[en] We study the opening of the Haldane gap in two-leg and four-leg anisotropic spin ladders using bosonization and renormalization group methods, and we determine the phase diagram as a function of the interchain coupling and relative anisotropy J^z/J^xy. It is found that the opening of the Haldane gap is qualitatively different for the two cases considered. For the two-leg ladder the Haldane gap opens for arbitrarily small interchain coupling, independent of J^z/J^xy, and the Haldane phase exists in a large region of parameter space. For the four-leg ladder the opening of the Haldane gap is strongly dependent on both the interchain coupling as well as J^z/J^xy, and the Haldane phase exists only in a narrow region about the isotropic antiferromagnet. (c) 1999 The American Physical Society

[en] An equation for the dependence of magnetization on magnetic field in the case of two-dimensional (base plane) anisotropy has been derived. The resulting equation is expressed as an infinite series of modified Bessel functions, unlike the elementary function expressions that are applicable to the one-dimensional (axially anisotropic) and three-dimensional (isotropic) cases. Nevertheless, in the low-field limit, the series can be effectively truncated to give an approximate solution, while, in the high-field limit, an alternative expression has been derived which represents the limiting function as the field strength tends to infinity. The resulting expressions can be used to describe the superparamagnetic magnetization and susceptibility as a function of magnetic field in situations where the magnetic moments are constrained to lie in a plane, with no preferred direction within the plane. This can therefore be applied to two-dimensional structures, such as magnetic thin films, where magnetostatic energy confines the moments to the plane of the film, or to three-dimensional structures with planar magnetocrystalline anisotropy. (c) 2000 American Institute of Physics

[en] Anisotropic bulk Sm₂(Fe,M)₁₇N_x magnets were prepared by rotation aligning easy plane Sm₂(Fe,M)₁₇ powders followed by sintering and nitriding. Vanadium substitution for iron can help the c-axis alignment compared to the binary Sm₂Fe₁₇ compound. Magnets with an energy product of 15 MGOe and an intrinsic coercivity of 7 kOe were obtained on a single phase Sm₂Fe_16.15V_0.85N_x compound with a density of 91% of theoretical density. copyright 1997 American Institute of Physics

[en] We present theoretical electronic structure calculations on the nature of electronic states and the magnetic coupling in the Mn₁₂O₁₂ free cluster and the Mn₁₂O₁₂(RCOO)₁₆(H₂O)₄ molecular magnetic crystal. The calculations have been performed with the all-electron full-potential NRLMOL code. We find that the free Mn₁₂O₁₂ cluster relaxes to an antiferromagnetic cluster with no net moment. However, when coordinated by sixteen HCOO ligands and four H₂O groups, as it is in the molecular crystal, we find that the ferrimagnetic ordering and geometrical and magnetic structure observed in the experiments is restored. Local Mn moments for the free and ligandated molecular magnets are presented and compared to experiment. We identify the occupied and unoccupied electronic states that are most responsible for the formation of the large anisotropy barrier and use a recently developed full-space and full-potential method for calculating the spin-orbit coupling interaction and anisotropy energies. Our calculated second-order anisotropy energy is in excellent agreement with experiment. (c) 2000 American Institute of Physics

[en] We used the technique of vector generalized magneto-optical ellipsometry to study the behavior of the magnetization vector of a 50 nm Co thin film as a function of external field magnitude and direction. We determined the relative contributions of magnetization rotation and domain formation to the reversal of M. The Co sample had a uniaxial in-plane anisotropy. When the angle between the applied field and the easy axis was greater than ∼40 degree sign , the reversal occurred primarily by rotation of the magnetization, accompanied by a small reduction of the magnitude of M. In this angular region, the field at which there is a large jump in the angle of M as a function of applied field angle followed a single domain coherent rotation model. However, at applied field angles less than 40 degree sign to the easy axis, a larger reduction in |M| occurred during the jump in the magnetization angle. The jump also occurred at fields much lower than those predicted by the coherent rotation model, indicating a reversal mode initiated by domain formation. (c) 2000 American Institute of Physics

