The origin of g $\approx$ 4 EPR line in magnetic nanocomposites: Manifestation of double quantum transitions in ferromagnetic granules

TL;DR

This study investigates the origin of a g ≈ 4 EPR line in magnetic nanocomposites, revealing it as a manifestation of double quantum transitions in ferromagnetic granules through electron magnetic resonance analysis.

Contribution

It introduces the quantum mechanical 'giant spin' model to explain the g ≈ 4 EPR line as double quantum transitions in ferromagnetic nanogranules.

Findings

Detection of an additional g ≈ 4 absorption peak below percolation threshold.

Explanation of the peak as forbidden double quantum transitions.

Validation of the 'giant spin' model for nanogranular magnetic systems.

Abstract

Films of metal-insulator nanogranular composites M$_x$D$_{100-x}$ with different compositions and atomic percentage of metal and dielectric phases (M = Fe, Co, Ni, CoFeB; D = Al$_2$O$_3$, SiO$_2$, ZrO$_2$; x $\approx$ 15-60 at.%) are investigated by electron magnetic resonance in a wide range of frequencies (f = 7-37 GHz) and temperatures (T = 4.2-360 K). At concentrations of the metallic ferromagnetic phase below the percolation threshold, the experimental spectra, besides the conventional ferromagnetic resonance signal, demonstrate an additional absorption peak characterized by a double effective g-factor g $\approx$ 4. The appearance of such a peak in the resonance spectra and its unusual properties are explained in the framework of the quantum mechanical "giant spin" model by the excitation of "forbidden" ("double quantum") transitions in magnetic nanogranules with a change of the spin projection $\Delta m = \pm2$.