Uniform spin wave modes in antiferromagnetic nanoparticles with uncompensated moments

In magnetic nanoparticles the uniform precession (q = 0 spin wave) mode gives the predominant contribution to the magnetic excitations. We have calculated the energy of the uniform mode in antiferromagnetic nanoparticles with uncompensated magnetic moments, using the coherent potential approximation. In the presence of uncompensated moments, an antiferromagnetic nanoparticle must be considered as a kind of a ferrimagnet. Two magnetic anisotropy terms are considered, a planar term confining the spins to the basal plane, and an axial term determining an easy axis in this plane. Excitation energies are calculated for various combinations of these two anisotropy terms, ranging from the simple uniaxial case to the planar case with a strong out-of-plane anisotropy. In the simple uniaxial case, the uncompensated moment has a large influence on the excitation energy, but in the planar case it is much less important. The calculations explain recent neutron scattering measurements on nanoparticles of antiferromagnetic α-Fe₂O₃ and NiO.     

Reciprocal propagation of surface modes in an antiferromagnetic film

Linear response theory is used to evaluate the Green functions describing the magnetic fluctuations in an antiferromagnetic film at zero applied field. Similarities between the dielectric and magnetic fluctuations are shown.     

Disordered loops in the two-dimensional antiferromagnetic spin–fermion model

The spin–fermion model has long been used to describe the quantum-critical behavior of 2d electron systems near an antiferromagnetic (AFM) instability. Recently, the standard procedure to integrate out the fermions and obtain an effective action for spin waves has been questioned in the clean case. We show that, in the presence of disorder, the single fermion loops display two crossover scales: upon lowering the energy, the singularities of the clean fermionic loops are first cut off, but below a second scale new singularities arise that lead again to marginal scaling. In addition, impurity lines between different fermion loops generate new relevant couplings which dominate at low energies. We outline a non-linear σ model formulation of the single-loop problem, which allows to control the higher singularities and provides an effective model in terms of low-energy diffusive as well as spin modes.     

A search for high frequency vibrational modes at a stepped surface

In this paper we present a theory of the vibrations of atoms in the vicinity of a stepped surface on a Bravais crystal. The static relaxations in the positions of the atoms in the crystal are determined, and the atomic force constants are then calculated in the relaxed atomic configuration. The general theory is applied to a simple stepped surface, and the local phonon density of states is carried out for atoms at several points on the stepped surface by the real space continued fraction recursion method. No evidence is found for high frequency surface phonons.     

Localised spin modes of decorated magnetic clusters on a magnetic surface

A method for the calculation of the energies of spin modes of surface magnetic clusters on a magnetic surface, using broken translation symmetry in three dimensions (3D), is employed to determine the spin mode energies for a variety of planar clusters. The cluster is considered to be supported on a magnetically ordered substrate such that the localised spins of the cluster and the substrate interact via magnetic exchange. No electronic effects are considered. The analytical approach solves for the 3D evanescent crystal spin field in the bulk and the surface domains around the cluster. This spin field arises owing to the breakdown of magnetic translation symmetry caused by the surface cluster. The analytical approach can be applied to any cluster configuration, underlying the general character of the calculation. In particular, we consider here a 4-, 5-, and 9-atoms planar clusters supported on the surface of a ferromagnetic simple cubic lattice, the spin order being in the direction normal to surface boundary. The method is applied to calculate the spin mode energies of these planar clusters consisting of Gd atoms interacting via Anti-ferromagnetic exchange with an Fe(1 0 0) surface. These results are compared with the calculated energies of the spin modes of the free clusters, and also with those for the same planar clusters when the cluster-substrate exchange is considered hypothetically ferromagnetic.     

Magnon spin Nernst effect on the antiferromagnetic checkerboard lattice

In this paper we study topological effects in an anisotropic checkerboard antiferromagnetic lattice in the presence of the Dzyaloshinskii-Moriya interaction (DMI). The time reversal symmetry is broken by anisotropies in the Hamiltonian. However, in contrast to the honeycomb lattice, the DMI does not open a gap here. We calculate the energy spectrum of the magnons and the spin Nernst coefficient in the presence of a longitudinal temperature gradient.     

