81 research outputs found
Structural anomalies, spin transitions and charge disproportionation in LnCoO3
The diamagnetic-paramagnetic and insulator-metal transitions in LnCoO3
perovskites (Ln = La, Y, rare earths) are reinterpreted and modeled as a
two-level excitation process. In distinction to previous models, the present
approach can be characterized as a LS-HS-IS (low-high-intermediate spin)
scenario. The first level is the local excitation of HS Co3+ species in the LS
ground state. The second excitation is based on the interatomic electron
transfer between the LS/HS pairs, leading finally to a stabilization of the
metallic phase based on IS Co3+. The model parameters have been quantified for
Ln = La, Pr and Nd samples using the powder neutron diffraction on the thermal
expansion of Co-O bonds, that is associated with the two successive spin
transitions. The same model is applied to interpret the magnetic susceptibility
of LaCoO3 and YCoO3.Comment: 52.Conference on Magnetism and Magnetic Materials, November 2007,
Tamp
Control of the colossal magnetoresistance by strain effect in NdCaMnO thin films
Thin films of NdCaMnO manganites with colossal
magnetoresistance (CMR) properties have been synthesized by the Pulsed Laser
Deposition technique on (100)-SrTiO. The lattice parameters of these
manganites and correlatively their CMR properties can be controlled by the
substrate temperature . The maximum CMR effect at 75K, calculated as the
ratio is 10 for a deposition temperature of
degC. Structural studies show that the
NdCaMnO film is single phase, [010]-oriented and has a
pseudocubic symmetry of the perovskite subcell with a=3.77 at room
temperature. We suggest that correlation between lattice parameters, CMR and
substrate temperature result mainly from substrate-induced strains
which can weaken the charge-ordered state at low temperature.Comment: 9 pages, 4 figures. To be published in Applied Physics Letter
Phase transition in Pr0.5Ca0.5CoO3 and related cobaltites
We present an extensive investigation (magnetic, electric and thermal
measurements and X-ray absorption spectroscopy) of the Pr0.5Ca0.5CoO3 and
(Pr1-yYy)0.7Ca0.3CoO3 (y=0.0625-0.15) perovskites, in which a peculiar
metal-insulator (M-I) transition, accompanied with pronounced structural and
magnetic anomalies, occurs at 76 K and 40-132 K, respectively. The inspection
of the M-I transition using the XANES data of Pr L3-edge and Co K-edge proofs
the presence of Pr4+ ions at low temperatures and indicates simultaneously the
intermediate spin to low spin crossover of Co species on lowering the
temperature. The study thus definitively confirms the synchronicity of the
electron transfer between Pr3+ ions and Co^(3+/4+)O3 subsystem and the
transition to the low-spin, less electrically conducting phase. The large
extent of the transfer is evidenced by the good quantitative agreement of the
determined amount of the Pr4+ species, obtained either from the temperature
dependence of the XANES spectra or via integration of the magnetic entropy
change over the Pr4+ related Schottky peak in the low-temperature specific
heat. These results show that the average valence of Pr3+/Pr4+ ions increases
(in concomitance with the decrease of the formal Co valence) below TMI for
(Pr0.925Y0.075)0.7Ca0.3CoO3 up to 3.16+ (the doping level of the CoO3 subsystem
decreases from 3.30+ to 3.20+), for (Pr0.85Y0.15)0.7Ca0.3CoO3 up to 3.28+ (the
decrease of doping level from 3.30+ to 3.13+) and for Pr0.5Ca0.5CoO3 up to
3.46+ (the decrease of doping level from 3.50+ to 3.27+).Comment: 19 pages, 11 figure
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.
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