216 research outputs found
Magnetic Relaxation Phenomena in a CuMn Spin Glass
Experiments on the temperature and time dependence of the response function
and the field cooled magnetisation of a Cu(Mn) spin glass at temperatures below
the zero field spin glass temperature are used to explore the non-equilibrium
nature of the underlying spin configuration. The results imply that a certain
spin configuration is imprinted on the system as the temperature is decreased
at a constant cooling rate. The cooling rate governs the magnitude of the FC
magnetisation (M_{FC}(H,T)). Any intermittent halt at a constant temperature,
T_{i}, imprints an extended spin configuration, a process that is reflected
e.g. in a downward relaxation of M_{FC}. On continued cooling at the same rate,
the magnitude of M_{FC}(T) remains at a lower level than that of a continuous
cooling curve. These results are put into the context of the corresponding
behaviour of the response function as observed in measurements of the
relaxation of the zero field cooled magnetisation.Comment: 6 pages, 7 figure
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
Critical dynamics of an interacting magnetic nanoparticle system
Effects of dipole-dipole interactions on the magnetic relaxation have been
investigated for three Fe-C nanoparticle samples with volume concentrations of
0.06, 5 and 17 vol%. While both the 5 and 17 vol% samples exhibit collective
behavior due to dipolar interactions, only the 17 vol% sample displays critical
behavior close to its transition temperature. The behaviour of the 5 vol%
sample can be attributed to a mixture of collective and single particle
dynamics.Comment: 19 pages, 8 figure
Comment on "Memory Effects in an Interacting Magnetic Nanoparticle System"
In Phys. Rev. Lett. 91 167206 (2003), Sun et al. study memory effects in an
interacting nanoparticle system with specific temperature and field protocols.
The authors claim that the observed memory effects originate from spin-glass
dynamics and that the results are consistent with the hierarchical picture of
the spin-glass phase. In this comment, we argue their claims premature by
demonstrating that all their experimental curves can be reproduced
qualitatively using only a simplified model of isolated nanoparticles with a
temperature dependent distribution of relaxation times.Comment: 1 page, 2 figures, slightly changed content, the parameters involved
in Figs. 1 and 2 are changed a little for a semi-quantitative comparision
with experimental result
Memory interference effects in spin glasses
When a spin glass is cooled down, a memory of the cooling process is
imprinted in the spin structure. This memory can be disclosed in a continuous
heating measurement of the ac-susceptibility. E.g., if a continuous cooling
process is intermittently halted during a certain aging time at one or two
intermediate temperatures, the trace of the previous stop(s) is recovered when
the sample is continuously re-heated [1]. However, heating the sample above the
aging temperature, but keeping it below Tg, erases the memory of the thermal
history at lower temperatures. We also show that a memory imprinted at a higher
temperature can be erased by waiting a long enough time at a lower temperature.
Predictions from two complementary spin glass descriptions, a hierarchical
phase space model and a real space droplet picture are contested with these
memory phenomena and interference effects.
[1] K. Jonason, E. Vincent, J. Hammann, J. P. Bouchaud and P. Nordblad, Phys.
Rev. Lett. 31, 3243 (1998).Comment: 7 pages, 1 LaTex file + 5 figures in EPS Revised version of June 17,
1999 (minor changes), to appear in EPJ B around November 9
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
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