128 research outputs found
Lattice dynamics and phonon softening in Ni-Mn-Al Heusler alloys
Inelastic and elastic neutron scattering have been used to study a single
crystal of the NiMnAl Heusler alloy over a broad
temperature range. The paper reports the first experimental determination of
the low-lying phonon dispersion curves for this alloy system. We find that the
frequencies of the TA modes are relatively low. This branch exhibits an
anomaly (dip) at a wave number , which softens with
decreasing temperature. Associated with this anomalous dip at , an
elastic central peak scattering is also present. We have also observed
satellites due to the magnetic ordering.Comment: 6 pages, 6 figures. Accepted for publication in the Physical Review
On the Nature of Memory and Rejuvenation in Glassy Systems
The memory effect in a single crystal spin glass
() has been measured using ac susceptibility techniques over a reduced temperature range of
and a model of the memory effect has been developed. A
double-waiting-time protocol is carried out where the spin glass is first
allowed to age at a temperature below , followed by a second aging at a
lower temperature after it has fully rejuvenated. The model is based on
calculating typical coincidences between the growth of correlated regions at
the two temperatures. It accounts for the absolute magnitude of the memory
effect as a function of both waiting times and temperatures. The data can be
explained by the memory loss being a function of the relative change in the
correlated volume at the first waiting temperature because of the growth in the
correlations at the second waiting temperature.Comment: 11 pages, 6 figure
Anomalous phonon behavior in the high temperature shape memory alloy: TiPd:Cr
Ti50 Pd50-xCrx is a high temperature shape memory alloy with a martensitic
transformation temperature strongly dependent on the Cr composition. Prior to
the transformation a premartensitic phase is present with an incommensurate
modulated cubic lattice with wave vector of q0=(0.22, 0.22, 0). The temperature
dependence of the diffuse scattering in the cubic phase is measured as a
function temperature for x=6.5, 8.5, and 10 at. %. The lattice dynamics has
been studied and reveals anomalous temperature and q-dependence of the
[110]-TA2 transverse phonon branch. The phonon linewidth is broad over the
entire Brillouin zone and increases with decreasing temperature, contrary to
the behavior expected for anharmonicity. No anomaly is observed at q0. The
results are compared with first principles calculation of the phonon structure.Comment: 26 pages, 11 figure
Anisotropic magnetic deflagration in single crystals of Gd5Ge4
Experimental evidence of the anisotropy of the magnetic deflagration
associated with the low-temperature first order antiferromagnetic (AFM) -->
ferromagnetic (FM) phase-transition in single crystals of Gd5Ge4 is reported.
The deflagrations have been induced by controlled pulses of surface acoustic
waves (SAW) allowing us to explore both the magnetic field and temperature
dependencies on the characteristic times of the phenomenon. The study was done
using samples with different geometries and configurations between the SAW
pulses and the direction of the applied magnetic field with respect to the
three main crystallographic directions of the samples. The effect of
temperature is nearly negligible, whereas observed strong magnetic field
dependence correlates with the magnetic anisotropy of the sample. Finally, the
role of the SAW pulses in both the ignition and formation of the deflagration
front was also studied, and we show that the thermal diffusivity of Gd5Ge4 must
be anisotropic, following \kappaa>\kappab>\kappac.Comment: 10 pages, 4 figure
Magnetic structure of Gd5Ge4
Gd5Ge4 crystallizes in the orthorhombic space group Pnma, and orders antiferromagnetically below the Néel temperatureTN∼127 K. We have employed x-ray resonant magnetic scattering to elucidate the details of the magnetic structure. The magnetic unit cell is the same as the chemical unit cell. From azimuth scans and the Q dependence of the magnetic scattering, all three Gd sites in the structure were determined to be in the same magnetic space group Pnm′a. The magnetic moments are primarily aligned along the c axis and the c components of the magnetic moments at the three different sites are equal. The ferromagnetic Gd-rich slabs are stacked antiferromagnetically along the b direction
Magnetism of (Dy0.5Er0.5)Al2 single crystal in ac and dc magnetic fields
The temperature (4.2–90 K), ac magnetic field (1.25–50 Oe), frequency (5–125 Hz), and bias dc magnetic field (0–10 kOe) dependencies of the real and imaginary components of the ac magnetic susceptibility, and the temperature (4.2–250 K) and dc magnetic field(0.1–50 kOe) dependencies of the dc magnetic susceptibility and magnetization of a(Dy0.5Er0.5)Al2 single crystal have been studied. Isothermal magnetization measurement in a dc magnetic field indicates that (Dy0.5Er0.5)Al2 orders ferromagnetically at 37 K. The ac and dc magnetic susceptibilities of (Dy0.5Er0.5)Al2 exhibit a similar behavior in the paramagnetic region but quite different behaviors in the ferromagnetic state. Both the real and imaginary components of the ac magnetic susceptibility are sensitive to the applied ac magnetic field, the crystallographic direction, and the bias magnetic field, showing that domain wall dynamics mainly account for the response to the ac magnetic field. The contributions to the magnetization process arise from the magnetically ordered Dy and Er sublattices and depend upon the single-ion anisotropy of the Dy and Er ions
Magnetic field induced phase transitions in Gd5(Si1.95Ge2.05) single crystal and the anisotropic magnetocaloric effect
Magnetization measurements using a Gd5(Si1.95Ge2.05) single crystal with the magnetic field applied along three crystallographic directions, [001], [010] and [100], were carried out as a function of the applied field (0–56 kOe) at various temperatures (∼5–320 K). The magnetic field (H)–temperature (T) phase diagrams were constructed for theGd5(Si1.95Ge2.05) single crystal with field along the three directions. A small anisotropy was observed. The magnetocaloric effect was calculated from isothermal magnetization data, and the observed anisotropy correlates with the H–T phase diagrams. The results are discussed in connection with the magnetic field induced martensitic-like structural transition observed in Gd5(Si2Ge2)-type compounds
- …