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 th
e 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.
We analyze theoretically the novel pathway of ultrafast spin dynamics for ferromagnets with high enough single-ion anisotropy (non-Heisenberg ferromagnets). This longitudinal spin dynamics includes the coupled oscillations of the modulus of the magne
tization 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 a dynamics can lead to an inertial magnetization reversal under the action of an ultrashort laser pulse.
We show that a magnetic vortex is the ground state of an array of magnetic particles shaped as a hexagonal fragment of a triangular lattice, even for an small number of particles in the array $N leq 100$. The vortex core appears and the symmetry of t
he vortex state changes with the increase of the intrinsic magnetic anisotropy of the particle $beta$; the further increase of $beta$ leads to the destruction of the vortex state. Such vortices can be present in arrays as small in size as dozen of nanometers.
We analyzed the ground state of the array of magnetic particles (magnetic dots) which form a two-dimensional triangular lattice, and magnetic moment of which is perpendicular to the plane of the lattice, in the presence of external magnetic field. In
the small fields long range dipole-dipole interaction leads to the specific antiferromagnetic order, where two out of six nearest neighbors of the particle have the same direction of magnetization moment and four - the opposite one. It is shown that magnetization process in such array of particles as opposed to the rectangular lattices results from the formation of the magnetized topological defects (dislocations) in the shape of the domain walls.
Here we consider micron-sized samples with any axisymmetric body shape and made with a canted antiferromagnet, like hematite or iron borate. We find that its ground state can be a magnetic vortex with a topologically non-trivial distribution of the s
ublattice magnetization $vec{l}$ and planar coreless vortex-like structure for the net magnetization $vec{M}$. For antiferromagnetic samples in the vortex state, in addition to low-frequency modes, we find high-frequency modes with frequencies over the range of hundreds of gigahertz, including a mode localized in a region of radius $sim$ 30--40 nm near the vortex core.
