The line shape of Raman scattering in a two-dimensional Heisenberg antiferromagnet at zero temperature is studied within spin-wave theory, by using the Dyson-Maleev transformation.Besides the wellknown dominant contribution coming from two-magnon scattering, a systematic analysis of the contribution coming from the production of four magnons is carried out. It is found that the four-magnon contribution is very small compared with the two-magnon intensity. However, it occurs at such large energies that the ratio of the first two frequency cumulants is enhanced by a factor of 2.5 for S= 2, in fairly good agreement with recent series-expansion estimates. This is true despite the fact that the total line shape that we obtain is still in rather poor agreement with the experimental line shape obtained in experiments on undoped La2Cu04. As a by-product of the analysis of the effects of spin-wave interactions on the magnon propagator, the renormalization factor of the spin-wave velocity is computed to O((1/S) ). For S=-', the number that we find is Z, =1. 177, in very good agreement with the seriesexpansion result.

Interest 2 in magnetic nanoparticles, which typically contain tens of thousands of magnetic atoms, has been spurred both by the crucial role that they play in advanced magnetic information storage devices, and by the light that investigating magnetism at the nanoscale sheds on the fundamental interactions responsible for the magnetic state.As the frontier advances, interest is shifting to still smaller size scales. Dimers represent the small size end point in the transition metal cluster crossover from nanoparticle 2 to molecular magnetic properties. The magnetic moments per atom in clusters of the 3d

Motivated by recent scanning tunnel microscopy ͑STM͒ experiments, we present a theoretical study of the electronic and magnetic properties of the Mn-induced acceptor level obtained by substituting a single Ga atom in the ͑110͒ surface layer of GaAs or in one of the atoms layers below the surface. We employ a kineticexchange tight-binding model in which the relaxation of the ͑110͒ surface is taken into account. The acceptor wave function is strongly anisotropic in space and its detailed features depend on the depth of the sublayer in which the Mn atom is located. The local-density-of-states ͑LDOS͒ on the ͑110͒ surface associated with the acceptor level is more sensitive to the direction of the Mn magnetic moment when the Mn atom is located further below the surface. We show that the total magnetic anisotropy energy of the system is due almost entirely to the dependence of the acceptor level energy on Mn spin orientation, and that this quantity is strongly dependent on the depth of the Mn atom.

We present a theory of the low-energy excitations of a ferromagnetic metal nanoparticle. In addition to the particle-hole excitations, which occur in a paramagnetic metal nanoparticle, we predict a branch of excitations involving the magnetization-orientation collective coordinate. Tunneling matrix elements are in general sizable for several different collective states associated with the same band configuration. We point out that the average change in ground state spin per added electron differs from noninteracting quasiparticle expectations, and that the change in the spin polarization, due to Zeeman coupling, is strongly influenced by Coulomb blockade physics.

We use a microscopic Slater-Koster tight-binding model with short-range exchange and atomic spin-orbit interactions that realistically captures generic features of ferromagnetic metal nanoparticles to address the mesoscopic physics of magnetocrystalline anisotropy and hysteresis in nanoparticle quasiparticle excitation spectra. Our analysis is based on qualitative arguments supported by self-consistent Hartree-Fock calculations for nanoparticles containing up to 260 atoms. Calculations of the total energy as a function of magnetization direction demonstrate that the magnetic anisotropy per atom fluctuates by several percents when the number of electrons in the particle changes by one, even for the largest particles we consider. Contributions of individual orbitals to the magnetic anisotropy are characterized by a broad distribution with a mean more than two orders of magnitude smaller than its variance and with no detectable correlations between anisotropy contribution and quasiparticle energy. We find that the discrete quasiparticle excitation spectrum of a nanoparticle displays a complex non-monotonic dependence on an external magnetic field, with abrupt jumps when the magnetization direction is reversed by the field, explaining recent spectroscopic studies of magnetic nanoparticles. Our results suggests the existence of a broad cross-over from a weak spin-orbit coupling to a strong spin-orbit coupling regime, occurring over the range from approximately 200-to 1000-atom nanoparticles.2