The static and dynamic properties of hexagonal lattices of magnetic antidots have been studied using micromagnetic simulations and semianalytical modeling. The theoretical model is based on the Landau–Lifshitz equation and is developed for the case of comparable magnitudes of the field and linewidth of the ferromagnetic resonance (FMR). It is found that the antidot pattern induces an apparent sixfold configurational anisotropy manifesting itself via an anisotropic resonant response of the patterned film. However, calculations performed with a reduced damping reveal that the resonance peak consists of three different quasiuniform modes of the magnetization dynamics, with the resonant field of each of them showing a twofold variation with respect to the in-plane orientation of the applied magnetic field. The easy axes of the resonant modes are mutually rotated by 60° and combine to yield the observed sixfold configurational anisotropy. Micromagnetic calculations of the local dynamic susceptibility allow us to attribute each absorption line to a different area of the sample. Finally, we analyze the effect of the antidot radius and the lattice period on the broadening of the uniform FMR line in comparison with the uniform FMR mode of the corresponding continuous film.
Static and resonance properties of ferromagnetic films with a hexagonal lattice of antidots (pores in the film) were studied. The description of the system is based on micromagnetic modeling and analytical solutions of the Landau–Lifshitz equation. The dependences of ferromagnetic resonance spectra on the in-plane direction of applied magnetic field and on the lattice parameters were investigated. The nature of the dependences of a dynamic system response on the frequency at fixed magnetic fields and on the field at fixed frequency when the field changes were explored. They cause the static magnetic order to change. It was found that the specific peculiarities of the system dynamics remain unchanged for both of these experimental conditions. Namely, for low damping the resonance spectra contain three quasi-homogeneous modes which are due to the resonance of different regions (domains) of the antidot lattice cell. It is shown that the angular field dependences of each mode are characterized by a twofold symmetry, and the related easy axes are mutually rotated by 60 °. As a result, a hexagonal symmetry of the system’s static and dynamic magnetic characteristics is realized. The existence in the resonance spectrum of several quasi-homogeneous modes related to different regions of the unit cell could be fundamental for the function of the working elements of magnonics devices.
The microwave absorption of an ensemble of single-domain nanoparticles of La0.7Sr0.3MnO3 is investigated in the temperature range 5–300K. At low temperatures the resonance spectra demonstrate the appreciable increase in linewidth and downward shift of the resonance field which are typical for superparamagnetic resonance. With increasing temperature the line shape changes, and at T>TB (TB∼100K) one observes a narrowing of the resonance curve and saturation of the resonance field. The line shape of an individual nanoparticle is described by the dynamic Landau–Lifshitz equations with damping. Modeling of the magnetic dynamics of the system is based on the assumption of a random distribution of the directions of the magnetic moments and of the thermal fluctuations of the direction of the anisotropy axis of the particles. The theory takes into account the dependence of the value of the resonance field on the linewidth.
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