The magnetization in a magnetic microdot made from soft magnetic materials can have a vortexlike ground state structure resulting from competition between the exchange and dipolar interactions. Normal mode magnon frequencies for such dots are calculated taking into account both exchange and magnetostatic effects. The presence of a low-lying mode as well as doublet structure with small splitting is demonstrated. Estimates of the mode frequencies for permalloy dots are obtained, and the possibility of experimental detection of such modes is discussed.

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The magnon mode excitation spectrum is obtained from a linearized set of Landau-Lifshitz equations for vortex ground state cylindrical nanomagnets in an external magnetic field. It is shown that there is a rich spectrum of doublet states, and the splitting can be amplified in an external magnetic field.

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An observed exponential decay in the electron paramagnetic resonance linewidth of the nearly classical two-dimensional antiferromagnet as a function of temperature is shown to be the result of solitons interacting with magnons, and provides an experimental confirmation of these excitations. The temperature dependence of the linewidth is calculated using the dynamic spin correlation function derived from soliton-magnon scattering in the Born approximation. Data in the critical regime for (n-propylammonium) 2 tetrachloromanganese (II) are presented and compared to the theory.

It is shown that the introduction of a very small amount of nonmagnetic impurities into the magnetic sites of a classical two-dimensional antiferromagnet creates a new type of static (impuritypinned) soliton that affects the Arrhenius, exp͑2E͞T͒, temperature-dependent electron paramagnetic resonance linewidth by drastically changing the parameter E. Data just above the transition temperature for ͑C 3 H 7 NH 3 ͒ 2 M x Mn 12x Cl 4 confirm the existence of these impurity-pinned solitons.[S0031-9007(98)05498-2] PACS numbers: 75.10. Hk, 75.40.Gb, 76.30.Fc Two-dimensional magnetic systems support interesting nonlinear excitations including solitons and vortices. For the two-dimensional (2D) isotropic ferromagnetic Belavin and Polyakov [1] obtained these solitonlike solutions (BP solitons) from topological considerations. The energy of this excitation is found to be independent of the soliton size resulting from scale invariance of the continuum Heisenberg Hamiltonian. The significance of these excitations was recognized early in connection with the critical properties of 2D magnets. For example, in [1] it was shown that the existence of large localized excitations will cause the correlation length to remain finite at any nonzero temperature as expected from the Mermin-Wagner theorem [2].Recently we have shown [3,4] that BP solitons dominate the thermodynamics in the fluctuation region immediately above the Néel temperature of a large class of nearly classical 2D antiferromagnets. Experimentally this is observed as an Arrhenius behavior of the temperature-dependent electron paramagnetic resonance (EPR) linewidth in layered manganese systems which was first predicted by Waldner [5,6]. In [3,4] the EPR linewidth was calculated from the dynamic spin correlation function with the time dependence from the solitonmagnon interaction; moveover, it was shown that the calculated linewidth matched the observed Arrhenius behavior.In this Letter we show that a new type of soliton pinned to a nonmagnetic impurity will form, and this pinned soliton has a lower energy than a large pinned soliton with a corresponding larger density in the lattice. This lowering of energy for the impurity solitons occurs simply because of elimination of exchange bonds at the impurity, which is a significant effect in the small and a negligible effect in the large impurity solitons. Because of this energy difference, the smaller pinned soliton will dominate the BP soliton in the fluctuation region. In order to relate these small impurity-pinned solitons to experimental data, we first obtain the temperature-dependent EPR linewidth resulting from these structures as a function of impurity concentration. This calculation shows that there will be large changes in the temperature dependence of the EPR linewidth as the impurity concentration is varied in a small (less than 1%) range. Finally, this effect is observed by EPR measurements on manganese compounds with nonmagnetic impurities where the calculated impurity dependence is indeed observed.We begin with...

The dispersion relations of collective oscillations of the magnetic moment of magnetic dots arranged in square-planar arrays and having magnetic moments perpendicular to the array plane are calculated. The presence of the external magnetic field perpendicular to the plane of array, as well as the uniaxial anisotropy for single dot are taken into account. The ferromagnetic state with all the magnetic moments parallel, and chessboard antiferromagnetic state are considered. The dispersion relation yields information about the stability of different states of the array. There is a critical magnetic field below which the ferromagnetic state is unstable. The antiferromagnetic state is stable for small enough magnetic fields. The dispersion relation is non-analytic as the value of the wave vector approaches zero. Non-trivial Van Hove anomalies are also found for both ferromagnetic and antiferromagnetic states.

The dispersion relations for collective magnon modes for square-planar arrays of vortex-state magnetic dots, having closure magnetic flux are calculated. The array dots have no direct contact between each other, and the sole source of their interaction is the magnetic dipolar interaction.The magnon formalism using Bose operators along with translational symmetry of the lattice, with the knowledge of mode structure for the isolated dot, allows the diagonalization of the system Hamiltonian giving the dispersion relation. Arrays of vortex-state dots show a large variety of collective mode properties, such as positive or negative dispersion for different modes.For their description, not only dipolar interaction of effective magnetic dipoles, but non-dipolar terms common to higher multipole interaction in classical electrodynamics can be important. The dispersion relation is shown to be non-analytic as the value of the wavevector approaches zero for all dipolar active modes of the single dot. For vortex-state dots the interdot interaction is not weak, because, the dynamical part (in contrast to the static magnetization of the vortex state) dot does not contain the small parameter, the ratio of vortex core size to the dot radius. This interaction can lead to qualitative effects like the formation of modes of angular standing waves instead of modes with definite azimuthal number known for the insolated vortex state dot.