K. P. Rajeev

Signatures of spin-glass freezing in NiO nanoparticles

We present a detailed study of the magnetic properties of sol-gel prepared nickel oxide nanoparticles of different sizes. We report various measurements such as frequency, field, and temperature dependence of ac susceptibility, temperature and field dependence of dc magnetization, and time decay of thermoremanent magnetization. Our results and analysis show that the system behaves as a spin glass.

Low-temperature electronic properties of a normal conducting perovskite oxide (LaNiO3)

Abstract We report here the measurements of specific heat (10 K >T > 1.5 K), thermopower (300 K >T > 4 K), electrical conductivity (300 K >T > 0.4 K) and tunneling conductance (T = 4.2 K) of the perovskite oxide metal LaNiO3. We show that this oxide metal has a fairly large electron-electron interaction which gives rise to a T 1 2 correction to the conductivity for T V 1 2 correction to the single particle density of states near the Fermi energy. The strong interaction effects in this metal arise from the low electron diffusivity.

Memory and aging effects in NiO nanoparticles.

We report studies on magnetization dynamics in NiO nanoparticles of average size 5 nm. Temperature and time dependence of dc magnetization, wait time dependence of magnetic relaxation (aging) and memory phenomena in dc magnetization are studied with various temperature and field protocols. We observe that the system shows memory and aging in field-cooled and zero-field-cooled magnetization measurements. These experiments show that the magnetic behavior of NiO nanoparticles is similar to spin glasses. We argue that the spin glass behavior originates from the freezing of spins at the surface of the individual particles.

Effect of oxygen stoichiometry on the electrical resistivity behaviour of NdNiO3−δ

Abstract The effect of oxygen deficiency on the electrical resistivity of NdNiO3−δ(0.08 A nonzero conductivity was observed at T=0 K which shows that the material is not fully insulating and a large amount of metallic phase coexists with the insulating phase.

Anomalous magnetic behavior of CuO nanoparticles

Abstract We report studies on temperature, field and time dependence of magnetization on cupric oxide nanoparticles of sizes 9 nm, 13 nm and 16 nm. The nanoparticles show unusual features in comparison to other antiferromagnetic nanoparticle systems. The field cooled (FC) and zero field cooled (ZFC) magnetization curves bifurcate well above the Neel temperature and the usual peak in the ZFC magnetization curve is absent. The system does not show any memory effects which is in sharp contrast to the usual behavior shown by other antiferromagnetic nanoparticles. It turns out that the non-equilibrium behavior of CuO nanoparticles is very strange and is neither superparamagnetic nor spin glass like.

Heterogeneous nucleation and metal-insulator transition in epitaxial films of NdNiO3

We have investigated the temperature driven first order metal-insulator (M-I) transition in thin films of NdNiO3 and have compared it with the bulk behavior. The M-I transition of thin films is sensitive to epitaxial strain and its partial relaxation creates an inhomogeneous strain field in the films which broadens the M-I transition. Both the thin film and the bulk samples exhibit nonequilibrium features in the transition regime which are attributed to the presence of high temperature metallic phases in their supercooled state. The degree of supercooling in the thin films is found to be much smaller than in the bulk which suggests that the M-I transition in the thin film occurs through heterogeneous nucleation.

Electrical transport in

We report a systematic study of the effect of increasing oxygen deficiency on the electrical resistivity, tunnelling conductance and thermopower of . Stoichiometric behaves like a normal metal with a positive temperature coefficient of resistivity (TCR) and a negative thermopower which is proportional to temperature over the range 5 K-300 K. As the oxygen deficiency increases, the electrical resistivity shows a minimum at low temperature which progressively shifts to higher temperature with increasing . In the low-temperature limit the correction to the electrical conductivity follows a power-law behaviour with an exponent . We also find a cusp-like dip in the tunnelling conductance near zero bias voltage. We attribute both of these behaviours to enhanced interaction between the charge carriers in the system occurring because of the disorder. The thermopower of the oxygen-deficient samples is found to vary systematically with , showing a sign reversal at low temperature.

Metal-insulator transition in La0.7Sr0.3Mn1−xFexO3

We report the effect of Fe doping at the Mn site in La0.7Sr0.3MnO3 oxides. We find that the doping of Fe does not cause any structural change, but the electrical transport in the system is strongly affected. The parent compound La0.7Sr0.3MnO3 shows a resistivity peak at T=Tp (365 K) and behaves as a ferromagnetic metal at low temperature. Double exchange (DE) interaction between Mn3+ and Mn4+ is understood to be the cause of ferromagnetism and metallicity of the material. As Fe is doped into the system a depletion in the number of hopping electrons occurs and consequently the DE interaction becomes weak, however, as long as x⩽0.2 the system shows metallic behavior at low temperatures. For higher Fe doping the system becomes insulating throughout the whole temperature range. Electron tunneling conductance measurements show that the density of states of metallic samples exhibits a dip at EF. As the value of x increases the dip deepens and finally at x=0.25 a gap opens up at the Fermi level.

Electrical resistivity study of La, B doped nanocrystalline superconducting vanadium nitride

A class of nanocrystalline superconducting vanadium nitrides has been prepared by a chemical synthesis route. This method involves the simultaneous decomposition of [VO(NH2O)2Gly]⋅H2O complex and nitridation at 973 K in ammonia atmosphere resulting pure and doped VN with controlled stoichiometry. The structure and stoichiometry of these samples were characterized by x-ray diffraction, scanning electron microscope, energy dispersive analysis of x-ray, transmission electron microscope, and chemical analysis. The VN materials crystallize in the cubic structure. The crystallite size was found to decrease with doping. We obtained samples having crystallite sizes between 8 and 32 nm. The electrical resistivity variation in these materials was carefully measured and the results are presented. The resistivity data in the range Tc<T< 30 K have been fitted to the power law ρ=ρ0+ATn and analyzed the results in the light of the existing theories. It was found that the resistivity follows a T3 law rather than the expe...

Effect of distributed particle magnetic moments on the magnetization of NiO nanoparticles

Abstract Magnetization of NiO nanoparticles, at different temperatures, is measured and analyzed taking into account a distribution in particle magnetic moment. We find that disregarding this distribution in the analysis is the reason for the many anomalous observations reported on this system in the literature.