63 research outputs found
Spin Seebeck effect in Y-type hexagonal ferrite thin films
Spin Seebeck effect (SSE) has been investigated in thin films of two
Y-hexagonal ferrites BaZnFeO (Zn2Y) and
BaCoFeO (Co2Y) deposited by a spin-coating method on
SrTiO(111) substrate. The selected hexagonal ferrites are both
ferrimagnetic with similar magnetic moments at room temperature and both
exhibit easy magnetization plane normal to -axis. Despite that, SSE signal
was only observed for Zn2Y, whereas no significant SSE signal was detected for
Co2Y. We tentatively explain this different behavior by a presence of two
different magnetic ions in Co2Y, whose random distribution over octahedral
sites interferes the long range ordering and enhances the Gilbert damping
constant. The temperature dependence of SSE for Zn2Y was measured and analyzed
with regard to the heat flux and temperature gradient relevant to the SSE
signal.Comment: 7 pages, 9 figure
Anisotropic phonon conduction and lattice distortions in CMR-type bilayer manganite (LaPr)SrMnO (z=0,0.2,0.4 and 0.6) single crystals
We have undertaken a systematic study of thermal conductivity as a function
of temperature and magnetic field of single crystals of the compound
(LaPr)SrMnO for (Pr) =0.2,0.4. and
0.6. The lattice distortion due to Pr-substitution and anisotropic thermal
conductivity in bilayer manganites are discussed on the basis of different
relaxation models of local lattice distortions in metal and insulating states
proposed by Maderda et al. The giant magnetothermal effect is scaled as a
function of magnetization and discussed on the basis of a systematic variation
of the occupation of the -electron orbital states due to Pr-substitution.Comment: 7 pages, 6 figures, in press in Phys.Rev.
Low temperature anomalies in the specific heat and magnetic susceptibility of Na0.7CoO2 samples
We present a comparative study of Na0.7CoO2 samples obtained from three different sources and prepared by different methods. The specific heat and magnetic susceptibility measurements in the temperature range 2-300 K show substantial influence on the observed anomalies, which underlines that the system is extremely sensitive to preparation protocols.13th Czech and Slovak Conference on Magnetism (CSMAG07), Jul 09-12, 2007, Kosice, Slovaki
Strong spin-phonon coupling in infrared and Raman spectra of SrMnO3
Infrared reflectivity spectra of cubic SrMnO3 ceramics reveal 18% stiffening of the lowest-frequency phonon below the antiferromagnetic phase transition occurring at T-N = 233 K. Such a large temperature change of the polar phonon frequency is extraordinary and we attribute it to an exceptionally strong spin-phonon coupling in this material. This is consistent with our prediction from first-principles calculations. Moreover, polar phonons become Raman active below T-N, although their activation is forbidden by symmetry in the Pm (3) over barm space group. This gives evidence that the cubic Pm (3) over barm symmetry is locally broken below T-N due to a strong magnetoelectric coupling. Multiphonon and multimagnon scattering is also observed in Raman spectra. Microwave and THz permittivity is strongly influenced by hopping electronic conductivity, which is caused by small nonstoichiometry of the sample. Thermoelectric measurements show room-temperature concentration of free carriers n(e) = 3.6 x 10(20) cm(-3) and the sample composition Sr2+Mn0.984+Mn0.023+O2.992-. The conductivity exhibits very unusual temperature behavior: THz conductivity increases on cooling, while the static conductivity markedly decreases on cooling. We attribute this to different conductivity of the ceramic grains and grain boundariesclose
Magnetic, orbital and charge ordering in the electron-doped manganites
The three dimensional perovskite manganites in the range of hole-doping are studied in detail using a double exchange model with degenerate
orbitals including intra- and inter-orbital correlations and near-neighbour
Coulomb repulsion. We show that such a model captures the observed phase
diagram and orbital-ordering in the intermediate to large band-width regime. It
is argued that the Jahn-Teller effect, considered to be crucial for the region
, does not play a major role in this region, particularly for systems
with moderate to large band-width. The anisotropic hopping across the
degenerate orbitals are crucial in understanding the ground state phases
of this region, an observation emphasized earlier by Brink and Khomskii. Based
on calculations using a realistic limit of finite Hund's coupling, we show that
the inclusion of interactions stabilizes th e C-phase, the antiferromagnetic
metallic A-phase moves closer to while th e ferromagnetic phase shrinks
in agreement with recent observations. The charge ordering close to and
the effect of reduction of band-width are also outlined. The effect of disorder
and the possibility of inhomogeneous mixture of competing states have been
discussed.Comment: 42 pages, 16 figure
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