173 research outputs found
On the reorientation transition of ultra-thin Ni/Cu(001) films
The reorientation transition of the magnetization of ferromagnetic films is
studied on a microscopic basis within a Heisenberg spin model. Using a modified
mean field formulation it is possible to calculate properties of magnetic thin
films with non-integer thicknesses. This is especially important for the
reorientation transition in Ni/Cu(001), as there the magnetic properties are a
sensitive function of the film thickness. Detailed phase diagrams in the
thickness-temperature plane are calculated using experimental parameters and
are compared with experimental measurements by Baberschke and Farle (J. Appl.
Phys. 81, 5038 (1997)).Comment: 7 pages(LaTeX2e) with one figure(eps), accepted for publication in
JMMM. See also http://www.thp.Uni-Duisburg.DE/Publikationen/Publist_Us_R.htm
Spin wave excitations: The main source of the temperature dependence of Interlayer exchange coupling in nanostructures
Quantum mechanical calculations based on an extended Heisenberg model are
compared with ferromagnetic resonance (FMR) experiments on prototype trilayer
systems Ni_7/Cu_n/Co_2/Cu(001) in order to determine and separate for the first
time quantitatively the sources of the temperature dependence of interlayer
exchange coupling. Magnon excitations are responsible for about 75% of the
reduction of the coupling strength from zero to room temperature. The remaining
25% are due to temperature effects in the effective quantum well and the
spacer/magnet interfaces.Comment: accepted for publication in PR
Ferromagnetic resonance in nanostructures, rediscovering its roots in paramagnetic resonance
Abstract. Both techniques went different routes: The EPR explored an enormous variety of paramagnets in solids, liquids, and gas phase. The focus was to determine orbital-and spin-magnetic moments (g-tensor), hyperfine interactions, and from the linewidth the spin dynamics (T 1 , T 2 relaxation). In FMR most of the experiments and theory assumed the total value M to be constant in the equation of motion and used only one effective damping parameter (Gilbert). This is an enormous, unnecessary limitation for today's analysis of magnetism in nanostructures and ultrathin films. To assume M = const ignores spin wave excitations, scattering between longitudinal and transverse components of M. Moreover, in the framework of itinerant ferromagnetism, the magnetic moment/atom μ was assumed to be isotropic with g ≈ 2! That ignores the anisotropy of μ in nanostructures and the importance of the orbital magnetic moments with μ L /μ S = (g − 2)/2. Without finite μ L we would have no magnetic anisotropy energy (MAE), no hard magnets, no magnetic storage media. Only recently the "language" of EPR was adapted to FMR in ultrathin films. A g-tensor is discussed and its interrelation with the MAE is pointed out. Also recent theory points out, that "there is no reason to assume a fixed magnetization length for nanoelements". This allows a detailed discussion of magnon-magnon scattering, spin-spin, and spin-lattice relaxation -useful, for example, for fs spin dynamics. Recent FMR experiments using frequencies from 1 GHz up to several hundred GHz, will allow measuring the proper g-factor components and μ L , μ S . From the frequency dependent linewidth magnon-magnon scattering can be separated from dissipative spin-lattice damping
Mechanism of temperature dependence of the magnetic anisotropy energy in ultrathin Cobalt and Nickel films
Temperature dependent FMR-measurements of Ni and Co films are analysed using
a microscopic theory for ultrathin metallic systems. The mechanism governing
the temperature dependence of the magnetic anisotropy energy is identified and
discussed. It is reduced with increasing temperature. This behavior is found to
be solely caused by magnon excitations.Comment: 3 pages, 4 figures III Joint European Magnetic Symposia, San
Sebastian, Spai
Nonlocal feedback in ferromagnetic resonance
Ferromagnetic resonance in thin films is analyzed under the influence of
spatiotemporal feedback effects. The equation of motion for the magnetization
dynamics is nonlocal in both space and time and includes isotropic, anisotropic
and dipolar energy contributions as well as the conserved Gilbert- and the
non-conserved Bloch-damping. We derive an analytical expression for the
peak-to-peak linewidth. It consists of four separate parts originated by
Gilbert damping, Bloch-damping, a mixed Gilbert-Bloch component and a
contribution arising from retardation. In an intermediate frequency regime the
results are comparable with the commonly used Landau-Lifshitz-Gilbert theory
combined with two-magnon processes. Retardation effects together with Gilbert
damping lead to a linewidth the frequency dependence of which becomes strongly
nonlinear. The relevance and the applicability of our approach to ferromagnetic
