R. Ganguly

Magnetic and electrical properties of La0.67Ca0.33MnO3 as influenced by substitution of Cr

Abstract The compounds La 0.67 Ca 0.33 Mn 1− x Cr x O 3 ( x =0.00, 0.05, 0.10, 0.15, 0.20, 0.35, 0.50 and 0.60) have been studied by X-ray diffraction (XRD), AC and DC magnetization, and DC electrical resistivity measurement techniques. Ferromagnetism is observed up to as high as 60% of Cr substitution. Electrical resistivity behavior of the compounds as a function of temperature could be explained on the basis of adiabatic small polaron model.

Synthesis of surfactant encapsulated nickel hexacyanoferrate nanoparticles and deposition of their Langmuir-Blodgett film

Sodium hexametaphosphate (HMP) stabilized nickel hexacyanoferrate (NiHCF) nanoparticles were prepared in aqueous solution and were successfully extracted into an organic phase using cetyltrimethylammonium bromide (CTAB) as the surfactant. Dynamic Light Scattering (DLS) studies suggest that the average size of the nanoparticles is retained during the extraction process from the aqueous to the organic phase. X-Ray diffraction, cyclic voltammetry, IR spectroscopy and magnetic measurements carried on the organic phase shows specific signatures of the presence of the surfactant encapsulated NiHCF nanoparticles. Transmission Electron Microscopy (TEM) measurements show that the average size of these surfactant encapsulated nanoparticles in the organic phase is about 22 nm, and as has been suggested by DLS studies, it does not change with respect to repeated evaporation and re-extraction processes of the organic phase. Pressure–area isotherms of the organic phase in a Langmuir–Blodgett (LB) trough, with water as subphase, indicate stable monolayer formation of the surfactant-encapsulated NiHCF nanoparticles at the air–water interface. Multi-layered deposition of the surfactant-encapsulated nanoparticles onto an indium tin oxide (ITO) coated glass slide could also be carried out using the LB technique. Cyclic voltammetry studies on these LB multilayers confirm regular and systematic transfer of the NiHCF nanoparticles on the ITO substrate. The method described here is the first of its kind with respect to the synthesis of surfactant encapsulated molecular magnetic nanoparticles and subsequent deposition of their LB films.

Stability of the layered Sr3Ti2O7 structure in La1.2(Sr1-xCax)1.8Mn2O7

Samples with nominal compositions La1.2 (Sr1-x Cax )1.8 Mn2 O7 (x = 0.00, 0.25, 0.50, 0.65, 0.80 and 1.00) have been prepared and characterized by x-ray diffraction (XRD) and ac susceptibility techniques. Drastic changes in the XRD pattern and in the ac susceptibility versus temperature behaviour at x = 0.65 indicate that single phase compounds with layered Sr3 Ti2 O7 structure are formed only up to x = 0.50. Rietveld refinement of the XRD patterns of x = 1.00 composition suggests that the compositions with x 0.65 form multiphase mixtures comprising hole doped perovskite manganates as majority phases and CaO as the minority phase. These results are at variance with the literature reports that the compound La1.2 Ca1.8 Mn2 O7 forms with layered Sr3 Ti2 O7 structure.

Does the LaMnO3 phase accept Ce-doping?

Recent reports, that the samples of nominal compositions, La1-xCexMnO3, form single-phase compounds with orthorhombically distorted perovskite structure, are questionable. Our studies on the sample of nominal composition, La0.7Ce0.3MnO3, and careful analysis of the structural data available in the literature, suggest that the above mentioned samples actually form multi-phase mixtures comprising hole doped lanthanum deficient lanthanum manganate phases and cerium oxide (CeO2).

The magnetic properties of the hole-doped cobaltites R0.5A0.5CoO3 (R = La, rare earth and A = Ca, Sr, Ba)

The ac and dc magnetic properties of hole-doped cobaltites with compositions La0.5Sr0.5-xAxCoO3 (A = Ca, Ba) and La0.5-xRxSr0.5CoO3 (R = Nd, Eu) have been studied. The suppression of the cluster glass behaviour of the parent compound, La0.5Sr0.5CoO3, by substitutions for either the lanthanum or the strontium ions is accompanied by an increase in coercive field (HC) and thermomagnetic irreversibility. The results suggest that the large thermomagnetic irreversibility and coercive fields observed in the hole-doped perovskite cobaltites, arising due to the anisotropic nature of their ferromagnetic character, cannot be correlated with the cluster glass freezing observed in them.

