122 research outputs found
Specific heat and magnetocaloric effect in Pr1-xAgxMnO3 manganites
The magnetocaloric effect in alternating magnetic fields has been
investigated in Pr1-xAgxMnO3 manganites with x=0.05-0.25. The stepwise reversal
of the sign of the magnetocaloric effect has been revealed in a weakly doped
sample (x=0.05) at low temperatures (~80 K). This reversal is attributed to the
coexistence of the ferromagnetic and canted antiferromagnetic phases with
different critical temperatures.Comment: 4 pages, 4 figure
Phase Separation and the Low-Field Bulk Magnetic Properties of Pr0.7Ca0.3MnO3
We present a detailed magnetic study of the perovskite manganite
Pr0.7Ca0.3MnO3 at low temperatures including magnetization and a.c.
susceptibility measurements. The data appear to exclude a conventional spin
glass phase at low fields, suggesting instead the presence of correlated
ferromagnetic clusters embedded in a charge-ordered matrix. We examine the
growth of the ferromagnetic clusters with increasing magnetic field as they
expand to occupy almost the entire sample at H ~ 0.5 T. Since this is well
below the field required to induce a metallic state, our results point to the
existence of a field-induced ferromagnetic insulating state in this material.Comment: 15 pages with figures, submitted to Physical Review
X-ray Resonant Scattering Studies of Orbital and Charge Ordering in PrCaMnO
We present the results of a systematic x-ray scattering study of the charge
and orbital ordering in the manganite series PrCaMnO with
=0.25, 0.4 and 0.5. The temperature dependence of the scattering at the
charge and orbital wavevectors, and of the lattice constants, was characterized
throughout the ordered phase of each sample. It was found that the charge and
orbital order wavevectors are commensurate with the lattice, in striking
contrast to the results of earlier electron diffraction studies of samples with
=0.5. High momentum-transfer resolution studies of the x=0.4 and 0.5 samples
further revealed that while long-range charge order is present, long-range
orbital order is never established. Above the charge/orbital ordering
temperature T, the charge order fluctuations are more highly correlated
than the orbital fluctuations. This suggests that charge order drives orbital
order in these samples. In addition, a longitudinal modulation of the lattice
with the same periodicity as the charge and orbital ordering was discovered in
the x=0.4 and 0.5 samples. For x=0.25, only long-range orbital order was
observed with no indication of charge ordering, nor of an additional lattice
modulation. We also report the results of a preliminary investigation of the
loss of charge and orbital ordering in the x=0.4 sample by application of a
magnetic field. Finally, the polarization and azimuthal dependence of the
charge and orbital ordering in these compounds is characterized both in the
resonant and nonresonant limits, and compared with the predictions of current
theories. The results are qualitatively consistent with both cluster and LDA+U
calculations of the electronic structure.Comment: 37 pages, 22 figure
Formation of finite antiferromagnetic clusters and the effect of electronic phase separation in Pr{_0.5}Ca{_0.5}Mn{_0.975}Al{_0.025}O{_3}
We report the first experimental evidence of a magnetic phase arising due to
the thermal blocking of antiferromagnetic clusters in the weakened charge and
orbital ordered system Pr{_0.5}Ca{_0.5}Mn{_0.975}Al{_0.025}O{_3}. The third
order susceptibility (\chi_3) is used to differentiate this transition from a
spin or cluster glass like freezing mechanism. These clusters are found to be
mesoscopic and robust to electronic phase separation which only enriches the
antiphase domain walls with holes at the cost of the bulk, without changing the
size of these clusters. This implies that Al substitution provides sufficient
disorder to quench the length scales of the striped phases.Comment: 4 Post Script Figure
A Resonant X-ray Scattering Study of Octahedral Tilt Ordering in LaMnO and PrCaMnO
We report an x-ray scattering study of octahedral tilt ordering in the
manganite series PrCaMnO with x=0.4 and 0.25 and in LaMnO.
The sensitivity to tilt ordering is achieved by tuning the incident x-ray
energy to the L, L and L absorption edges of Pr and La,
respectively. The resulting energy-dependent profiles are characterized by a
dipole-resonant peak and higher energy fine structure. The polarization
dependence is predominantly -to- and the azimuthal dependence
follows a sin-squared behavior. These results are similar to those obtained in
recent x-ray scattering studies of orbital ordering carried out in these same
materials at the Mn K edge. They lead to a description of the cross-section in
terms of Templeton scattering in which the tilt ordering breaks the symmetry at
the rare earth site. The most interesting result of the present work is our
observation that octahedral tilt ordering persists above the orbital ordering
transition temperatures in all three samples. Indeed, we identify separate
structural transitions which may be associated with the onset of orbital and
tilt ordering, respectively, and characterize the loss of tilt ordering versus
temperature in LaMnO.Comment: 24 pages, 8 figure
Correlated Polarons in Dissimilar Perovskite Manganites
We report x-ray scattering studies of broad peaks located at a (0.5 0 0)/(0
0.5 0)-type wavevector in the paramagnetic insulating phases of
La_{0.7}Ca_{0.3}MnO_{3} and Pr_{0.7}Ca_{0.3}MnO_{3}. We interpret the
scattering in terms of correlated polarons and measure isotropic correlation
lengths of 1-2 lattice constants in both samples. Based on the wavevector and
correlation lengths, the correlated polarons are found to be consistent with
CE-type bipolarons. Differences in behavior between the samples arise as they
are cooled through their respective transition temperatures and become
ferromagnetic metallic (La_{0.7}Ca_{0.3}MnO_{3}) or charge and orbitally
ordered insulating (Pr_{0.7}Ca_{0.3}MnO_{3}). Since the primary difference
between the two samples is the trivalent cation size, these results illustrate
the robust nature of the correlated polarons to variations in the relative
strength of the electron-phonon coupling, and the sensitivity of the
low-temperature ground state to such variations.Comment: 13 pages, 6 figure
Multiphase segregation and metal-insulator transition in single crystal La(5/8-y)Pr(y)Ca(3/8)MnO3
The insulator-metal transition in single crystal La(5/8-y)Pr(y)Ca(3/8)MnO3
with y=0.35 was studied using synchrotron x-ray diffraction, electric
resistivity, magnetic susceptibility, and specific heat measurements. Despite
the dramatic drop in the resistivity at the insulator-metal transition
temperature Tmi, the charge-ordering (CO) peaks exhibit no anomaly at this
temperature and continue to grow below Tmi. Our data suggest then, that in
addition to the CO phase, another insulating phase is present below Tco. In
this picture, the insulator-metal transition is due to the changes within this
latter phase. The CO phase does not appear to play a major role in this
transition. We propose that a percolation-like insulator-metal transition
occurs via the growth of ferromagnetic metallic domains within the parts of the
sample that do not exhibit charge ordering. Finally, we find that the
low-temperature phase-separated state is unstable against x-ray irradiation,
which destroys the CO phase at low temperatures.Comment: 9 pages, 9 encapsulated eps figure
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