A. Sendil Kumar

Surface driven effects on magnetic properties of antiferromagnetic LaFeO3 nanocrystalline ferrite

LaFeO3 nanocrystalline ferrites were synthesized through sol-gel method in different size distributions and the effect of finite size on magnetic properties is investigated. Results of magnetization and Mossbauer measurements show that superparamagnetism and weak ferromagnetic behavior in some of the size distributions. The origin of the superparamagnetism is from fine particles similar to ferromagnetic single domains and the weak ferromagnetism comes from surface spin disorder caused by Dzyaloshinskii-Moriya interaction. The magnetic ground state of LaFeO3 nanoparticles differs from that of bulk, and the ground state is dictated by the finite size effect because density of states depends on the dimensionality of the sample.

FMR studies and magnetic properties of Mg doped La0.67Ca0.33MnO3

Temperature dependence of Ferromagnetic Resonance (FMR) and DC magnetization measurements were carried out on Mg doped orthorhombic La0.67Ca0.33MnO3manganite. All FMR spectra are Lorentzian lineshape, however, inhomogeneous broadening with asymmetry in the signal is observed below and near Curie temperature Tc. The resonance field below TΔHPP* decreases with decreasing temperature. The temperature dependence of FMR signal intensity follows the Arrhenius behavior.

Spontaneous exchange bias effect in superparamagnetic LaFeO3 nanoparticles

Observation of spontaneous exchange bias effect in superparamagnetic LaFeO3 nanoparticles is reported in this work. Magnetization vs temperature exhibits a minimum at ~ 35 K in zero field cooled condition and below this temperature, magnetization increases sharply. At room temperature nanoparticles are non-interacting to each other which behave as ‘a pure superparamagnet’ and when temperature is reduced the system is frozen into ‘the super-spin glass state’. The core shell model is employed to explain the phenomenon of the exchange bias. Relatively large size LaFeO3 nanoparticles behave as antiferromagnetic cores and small size LaFeO3 nanoparticles behave as weak ferromagnetic shells. The interface of the core shell nanostructure experiences the pinning of weak ferromagnetic spin which induces the unidirectional anisotropy which may be the origin for the shift in hysteresis and the observed exchange bias.Graphical Abstract

Magnetism from Fe 2 O 3 nanoparticles embedded in amorphous SiO 2 matrix

Fe2O3 nanoparticles are embedded in amorphous SiO2 matrix by coprecipitation method with varying concentrations. Conditions are optimized to get almost monodispersed Fe2O3 nanoparticles with high chemical stability. Microstructure of synthesized nanoparticles is well characterized and found that Fe2O3 is in nanocrystalline form and embedded uniformly in amorphous SiO2 matrix. Enhanced surface reactivity is found for nanoparticles which influences physical properties of the SiO2 supported Fe2O3 system due to adsorption. In oxide nanoparticles, significant number of defect sites at the surface is expected but when supported medium such as SiO2 it reduces this defect concentration. Field- and temperature-dependent magnetisation studies on these samples show superparamagnetic behaviour. Superparamagnetic behaviour is seen in all the concentration systems but the coercivity observed in the lower concentration systems is found to be anomalous compared to that of higher concentrations. The observed magnetic behaviour comes from either unsaturated bond existing due to the absence of anions at the surface of nanoparticles or reconstruction of atomic orbitals taking place at interface of Fe2O3–SiO2 system.

ESR and dielectric studies on superparamagnetic LaFeO3 nanoparticles

Superparamagnetic LaFeO3 nanoparticles are synthesized through sol gel method. Structural, magnetic and dielectric studies are carried out. Temperature dependent ESR studies show that the intensity of the ESR spectra increases at higher temperatures. The line shape at low temperature has inhomogeneous broadening and at higher temperature more symmetry is developed and it fits better with the Lorentzian. Resonance field decreases when temperature is lowered due to dipolar interactions and there are no extrema in linewidth are observed. Single semi-circular arc in the Cole-Cole plot shows that the AC conductivity is from grains only.

AC conductivity studies of La doped Ba0.5Sr0.5TiO3

Ferroelectric material with high dielectric constant of Ba0.5Sr0.5TiO3 is synthesized through Solid State Reaction and fraction of Lanthanum is substituted to introduce hole concentration. XRay Diffraction shows all the samples are stabilized in cubic crystal structure. With La doped samples the Cole-Cole plot is modified and AC conductivity increases at higher temperatures as well as higher frequencies compared to undoped sample.

Synthesis and Magnetic Properties of FeTiO 3 Nanoparticles

This paper reports the synthesis and magnetic properties of single phase FeTiO3 nanoparticles. Ilmenite nanoparticles are prepared through coprecipitation method. As prepared powder is amorphous in nature. When annealed inair and inert atmosphere, we get almost single phase pure crystalline FeTiO3. Our results show that FeTiO3 in nanocrystalline form behaves like a weak ferromagnet below ~50 K while it is known that ilmenite is an antiferromagnet below ~55 K.