Dipolar interactions and spin waves in periodic arrays of micron-size magnetic dots.

While interest in arrays of magnetic submicron dots is stimulated by their possible applications in high density recording media and magnetic random access memory they are very attractive as model systems to study fundamental physical properties of small magnetic particles. Recently it has been shown that magnetostatic interactions play an important role in the magnetization reversal process for ferromagnetic submicron dot arrays with small interdot distances. One important parameter that characterizes such interactions is the uniaxial (for rectangular arrays) or four-fold (for square arrays) anisotropy field induced by the dipole-dipole interaction of nearest neighbors in the film plane.

Ferromagnetic resonance was used to measure the anisotropies including the ones due to interdot interactions in rectangular and square arrays of circular dots. Unlike a hysteresis loop method, often used for such measurements, FMR allows the investigation of samples at their magnetic saturation state and excludes any influence of the domain structure or fluctuations of the magnetization near a critical point. In the case of narrow resonance lines (~ 100 Oe), a very small shift of the resonance field (~5 Oe) with the rotation of the applied field, was detected.

When the applied field was close to the sample normal, in addition to the main FMR peak, multiple additional sharp resonant peaks were also observed at lower fields. For all the samples of the same material the differences between corresponding neighbor spin wave modes were practically the same, indicating that the interdot interactions modify the relative positions of the modes but do not affect structure of the resonance spectra. No such periodic spectra was found for the reference continuous films, supporting the idea that modes with discrete frequencies are due to the restricted sample geometry of the patterned films and have magnetostatic nature. To fit the experimental spectra, the model was developed, that takes into account both dipolar and exchange contributions.

FMR is also the most judicious technique to investigate the dynamics of internal spin interactions in ferromagnets that determine the relaxation processes. Understanding the nature of relaxation losses is important from the standpoint of determining switching times for magnetic dots and arrays.