Wagga2026/AC2MP2026

Confinement-driven renormalization of magnon and phonon spectra in Fe3O4 nanoparticles

Not scheduled

20m

Convention Centre (Charles Sturt University, Wagga Wagga NSW Australia)

Contributed Oral Magnetism Magnetism

Speaker

Mr Kyle Portwin (Institute for Superconducting and Electronic Materials, Faculty of Engineering and Information Science, University of Wollongong, Innovation Campus, Squires Way, North Wollongong, NSW 2500, Australia)

Description

Magnetite (Fe$_3$O$_4$) nanoparticles exhibit pronounced size-dependent magnetic and thermal properties relevant to spintronics, thermoelectrics, biomedical heating, and catalytic applications. In bulk Fe$_3$O$_4$, magnons and phonons govern key behaviours such as the Verwey transition, spin-lattice coupling, and thermal transport. However, at the nanoscale, their behaviour remains poorly understood due to the combined influence of finite-size confinement, surface strain, and heterogeneous exchange interactions. Despite extensive interest in nanoscale magnetism, a unified framework linking particle size to quasiparticle renormalisation has yet to be established.

In this presentation, I will show how time-of-flight inelastic neutron scattering, combined with molecular dynamics and linear spin-wave theory, resolves the full magnon and phonon renormalisation across Fe$_3$O$_4$ nanoparticles ranging from 100 nm (bulk-like) to 8 nm. We find that nanoscale confinement universally alters quasiparticle dynamics: optical phonons soften by $\sim 0.5$ meV and broaden significantly due to surface-induced strain, while acoustic magnons display a pronounced 36\% reduction in group velocity (14\,000 $\rightarrow$ 10\,000 m s$^{-1}$) arising from bond-angle disorder. These magnetic excitations are quantitatively described by a heterogeneous exchange model comprising bulk-like interactions ($J$) and weakened surface interactions ($J' = 0.7J$), which collectively account for the observed spectral broadening and magnon softening.

These results establish finite-size confinement as a general mechanism for renormalising vibrational and spin excitations in strongly correlated oxides. By directly connecting surface strain and exchange disorder to measurable changes in quasiparticle spectra, this work provides a foundation for engineering spin-wave coherence in spintronic devices and tailoring phonon transport in thermoelectric materials. The principles demonstrated here extend broadly to functional oxides such as CoFe$_2$O$_4$ and Mn$_3$O$_4$, offering a pathway for the rational design of nanoscale materials with tuneable magnetic and thermal properties.