1,877 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
Spin glass behavior in an interacting gamma-Fe2O3 nanoparticle system
In this paper we investigate the superspin glass behavior of a concentrated
assembly of interacting maghemite nanoparticles and compare it to that of
canonical atomic spin glass systems. ac versus temperature and frequency
measurements show evidence of a superspin glass transition taking place at low
temperature. In order to fully characterize the superspin glass phase, the
aging behavior of both the thermo-remanent magnetization (TRM) and ac
susceptibility has been investigated. It is shown that the scaling laws obeyed
by superspin glasses and atomic spin glasses are essentially the same, after
subtraction of a superparamagnetic contribution from the superspin glass
response functions. Finally, we discuss a possible origin of this
superparamagnetic contribution in terms of dilute spin glass models
Dynamical breakdown of the Ising spin-glass order under a magnetic field
The dynamical magnetic properties of an Ising spin glass
FeMnTiO are studied under various magnetic fields. Having
determined the temperature and static field dependent relaxation time
from ac magnetization measurements under a dc bias field by a
general method, we first demonstrate that these data provide evidence for a
spin-glass (SG) phase transition only in zero field. We next argue that the
data of finite can be well interpreted by the droplet theory
which predicts the absence of a SG phase transition in finite fields.Comment: 4 pages, 5 figure
Electron Correlation Driven Heavy-Fermion Formation in LiV2O4
Optical reflectivity measurements were performed on a single crystal of the
d-electron heavy-fermion (HF) metal LiV2O4. The results evidence the highly
incoherent character of the charge dynamics for all temperatures above T^*
\approx 20 K. The spectral weight of the optical conductivity is redistributed
over extremely broad energy scales (~ 5 eV) as the quantum coherence of the
charge carriers is recovered. This wide redistribution is, in sharp contrast to
f-electron Kondo lattice HF systems, characteristic of a metallic system close
to a correlation driven insulating state. Our results thus reveal that strong
electronic correlation effects dominate the low-energy charge dynamics and
heavy quasiparticle formation in LiV2O4. We propose the geometrical
frustration, which limits the extension of charge and spin ordering, as an
additional key ingredient of the low-temperature heavy-fermion formation in
this system.Comment: 5 pages, 3 figure
Finite-size effects in amorphous Fe90Zr10/Al75Zr25 multilayers
The thickness dependence of the magnetic properties of amorphous Fe90Zr10
layers has been explored using Fe90Zr10/Al75Zr25 multilayers. The Al75Zr25
layer thickness is kept at 40 \AA, while the thickness of the Fe90Zr10 layers
is varied between 5 and 20 \AA. The thickness of the Al75Zr25 layers is
sufficiently large to suppress any significant interlayer coupling. Both the
Curie temperature and the spontaneous magnetization decrease non-linearly with
decreasing thickness of the Fe90Zr10 layers. No ferromagnetic order is observed
in the multilayer with 5 {\AA} Fe90Zr10 layers. The variation of the Curie
temperature with the Fe90Zr10 layer thickness is fitted with a
finite-size scaling formula [1-\Tc(t)/\Tc(\infty)]=[(t-t')/t_0]^{-\lambda},
yielding , and a critical thickness \AA, below which the
Curie temperature is zero.Comment: 8 pages, 8 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
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
Absence of Conventional Spin-Glass Transition in the Ising Dipolar System LiHo_xY_{1-x}F_4
The magnetic properties of single crystals of LiHo_xY_{1-x}F_4 with x=16.5%
and x=4.5% were recorded down to 35 mK using a micro-SQUID magnetometer. While
this system is considered as the archetypal quantum spin glass, the detailed
analysis of our magnetization data indicates the absence of a phase transition,
not only in a transverse applied magnetic field, but also without field. A
zero-Kelvin phase transition is also unlikely, as the magnetization seems to
follow a non-critical exponential dependence on the temperature. Our analysis
thus unmasks the true, short-ranged nature of the magnetic properties of the
LiHo_xY_{1-x}F_4 system, validating recent theoretical investigations
suggesting the lack of phase transition in this system.Comment: 5 pages, 4 figure
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