Resonance Absorption and Transverse Magnetization of a Ferromagnetic Spin System Interacting with a Phonon Reservoir in the Spin-Wave Region

Abstract
The linear response of a ferromagnetic spin system interacting with a phonon reservoir in the spin-wave region, is discussed employing the TCLE method (a method in which the admittance of a physical system interacting with a heat reservoir is directly derived from time-convolutionless equations with external driving terms) in terms of the non-equilibrium thermo-field dynamics (NETFD). The power absorption and the amplitude of the expectation value of the transverse magnetization, which is referred as “the magnetization-amplitude”, for the ferromagnetic system, are studied including not only the low-order parts but also the next higher-order parts in the spin-wave approximation. The approximate formulas of the resonance frequencies, peak-heights (heights of peak) and line half-widths in the resonance region of the power absorption and magnetization-amplitude, are derived for the ferromagnetic system interacting with the phonon reservoir in a transversely rotating magnetic-field. The power absorption and magnetization-amplitude are investigated numerically for a ferromagnetic system of one-dimensional infinite spins. The approximate formulas of the resonance frequencies, peak-heights and line half-widths, are shown to coincide well or nearly with the results investigated calculating numerically the analytic results of the power absorption and magnetization-amplitude in the resonance region, and also are shown to satisfy “the narrowing condition” that as the phonon reservoir is damped quickly, the peak-heights increase and the line half-widths decrease. Thus, the approximate formulas are verified numerically. The effects of the memory and initial correlation for the spin system and phonon reservoir, which are represented by the interference terms in the TCLE method and are referred as “the interference effects”, are confirmed to increase the power absorption and magnetizationamplitude in the resonance region, and are shown to produce effects that cannot be disregarded for the high temperature, for the non-quickly damped reservoir or for the small wave-number.

Keywords
Ferromagnetic spin system; Spin-wave method; Transverse magnetic susceptibility; The TCLE method of linear response; Resonance asorption; Non-equilibrium thermo-field dynamics

Cite this paper
Mizuhiko Saeki, Resonance Absorption and Transverse Magnetization of a Ferromagnetic Spin System Interacting with a Phonon Reservoir in the Spin-Wave Region , SCIREA Journal of Physics. Volume 10, Issue 6, December 2025 | PP. 267-302. 10.54647/physics140706

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