resonance experiments is discussed.Comment: 22 pages, 9 figure
Subgap structures in the current-voltage characteristic of the intrinsic Josephson effect due to phonons
A modified RSJ-model for the coupling of intrinsic Josephson oscillations and
c-axis phonons in the high-T_c superconductors Tl_2Ba_2Ca_2Cu_3O_{10+\delta}
and Bi_2Sr_2CaCu_2O_{8+\delta} is deveoped. This provides a very good
explanation for recently reported subgap structures in the I-V-characteristic
of the c-axis transport. It turns out that the voltages of these structures
coincide with the eigenfrequencies of longitudinal optical phonons, providing a
new measurement technique for this quantity. The significantly enhanced
microwave emission at the subgap structures in both the GHz and THz region is
discussed.Comment: correction of minor misprints, revtex, 3 pages, two postscript
figures, aps, epsf, Contributed Paper to the "International Symposion on the
Intrinsic Josphson effect and THz Plasma Oscillations", 22-25 February 1997,
Sendai, Japan; to be published in Physica
Magnetoelastic mechanism of spin-reorientation transitions at step-edges
The symmetry-induced magnetic anisotropy due to monoatomic steps at strained
Ni films is determined using results of first - principles relativistic
full-potential linearized augmented plane wave (FLAPW) calculations and an
analogy with the N\'eel model. We show that there is a magnetoelastic
anisotropy contribution to the uniaxial magnetic anisotropy energy in the
vicinal plane of a stepped surface. In addition to the known spin-direction
reorientation transition at a flat Ni/Cu(001) surface, we propose a
spin-direction reorientation transition in the vicinal plane for a stepped
Ni/Cu surface due to the magnetoelastic anisotropy. We show that with an
increase of Ni film thickness, the magnetization in the vicinal plane turns
perpendicular to the step edge at a critical thickness calculated to be in the
range of 16-24 Ni layers for the Ni/Cu(1,1,13) stepped surface.Comment: Accepted for publication in Phys. Rev.
Anisotropy of ultra-thin ferromagnetic films and the spin reorientation transition
The influence of uniaxial anisotropy and the dipole interaction on the
direction of the magnetization of ultra-thin ferromagnetic films in the
ground-state is studied. The ground-state energy can be expressed in terms of
anisotropy constants which are calculated in detail as function of the system
parameters and the film thickness. In particular non-collinear spin
arrangements are taken into account. Conditions for the appearance of a spin
reorientation transition are given and analytic results for the width of the
canted phase and its shift in applied magnetic fields associated with this
transition are derived.Comment: 6 pages, RevTeX
Micromagnetic simulations of spinel ferrite particles
This paper presents the results of simulations of the magnetization field
{\it ac} response (at to GHz) of various submicron ferrite particles
(cylindrical dots). The ferrites in the present simulations have the spinel
structure, expressed here by MZnFeO (where M stands for a
divalent metal), and the parameters chosen were the following: (a) for : M
= \{ Fe, Mn, Co, Ni, Mg, Cu \}; (b) for : M = \{ Fe, Mg \} (mixed
ferrites). These runs represent full 3D micromagnetic (one-particle) ferrite
simulations. We find evidences of confined spin waves in all simulations, as
well as a complex behavior nearby the main resonance peak in the case of the M
= \{ Mg, Cu \} ferrites. A comparison of the and cases for fixed
M reveals a significant change in the spectra in M = Mg ferrites, but only a
minor change in the M = Fe case. An additional larger scale simulation of a
by particle array was performed using similar conditions of the FeO
(magnetite; , M = Fe) one-particle simulation. We find that the main
resonance peak of the FeO one-particle simulation is disfigured in the
corresponding 3 by 3 particle simulation, indicating the extent to which
dipolar interactions are able to affect the main resonance peak in that
magnetic compound.Comment: 35 pages, 11 figures, Journal of Magnetism and Magnetic Materials, in
press
Reorientation transition of ultrathin ferromagnetic films
We demonstrate that the reorientation transition from out-of-plane to
in-plane magnetization with decreasing temperature as observed experimentally
in Ni-films on Cu(001) can be explained on a microscopic basis. Using a
combination of mean field theory and perturbation theory, we derive an analytic
expression for the temperature dependent anisotropy. The reduced magnetization
in the film surface at finite temperatures plays a crucial role for this
transition as with increasing temperature the influence of the uniaxial
anisotropies is reduced at the surface and is enhanced inside the film.Comment: 4 pages(RevTeX), 3 figures (EPS
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