J. Sannigrahi

Magnetic and electric properties of CaMn7O12 based multiferroic compounds: effect of electron doping

The mixed valent multiferroic compound CaMn7O12 is studied for its magnetic and electric properties. The compound undergoes magnetic ordering below 90 K with a helimagnetic structure followed by a low temperature magnetic anomaly observed around 43 K. This study shows that the magnetic anomaly at 43 K is associated with thermal hysteresis indicating the first order nature of the transition. The compound also shows field-cooled magnetic memory and relaxation below 43 K, although no zero-field-cooled memory is present. A clear magnetic hysteresis loop is present in the magnetization versus field measurements, signifying the presence of some ferromagnetic clusters in the system. We doped trivalent La at the site of divalent Ca expecting to enhance the fraction of Mn(3+) ions. The La doped samples show reduced magnetization, although the temperatures associated with the magnetic anomalies remain almost unaltered. Interestingly, the spontaneous electrical polarization below 90 K increases drastically on La substitution. We propose that the ground states of the pure as well as the La doped compositions contain isolated superparamagnetic like clusters, which can give rise to metastability in the form of field-cooled memory and relaxation. The ground state is certainly not spin glass type, as is evident from the absence of zero-field-cooled memory and frequency shift in the ac susceptibility measurements.

Aromatic bi-, tri- and tetracarboxylic acid doped polyaniline nanotubes: effect on morphologies and electrical transport properties

Polyaniline nanotubes have been prepared in a facile manner via the chemical polymerization of aniline in an aqueous medium using ammonium persulphate (APS) and ten different dopant acids. The formation of PANI in the composite was confirmed using 1H-NMR, UV-Vis and FTIR spectroscopic studies. An intensive FESEM investigation indicated that the dopant acid plays an important role in the formation of the PANI nanotubes. In particular, the symmetrical positioning of –COOH groups in the aromatic dopant acid is key for forming PANI nanotubes. Symmetrical BTA-1 produces PANI nanotubes, whereas BTA-2 and BTA-3 only generate granular aggregates of PANI. The diameter of the observed nanotubes varies with the molar ratio of BTA-1 to aniline and the diameter of the nanotubes increases with the increasing number of –COOH groups in dopant acids that have the same central core. A comparative study with other tricarboxylic acids shows that the outer diameter (OD) of the PANI nanotubes depends on the size of the dopant under identical reaction conditions and follows the order: ODTPCA > ODBPCA > ODBTA-1. Low temperature charge transport in these composites mainly follows a three dimensional variable range hopping mechanism.

Two dimensional magnetic correlation in the unconventional corrugated layered oxides (Ba,Sr)4Mn3O10

Both Ba4Mn3O10 and Sr4Mn3O10 crystallize in an orthorhombic crystal structure consisting of corrugated layers containing Mn3O12 polydedra. The thermal variation of magnetic susceptibility of the compositions consists of a broad hump like feature indicating the presence of low dimensional magnetic correlation. We have systematically investigated the magnetic data of these compounds and found that the experimental results match quite well with the two dimensional Heisenberg model of spin-spin interaction. The two dimensional nature of the magnetic spin-spin interaction is supported by the low temperature heat capacity data of Ba4Mn3O10. Interestingly, both the samples show dielectric anomaly near the magnetic ordering temperature indicating multiferroic behavior.

Anomalous transport properties of graphene oxide magnetic ferrite nanocomposites

Magnetic zinc ferrite (ZnFe2O4) nanoparticles (MZFN) and Graphene Oxide (GO) composites were prepared through low temperature reaction process. X-ray Diffraction and transmission electron microscopic studies of the individuals constituents reveal the crystalline nanocomposite nature. Anomalous transport properties with metal like to thermally activated semiconducting transition is observed around 160K for a typical MZFN/GO ratio (1: 0.25) which is explained by considering the formation of percolated GO clusters.

Metal-semiconductor transition and ferroelectricity in charge ordered La0.5Ba0.5FeO3 perovskite

The charge disproportionate correlated electronic system La0.5Ba0.5FeO3; synthesized by conventional solid state reaction route, exhibits novel properties. The sample undergoes semiconductor-metal type transition at TSM=102K. A crossover of electrical conduction mechanism from variable range hopping to small polaron hopping occurs around 210K, well above TSM. Analysis of the transport, magnetic susceptibility and magnetization data in the light of De Gennes proposal indicates the formation of canted antiferromagnetic spin order preceded by charge ordering in La0.5Ba0.5FeO3 perovskite. The observed resistive transition is considered to be associated with the onset of weak ferromagnetic ordering in the canted antiferromagnetic spin state. Another noteworthy feature is the observation of Polarization-Electric field (P-E) hysteresis loop signifying the appearance of ferroelectricity in the charge ordered state.