3,741 research outputs found
Structural and Magnetic Characteristics of MnAs Nanoclusters Embedded in Be-doped GaAs
We describe a systematic study of the synthesis, microstructure and
magnetization of hybrid ferromagnet-semiconductor nanomaterials comprised of
MnAs nanoclusters embedded in a p-doped GaAs matrix. These samples are created
during the in situ annealing of Be-doped (Ga,Mn)As heterostructures grown by
molecular beam epitaxy. Transmission electron microscopy and magnetometry
studies reveal two distinct classes of nanoclustered samples whose structural
and magnetic properties depend on the Mn content of the initial (Ga,Mn)As
layer. For Mn content in the range 5% - 7.5%, annealing creates a
superparamagnetic material with a uniform distribution of small clusters
(diameter around 6 nm) and with a low blocking temperature (T_B approximately
10 K). While transmission electron microscopy cannot definitively identify the
composition and crystalline phase of these small clusters, our experimental
data suggest that they may be comprised of either zinc-blende MnAs or Mn-rich
regions of (Ga,Mn)As. At higher Mn content (> 8 %), we find that annealing
results in an inhomogeneous distribution of both small clusters as well as much
larger NiAs-phase MnAs clusters (diameter around 25 nm). These samples also
exhibit supermagnetism, albeit with substantially larger magnetic moments and
coercive fields, and blocking temperatures well above room temperature
Quantum and thermal spin relaxation in diluted spin ice: Dy(2-x)MxTi2O7 (M = Lu, Y)
We have studied the low temperature a.c. magnetic susceptibility of the
diluted spin ice compound Dy(2-x)MxTi2O7, where the magnetic Dy ions on the
frustrated pyrochlore lattice have been replaced with non-magnetic ions, M = Y
or Lu. We examine a broad range of dilutions, 0 <= x <= 1.98, and we find that
the T ~ 16 K freezing is suppressed for low levels of dilution but re-emerges
for x > 0.4 and persists to x = 1.98. This behavior can be understood as a
non-monotonic dependence of the quantum spin relaxation time with dilution. The
results suggest that the observed spin freezing is fundamentally a single spin
process which is affected by the local environment, rather than the development
of spin-spin correlations as earlier data suggested.Comment: 26 pages, 9 figure
Quenching of Cross Sections in Nucleon Transfer Reactions
Cross sections for proton knockout observed in (e,e'p) reactions are
apparently quenched by a factor of ~0.5, an effect attributed to short-range
correlations between nucleons. Here we demonstrate that such quenching is not
restricted to proton knockout, but a more general phenomenon associated with
any nucleon transfer. Measurements of absolute cross sections on a number of
targets between 16O and 208Pb were analyzed in a consistent way, with the cross
sections reduced to spectroscopic factors through the distorted-wave Born
approximation with global optical potentials. Across the 124 cases analyzed
here, induced by various proton- and neutron-transfer reactions and with
angular momentum transfer l=0-7, the results are consistent with a quenching
factor of 0.55. This is an apparently uniform quenching of single-particle
motion in the nuclear medium. The effect is seen not only in (d,p) reactions
but also in reactions with A=3 and 4 projectiles, when realistic wave functions
are used for the projectiles.Comment: 5 pages, 3 figures, accepted to Physical Review Letter
Time Dependent Effects and Transport Evidence for Phase Separation in La_{0.5}Ca_{0.5}MnO_{3}
The ground state of La_{1-x}Ca_{x}MnO_{3} changes from a ferromagnetic
metallic to an antiferromagnetic charge-ordered state as a function of Ca
concentration at x ~ 0.50. We present evidence from transport measurements on a
sample with x = 0.50 that the two phases can coexist, in agreement with other
observations of phase separation in these materials. We also observe that, by
applying and then removing a magnetic field to the mainly charge-ordered state
at some temperatures, we can "magnetically anneal" the charge order, resulting
in a higher zero-field resistivity. We also observe logarithmic time dependence
in both resistivity and magnetization after a field sweep at low temperatures.Comment: 9 pages, LATEX, 3 postscript figure
Slow spin relaxation in a highly polarized cooperative paramagnet
We report measurements of the ac susceptibility of the cooperative paramagnet
Tb2Ti2O7 in a strong magnetic field. Our data show the expected saturation
maximum in chi(T) and also an unexpected low frequency dependence (< 1 Hz) of
this peak, suggesting very slow spin relaxations are occurring. Measurements on
samples diluted with nonmagnetic Y3+ or Lu3+ and complementary measurements on
pure and diluted Dy2Ti2O7 strongly suggest that the relaxation is associated
with dipolar spin correlations, representing unusual cooperative behavior in a
paramagnetic system.Comment: Accepted for publication in Physical Review Letter
Spin Freezing in Geometrically Frustrated Antiferromagnets with Weak Disorder
We investigate the consequences for geometrically frustrated antiferromagnets
of weak disorder in the strength of exchange interactions. Taking as a model
the classical Heisenberg antiferromagnet with nearest neighbour exchange on the
pyrochlore lattice, we examine low-temperature behaviour. We show that random
exchange generates long-range effective interactions within the extensively
degenerate ground states of the clean system. Using Monte Carlo simulations, we
find a spin glass transition at a temperature set by the disorder strength.
Disorder of this type, which is generated by random strains in the presence of
magnetoelastic coupling, may account for the spin freezing observed in many
geometrically frustrated magnets.Comment: 4 pages, 5 figure
Geometrical Magnetic Frustration in Rare Earth Chalcogenide Spinels
We have characterized the magnetic and structural properties of the CdLn2Se4
(Ln = Dy, Ho), and CdLn2S4 (Ln = Ho, Er, Tm, Yb) spinels. We observe all
compounds to be normal spinels, possessing a geometrically frustrated
sublattice of lanthanide atoms with no observable structural disorder. Fits to
the high temperature magnetic susceptibilities indicate these materials to have
effective antiferromagnetic interactions, with Curie-Weiss temperatures theta ~
-10 K, except CdYb2S4 for which theta ~ -40 K. The absence of magnetic long
range order or glassiness above T = 1.8 K strongly suggests that these
materials are a new venue in which to study the effects of strong geometrical
frustration, potentially as rich in new physical phenomena as that of the
pyrochlore oxides.Comment: 17 pages, 5 figures, submitted to Phys Rev B; added acknowledgement
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