We investigate analytically and numerically the ground and metastable states for easy-plane Heisenberg magnets with single-ion surface anisotropy and disk geometry. The configurations with two half-vortices at the opposite points of the border are shown to be preferable for strong anisotropy. We propose a simple analytical description of the spin configurations for all values of a surface anisotropy. The effects of lattice pinning leads to appearance of a set of metastable configurations.

We consider a domain wall in the mesoscopic quasi-one-dimensional sample (wire or stripe) of weakly anisotropic two-sublattice antiferromagnet, and estimate the probability of tunneling between two domain wall states with different chirality. Topological effects forbid tunneling for the systems with half-integer spin S of magnetic atoms which consist of odd number of chains N. External magnetic field yields an additional contribution to the Berry phase, resulting in the appearance of two different tunnel splittings in any experimental setup involving a mixture of odd and even N, and in oscillating field dependence of the tunneling rate with the period proportional to 1/N.Comment: 4 pages + 2 figures, references correcte

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In the crystal lattice of an antiferromagnet, dislocations are the origin of specific lines in the field of antiferromagnetic vector I, resembling disclinations in the field of the vector-director for nematic liquid crystals. A single atomic dislocation creates a delocalized non-uniform state – a spin disclination. A “compensated” system of dislocations, a closed dislocation loop in a three-dimensional antiferromagnet or a pair of point dislocations in a two-dimensional antiferromagnet, are shown to form a localized spin non-uniformity, similar to a soliton. For an isotropic or easy-plane antiferromagnet the shape of these solitons is ellipsoidal or circular in three- or two-dimensional cases, respectively. The geometry of a lattice defect differs from that of a soliton; for example, a planar lattice defect, a dislocation loop, produces a nearly spherical three-dimensional spin non-uniformity. In the presence of in-plane anisotropy, a domain wall forms in the easy-plane and ends on the dislocation line (points).

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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 ≤ 100. The vortex core appears and the symmetry of the vortex state changes with the increase of the intrinsic magnetic anisotropy of the particle β; the further increase of β leads to the destruction of the vortex state. Such vortices can be present in arrays as small in size as dozen of nanometers.

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The regular array of magnetic particles (magnetic dots) of the form of a two-dimensional triangular lattice in the presence of external magnetic field demonstrates complicated magnetic structures. The magnetic symmetry of the ground state for such a system is lower than that for the underlying lattice. Long range dipole-dipole interaction leads to a specific antiferromagnetic order in small fields, whereas a set of linear topological defects appears with the growth of the magnetic field. Self-organization of such defects determines the magnetization process for a system within a wide range of external magnetic fields.