Field Dependence of Magnetic Disorder in Nanoparticles

TL;DR

This study reveals that magnetic nanoparticles exhibit a significant increase in magnetic moment under applied magnetic fields due to polarization of surface spins, challenging the traditional view of a constant magnetic moment.

Contribution

It demonstrates the field-dependent increase of magnetic moments in ferrite nanoparticles caused by surface spin polarization, introducing a new understanding of intraparticle magnetization.

Findings

Magnetic moment increases by over 20% with applied field.

Surface spins become polarized, enlarging the magnetic volume.

Magnetic disorder energy rises sharply near the surface.

Abstract

The performance characteristics of magnetic nanoparticles towards application, e.g. in medicine, imaging, or as sensors, is directly determined by their magnetization relaxation and total magnetic moment. In the commonly assumed picture, nanoparticles have a constant overall magnetic moment originating from the magnetization of the single-domain particle core surrounded by a surface region hosting spin disorder. In contrast, this work demonstrates the significant increase of the magnetic moment of ferrite nanoparticles with applied magnetic field. At low magnetic field, the homogeneously magnetized particle core initially coincides in size with the structurally coherent grain of 12.8(2) nm diameter, indicating a strong coupling between magnetic and structural disorder. Applied magnetic fields gradually polarize the uncorrelated, disordered surface spins, resulting in a magnetic volume more than 20\% larger than the structurally coherent core. The intraparticle magnetic disorder energy increases sharply towards the defect-rich surface as established by the field-dependence of the magnetization distribution. In consequence, these findings illustrate how the nanoparticle magnetization overcomes structural surface disorder. This new concept of intraparticle magnetization is deployable to other magnetic nanoparticle systems, where the in-depth knowledge of spin disorder and associated magnetic anisotropies will be decisive for a rational nanomaterials design.