142 research outputs found
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
Symmetrical Temperature-Chaos Effect with Positive and Negative Temperature Shifts in a Spin Glass
The aging in a Heisenberg-like spin glass Ag(11 at% Mn) is investigated by
measurements of the zero field cooled magnetic relaxation at a constant
temperature after small temperature shifts . A
crossover from fully accumulative to non-accumulative aging is observed, and by
converting time scales to length scales using the logarithmic growth law of the
droplet model, we find a quantitative evidence that positive and negative
temperature shifts cause an equivalent restart of aging (rejuvenation) in terms
of dynamical length scales. This result supports the existence of a unique
overlap length between a pair of equilibrium states in the spin glass system.Comment: 4 page
Atomistic spin dynamics of the CuMn spin glass alloy
We demonstrate the use of Langevin spin dynamics for studying dynamical
properties of an archetypical spin glass system. Simulations are performed on
CuMn (20% Mn) where we study the relaxation that follows a sudden quench of the
system to the low temperature phase. The system is modeled by a Heisenberg
Hamiltonian where the Heisenberg interaction parameters are calculated by means
of first-principles density functional theory. Simulations are performed by
numerically solving the Langevin equations of motion for the atomic spins. It
is shown that dynamics is governed, to a large degree, by the damping parameter
in the equations of motion and the system size. For large damping and large
system sizes we observe the typical aging regime.Comment: 18 pages, 9 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
Memory and superposition in a spin glass
Non-equilibrium dynamics in a Ag(Mn) spin glass are investigated by
measurements of the temperature dependence of the remanent magnetisation. Using
specific cooling protocols before recording the thermo- or isothermal remanent
magnetisations on re-heating, it is found that the measured curves effectively
disclose non-equilibrium spin glass characteristics such as ageing and memory
phenomena as well as an extended validity of the superposition principle for
the relaxation. The usefulness of this "simple" dc-method is discussed, as well
as its applicability to other disordered magnetic systems.Comment: REVTeX style; 8 pages, 4 figure
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