51 research outputs found
Properties of the ferrimagnetic double-perovskite A_{2}FeReO_{6} (A=Ba and Ca)
Ceramics of A_{2}FeReO_{6} double-perovskite have been prepared and studied
for A=Ba and Ca. Ba_{2}FeReO_{6} has a cubic structure (Fm3m) with 8.0854(1) \AA whereas Ca_{2}FeReO_{6} has a distorted monoclinic symmetry with
and
. The barium compound is metallic from 5 K to 385
K, i.e. no metal-insulator transition has been seen up to 385 K, and the
calcium compound is semiconducting from 5 K to 385 K. Magnetization
measurements show a ferrimagnetic behavior for both materials, with T_{c}=315 K
for Ba_{2}FeReO_{6} and above 385 K for Ca_{2}FeReO_{6}. A specific heat
measurement on the barium compound gave an electron density of states at the
Fermi level, N(E_{F}) equal to 6.1. At 5 K, we
observed a negative magnetoresistance of 10 % in a magnetic field of 5 T, but
only for Ba_{2}FeReO_{6}. Electrical, thermal and magnetic properties are
discussed and compared to the analogous compounds Sr_{2}Fe(Mo,Re)O_{6}.Comment: 5 pages REVTeX, 7 figures included, submitted to PR
Mesoscopic, Non-equilibrium Fluctuations of Inhomogeneous Electronic States in Manganites
By using the dark-field real-space imaging technique of transmission electron
microscopy (TEM), we have observed slow 200 A-scale fluctuations of
charge-ordered (CO) phase in mixed-valent manganites under a strong electron
beam irradiation. In addition to these unusual fluctuations of the CO phase, we
observed the switching-type fluctuations of electrical resistivity in the same
sample, which were found to be as large as several percents. Systematic
analysis indicates that these two different types of fluctuations with a
similar time scale of the order of seconds are interconnected through a
meta-stable insulating charge-disordered state. Current dependence of the
fluctuations suggests a non-equilibrium nature of this slow dynamics.Comment: To appear in Europhysics Letter
Polaronic Heat Capacity in The Anderson - Hasegawa Model
An exact treatment of the Anderson - Hasegawa two - site model, incorporating
the presence of superexchange and polarons, is used to compute the heat
capacity. The calculated results point to the dominance of the lattice
contribution, especially in the ferromagnetic regime. This behavior is in
qualitative agreement with experimental findings.Comment: 9 pages, Revtex, 4 postscript figure
Specific heat and magnetic measurements in Nd0.5Sr0.5MnO3, Nd0.5Ca0.5MnO3 and Ho0.5Ca0.5MnO3 samples
We studied the magnetization as a function of temperature and magnetic field
in the compounds Nd0.5Sr0.5MnO3, Nd0.5Ca0.5MnO3 and Ho0.5Ca0.5MnO3. It allowed
us to identify the ferromagnetic, antiferromagnetic and charge ordering phases
in each case. The intrinsic magnetic moments of Nd3+ and Ho3+ ions experienced
a short range order at low temperatures. We also did specific heat measurements
with applied magnetic fields between 0 and 9 T and temperatures between 2 and
300 K in all three samples. Close to the charge ordering and ferromagnetic
transition temperatures the specific heat curves showed peaks superposed to the
characteristic response of the lattice oscillations. Below 10 K the specific
heat measurements evidenced a Schottky-like anomaly for all samples. However,
we could not successfully fit the curves to either a two level nor a
distribution of two-level Schottky anomaly. Our results indicated that the peak
temperature of the Schottky anomaly was higher in the compounds with narrower
conduction band.Comment: submitted to PR
Soft spin waves in the low temperature thermodynamics of Pr_{0.7}Ca_{0.3}MnO_{3}
We present a detailed magnetothermal study of Pr(0.7)Ca(0.3)MnO(3), a
perovskite manganite in which an insulator-metal transition can be driven by
magnetic field, but also by pressure, visible light, x-rays, or high currents.
