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In the exchange approximation, an exact solution is obtained for the sublattice magnetizations evolution in a two-sublattice ferrimagnet. Nonlinear regimes of spin dynamics are found that include both the longitudinal and precessional evolution of the sublattice magnetizations, with the account taken of the exchange relaxation. In particular, those regimes describe the spin switching observed in the GdFeCo alloy under the influence of a femtosecond laser pulse.PACS numbers: 75.10. Hk, Magnetic materials have various applications in modern electronics and informatics, but probably the most important research direction is still the creation of information storage and processing systems. The challenge of designing magnetic devices with ever increasing information density and recording speed requires solving certain fundamental problems of the magnetism dynamics. The possibility to manipulate the magnetization by means of femtosecond laser pulses opens wide opportunities in this direction. This field has been incepted by the work [1], where a fast (within a time shorter than a picosecond) reduction of nickel magnetization after the exposure to a 100 femtosecond laser pulse has been observed, as well as the subsequent relaxation of the magnetization with a characteristic time of the order of picoseconds. The authors explained the initial drop in the magnetization either by an extremely rapid heating of the sample above the Curie point, see review [2], or by spin-dependent super-diffusive electron transfer in the laser-excited metal [3]. Further work in this area followed for various materials, and unexpected and rather unusual effects were discovered. In the ferrimagnetic rare earth and transition metal alloy GdFeCo, a femtosecond pulse lead, in the first stage, to a similar spin reduction (i.e., the reduction of the magnetization of sublattices) as for nickel, but the subsequent evolution turned out to be fundamentally different. Instead of a simple relaxation to the initial value, within about the same time (a few picoseconds), both sublattice magnetizations changed their signs, i.e., a switching of the net magnetic moment took place [4], and during this picosecond-scale evolution there occurred an a priori energetically unfavorable state with parallel sublattice moments. Such a magnetization switching effect is of a threshold type, and is observed only for sufficiently strong pulses. It has been detected in films as well as in microparticles [5] and nanoparticles [6], both for ferromagnets with and without a compensation point [5]. There is also a way of "selective" switching: due to the magnetic dichroism, the absorbed energy of a circularly polarized pulse depends on the direction of the magnetic moment of the particles, and a pulse of certain polarization would only switch the moments of the particles which are in a matching state [7]. All that makes possible to create a purely optically-controlled magnetic memory with a picosecond recording speed.Although an analytical explanation of this effect is highly desi...

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A general method of constructing a dissipation function is developed for disordered magnetic media and for magnetically ordered systems. As an example it is shown for a ferromagnet that not only the invariance with respect to uniform rotations of the body but also the law of conservation of magnetization must be taken into account in order to construct a dissipation function. It is found that in ferromagnets the dissipation term in the equations of motion for the magnetization is a sum of Bloch and Landau-Lifshitz-Gilbert relaxation terms. The region of applicability of the relaxation term in the Landau-Lifshitz form is determined. The damping of spin waves in a ferromagnet with tetragonal symmetry is calculated. A procedure is formulated for transitioning from a ferromagnet with lower symmetry to a ferromagnet with a continuous degeneracy parameter. In this case the relaxation process can be systematically described by means of the dissipation function described in this article. It is shown how the relaxation term of a general form for ferromagnets becomes the Bloch relaxation term for paramagnets. It is shown that the magnetization vector relaxes in two stages. First the magnetic moment relaxes in magnitude quite rapidly as a result of exchange enhancement and then the magnetization relaxes slowly to its equilibrium direction. The second stage qualitatively corresponds to the relaxation picture described by the Landau-Lifshitz model.