33 research outputs found
Electric field driven destabilization of the insulating state in nominally pure LaMnO3
We report an electric field driven destabilization of the insulating state in
nominally pure LaMnO3 single crystal with a moderate field which leads to a
resistive state transition below 300 K. The transition is between the
insulating state in LaMnO3 and a high resistance bad metallic state that has a
temperature independent resistivity. The transition occurs at a threshold field
(Eth) which shows a steep enhancement on cooling. While at lower temperatures
the transition is sharp and involves large change in resistance but it softens
on heating and eventually absent above 280K. When the Mn4+ content is increased
by Sr substitution up to x=0.1, the observed transition though observable in
certain temperature range, softens considerably. The observation has been
explained as bias driven percolation type transition between two coexisting
phases, where the majority phase is a charge and orbitally ordered polaronic
insulating phase and the minority phase is a bad metallic phase. The mobile
fraction f of the bad metallic phase deduced from the experimental data follows
an activated kinetics with the activation energy nearly equal to 200 meV and
the prefactor fo is a strong function of the field that leads to a rapid
enhancement of f on application of field leading to the resistive state
transition. We suggest likely scenarios for such co-existing phases in
nominally pure LaMnO3 that can lead to the bias driven percolation type
transition.Comment: Accepted in JPC
Dielectric anomaly at the orbital order-disorder transition in LaMnO3+δ
We report a novel dielectric anomaly around the Jahn-Teller orbital orderdisorder transition temperature TJT in LaMnO3+δ. The transition has been characterized by resistivity (ρ) vs. temperature (T), calorimetry, and
temperature dependent X-ray diffraction studies. Measurements of complex dielectric permittivity ε* (= ε'+ iε") over a low frequency range (1 Hz -10 MHz) across TJT reveal distinct anomaly. This observation and the reported
relatively high static dielectric constant at T = 0 (ε0 ~ 18-20), possibly, indicate that the orbital order gives rise to intrinsic polarization that undergoes transition at TJT. The frequency dispersion of the dielectric response at any given temperature, however, reveals that the dielectric response consists of Maxwell-Wagner component, due to interfaces, within such low frequency range. The TJT and the nature of the anomaly in ε'(ω,T), ε"(ω,T) at TJT, of
course, vary - from sharp upward feature to a smeared plateau and then a downward trend - depending on the Mn4+ concentration of the sample. The observation of intrinsic dielectric response due to long-range orbital order in
LaMnO3 - where no ferroelectric order is possible due to the absence of offcenter distortion in MnO6 octahedra - may throw a new light on these classes of materials vis-à-vis multiferroic materials
Current-driven orbital order-disorder transition in LaMnO3
We report significant influence of electric current on the orbital
order-disorder transition in LaMnO3. The transition temperature T_OO, thermal
hysteresis in the resistivity (rho) versus temperature (T) plot around T_OO,
and latent heat L associated with the transition decrease with the increase in
current density. Eventually, at a critical current density, L reaches zero. The
transition zone, on the other hand, broadens with the increase in current
density. The states at ordered, disordered, and transition zone are all found
to be stable within the time window from ~10^-3 to ~10^4 seconds.Comment: 7 pages including 5 figures; resolution of Fig.1 is better here than
the published versio
Dielectric anomaly at the orbital order-disorder transition in LaMnO_(3+delta)
We report a novel dielectric anomaly around the Jahn-Teller orbital
order-disorder transition temperature T_JT in LaMnO_(3+delta). The transition
has been characterized by resistivity (rho)versus temperature (T), calorimetry,
and temperature-dependent X-ray diffraction studies. Measurements of complex
dielectric permittivity epsilon* (= epsilon'-i.epsilon'') over a low-frequency
range (1 Hz - 10 MHz)across T_JT reveal a distinct anomaly. This observation,
and the reported relatively high static dielectric constant at T = 0 (epsilon0
