62 research outputs found
Macrospin limit and configurational anisotropy in nanoscale Permalloy triangles
In Permalloy submicron triangles, configurational anisotropy - a higher-order
form of shape anisotropy - yields three equivalent easy axes, imposed by the
structures' symmetry order. Supported by micromagnetic simulations, an
experimental method was devised to evaluate the nanostructure dimensions for
which a Stoner-Wohlfarth type of reversal could be used to describe this
particular magnetic anisotropy. In this regime, a straightforward procedure
using an in-plane rotating field allowed us to quantify experimentally the
six-fold anisotropy fields for triangles of different thicknesses and sizes
Sputter grown Fe and Fe/Cr multilayers with fourfold magnetic anisotropy on GaAs
Thin films of Fe have been epitaxially sputtered on GaAs substrates with native oxide removal prior to the deposition carried out by an Ar ion milling. Films grown at substrate temperatures above 100 °C show well-defined fourfold anisotropies. The onset of epitaxial growth is accompanied by an increase in the surface roughness with growth occurring in a distinct island-like pattern. The Fe layers show significantly reduced moments, which decrease with increasing temperature. Antiferromagnetic coupling between Fe layers with Cr spacers was measured in a multilayer with a Cr thickness of 2.7 nm, around the second antiferromagnetic peak. The magnetic properties of the films are discussed in the context of multilayer storage applications
Controlling the canted state in antiferromagnetically coupled magnetic bilayers close to the spin reorientation transition
Canted magnetization is obtained in ultrathin, antiferromagnetically coupled magnetic bilayers with thicknesses around the spin reorientation transition. The canting angle is controlled by both the magnetic layer thickness and interlayer coupling strength, which are tuned independently. Hysteresis loops are obtained, where magnetization components parallel and transverse to the applied field are measured, and analyzed by comparison to micromagnetic simulations. This enables the canting angle to be extracted and the behavior of the individual layers to be distinguished. Two types of canted systems are obtained with either single-layer reversal or complex, coupled two-layer reversal, under moderate external magnetic fields. Controlling the magnetization canting and reversal behavior of ultra-thin layers is relevant for the development of magnetoresistive random-access memory and spin-torque oscillator devices
Zigzag domain wall mediated reversal in antiferromagnetically coupled layers
The Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling between two magnetic layers leads to many important technological applications. Here, the interaction between changing antiferromagnetic RKKY coupling and domain structure is studied in a sample consisting of two 5 nm thick CoFeB layers separated by a wedge of Cu up to 4 nm thick. Magnetic reversal occurs via the propagation of a zigzag domain wall front along the wedge. The modification of domain patterns created in the reversal of a coupled layers in the presence of antiferromagnetic RKKY coupling and coupling gradients is demonstrated. Firstly, the coupling leads to a smaller amplitude of the zigzag wall, which is aligned perpendicular to the easy axis, followed by elongation of the walls at higher coupling strength. The antiferromagnetic RKKY coupling, while not strong enough to cause antiparallel alignment of the layers, is argued to lead to coupling between the spins in the domain walls in the two layers, lowering their energy and driving the reversal behavior of the film
Inhomogeneous States in a Small Magnetic Disk with Single-Ion Surface Anisotropy
We investigate analytically and numerically the ground and metastable states
for easy-plane Heisenberg magnets with single-ion surface anisotropy and disk
geometry. The configurations with two half-vortices at the opposite points of
the border are shown to be preferable for strong anisotropy. We propose a
simple analytical description of the spin configurations for all values of a
surface anisotropy. The effects of lattice pinning leads to appearance of a set
of metastable configurations.Comment: 10 pages, 7 figures; submitted to Phys. Rev.
