296 research outputs found
Magnetization reversal and anomalous coercive field temperature dependence in MnAs epilayers grown on GaAs(100) and GaAs(111)B
The magnetic properties of MnAs epilayers have been investigated for two
different substrate orientations: GaAs(100) and GaAs(111). We have analyzed the
magnetization reversal under magnetic field at low temperatures, determining
the anisotropy of the films. The results, based on the shape of the
magnetization loops, suggest a domain movement mechanism for both types of
samples. The temperature dependence of the coercivity of the films has been
also examined, displaying a generic anomalous reentrant behavior at T200 K.
This feature is independent of the substrate orientation and films thickness
and may be associated to the appearance of new pinning centers due to the
nucleation of the -phase at high temperatures.Comment: 9 pages, 7 figure
J Acquir Immune Defic Syndr
We describe HIV-1 evolutionary dynamics in the 4 participants from the TDF2-PrEP trial who became HIV-1 infected while prescribed emtricitabine and tenofovir disoproxil fumarate (FTC/TDF). At seroconversion, virus diversity in the 2 participants with detectable drug was only 0.05% (95% confidence intervals: 0.04 to 0.06) and 0.07% (0.06 to 0.08) compared with 2.25% (1.95 to 2.6) and 0.42% (0.36 to 0.49) in those with no detectable drug and 0.07%-0.69% in 5 placebo recipients (P > 0.5). At 10 months, diversity in adherent participants was only 0.37% (0.31 to 0.41) and 0.86% (0.82 to 0.90) compared with 0.5%-1.7% among participants who did not take FTC/TDF (P > 0.5). Although limited by the small number of infections that reduced the power to detect differences, we found that sequences from seroconverters with detectable drug were more homogeneous than those from placebo or nonadherent seroconverters.26689970PMC487657
Ultrasonic triggering of giant magnetocaloric effect in MnAs thin films
Mechanical control of magnetic properties in magnetostrictive thin films
offers the unexplored opportunity to employ surface wave acoustics in such a
way that acoustic triggers dynamic magnetic effects. The strain-induced
modulation of the magnetic anisotropy can play the role of a high frequency
varying effective magnetic field leading to ultrasonic tuning of electronic and
magnetic properties of nanostructured materials, eventually integrated in
semiconductor technology. Here, we report about the opportunity to employ
surface acoustic waves to trigger magnetocaloric effect in
MnAs(100nm)/GaAs(001) thin films. During the MnAs magnetostructural phase
transition, in an interval range around room temperature (0{\deg}C -
60{\deg}C), ultrasonic waves (170 MHz) are strongly attenuated by the phase
coexistence (up to 150 dB/cm). We show that the giant magnetocaloric effect of
MnAs is responsible of the observed phenomenon. By a simple anelastic model we
describe the temperature and the external magnetic field dependence of such a
huge ultrasound attenuation. Strain-manipulation of the magnetocaloric effect
could be a further interesting route for dynamic and static caloritronics and
spintronics applications in semiconductor technology
Biaxial Strain in the Hexagonal Plane of MnAs Thin Films: The Key to Stabilize Ferromagnetism to Higher Temperature
The alpha-beta magneto-structural phase transition in MnAs/GaAs(111)
epilayers is investigated by elastic neutron scattering. The in-plane parameter
of MnAs remains almost constant with temperature from 100 K to 420 K, following
the thermal evolution of the GaAs substrate. This induces a temperature
dependent biaxial strain that is responsible for an alpha-beta phase
coexistence and, more important, for the stabilization of the ferromagnetic
alpha-phase at higher temperature than in bulk. We explain the premature
appearance of the beta-phase at 275 K and the persistence of the ferromagnetic
alpha-phase up to 350 K with thermodynamical arguments based on the MnAs phase
diagram. It results that the biaxial strain in the hexagonal plane is the key
parameter to extend the ferromagnetic phase well over room temperature.Comment: 4 pages, 3 figures, accepted for publication in Physical Review
Letter
Biaxial Strain in the Hexagonal Plane of MnAs Thin Films: The Key to Stabilize Ferromagnetism to Higher Temperature
The alpha-beta magneto-structural phase transition in MnAs/GaAs(111)
epilayers is investigated by elastic neutron scattering. The in-plane parameter
of MnAs remains almost constant with temperature from 100 K to 420 K, following
the thermal evolution of the GaAs substrate. This induces a temperature
dependent biaxial strain that is responsible for an alpha-beta phase
coexistence and, more important, for the stabilization of the ferromagnetic
alpha-phase at higher temperature than in bulk. We explain the premature
appearance of the beta-phase at 275 K and the persistence of the ferromagnetic
alpha-phase up to 350 K with thermodynamical arguments based on the MnAs phase
diagram. It results that the biaxial strain in the hexagonal plane is the key
parameter to extend the ferromagnetic phase well over room temperature.Comment: 4 pages, 3 figures, accepted for publication in Physical Review
Letter
Interface bonding of a ferromagnetic/semiconductor junction : a photoemission study of Fe/ZnSe(001)
We have probed the interface of a ferromagnetic/semiconductor (FM/SC)
heterojunction by a combined high resolution photoemission spectroscopy and
x-ray photoelectron diffraction study. Fe/ZnSe(001) is considered as an example
of a very low reactivity interface system and it expected to constitute large
Tunnel Magnetoresistance devices. We focus on the interface atomic environment,
on the microscopic processes of the interface formation and on the iron
valence-band. We show that the Fe contact with ZnSe induces a chemical
conversion of the ZnSe outermost atomic layers. The main driving force that
induces this rearrangement is the requirement for a stable Fe-Se bonding at the
interface and a Se monolayer that floats at the Fe growth front. The released
Zn atoms are incorporated in substitution in the Fe lattice position. This
formation process is independent of the ZnSe surface termination (Zn or Se).
The Fe valence-band evolution indicates that the d-states at the Fermi level
show up even at submonolayer Fe coverage but that the Fe bulk character is only
recovered above 10 monolayers. Indeed, the Fe 1-band states,
theoretically predicted to dominate the tunneling conductance of Fe/ZnSe/Fe
junctions, are strongly modified at the FM/SC interface.Comment: 23 pages, 5 figures, submitted to Physical review
Modulating the phase transition temperature of giant magnetocaloric thin films by ion irradiation
Magnetic refrigeration based on the magnetocaloric effect at room temperature
is one of the most attractive alternative to the current gas
compression/expansion method routinely employed. Nevertheless, in giant
magnetocaloric materials, optimal refrigeration is restricted to the narrow
temperature window of the phase transition (Tc). In this work, we present the
possibility of varying this transition temperature into a same giant
magnetocaloric material by ion irradiation. We demonstrate that the transition
temperature of iron rhodium thin films can be tuned by the bombardment of ions
of Ne 5+ with varying fluences up to 10 14 ions cm --2 , leading to optimal
refrigeration over a large 270--380 K temperature window. The Tc modification
is found to be due to the ion-induced disorder and to the density of new
point-like defects. The variation of the phase transition temperature with the
number of incident ions opens new perspectives in the conception of devices
using giant magnetocaloric materials
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