Melting and freezing of a skyrmion lattice.

We report comprehensive Monte-Carlo studies of the melting of skyrmion lattices (SkL) in systems of small, medium, and large sizes with the number of skyrmions ranging from 10to over 10. Large systems exhibit hysteresis similar to that observed in real experiments on the melting of SkLs. For sufficiently small systems which achieve thermal equilibrium, a fully reversible sharp solid-liquid transition on temperature with no intermediate hexatic phase is observed.

Random anisotropy magnet at finite temperature.

We present finite-temperature Monte Carlo studies of a 2D random-anisotropy (RA) magnet on lattices containing one million spins. The correlated spin-glass state predicted by analytical theories is reproduced in simulations, as are the field-cooled and zero-field-cooled magnetization curves observed in experiments. The orientations of lattice spins begin to freeze when the temperature is lowered.

Energy balance and energy correction in dynamics of classical spin systems.

An energy-correction method is proposed as an addition to mainstream integrators for equations of motion of systems of classical spins. This solves the problem of nonconservation of energy in long computations and makes mainstream integrators competitive with symplectic integrators for spin systems that for different-site interactions conserve the energy explicitly. The proposed method is promising for spin systems with single-site interactions for which symplectic integrators do not conserve energy and thus have no edge against mainstream integrators.

Topological Order Generated by a Random Field in a 2D Exchange Model.

We study a 2D exchange model with a weak static random field on lattices containing over 10^{8} spins. Ferromagnetic correlations persist on the Imry-Ma scale inversely proportional to the random-field strength and decay exponentially at greater distances. We find that the average energy of the correlated area is close to the ground-state energy of a Skyrmion, while the topological charge of the area is close to ±1.

Pulse-noise approach for classical spin systems.

For systems of classical spins interacting with the bath via damping and thermal noise, an approach is suggested to replace the white noise by a pulse noise acting at regular time intervals Δt, within which the system evolves conservatively. The method is working well in the typical case of a small dimensionless damping constant λ and allows a considerable speedup of computations by using high-order numerical integrators with a large time step δt (up to a fraction of the precession period), while keeping δt≪Δt to reduce the relative contribution of noise-related operations. In cases when precession can be discarded, δt can be increased up to a fraction of the relaxation time ∝1/λ that leads to a further speedup.

Surface effects on ferromagnetic resonance in magnetic nanocubes.

We study the effect of surface anisotropy on the spectrum of spin-wave excitations in a magnetic nanocluster and compute the corresponding absorbed power. For this, we develop a general numerical method based on the (undamped) Landau-Lifshitz equation, either linearized around the equilibrium state leading to an eigenvalue problem or solved using a symplectic technique. For box-shaped clusters, the numerical results are favorably compared to those of the finite-size linear spin-wave theory.

Random fields, topology, and the Imry-Ma argument.

We consider an n-component fixed-length order parameter interacting with a weak random field in d=1, 2, 3 dimensions. Relaxation from the initially ordered state and spin-spin correlation functions are studied on lattices containing hundreds of millions of sites. At n ≤ d the presence of topological defects leads to strong metastability and glassy behavior, with the final state depending on the initial condition.

Self-organized patterns of macroscopic quantum tunneling in molecular magnets.

We study low temperature resonant spin tunneling in molecular magnets induced by a field sweep with account of dipole-dipole interactions. Numerical simulations uncovered formation of self-organized patterns of the magnetization and of the ensuing dipolar field that provide resonant conditions inside a finite volume of the crystal. This effect is robust with respect to disorder and should be relevant to the dynamics of the magnetization steps observed in molecular magnets.

Field dependence of the electron spin relaxation in quantum dots.

The interaction of the electron spin with local elastic twists due to transverse phonons is studied. The universal dependence of the spin-relaxation rate on the strength and direction of the magnetic field is obtained in terms of the electron gyromagnetic tensor and macroscopic elastic constants of the solid. The theory contains no unknown parameters and it can be easily tested in experiment.

Phonon superradiance and phonon laser effect in nanomagnets.

We show that the theory of spin-phonon processes in paramagnetic solids must take into account the coherent generation of phonons by the magnetic centers. This effect should drastically enhance spin-phonon rates in nanoscale paramagnets and in crystals of molecular nanomagnets.

Saddle index properties, singular topology, and its relation to thermodynamic singularities for a phi4 mean-field model.

We investigate the potential energy surface of a phi(4) model with infinite range interactions. All stationary points can be uniquely characterized by three real numbers alpha(+) , alpha(0) , alpha(-) with alpha(+) + alpha(0) + alpha(-) =1 , provided that the interaction strength mu is smaller than a critical value. The saddle index n(s) is equal to alpha(0) and its distribution function has a maximum at n(max)(s) =1/3 .

Surface contribution to the anisotropy of magnetic nanoparticles.

We calculate the contribution of the Néel surface anisotropy to the effective anisotropy of magnetic nanoparticles of spherical shape cut out of a simple cubic lattice. The effective anisotropy arises because deviations of atomic magnetizations from collinearity and thus the energy depends on the orientation of the global magnetization. The result is second order in the Néel surface anisotropy, scales with the particle's volume, and has cubic symmetry with preferred directions [+/- 1, +/-1 , +/-1].

Superradiance from crystals of molecular nanomagnets.

We show that crystals of molecular nanomagnets can exhibit giant magnetic relaxation due to the Dicke superradiance of electromagnetic waves. Rigorous theory is presented that combines superradiance with the Landau-Zener effect.

Thermally activated escape rates of uniaxial spin systems with transverse field: uniaxial crossovers.

Classical escape rates of uniaxial spin systems are characterized by a prefactor differing from and much smaller than that of the particle problem, since the maximum of the spin energy is attained everywhere on the line of constant latitude: theta =const, 0 < or= phi < or = 2 pi. If a transverse field is applied, a saddle point of the energy is formed, and high, moderate, and low damping regimes (similar to those for particles) appear. Here we present the first analytical and numerical study of crossovers between the uniaxial and other regimes for spin systems.

[A postoperative osteosarcoma recurrence after bone distraction by Ilizarov method in a dog].

Distraction osteogenesis by Ilizarov's method is becoming popular for limb saving surgery today. In this article we report a case of osteosarcoma recurrence after using bone distraction in a dog with osteogenic sarcoma. This case suggests that stimulation of osteogenesis by bone distraction promotes tumor recurrence and metastases in osteosarcoma.

[Secondary osteosarcoma (in a dog) after bone fracture healed on the secondary type].

A case is reported of osteosarcoma arising in a dog after the fracture of the femur. The tumor was diagnosed 6 years after the trauma. We suppose that the cause of malignant transformation in this case was a permanent mechanical trauma, restructuration and regeneration of the bone.