Influence of the size of dopant ion on ferromagnetic behavior of Ln0.7A0.3CoO3 system [Ln=La, Nd; and A=Ca, (Ca, Sr), Sr, (Sr, Ba), Ba]

Abstract The compounds La 0.7 A 0.3 CoO 3 [A=Ca, (Ca, Sr), Sr, (Sr, Ba), Ba] and La 0.5 Nd 0.2 Sr 0.234 Ba 0.066 CoO 3 have been studied by X-ray diffraction, AC and DC magnetization and DC electrical resistivity techniques. DC magnetization studies on these compounds suggest that large thermomagnetic irreversibility observed in hole-doped cobaltates arises not due to cluster glass behavior but from ferromagnetic behavior with exchange anisotropy. It is also observed that the size of the ions occupying the rare earth site has significant effect on the structural parameters, ferromagnetic transition temperature and electrical resistivity of these compounds.

Magnetization behavior of (NBu4)2Mn2[Cu(opba)]3 and related solvated ferromagnets

Direct current magnetization studies are reported for the organic ferromagnet (NBu4)2Mn2[Cu(opba)]3 (TC=22 K), where opba stands for o‐phenylenebis (oxamato), and related compounds obtained by solvating it with dimethylsulphoxide, methanol, and acetonitrile, which have TC values of 15, 12, and 14.5 K, respectively. M(H) plots for the first two compounds show very little width and criss‐cross at low applied field values (<50 Oe). Zero field‐cooled magnetization and field‐cooled magnetization curves for (NBu4)2Mn2[Cu(opba)]3 nearly overlap and its remanence plot changes sign at 11.5 K, exhibiting a compensation behavior. Negative remanent magnetization has also been observed for the other three compounds. The results are discussed in terms of the ferrimagnetic nature of the building blocks.

Effect of Dy substitution for La in La0.7Ca0.3MnO3 perovskite

AC susceptibility, resistivity and dc magnetization measurements on the (La 0.757 Dy 0.243 ) 0.7 Ca 0.3 MnO 3 perovskite reveal a clear departure from the well-behaved (collinear) ferromagnetic nature observed in the parent material La 0.7 Ca 0.3 MnO 3 at low temperatures. Neutron diffraction structural study shows that the buckling of the MnO 6 octahedra increases with the Dy substitution. This in turn gives a weaker double exchange (ferromagnetic) interaction, as the transfer integral, t o, describing the hopping of eg electrons between Mn 3+ and Mn 4+ decreases resulting in the observed deterioration of ferromagnetism and metallic conduction.

The structural, magnetic and electrical properties of the hole-doped cobaltites La0.7(Ca1-xBax)0.3CoO3 (x = 0.0, 0.5 and 1.0)

We have studied the structural, magnetic and electrical properties of hole-doped cobaltites with compositions, La0.7(Ca1-xBax)0.3CoO3 (x = 0.0, 0.5 and 1.0). Due to the large difference in size between the Ca and Ba ions (size mismatch), these compounds show interesting changes in the magnetic and transport behaviours with an increase in the concentration of Ba. The distortion in the perovskite structure decreases with the substitution of Ba for Ca. Magnetic studies indicate that the anisotropy in their ferromagnetic characters, which contribute to the observations of large thermomagnetic irreversibilities and large coercive fields in these compounds, change significantly with the substitution of Ba for Ca. The unusual magnetic behaviour of the La0.7Ba0.3CoO3 compound, as reflected in its M-H behaviour, has been explained on the basis of a possible coexistence of different magnetic phases. Observations of the low-temperature resistivity minimum, and negative magnetoresistance up to 26% in the low-temperature insulating regions of these polycrystalline compounds, have been ascribed to grain boundary effects.

Electrical conductivity and magnetic behavior of La0.67Ca0.33MnO3 as influenced by substitution of Co

Abstract La 0.67 Ca 0.33 Mn 1− x Co x O 3 ( x =0.00, 0.05, 0.075, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40) have been investigated by AC susceptibility, DC magnetization and DC electrical resistivity measurements. Substitution of Co for Mn did not cause any change in crystal symmetry, yet electrical conductivity as well as ferromagnetism have been consistently suppressed by the dopant. The results can be explained on the basis of weakening of the double exchange process and an increase in anisotropy of the exchange interaction consequent upon Co doping.