Atomic-scale magnetic modeling of oxide nanoparticles

TLDR

The study presents a method for atomic‑scale modeling of the magnetic behavior of ionic magnetic solids. The method computes spin distributions and net magnetic moments for ferrimagnetic NiFe₂O₄, γ‑Fe₂O₃, and antiferromagnetic NiO nanoparticles under applied fields, using bulk‑derived crystal structures, exchange parameters, core and surface anisotropies, and temperature effects modeled by random spin perturbations. The calculations reveal surface spin disorder in ferrimagnetic spinel nanoparticles, show that surface anisotropy boosts coercivity only when disorder exists, explain low‑temperature quantum‑tunneling‑like relaxation via barrier distributions, predict multi‑sublattice configurations for NiO nanoparticles, and attribute their large coercivities and loop shifts to weak inter‑sublattice coupling, all in agreement with experiment.

Abstract

We present a method for atomic-scale modeling of the magnetic behavior of ionic magnetic solids. Spin distributions and net magnetic moments are calculated for nanoparticles of ferrimagnetic ${\mathrm{NiFe}}_{2}{\mathrm{O}}_{4}$ and $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3},$ and antiferromagnetic NiO as a function of applied field. Calculations incorporate crystal structures and exchange parameters determined from bulk data, bulk anisotropy for core spins, reasonable estimates for the anisotropy of surface spins, and finite temperatures simulated by random perturbations of spins. Surface spin disorder was found in the case of ferrimagnetic spinel nanoparticles, due to broken exchange bonds at the surface. The calculations also demonstrate that surface anisotropy enhances the coercivity of such particles only when surface spin disorder is present. Simulated thermal perturbations were used to characterize the distribution of energy barriers between surface spin states of such particles. The distribution of barriers can explain the macroscopic quantum tunneling like magnetic relaxation at low temperatures found experimentally. Calculations on NiO nanoparticles predict eight, six, or four-sublattice spin configurations in contrast to the two-sublattice configuration accepted for bulk NiO. Relatively weak coupling between the multiple sublattices allows a variety of reversal paths for the spins upon cycling the applied field, resulting in large coercivities and loop shifts, in qualitative agreement with experiment.