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Magnetization, excitations, and microwave power absorption in transition-metal/rare-earth ferrites with disorder

D. A. Garanin, E. M. Chudnovsky

TL;DR

This work develops efficient numerical tools to study temperature- and composition-driven magnetization, uniform spin excitations, and microwave absorption in disordered two-sublattice ferrimagnets with RE dilution. By combining zero- and finite-temperature energy minimization, adaptive Monte Carlo, and high-accuracy dynamical integration, the authors derive and verify analytic expressions for uniform-mode frequencies $\varepsilon_{\pm}$ and map their evolution across RE concentration and temperature, including the angular-momentum and magnetic-moment compensation points at $c\Sigma=S$ and $c\Sigma=(g/g')S$. Disorder from random RE positions yields localized modes that broaden into a continuum in large systems, while the total integrated absorption exhibits a minimum near the compensation region and is strongly influenced by spin-flip transitions. The results provide a coherent framework linking compensation physics, spin dynamics, and microwave absorption, with applicability to CoGd-type ferrimagnets and related materials.

Abstract

Efficient numerical routines are developed for numerical studies of the dependence of the equilibrium magnetic states, excitations, and microwave power absorption on temperature and composition in transition-metal/rare-earth ferrites, including the reversal of the Néel vector occurring on both temperature and the concentration of the rare-earth atoms. It results in a drastic change in the behavior at the magnetization and angular-momentum compensation points. Dominant uniform oscillation modes are obtained by computing the magnetization correlation function. They are compared with the analytical solution, which is analyzed in detail. The fluctuation-dissipation theorem is used to compute the frequency dependence of the absorbed microwave power. A good agreement with analytical results is demonstrated. Disorder caused by random positions of rare-earth atoms in a diluted RE system leads to multiple localized modes that converge into broad absorption maxima as the size of the system increases. The power absorption integrated over frequency exhibits a minimum at the compensation point.

Magnetization, excitations, and microwave power absorption in transition-metal/rare-earth ferrites with disorder

TL;DR

This work develops efficient numerical tools to study temperature- and composition-driven magnetization, uniform spin excitations, and microwave absorption in disordered two-sublattice ferrimagnets with RE dilution. By combining zero- and finite-temperature energy minimization, adaptive Monte Carlo, and high-accuracy dynamical integration, the authors derive and verify analytic expressions for uniform-mode frequencies and map their evolution across RE concentration and temperature, including the angular-momentum and magnetic-moment compensation points at and . Disorder from random RE positions yields localized modes that broaden into a continuum in large systems, while the total integrated absorption exhibits a minimum near the compensation region and is strongly influenced by spin-flip transitions. The results provide a coherent framework linking compensation physics, spin dynamics, and microwave absorption, with applicability to CoGd-type ferrimagnets and related materials.

Abstract

Efficient numerical routines are developed for numerical studies of the dependence of the equilibrium magnetic states, excitations, and microwave power absorption on temperature and composition in transition-metal/rare-earth ferrites, including the reversal of the Néel vector occurring on both temperature and the concentration of the rare-earth atoms. It results in a drastic change in the behavior at the magnetization and angular-momentum compensation points. Dominant uniform oscillation modes are obtained by computing the magnetization correlation function. They are compared with the analytical solution, which is analyzed in detail. The fluctuation-dissipation theorem is used to compute the frequency dependence of the absorbed microwave power. A good agreement with analytical results is demonstrated. Disorder caused by random positions of rare-earth atoms in a diluted RE system leads to multiple localized modes that converge into broad absorption maxima as the size of the system increases. The power absorption integrated over frequency exhibits a minimum at the compensation point.
Paper Structure (9 sections, 53 equations, 8 figures)

This paper contains 9 sections, 53 equations, 8 figures.

Table of Contents

  1. Introduction
  2. The model
  3. Excitation modes in the uniform state
  4. Numerical methods
  5. Inversion of the Néel vector on the RE concentration and temperature
  6. Uniform excitation modes at different RE concentrations
  7. The absorption spectrum
  8. The integral absorption
  9. Conclusions

Figures (8)

  • Figure 1: Spin-flip transition on the RE concentration $c$. Top: Angular momentum of the TM sublattice, $m_{z}$, and dispersion of the longitudinal and transverse fluctuations. Bottom: Total angular momentum ($m_{z}$) and total magnetic moment ($\mu_{z}$).
  • Figure 2: Spin-flip transition on temperature. Top: Angular momentum of the TM sublattice, $m_{z}$, and dispersion of its fluctuations. Bottom: Total angular momentum ($m_{z}$) and total magnetic moment ($\mu_{z}$).
  • Figure 3: Energy spectrum of uniform modes at different RE concentrations, obtained from the first period of the spin correlation function, compared with the theory in Sec. \ref{['Sec_Excitation-modes']}.
  • Figure 4: The magnetization CF close to the compensation point (here $c=0.48)$ has a complicated form.
  • Figure 5: The absorption spectrum of an undiluted ferrite at low temperatures, $T/(SJ)=0.01$ and 0.03.
  • ...and 3 more figures