264 research outputs found
Dynamics of diluted magnetic semiconductors from atomistic spin dynamics simulations: Mn doped GaAs as a case study
The dynamical behavior of the magnetism of diluted magnetic semiconductors
(DMS) has been investigated by means of atomistic spin dynamics simulations.
The conclusions drawn from the study are argued to be general for DMS systems
in the low concentration limit, although all simulations are done for 5%
Mn-doped GaAs with various concentrations of As antisite defects. The
magnetization curve, , and the Curie temperature have been
calculated, and are found to be in good correspondence to results from Monte
Carlo simulations and experiments. Furthermore, equilibrium and non-equilibrium
behavior of the magnetic pair correlation function have been extracted. The
dynamics of DMS systems reveals a substantial short ranged magnetic order even
at temperatures at or above the ordering temperature, with a non-vanishing pair
correlation function extending up to several atomic shells. For the high As
antisite concentrations the simulations show a short ranged anti-ferromagnetic
coupling, and a weakened long ranged ferromagnetic coupling. For sufficiently
large concentrations we do not observe any long ranged ferromagnetic
correlation. A typical dynamical response shows that starting from a random
orientation of moments, the spin-correlation develops very fast ( 1ps)
extending up to 15 atomic shells. Above 10 ps in the simulations, the
pair correlation is observed to extend over some 40 atomic shells. The
autocorrelation function has been calculated and compared with ferromagnets
like bcc Fe and spin-glass materials. We find no evidence in our simulations
for a spin-glass behaviour, for any concentration of As antisites. Instead the
magnetic response is better described as slow dynamics, at least when compared
to that of a regular ferromagnet like bcc Fe.Comment: 24 pages, 15 figure
Relaxation of the field-cooled magnetization of an Ising spin glass
The time and temperature dependence of the field-cooled magnetization of a
three dimensional Ising spin glass, Fe_{0.5}Mn_{0.5}TiO_{3}, has been
investigated. The temperature and cooling rate dependence is found to exhibit
memory phenomena that can be related to the memory behavior of the low
frequency ac-susceptibility. The results add some further understanding on how
to model the three dimensional Ising spin glass in real space.Comment: 8 pages RevTEX, 5 figure
Interaction effects and transport properties of Pt capped Co nanoparticles
We studied the magnetic and transport properties of Co nanoparticles (NPs)
being capped with varying amounts of Pt. Beside field and temperature dependent
magnetization measurements we performed delta-M measurements to study the
magnetic interactions between the Co NPs. We observe a transition from
demagnetizing towards magnetizing interactions between the particles for an
increasing amount of Pt capping. Resistivity measurements show a crossover from
giant magnetoresistance towards anisotropic magnetoresistance
Phase transition in a super superspin glass
We here confirm the occurrence of spin glass phase transition and extract
estimates of associated critical exponents of a highly monodisperse and densely
compacted system of bare maghemite nanoparticles. This system has earlier been
found to behave like an archetypal spin glass, with e.g. a sharp transition
from paramagnetic to non-equilibrium behavior, suggesting that this system
undergoes a spin-glass phase transition at a relatively high temperature,
140 K.Comment: 4 pages, 3 figure
The Memory Effect in Electron Glasses
We present a theory for the memory effect in electron glasses. In fast gate
voltage sweeps it is manifested as a dip in the conductivity around the
equilibration gate voltage. We show that this feature, also known as anomalous
field effect, arises from the long-time persistence of correlations in the
electronic configuration. We argue that the gate voltage at which the memory
dip saturates is related to an instability caused by the injection of a
critical number of excess carriers. This saturation threshold naturally
increases with temperature. On the other hand, we argue that the gate voltage
beyond which memory is erased, is temperature independent. Using standard
percolation arguments, we calculate the anomalous field effect as a function of
gate voltage, temperature, carrier density and disorder. Our results are
consistent with experiments, and in particular, they reproduce the observed
scaling of the width of the memory dip with various parameters.Comment: Accepted version, to be published in PR
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