Magnetization dynamics with a spin-transfer torque
The magnetization reversal and dynamics of a spin valve pillar, whose lateral
size is 6464 nm, are studied by using micromagnetic simulation in
the presence of spin transfer torque. Spin torques display both characteristics
of magnetic damping (or anti-damping) and of an effective magnetic field. For a
steady-state current, both M-I and M-H hysteresis loops show unique features,
including multiple jumps, unusual plateaus and precessional states. These
states originate from the competition between the energy dissipation due to
Gilbert damping and the energy accumulation due to the spin torque supplied by
the spin current. The magnetic energy oscillates as a function of time even for
a steady-state current. For a pulsed current, the minimum width and amplitude
of the spin torque for achieving current-driven magnetization reversal are
quantitatively determined. The spin torque also shows very interesting thermal
activation that is fundamentally different from an ordinary damping effect.Comment: 15 figure
Langevin Simulation of Thermally Activated Magnetization Reversal in Nanoscale Pillars
Numerical solutions of the Landau-Lifshitz-Gilbert micromagnetic model
incorporating thermal fluctuations and dipole-dipole interactions (calculated
by the Fast Multipole Method) are presented for systems composed of nanoscale
iron pillars of dimension 9 nm x 9 nm x 150 nm. Hysteresis loops generated
under sinusoidally varying fields are obtained, while the coercive field is
estimated to be 1979 14 Oe using linear field sweeps at T=0 K. Thermal
effects are essential to the relaxation of magnetization trapped in a
metastable orientation, such as happens after a rapid reversal of an external
magnetic field less than the coercive value. The distribution of switching
times is compared to a simple analytic theory that describes reversal with
nucleation at the ends of the nanomagnets. Results are also presented for
arrays of nanomagnets oriented perpendicular to a flat substrate. Even at a
separation of 300 nm, where the field from neighboring pillars is only 1
Oe, the interactions have a significant effect on the switching of the magnets.Comment: 19 pages RevTeX, including 12 figures, clarified discussion of
numerical technique
Magnetic Reversal on Vicinal Surfaces
We present a theoretical study of in-plane magnetization reversal for vicinal
ultrathin films using a one-dimensional micromagnetic model with
nearest-neighbor exchange, four-fold anisotropy at all sites, and two-fold
anisotropy at step edges. A detailed "phase diagram" is presented that catalogs
the possible shapes of hysteresis loops and reversal mechanisms as a function
of step anisotropy strength and vicinal terrace length. The steps generically
nucleate magnetization reversal and pin the motion of domain walls. No sharp
transition separates the cases of reversal by coherent rotation and reversal by
depinning of a ninety degree domain wall from the steps. Comparison to
experiment is made when appropriate.Comment: 12 pages, 8 figure
Imaging of Spin Dynamics in Closure Domain and Vortex Structures
Time-resolved Kerr microscopy is used to study the excitations of individual
micron- scale ferromagnetic thin film elements in their remnant state. Thin (18
nm) square elements with edge dimensions between 1 and 10 m form closure
domain structures with 90 degree Neel walls between domains. We identify two
classes of excitations in these systems. The first corresponds to precession of
the magnetization about the local demagnetizing field in each quadrant, while
the second excitation is localized in the domain walls. Two modes are also
identified in ferromagnetic disks with thicknesses of 60 nm and diameters from
2 m down to 500 nm. The equilibrium state of each disk is a vortex with a
singularity at the center. As in the squares, the higher frequency mode is due
to precession about the internal field, but in this case the lower frequency
mode corresponds to gyrotropic motion of the entire vortex. These results
demonstrate clearly the existence of well-defined excitations in
inhomogeneously magnetized microstructures.Comment: PDF File (Figures at reduced resolution
Structural and magnetic properties of ultra-thin Fe films on metal-organic chemical vapour deposited GaN(0001)
Structural and magnetic properties of 1-10 nm thick Fe films deposited on GaN(0001) were investigated. In-situ reflecting high energy electron diffraction images indicated a α-Fe(110)/GaN(0001) growth of the 3D Volmer-Weber type. The α-Fe(110) XRD peak showed a 1° full-width at half-maximum, indicating ≈ 20 nm grain sizes. A significant reduction in Fe atomic moment from its bulk value was observed for films thinner than 4 nm. Both GaN/Fe interface roughness and Fe film coercivity increased with Fe thickness, indicating a possible deterioration of Fe crystalline quality. Magnetic anisotropy was mainly uniaxial for all films while hexagonal anisotropies appeared for thicknesses higher than 3.7 nm
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