Rigorous broadband study of the intrinsic ferromagnetic linewidth of monocrystalline garnet spheres

Adam Pacewicz,Bartlomiej Salski,Pavlo Aleshkevych,Pawel Kopyt,Jerzy Krupka

doi:10.1038/s41598-019-45699-7

Abstract

This work demonstrates the first application of direct broadband (1 GHz–30 GHz) quality (Q) factor measurements of the uniform precession mode in magnetised garnet spheres for the accurate determination of the room-temperature intrinsic ferromagnetic linewidth (ΔH). The spheres were enclosed in a subwavelength cavity, so that the measured Q-factor depended mainly on their magnetic losses and the conduction losses of the cavity walls. The contribution of the latter is assessed by means of the recently proposed magnetic plasmon resonance model and has been found to be negligible. A total of 10 samples made from commercially available pure yttrium iron garnet (YIG) and gallium-substituted YIG have been measured, differing in diameter and/or saturation magnetisation Ms. The dependence of the intrinsic ΔH on the internal magnetic field is found to have near-perfect linear dependence, which cannot be said about the typically studied extrinsic ΔH even at high frequencies. It is found that the difference between the two linewidths, which becomes significant at low frequencies, can be attributed to a geometric effect. Due to its fundamental nature, this work is applicable not only to magnetic material characterization, but also to the study of the origins of losses in magnetic materials.

Highlights

The phenomenon of resonant absorption of radio frequency (RF) and microwave radiation in ferromagnetic materials has been heavily studied since the 1940s1,2
Measurements of the Q-factor vs. magnetic bias were performed in the setup shown schematically in Fig. 1, consisting of a brass subwavelength cylindrical cavity loaded with the spherical sample, an electromagnet and a vector network analyzer (VNA)
A method enabling the determination of the intrinsic ferromagnetic linewidth (ΔHint) of the monocrystalline spheres from the accurate broadband measurements of the Q-factor has been demonstrated

Summary

Introduction

The phenomenon of resonant absorption of radio frequency (RF) and microwave radiation in ferromagnetic materials has been heavily studied since the 1940s1,2. The presented theoretical analysis and experiments are restricted to the mode of uniform precession, for which a rigorous electrodynamic model has been recently introduced and validated[7,8,9] Such an electrodynamic approach has led to the discovery that the resonance observed in bulk ferromagnetic samples is a magnetic plasmon resonance (MPR), but has misfortunately been called the ferromagnetic resonance (FMR), which it is not. The intrinsic permeability of a ferromagnetic (gyrotropic) material saturated along the +z axis is a tensor of the following form[7]: www.nature.com/scientificreports/. Diagonal and off-diagonal components of the tensor, μ and κ, respectively, of a saturated ferromagnet in the low magnetic loss regime (Gilbert damping factor α ≪ 1) depend on the excitation frequency and internal magnetic field Hint and can be conveniently expressed as follows[7,10]: μ. Ms is the saturation magnetisation of the sample, γ ≈ 2.8 MHzOe−1 is the gyromagnetic ratio, and Q0 is the unloaded Q-factor of the resonant system

Methods

Results

Conclusion