We find that the field-induced transition is associated with an enormous
release of energy which accounts for its strong irreversibility. In the
ferromagnetic metallic state, specific heat and magnetization measurements
indicate a much smaller spin wave stiffness than that seen in any other
manganite, which we attribute to spin waves among the ferromagnetically ordered
Pr moments. The coupling between the Pr and Mn spins may also provide a basis
for understanding the low temperature phase diagram of this most unusual
manganite.Comment: 10 pages, LATEX, 5 PDF figures, corrected typo
Anomalous field-dependent specific heat in charge-ordered PrCaMnO and LaCaMnO
We report low temperature specific heat measurements of
PrCaMnO () and
LaCaMnO with and without applied magnetic field. An
excess specific heat, , of non-magnetic origin associated with
charge ordering is found for all the samples. A magnetic field sufficient to
induce the transition from the charge-ordered state to the ferromagnetic
metallic state does not completely remove the contribution. This
suggests that the charge ordering is not completely destroyed by a "melting"
magnetic field. In addition, the specific heat of the
PrCaMnO compounds exhibit a large contribution linear in
temperature () originating from magnetic and charge disorder.Comment: submitted to PRL 5 pages, 3 figures include
Electrical transport and magnetic properties of a possible electron-doped layered manganese oxide
We report on the structural, transport, and magnetic properties of La0.67Sr0.33MnOx thin films grown in vacuum by pulsed-laser deposition. The as-grown thin films have both the matrix La1.34Sr0.66MnO4 phase with K2NiF4 structure and an embedded MnO phase. The electrical transport and magnetic properties of the films are determined mainly by those of the matrix phase. By annealing, the as-grown thin films can be transformed into the normal La0.67Sr0.33MnO3 single phase, which shows the expected colossal magnetoresistance effect. Based on the composition of the matrix phase, and the structural, electrical, and magnetic properties of the films, we propose that the matrix phase is possibly electron doped with a mixed valence of Mn2+/Mn3+ instead of the Mn3+/Mn4+ as in the hole-doped case
Specific heat and magnetization study on single crystals of a frustrated, quasi one-dimensional oxide: Ca3Co2O6
Specific heat and magnetization measurements have been carried out under a
range of magnetic fields on single crystals of Ca3Co2O6. This compound is
composed of Ising magnetic chains that are arranged on a triangular lattice.
The intrachain and interchain couplings are ferromagnetic and
antiferromagnetic, respectively. This situation gives rise to geometrical
frustration, that bears some similarity to the classical problem of a
two-dimensional Ising triangular antiferromagnet. This paper reports on the
ordering process at low-T and the possibility of one-dimensional features at
high-T.Comment: 7 pages, 6 figures, accepted for publication in PR
Non-linear electrical response in a charge/orbital ordered CaMnO crystal : the charge density wave analogy
Non-linear conduction in a charge-ordered manganese oxide
PrCaMnO is reported. To interpret such a feature, it is
usually proposed that a breakdown of the charge or orbitally ordered state is
induced by the current. The system behaves in such a way that the bias current
may generate metallic paths giving rise to resistivity drop. One can describe
this feature by considering the coexistence of localized and delocalized
electron states with independent paths of conduction. This situation is
reminiscent of what occurs in charge density wave systems where a similar
non-linear conduction is also observed. In the light of recent experimental
results suggesting the development of charge density waves in charge and
orbitally ordered manganese oxides, a phenomenological model for charge density
waves motion is used to describe the non-linear conduction in
PrCaMnO. In such a framework, the non-linear conduction
arises from the motion of the charge density waves condensate which carries a
net electrical current.Comment: 13 pages, 6 figure
Magnetic Field Dependence of Electronic Specific Heat in Pr_{1.85} Ce_{0.15} CuO_4
The specific heat of electron-doped Pr_{1.85} Ce_{0.15} CuO_4 single crystals
is reported for the temperature range 2 - 10 K and magnetic field range 0 - 10
T. A non-linear magnetic field dependence is observed for the field range 0 - 2
T. Our data supports a model with lines of nodes in the gap function of these
superconductors. Theoretical calculations of the electronic specific heat for
dirty d-wave, clean d-wave, and s-wave symmetries are compared to our data.Comment: 10 pages Latex and 4 eps figures, submitted to Phys. Rev.
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