2,428 research outputs found
Single nanoparticle measurement techniques
Various single particle measuring techniques are briefly reviewed and the
basic concepts of a new micro-SQUID technique are discussed. It allows
measurements of the magnetization reversal of single nanometer-sized particles
at low temperature. The influence of the measuring technique on the system of
interest is discussed.Comment: 3 pages, 3 figures, conference proceedings of MMM 1999, San Jose,
15-18 Nov., session number BE-0
Field sweep rate dependence of the coercive field of single-molecule magnets: a classical approach with applications to the quantum regime
A method, based on the Neel-Brown model of thermally activated magnetization
reversal of a magnetic single-domain particle, is proposed to study the field
sweep rate dependence of the coercive field of single-molecule magnets (SMMs).
The application to Mn12 and Mn84 SMMs allows the determination of the important
parameters that characterize the magnetic properties: the energy barrier, the
magnetic anisotropy constant, the spin, tau_0, and the crossover temperature
from the classical to the quantum regime. The method may be particularly
valuable for large SMMs that do not show quantum tunneling steps in the
hysteresis loops.Comment: 6 pages, 6 figure
Magnetization Switching in Small Ferromagnetic Particles: Nucleation and Coherent Rotation
The mechanisms of thermally activated magnetization switching in small
ferromagnetic particles driven by an external magnetic field are investigated.
For low uniaxial anisotropy the spins rotate coherently while for sufficiently
large uniaxial anisotropy they behave Ising-like, i.e. the switching then is
due to nucleation. The crossover from coherent rotation to nucleation is
studied for the classical three-dimensional Heisenberg model with uniaxial
anisotropy by Monte Carlo simulations. From the temperature dependence of the
metastable lifetime the energy barrier of a switching process can be
determined. For the case of infinite anisotropy we compare numerical results
from simulations of the Ising model with theoretical results for energy
barriers for both, single-droplet and multi-droplet nucleation. The simulated
barriers are in agreement with the theoretical predictions.Comment: 3 pages, Revtex, 4 Figures include
Non-converging hysteretic cycles in random spin networks
Behavior of hysteretic trajectories for cyclical input is investigated as a
function of the internal structure of a system modeled by the classical random
network of binary spins. Different regimes of hysteretic behavior are
discovered for different network connectivity and topology. Surprisingly,
hysteretic trajectories which do not converge at all are observed. They are
shown to be associated with the presence of specific topological elements in
the network structure, particularly with the fully interconnected spin groups
of size equal or greater than 4.Comment: 4 pages, 3 figure
Temperature dependence of antiferromagnetic susceptibility in ferritin
We show that antiferromagnetic susceptibility in ferritin increases with
temperature between 4.2 K and 180 K (i. e. below the N\'{e}el temperature) when
taken as the derivative of the magnetization at high fields (
Oe). This behavior contrasts with the decrease in temperature previously found,
where the susceptibility was determined at lower fields ( Oe). At
high fields (up to Oe) the temperature dependence of the
antiferromagnetic susceptibility in ferritin nanoparticles approaches the
normal behavior of bulk antiferromagnets and nanoparticles considering
superantiferromagnetism, this latter leading to a better agreement at high
field and low temperature. The contrast with the previous results is due to the
insufficient field range used ( Oe), not enough to saturate the
ferritin uncompensated moment.Comment: 7 pages, 7 figures, accepted in Phys. Rev.
Surface contribution to the anisotropy of magnetic nanoparticles
We calculate the contribution of the Neel surface anisotropy to the effective
anisotropy of magnetic nanoparticles of spherical shape cut out of a simple
cubic lattice. The effective anisotropy arises because deviations of atomic
magnetizations from collinearity and thus the energy depends on the orientation
of the global magnetization. The result is second order in the Neel surface
anisotropy, scales with the particle volume and has cubic symmetry with
preferred directions [+-1,+-1,+-1].Comment: 4 PR pages, 3 figure
A continuous time random walk model of transport in variably saturated heterogeneous porous media
We propose a unified physical framework for transport in variably saturated
porous media. This approach allows fluid flow and solute migration to be
treated as ensemble averages of fluid and solute particles, respectively. We
consider the cases of homogeneous and heterogeneous porous materials. Within a
fractal mobile-immobile (MIM) continuous time random walk framework, the
heterogeneity will be characterized by algebraically decaying particle
retention-times. We derive the corresponding (nonlinear) continuum limit
partial differential equations and we compare their solutions to Monte Carlo
simulation results. The proposed methodology is fairly general and can be used
to track fluid and solutes particles trajectories, for a variety of initial and
boundary conditions.Comment: 12 pages, 9 figure
Diffusion based degradation mechanisms in giant magnetoresistive spin valves
Spin valve systems based on the giant magnetoresistive (GMR) effect as used
for example in hard disks and automotive applications consist of several
functional metallic thin film layers. We have identified by secondary ion mass
spectrometry (SIMS) two main degradation mechanisms: One is related to oxygen
diffusion through a protective cap layer, and the other one is interdiffusion
directly at the functional layers of the GMR stack. By choosing a suitable
material as cap layer (TaN), the oxidation effect can be suppressed.Comment: 3 pages, 3 figures. to be published in Appl. Phys. Let
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