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...

Ultrafast control of magnets using femtosecond light pulses attracts interest regarding applications and fundamental physics of magnetism. Antiferromagnets are promising materials with magnon frequencies extending into the terahertz range. Visible or near-infrared light interacts mainly with the electronic orbital angular momentum. In many magnets, however, in particular with iron-group ions, the orbital momentum is almost quenched by the crystal field. Thus, the interaction of magnons with light is hampered, because it is only mediated by weak unquenching of the orbital momentum by spin–orbit interactions. Here we report all-optical excitation of magnons with frequencies up to 9 THz in antiferromagnetic CoO with an unquenched orbital momentum. In CoO, magnon modes are coupled oscillations of spin and orbital momenta with comparable amplitudes. We demonstrate excitations of magnon modes by directly coupling light with electronic orbital angular momentum, providing possibilities to develop magneto-optical devices operating at several terahertz with high output-to-input ratio.

We study dynamics and relaxation of elementary excitations (magnons) in the spin nematic (quadrupole ordered) phase of S=1 magnets. We develop a general phenomenological theory of spin dynamics and relaxation for spin-1 systems. Results of the phenomenological approach are compared to those obtained by microscopic calculations for the specific S=1 model with isotropic bilinear and biquadratic exchange interactions. This model exhibits a rich behavior depending on the ratio of bilinear and biquadratic exchange constants, including several points with an enhanced symmetry. It is shown that symmetry plays an important role in relaxation. Particularly, at the SU(3) ferromagnetic point the magnon damping $\Gamma$ depends on its wavevector k as $\Gamma\propto k^{4}$, while a deviation from the high-symmetry point changes the behavior of the leading term to $\Gamma\propto k^{2}$. We point out a similarity between the behavior of magnon relaxation in spin nematics to that in an isotropic ferromagnet.Comment: the final published versio

We analyze theoretically the novel pathway of ultrafast spin dynamics for ferromagnets with high enough single-ion anisotropy. This longitudinal spin dynamics includes the coupled oscillations of the modulus of the magnetization together with the quadrupolar spin variables, which are expressed through quantum expectation values of operators bilinear on the spin components. Even for a simple single-element ferromagnet, such dynamics can lead to a magnetization reversal under the action of an ultrashort laser pulse.

Magnon relaxation processes in the nematic phase of a magnet with spin S=1 are investigated for a general form of the isotropic exchange interaction, including bilinear and biquadratic interactions in respect to the site spin operators. The temperature dependence and momentum dependence of the magnetic decrement are found in the long-wavelength approximation. It is shown that the elementary excitations in a spin nematic (magnons) have all the properties of Goldstone excitations; in the limit of small wave vectors they have a linear dispersion law, while the damping is quadratic in the wave vector. The similarity of magnon behavior in a spin nematic to that in an antiferromagnet is noted.

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The presence of single-ion anisotropy leads to the appearance of the effect of quantum spin reduction. As a consequence, purely longitudinal magnetization dynamics arises, which involves coupled oscillations of the mean spin modulus and the quadrupole mean values constructed on spin operators. In nickel fluorosilicate, the effect of quantum spin reduction may be controlled by changing pressure. The study of nonlinear longitudinal spin dynamics and the analysis of possible photomagnetic effects showed that this compound is a convenient model system to implement switching of the magnetization direction by femtosecond laser pulses.