Ferromagnetic resonance in uniaxial superparamagnetic particlesстатья
Статья опубликована в высокорейтинговом журнале
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Статья опубликована в журнале из списка Web of Science и/или Scopus
Дата последнего поиска статьи во внешних источниках: 16 января 2019 г.
Аннотация:A consistent theory of ferromagnetic resonance in superparamagnetic nanoparticles with uniaxial anisotropy of arbitrary strength is developed. Being free from restrictions on the material parameters of the particles and on the spectrometer characteristics, the approach is well suitable for the interpretation of experimental data as well as for cross-checking approximate models. Ferromagnetic resonance absorption lines of single particles and noninteracting random ensembles of those are presented. It is shown that the multicomponent spectra inherent to highly anisotropic particles in the athermic limit persist for the superparamagnetic ones until temperature attains a certain level defined by the interplay between the intensity of thermal fluctuations and the anisotropy magnitude. The inference that orientational spread of the particle easy axes is a precondition for the occurrence of multipeak absorption lines in superparamagnetic systems is refuted: We demonstrate that ensembles of coherently oriented nanoparticles are able to render well-defined two-component spectra provided their anisotropy is considerable and the tilt of the easy axes to the magnetizing field is close to 90 degrees. However, the orientational spread of the easy axes, if it exists, sharpens the spectrum contour. Concerning the role of polydispersity, it is found that a fraction of small particles always smoothes the spectrum as those particles produce wide, nearly symmetrical absorption lines with a single maximum.