Esa Bose

Magnetocaloric effect in charge ordered Nd0.5Ca0.5MnO3 manganite

The magnetocaloric effect (MCE) has been observed in three well-defined temperature intervals (region I: 2–115 K, region II: around charge order transition TCO, and region III: around room temperature) in polycrystalline Nd0.5Ca0.5MnO3 (NCMO) manganite system showing a first order (CO) transition at a temperature, TCO∼250 K. The magnitude of ΔSM(H) increases monotonically with applied magnetic field but does not reach saturation even at fields as high as 60 kOe. In the three given temperature regions, negligible magnetic and thermal hysteresis are found, which satisfies the requirements of using NCMO as an effective magnetocaloric material for magnetic refrigeration.

Small polaron hopping conduction and magnetic frustration in the electron-doped charge ordered Ca0.85La0.15MnO3 system

The electron-doped Ca0.85La0.15MnO3 system exhibits a charge ordering transition at ~135 K (TCO) with the antiferromagnetic Neel temperature at 160 K (TN). The resistivity data above and below TN (down to 90 K) are found to follow the small polaron hopping conduction mechanism. The corresponding activation energy, estimated from the theoretical fitting of resistivity data, shows an anomalous increase below TN, supporting the aforementioned charge ordering phenomenon. This result is further confirmed from fitting of the current–voltage (I–V) data with the small polaron hopping model. As revealed from the field cooled and zero field cooled magnetization measurements, this electron-doped system exhibits the signature of magnetic frustration, below TCO associated with phase separation behavior.

Anomalous thermal properties and local lattice effects in low doped manganites La0.875Ca0.125MnO3 and La0.875 Ca0.0625 Sr0.0625MnO3

The results of a comparative study of the magnetic, transport and thermal properties of two ferromagnetic La0.875Ca0.125MnO3(LCMO) and La0.875Ca0.0625Sr0.0625MnO3 (LCSMO) samples with the same Mn3+/Mn4+ ratio and identical crystallographic structure have been presented. While LCSMO exhibits magnetic transition accompanied by an insulator to metal transition, no such transition is observed in LCMO. Moreover, unlike LCMO, only in the case of LCSMO, is a sudden drop in the thermoelectric power S(T) along with an enhancement in the value of thermal conductivity λ(T) observed below the Curie temperature (TC). This confirms that not only the Mn3+/Mn4+ ratio but also local lattice effects and carrier transport play a vital role in determining in the physical properties of these low doped manganite systems.

Metal-insulator transition and non-adiabatic small polaron hopping conduction in electron-doped Ca0.85Pr0.15MnO3 manganite

We report the detailed analysis of magnetic-field-dependent electrical transport properties of Ca0.85Pr0.15MnO3 (CPMO) over a wide temperature range (5–300 K) in absence and presence of applied magnetic fields as high as 15 T. The resistivity undergoes a metal-insulator (M-I) transition. We report the effect of magnetic field on the transition temperature that decreases with the applied field. The M-I transition in this compound may be due to the spin-state transition from a high spin state to a low spin state of Mn3+ ions. Temperature-dependent resistivity fitting above TMI (M-I transition temperature) suggests that electrical-conductivity carriers are mainly scattered by the electron–phonon interaction. In the semiconducting region, the conduction mechanism follows nonadiabatic, small-polaron hopping (SPH) above θD/2 (θD is the Debye temperature) and Mott-type variable range hopping (VRH) below θD/2. It has also been found that the density of states (DOS) at the Fermi level increases, whereas the hopping energy Whop decreases with external magnetic field.

Nonlinear behavior and universal scaling of ac conductivity in (100???x) BaTiO3/(x) La0.7A0.3MnO3 (A?=?Pb/Ca) composites

We have measured the ac conductance ? (x,f) of the ferroelectric?ferromagnetic (100???x) BaTiO3/(x) La0.7A0.3MnO3 (A?=?Pb/Ca) composites as a function of frequency (f) by varying x in wt% (20, 30, 40 and 50%) at a constant temperature T (300?K). ? (x,f) of the sample with fixed x remains at an almost constant dc value ? 0 at a lower frequency at a fixed temperature T. With the increase of f, ? (x,f) increases slowly from ? 0. Finally, ? (x,f) increases rapidly with ffollowing a power law with an exponent n in the high frequency regime. In the scaled coordinate, the data for ?(x, f) fall on the same universal master curve, indicating the existence of a general scaling formalism for the ac conductivity in composite systems. The characteristic frequency fc scales with ? 0 as fc???? (0,x)xf , where xf is the onset exponent. We have also shown that fc, scaling formalism of ac conductivity data can be successfully applied in insulating/conducting composite systems irrespective of the conducting phase.

