Dynamic rotor mode in antiferromagnetic nanoparticles

TL;DR

This paper uncovers a new coherent magnetic precession mode, called the rotor mode, in antiferromagnetic nanoparticles, supported by experimental neutron scattering, numerical simulations, and theoretical modeling, revealing high-temperature spin dynamics.

Contribution

It introduces the rotor mode as a novel magnetic excitation in antiferromagnetic nanoparticles, supported by combined experimental, numerical, and analytical evidence.

Findings

Neutron scattering shows loss of diffraction intensity above 150 K.

Inelastic neutron scattering indicates persistent magnetic motion above 150 K.

Numerical simulations reproduce the observed neutron data and reveal the rotor mode.

Abstract

We present experimental, numerical, and theoretical evidence for a new mode of antiferromagnetic dynamics in nanoparticles. Elastic neutron scattering experiments on 8 nm particles of hematite display a loss of diffraction intensity with temperature, the intensity vanishing around 150 K. However, the signal from inelastic neutron scattering remains above that temperature, indicating a magnetic system in constant motion. In addition, the precession frequency of the inelastic magnetic signal shows an increase above 100 K. Numerical Langevin simulations of spin dynamics reproduce all measured neutron data and reveal that thermally activated spin canting gives rise to a new type of coherent magnetic precession mode. This "rotor" mode can be seen as a high-temperature version of superparamagnetism and is driven by exchange interactions between the two magnetic sublattices. The frequency of the rotor mode behaves in fair agreement with a simple analytical model, based on a high temperature approximation of the generally accepted Hamiltonian of the system. The extracted model parameters, as the magnetic interaction and the axial anisotropy, are in excellent agreement with results from Mossbauer spectroscopy.