涡流对侧向反铁磁/非磁超晶格表面推迟模的影响

使用等效介质理论研究了在Voigt位形下,由反铁磁半导体和绝缘非磁物质构成的半无限侧向反铁磁/非磁超晶格中的表面推迟模.讨论了在外磁场为零和非零两种情况下涡流对推迟模的影响,给出了同时考虑传导电流和位移电流时,侧向反铁磁/非磁超晶格的频率与波矢的关系曲线以及自旋波衰减随波矢k的变化曲线. 关键词： 推迟模 侧向超晶格 自旋波衰减     

Magnetic properties in a transverse Ising nanowire with an antiferromagnetic spin configuration at the surface

The phase diagrams and temperature dependences of magnetizations in a transverse Ising nanowire with an antiferromagnetic spin configuration at the surface are studied by the use of the effective-field theory with correlations. A number of characteristic features, such as the possibility of reentrant phenomena being free from disorder-induced frustration, are obtained for the magnetic properties in the system.     

The magnetic field effects on the frustrated antiferromagnetic spin chain

We presented a numerical study of the magnetization process of frustrated spin-1/2 chain. Using the exact diagonalization technique, we provided the evidence that a plateau at one-third of the saturation magnetization exists in the magnetization curve of frustrated antiferromagnetic spin-1/2 chain.     

五边形截面的Ag纳米线局域表面等离子体共振模式

五边形截面的单晶Ag纳米线对ZnO量子点荧光具有增强的现象. 为解释这一现象, 利用时域有限差分法对五边形截面的Ag纳米线的局域表面等离子体共振模式进行了理论模拟. 结果表明, 五边形截面的Ag纳米线在紫外区域存在两个消光峰, 分别由Ag纳米线的横向偶极共振(340 nm)和四极共振(375 nm)引起; 这两个消光峰与ZnO量子点荧光增强峰相一致, 而且随着Ag纳米线的半径增大而红移; 消光峰对应的共振模式取决于Ag纳米线的截面形状; 根据Ag纳米线电场增强倍数与激发光波长变化关系曲线可知, 最大增强电场位于五边形截面的顶点处, 而边线处电场增强较小. 理论模拟的结果较好地解释了Ag纳米线/ZnO量子点体系的荧光增强现象, 也为Ag纳米线在提高半导体材料发光效率、生物探测等方面的应用提供有益的参考.     

Frustrated antiferromagnetic spin 1 Heisenberg model with single ion anisotropy on a simple cubic lattice

We study the effects of frustration between nearest, next-nearest neighbor and next-next-nearest neighbors (NNN) of the quantum S=1 anisotropic antiferromagnetic Heisenberg model on a simple cubic lattice with single ion anisotropy using the bond operator technique. We calculate the phase diagram at zero temperature and the gap as a function of temperature in the disordered paramagnetic phase.     

Solitary waves on finite-size antiferromagnetic quantum Heisenberg spin rings

Motivated by the successful synthesis of several molecular quantum spin rings we are investigating whether such systems can host magnetic solitary waves. The small size of these spin systems forbids the application of a classical or continuum limit. We therefore investigate whether the time-dependent Schrödinger equation itself permits solitary waves. Example solutions are obtained via complete diagonalization of the underlying Heisenberg Hamiltonian.     

Localized modes on an Ablowitz-Ladik nonlinear impurity

We study localized modes on a single Ablowitz-Ladik impurity embedded in the bulk or at the surface of a one-dimensional linear lattice. Exact expressions are obtained for the bound state profile and energy. Dynamical excitation of the localized mode reveals exponentially-high amplitude oscillations of the spatial profile at the impurity location. The presence of a surface increases the minimum nonlinearity to effect a dynamical selftrapping.