Separating spin dynamics modes in iron oxide nanoparticles

Abstract

The different modes of the spin dynamics in ferrimagnetic magnetite iron oxide nanoparticles (Fe3O4) have been addressed by µeV neutron spectroscopy for particles with a diameter of 75 ± 18 Å. In an approach building on intensity considerations over a wide range of wave vectors Q in combination with diffusion dynamics, we identify three magnetic modes prevailing in distinct Q regimes up to 1.7 Å−1: (i) Evaluating the structure factor of quasielastic magnetic intensity enables to identify uniquely longitudinal superparamagnetic spin fluctuations in the small Q regime <0.5Å−1, (ii) individual spin relaxation with quasielastic intensity albeit of low intensity is present in two Q ranges of 0.5 Å−1<Q<1.0Å−1 and 1.5 Å−1<Q<1.7Å−1, and (iii) a surprisingly low-energy inelastic spin excitation of 23.2 µeV in magnetite IONPs is found at 150 K in the magnetic Bragg region between 1.1 Å−1<Q<1.4Å−1. Its temperature dependence suggests a collapse of the coherent transverse magnetic fluctuations and a transition into a generalized diffusive mode at about 500 K. Published by the American Physical Society 2025