\~18-20), possibly indicate that the orbital order gives rise to intrinsic
polarization that undergoes transition at T_JT. The frequency dispersion of the
dielectric response at any given temperature, however, reveals that the
dielectric response consists of Maxwell-Wagner component, due to interfaces,
within such a low frequency range. The T_JT and the nature of the anomaly in
epsilon'(omega,T), epsilon''(omega,T) at T_JT, of course, vary - from a sharp
upward feature to a smeared plateau and then a downward trend - depending on
the Mn^4+ concentration of the sample. The observation of an intrinsic
dielectric response due to long-range orbital order in LaMnO_3 - where no
ferroelectric order is possible due to the absence of off-centre distortion in
MnO_6 octahedra - may throw a new light onto these classes of materials
vis-a-vis multiferroic materials.Comment: 22 pages including 7 figures; pdf only; accepted for publication in
J.Phys.:Condens. Matte
Evolution of orbital phases with particle size in nanoscale stoichiometric LaMnO3
The thermodynamically stable long-range orbital order in bulk LaMnO3 becomes
metastable at nanoscale around a critical particle size d_C~20 nm. The orbital
order-disorder transition switches from reversible to irreversible at d_C while
the resistance in the orbital ordered state decays by 2-4% over a time scale of
~3000s. At well below d_C, of course, a stable orbital disordered phase
emerges. The orthorhombic distortion of the underlying crystallographic
structure (space group Pbnm) decreases systematically with the decrease in
particle size and at far below d_C (e.g., at ~10 nm), the structure becomes
cubic (space group Pm-3m). Using the crystallographic and electrical resistance
data, a phase diagram has been constructed showing the evolution of different
orbital phases as a function of particle size across ~10 nm to bulk for
stoichiometric LaMnO3.Comment: revised following referees' comments, accepted for publicaton in J.
Appl. Phy
Dielectric anomaly at T_N in LaMnO3
We observe a distinct anomaly in dielectric permittivity (epsilon') as well
as relaxation time (tao_0) versus temperature (T) pattern at the
antiferromagnetic transition point (T_N) in single crystal of LaMnO3. The
equivalent circuit analysis of the impedance spectra across T_N reveals clear
anomaly in the capacitive component C_0 at T_N. Since no structural transition
takes place across T_N, the anomaly in tao_0 and C_0 at T_N possibly signifies
multiferroicity stemming from coupling between orbital and spin order in
LaMnO3.Comment: 14 pages including 3 figures; pdf only; accepted for publication in
Phys. Rev.
Particle-size dependence of orbital order-disorder transition in LaMnO3
The latent heat (L) associated with the orbital order-disorder transition at
T_JT is found to depend significantly on the average particle size (d) of
LaMnO3. It rises slowly with the decrease in d down to ~100 nm and then jumps
by more than an order of magnitude in between d ~ 100 nm and ~30 nm. Finally, L
falls sharply to zero at a critical particle size d_c ~ 19 nm. The transition
temperature T_JT also exhibits an almost similar trend of variation with the
particle size, near d ~ 30 nm and below, even though the extent of variation is
relatively small. The zero-field-cooled (ZFC) and field-cooled (FC)
magnetization versus temperature study over a temperature range 10-300 K
reveals that the antiferromagnetic transition temperature decreases with d
while the temperature range, over which the ZFC and FC data diverge, increases
with the drop in d. The FC magnetization also is found to increase sharply with
the drop in particle size. A conjecture of nonmonotonic variation in orbital
domain structure with decrease in particle size - from smaller domains with
large number of boundaries to larger domains with small number of boundaries
due to lesser lattice defects and, finally, down to even finer domain
structures with higher degree of metastability - along with increase in surface
area in core-shell structure, could possibly rationalize the observed L versus
d and T_JT versus d patterns. Transmission electron microscopy data provide
evidence for presence of core-shell structure as well as for increase in
lattice defects in finer particles.Comment: 26 pages including 5 figures; pdf only; accepted for publication in
Phys. Rev.