Parthasarathi Mondal

Particle size dependence of magnetization and phase transition near TN in multiferroic BiFeO3

We report results of a comprehensive study of the phase transition at TN (∼643K) as a function of particle size in multiferroic BiFeO3 system. We employed electrical, thermal, and temperature dependent x-ray diffraction studies in order to characterize the transition in a host of samples. We also carried out detailed magnetic measurements over a temperature regime of 2–300K under a magnetic field of 100–10000Oe both on bulk and nanocrystalline systems. While in the bulk system a sharp endothermic peak at TN together with a broad feature, ranging over nearly ∼100K (ΔT), could be observed in calorimetry, the nanoscale systems exhibit only the broad feature. The characteristic dielectric anomaly, expected at TN, is found to occur both at TO and TN across ΔT in the bulk sample. The Maxwell-Wagner component due to interfaces between heterogenous regions with different conductivities is also present. The magnetic properties, measured at lower temperature, corroborate our observations in calorimetry. The metasta...

Particle-size dependence of orbital order-disorder transition in LaMnO3

The latent heat (L) associated with the orbital order-disorder transition at TJT 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 dc≈19 nm. The transition temperature TJT 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 TJT 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.

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) that shows a steep enhancement on cooling. While at lower temperatures the transition is sharp and involves a large change in resistance, it softens on heating and is eventually absent above 280 K. When the Mn(4+) content is increased by Sr substitution up to x = 0.1, the observed transition, although observable in a certain temperature range, softens considerably. This observation has been explained as a bias driven percolation type transition between two co-existing 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 as f = fo(E)exp(-Δ/kBT) with the activation energy Δ ≈ 200 meV, and the pre-factor fo(E) 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.

Current-driven orbital order-disorder transition in LaMnO(3)

We report a significant influence of electric current on the orbital order-disorder transition in LaMnO(3). 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 an increase in current density. Eventually, at a critical current density, L reaches zero. The transition zone, on the other hand, broadens with an increase in current density. The states at ordered, disordered, and transition zones are all found to be stable within the time window from similar to 10(-3) to similar to 10(4) s.

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 dC 20 nm. The orbital order–disorder transition switches from reversible to irreversible at dC while the resistance in the orbital ordered state decays by 2%–4% over a time scale of ∼3000s. At well below dC, 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 dC (e.g., at ∼10 nm), the structure becomes cubic (space group Pm3−m). 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.