic properties, supported by micromagnetic simulations.

Contribution

It provides new insights into the role of crystal defects and strain fields in inducing spin disorder in nanocrystalline Ni, using combined experimental and simulation approaches.

Findings

Defects cause significant spin-misalignment scattering.

Average defect size is about 11 nm, smaller than crystallite size.

Strain fields induce spin disorder with a penetration depth of 22 nm.

Abstract

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. As seen by magnetometry, the mechanical deformation leads to a significant increase of the coercivity compared to 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 around 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 particular 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 high-pressure torsion materials.