Ferromagnetic resonance in $\epsilon$-Co magnetic composites

TL;DR

This study explores the microwave-frequency ferromagnetic resonance behavior of nanoscale $ ext{epsilon}$-Co particles embedded in polystyrene, revealing how external magnetic fields and particle aggregation influence their electromagnetic properties.

Contribution

It provides new insights into how magnetic resonance shifts with field and aggregation in nanoscale cobalt composites, combining experimental measurements with theoretical modeling.

Findings

Resonant frequency shifts with magnetic field and aggregation.

Particles become decoupled at fields above 2.5 kOe.

Magnetic properties can be tuned by external fields and structure.

Abstract

We investigate the electromagnetic properties of assemblies of nanoscale $\epsilon$-cobalt crystals with size range between 5 nm to 35 nm, embedded in a polystyrene (PS) matrix, at microwave (1-12 GHz) frequencies. We investigate the samples by transmission electron microscopy (TEM) imaging, demonstrating that the particles aggregate and form chains and clusters. By using a broadband coaxial-line method, we extract the magnetic permeability in the frequency range from 1 to 12 GHz, and we study the shift of the ferromagnetic resonance with respect to an externally applied magnetic field. We find that the zero-magnetic field ferromagnetic resonant peak shifts towards higher frequencies at finite magnetic fields, and the magnitude of complex permeability is reduced. At fields larger than 2.5 kOe the resonant frequency changes linearly with the applied magnetic field, demonstrating the transition to a state in which the nanoparticles become dynamically decoupled. In this regime, the particles inside clusters can be treated as non-interacting, and the peak position can be predicted from Kittel's ferromagnetic resonance theory for non-interacting uniaxial spherical particles combined with the Landau-Lifshitz-Gilbert (LLG) equation. In contrast, at low magnetic fields this magnetic order breaks down and the resonant frequency in zero magnetic field reaches a saturation value reflecting the interparticle interactions as resulting from aggregation. Our results show that the electromagnetic properties of these composite materials can be tuned by external magnetic fields and by changes in the aggregation structure.