[en] Decoherence mechanisms in crystals of weakly anisotropic magnetic molecules, such as V₁₅ , are studied. We show that an important decohering factor is the rapid thermal fluctuation of dipolar interactions between magnetic molecules. A model is proposed to describe the influence of this source of decoherence. Based on the exact solution of this model, we show that at relatively high temperatures, about 0.5 K, the quantum coherence in a V₁₅ molecule is not suppressed and, in principle, can be detected experimentally. Therefore, these molecules may be suitable prototype systems for study of physical processes taking place in quantum computers. (c) 2000 The American Physical Society

[en] Magnetization reversal in transition-metal nanowires is investigated. Model calculations explain why magnetization reversal is localized, as opposed to the sometimes assumed delocalized coherent-rotation and curling modes. The localization is a quite general phenomenon caused by morphological inhomogenities and occurring in both polycrystalline and single-crystalline wires. In the polycrystalline limit, the competition between interatomic exchange and anisotropy gives rise to a variety of random-anisotropy effects, whereas nearly single-crystalline wires exhibit a weak localization of the nucleation mode. Model predictions are used to explain the coercive and magnetic-viscosity behavior of Co (and Ni) nanowires electrodeposited in self-assembled alumina pores. (c) 2000 The American Physical Society

[en] We report in-plane anisotropy in epitaxial Sm-Co(x)/Fe(y) bilayers as determined by ferromagnetic resonance (FMR). Four samples, (x,y)=(35,30) and (20, 20) nm each on MgO (110) and (100) substrates, have been prepared via magnetron sputtering. The two substrate orientations result in twofold and fourfold Sm-Co symmetry respectively, with the Sm-Co c-axis in-plane. Magnetization curves indicate elastic exchange spring Fe behavior in reversing fields up to the Sm-Co switching fields (6 and 8 kG at room temperature in the (35, 30) and (20, 20) nm films, respectively). 35 GHz in-plane FMR measurements were made in order to map the crystalline anisotropy of the Fe layer as well as the induced anisotropy from the exchange coupling to the Sm-Co layer. The twofold Sm-Co samples exhibit a clear superposition of the near fourfold Fe crystal field anisotropy (530 Oe) and the unidirectional exchange-bias anisotropy (≅650 Oe) arising from the Fe/Sm-Co interface. The crystalline Fe anisotropy in the fourfold Sm-Co samples is less well defined, presumably due to poorer epitaxy of the Fe layer for this orientation. (c) 2000 American Institute of Physics

[en] We observe a systematic increase in interface perpendicular magnetic anisotropy (PMA) with increasing Au-interlayer thickness t_Au ranging from 1 to 5 monolayers (MLs) in Cu/Co/Au/Cu(111), where misfit strain in Co due to the epitaxial growth increases with increasing t_Au. This result can be understood within the framework of the magneto-elastic contribution to the interface PMA. With a Cu overlayer, a constant contribution of interface PMA as well as of volume anisotropy is observed when the Co thicknesses are greater than 1.3 ML. However, we have found an unexpected suppression of this variation of interface PMA with the use of Au overlayers instead of Cu ones. With Au or Pd overlayers, the PMA starts to degrade as the Co thickness is reduced below 2.5 ML. (c) 2000 American Institute of Physics

[en] Structural and magnetic properties of the two-layered Ruddlesden-Popper phase SrO(La_1-xSr_xMnO₃)₂ with x=0.3-0.5 are highlighted. Intrinsic properties of these naturally layered manganites include a ''colossal'' magnetoresistance, a composition-dependent magnetic anisotropy, and almost no remanence. Above the Curie temperature there is a nonvanishing extrinsic magnetization attributed to intergrowths (stacking faults in the layered structure). These lattice imperfections consist of additional or missing manganite layers, as observed in transmission electron microscopy. Their role in influencing the properties of the host material is highlighted. (c) 2000 American Vacuum Society