Current switching and slow magnetic relaxation behaviour in phase-separated Ca0.85La0.15MnO3 system showing colossal electroresistance

From the study of current–voltage (I–V) characteristics, current switching (at temperatures ≤100 K) has been observed in the electron doped charge-ordered polycrystalline manganite Ca0.85La0.15MnO3. The threshold switching voltage is found to decrease with an increase in temperature. The sample also shows colossal electroresistance (ER) when resistance is measured using various measuring currents. The ER, interestingly, undergoes an unusual sign change from negative to positive at a temperature (Tt) ascending with increasing applied current. These effects are analysed in terms of electric field induced collapse of the charge-ordered state and modification of percolative conduction by the applied field, within the phase separation scenario. The phase separation hypothesis has been verified by magnetic relaxation measurements.

Study of magnetic and dielectric properties of polycrystalline Dy0.8La0.2Mn2O5

Polycrystalline Dy0.8La0.2Mn2O5 samples were prepared by standard solid state reaction method. Reitveld analysis of the X-ray Diffraction data confirmed single phase of the samples. Here, we are reporting the dielectric and magnetic properties of the Dy0.8La0.2Mn2O5 sample. It showed ferromagnetic ordering around 243K, a wide plateau like region in the magnetic susceptibility χ vs temperature plot was observed. Whether this region arises due to the presence of ferromagnetic clusters within the paramagnetic matrix or any other phase can only be confirmed by further study, which is at present going on. Magnetic field dependent dielectric properties were studied in wide temperature range. However no change in the dielectric constant was observed even in the high magnetic field ∼15T. Field dependent magnetization data within the aforementioned plateau region show no hysteresis. The modification in the magnetic properties could be attributed to the strengthening of Mn3+-O-Mn4+ double exchange interaction which...

Magnetization reversal properties of Pr1-x(Gd/Nd)xMnO3 (x=0.3, 0.5, 0.7)

We report measurements of the temperature dependent magnetic properties of single phase orthorhombic perovskites system associated with space group Pbnm compounds Pr1-x(Gd/Nd)xMnO3 (x=0.3, 0.5, 0.7). Magnetic properties radically changes with the doping of Gd or Nd. A magnetization reversal is observed below the Neel temperature (TN), in DC magnetization measurements (at 50 Oe) in the doped compounds. The reversal of magnetization may be due to the antiparallel coupling between the two magnetic sublattices (|Pr+ Gd/ Nd | and Mn). The hysteresis plot taken at 50K indicates a ferrimagnetic characteristic and existence of spin canting of ions in the magnetic sublattices.

Effects of rare-earth doping on transport properties in isoelectronic perovskite manganites

In this work, we explore the temperature dependence transport properties of isoelectronic manganites Ca0.85Re0.15MnO3 (Re = La, Pr, Nd, Gd). Substitution of Re3+ for Ca2+ will bring Mn3+ ions and hopping between Mn3+ and Mn4+ facilitated by double exchange (DE) interactions is responsible for augmentation the electrical conductivity. Resistivity shows insulating behavior with an inflection near charge ordering temperature (TCO). ρ (T) follows Mott-type variable range hopping (VRH) for all the samples in the low-T region.

Study of Dielectric Relaxation at Room Temperature of the Composite (100-x) LCMO/(x) BTO

The frequency dependent complex impedance and dielectric behavior of (100-x) La0.7Ca0.3MnO3 (LCMO)/(x) BaTiO3 (BTO) has been investigated at room temperature (300K). Separate contributions from various factors such as bulk effect, grain boundary and interface effect have been established by fitting the cole-cole function of real and imaginary part of complex impedance. Dielectric data were well fitted in the frequency domain with modified Curie-Von Schweider (CS) functions. Both formalisms support a non-Debye relaxation process in the composites.