Oscillating Solitons Pinned to a Nonmagnetic Impurity in Layered Antiferromagnets

We argue that an oscillatory motion of impurity-pinned solitons may occur in layered antiferromagnetic compounds. The characteristic frequencies of these modes, that may be detected by resonance or inelastic neutron scattering, are estimated analytically and depend on the soliton sizes and types .Comment: 11 pages, 1 figur

Vortex-antivortex dynamics driven by spin-torque in a nanocontact

A spin-polarized current in a nanocontact has been shown to induce the formation of a magnetic vortex at the nanocontact by the Oersted field, and spin-torque drives the vortex core in an elliptical orbit about the nanocontact. For the case of an external in-plane magnetic field in an extended free layer, the magnetization will be uniform far from the nanocontact, implying that vortex formation must be accompanied by the formation of an antivortex. Using the Thiele approach to describe the vortex-antivortex dynamics it is shown that the frequency of gyrotropic motion of the vortex is a function of the nanocontact current which is linear for large vortex-antivortex separations and it becomes nonlinear as the separation is decreased. The equilibrium vortexantivortex separation can be controlled by the nanocontact current as well as the external magnetic field

Few-cycle soliton propagation

Soliton propagation is usually described in the ``slowly varying envelope approximation'' (SVEA) regime, which is not applicable for ultrashort pulses. We present theoretical results and numerical simulations for both NLS and parametric ($\chi^{(2)}$) ultrashort solitons in the ``generalised few-cycle envelope approximation'' (GFEA) regime, demonstrating their altered propagation.Comment: 4 pages, 4 figure

Vortex behavior near a spin vacancy in 2D XY-magnets

The dynamical behavior of anisotropic two dimensional Heisenberg models is still a matter of controversy. The existence of a central peak at all temperatures and a rich structure of magnon peaks are not yet understood. It seems that the central peaks are related, in some way, to structures like vortices. In order to contribute to the discussion of the dynamical behavior of the model we use Monte Carlo and spin dynamics simulations as well analytical calculations to study the behavior of vortices in the presence of nonmagnetic impurities. Our simulations show that vortices are attracted and trapped by the impurities. Using this result we show that if we suppose that vortices are not very much disturbed by the presence of the impurities, then they work as an attractive potential to the vortices explaining the observed behavior in our simulations.Comment: 4 pages, 6 figure

Monte Carlo study of the critical temperature for the planar rotator model with nonmagnetic impurities

We performed Monte Carlo simulations to calculate the Berezinskii-Kosterlitz-Thouless (BKT) temperature $T_{BKT}$ for the two-dimensional planar rotator model in the presence of nonmagnetic impurity concentration $(\rho)$. As expected, our calculation shows that the BKT temperature decreases as the spin vacancies increase. There is a critical dilution $\rho_c \approx 0.3$ at which $T_{BKT} =0$. The effective interaction between a vortex-antivortex pair and a static nonmagnetic impurity is studied analytically. A simple phenomenological argument based on the pair-impurity interaction is proposed to justify the simulations.Comment: 5 pages, 5 figures, Revetex fil

Dark solitons in ferromagnetic chains with first- and second-neighbor interactions

We study the ferromagnetic spin chain with both first- and second-neighbor interactions. We obtained the condition for the appearance and stability of bright and dark solitons for arbitrary wave number inside the Brillouin zone. The influence of the second-neighbor interaction and the anisotropy on the soliton properties is considered. The scattering of dark solitons from point defects in the discrete spin chain is investigated numerically.Comment: 7 pages,5 figure

Magnetic properties of the quantum spin-1/2 XX diamond chain: The Jordan-Wigner approach

The Jordan-Wigner transformation is applied to study magnetic properties of the quantum spin-1/2 $XX$ model on the diamond chain. Generally, the Hamiltonian of this quantum spin system can be represented in terms of spinless fermions in the presence of a gauge field and different gauge-invariant ways of assigning the spin-fermion transformation are considered. Additionally, we analyze general properties of a free-fermion chain, where all gauge terms are neglected and discuss their relevance for the quantum spin system. A consideration of interaction terms in the fermionic Hamiltonian rests upon the Hartree-Fock procedure after fixing the appropriate gauge. Finally, we discuss the magnetic properties of this quantum spin model at zero as well as non-zero temperatures and analyze the validity of the approximation used through a comparison with the results of the exact diagonalization method for finite (up to 36 spins) chains. Besides the $m=1/3$ plateau the most prominent feature of the magnetization curve is a jump at intermediate field present for certain values of the frustrating vertical bond.Comment: 12 pages, 9 figures, accepted for publication in Eur. Phys. J.

Multi-plateau magnetization curves of one-dimensional Heisenberg ferrimagnets

Ground-state magnetization curves of ferrimagnetic Heisenberg chains of alternating spins $S$ and $s$ are numerically investigated. Calculating several cases of $(S,s)$, we conclude that the spin-$(S,s)$ chain generally exhibits $2s$ magnetization plateaux even at the most symmetric point. In the double- or more-plateau structure, the initial plateau is generated on a classical basis, whereas the higher ones are based on a quantum mechanism.Comment: 6 pages, 6 figures embedded, to appear in Phys. Rev. B 01 August 200

Normal modes for the vortex state magnetic dots

Submicron permalloy magnetic dots have a vortex ground state, and application of an in-plane magnetic pulse will result in precession of the vortex about the dot axis at a frequency in the sub GHz range. The precession frequency of this mode is calculated using a perturbation technique based on vortex-magnon scattering including the magnetostatic interaction. These calculations show that the frequencies vary between 0.2 and 0.8 GHz for 60 nm disks or radii between 250 and 1000 nm, which agrees with recent experimental data. There is also a higher frequency mode between 5 and 7 GHz.Субмикронные пермаллоевые магнитные точки имеют вихревое основное состояние, и приложение магнитного импульса в плоскости точки приводит к прецессии вихря вокруг оси точки с частотой в субгигагерцевом диапазоне. Частота этой прецессии вычислена с использованием теории возмущений, базирующейся на теории рассеяния магнонов на вихре, включающей магнитостатическое взаимодействие. Расчет показывает, что частоты изменяются в пределах от 0.2 до 0.8 GHz для 60 нм дисков с радиусом между 250 и 1000 нм, что согласуется с последними экспериментальными данными. Существует также мода с более высокой частотой, лежащей между 5 и 7 GHz.Субмікронні пермаллоєві магнітні точки мають вихровий основний стан, i додаток магнитного імпульсу в площині точки приводить до прецесії вихря навколо вісі точки з частотою у субгігагерцевому діапазоні. Частоту цієї прецесії обчислено з використанням теорії збурювань, що базується на теорії розсіювання магнонов на вихрі, що включає магнітостатичну взаємодію. Розрахунок показує, що частоти змінюються у межах від o.2 до o.8 GHz для 6o нм дисків з радіусом між 25o i 1ooo нм, що узгоджується з останніми експериментальними даними. Існує також мода з більш високою частотою, що лежить між 5 і 7 GHz