Neutron study of magnetic correlations in rare-earth-free Mn-Bi magnets

TL;DR

This study uses neutron scattering to analyze magnetic correlations in Mn-Bi magnets, revealing long-wavelength fluctuations and shape anisotropy in magnetic structures, which enhances understanding of rare-earth-free permanent magnets.

Contribution

It provides the first detailed neutron scattering analysis of magnetic correlations in Mn-Bi-based rare-earth-free magnets, highlighting their magnetic fluctuation characteristics and structural anisotropy.

Findings

Magnetic fluctuations are dominated by long-wavelength transversal modes.

The radius of gyration of magnetic structures is approximately 220-240 nm.

Magnetic scattering originates from slightly shape-anisotropic structures.

Abstract

We report the results of an unpolarized small-angle neutron scattering (SANS) study on Mn-Bi-based rare-earth-free permanent magnets. The magnetic SANS cross section is dominated by long-wavelength transversal magnetization fluctuations and has been analyzed in terms of the Guinier-Porod model and the distance distribution function. This provides the radius of gyration which, in the remanent state, ranges between about $220-240 \, \mathrm{nm}$ for the three different alloy compositions investigated. Moreover, computation of the distance distribution function in conjunction with results for the so-called $s$-parameter obtained from the Guinier-Porod model indicate that the magnetic scattering of a Mn$_{45}$Bi$_{55}$ sample has its origin in slightly shape-anisotropic structures.