P. Padhan

Dielectric relaxation and magnetodielectric response in epitaxial thin films of La2NiMnO6

Frequency and magnetic field dependent dielectric measurements have been performed on epitaxial thin films of the double perovskite La2NiMnO6, revealing a dielectric relaxation and magnetodielectric effect. The films are grown on Nb-doped and SrRuO3-coated SrTiO3 substrates using the pulsed laser deposition technique. While a rapid dielectric relaxation is observed at ∼300K, the relaxation rate increases dramatically at lower temperatures. Below the Curie temperature of La2NiMnO6, the dielectric constant increases in a magnetic field for a range of temperature. This temperature range depends on magnetic field and measurement frequency. The results are explained by the influence of a magnetic field on the dipolar relaxation.

Frequency-dependent magnetoresistance and magnetocapacitance properties of magnetic tunnel junctions with MgO tunnel barrier

The frequency-dependent impedance of magnetic tunnel junctions (MTJs) with MgO barriers was investigated. The capacitance of the MTJs switches from high to low when the relative electrode magnetizations change from parallel to antiparallel, opposite the resistance change. Additionally, for parallel magnetizations, the capacitance varies with temperature though resistance remains approximately constant. The low frequency resistance and the tunneling magnetoresistance are in agreement with dc values. The capacitance is found to be larger than the expected (geometrical) capacitance, in contrast to MTJs with Al2O3 barriers. These results are explained by screening due to charge and spin accumulation at the interfaces.

Structural and magnetic properties of a series of low-doped Zn1−xCoxO thin films deposited from Zn and Co metal targets on (0001) Al2O3 substrates

We report on the synthesis of low doping Zn1−xCoxO (0<x<0.1) thin films on (0001)-Al2O3 substrates. The films were prepared in an oxidizing atmosphere, using the pulsed-laser deposition technique starting from Zn and Co metallic targets. We first studied the influence of the strains of ZnO and their structural properties. Second, we investigated the structural and the magnetic properties of the Zn1−xCoxO films. We show that at low doping, the lattice parameters and the magnetization of the Zn1−xCoxO films depend strongly on the Co concentration.

Magnetodielectric response in epitaxial thin films of multiferroic Bi2NiMnO6

Thin films of multiferroic Bi2NiMnO6 (BNMO) have been epitaxially stabilized on various lattice-matched substrates using the pulsed laser deposition technique. BNMO films deposited on conducting Nb doped SrTiO3 substrates exhibit a magnetodielectric effect, in which the effective dielectric constant is dependent on the applied magnetic field and measurement frequency. A rapid change in the effective dielectric constant with a corresponding maximum in the magnetodielectric effect is observed near the ferromagnetic transition temperature of BNMO. The temperature-dependent magnetodielectric effect is attributed to the coupling between electric dipole ordering and fluctuations and magnetic ordering and fluctuations.

Antiferromagnetic coupling and enhanced magnetization in all-ferromagnetic superlattices

The structural and magnetic properties of a series of superlattices consisting of two ferromagnetic metals La0.7Sr0.3MnO3 (LSMO) and SrRuO3 (SRO) grown on (001) oriented SrTiO3 are studied. Superlattices with a fixed LSMO layer thickness of 20unit cells and varying SRO layer thickness show a sudden drop in magnetization on cooling through a temperature where both LSMO and SRO layers are ferromagnetic. This behavior suggests an antiferromagnetic coupling between the layers. In addition, the samples having thinner SRO layers (n<6) exhibit enhanced saturation magnetization at 10K. These observations are attributed to the possible modification in the stereochemistry of the Ru and Mn ions in the interfacial region.

Magnetocaloric effect and improved relative cooling power in (La0.7Sr0.3MnO3/SrRuO3) superlattices

Magnetic properties of a series of (La(0.7)Sr(0.3)MnO(3)/SrRuO(3)) superlattices, where the SrRuO(3) layer thickness is varying, are examined. A room-temperature magnetocaloric effect is obtained owing to the finite size effect which reduces the T(C) of La(0.7)Sr(0.3)MnO(3) layers. While the working temperature ranges are enlarged, - ΔS(M)(max) values remain similar to the values in polycrystalline La(0.7)Sr(0.3)MnO(3). Consequently, the relative cooling powers are significantly improved, the microscopic mechanism of which is related to the effect of the interfaces at La(0.7)Sr(0.3)MnO(3)/SrRuO(3) and higher nanostructural disorder. This study indicates that artificial oxide superlattices/multilayers might provide an alternative pathway in searching for efficient room-temperature magnetic refrigerator for (nano) micro-scale systems.

Enhanced magnetoresistance in ferromagnetic Pr0.85Ca0.15MnO3∕ferroelectric Ba0.6Sr0.4TiO3 superlattice films

Artificial superlattices designed with a ferromagnetic Pr0.85Ca0.15MnO3 insulating layer and ferroelectric Ba0.6Sr0.4TiO3 layer were grown on (100) SrTiO3 substrates. The magnetotransport properties were measured with the current perpendicular to the plane geometry. An increase in magnetoresistance (MR), with no significant low-field effect, was observed as the amount of ferroelectric Ba0.6Sr0.4TiO3 layer thickness increases even up to 9 unit cells. For example, the superlattice [(Pr0.85Ca0.15MnO3)10(Ba0.6Sr0.4TiO3)9]25 shows a 35% MR at 100K, though the Pr0.85Ca0.15MnO3 film was a robust insulator with negligible MR even at a high applied magnetic field. Similar superlattice structure, (Pr0.85Ca0.15MnO3)∕(SrTiO3) did not show any enhancement in MR. We discussed that the enhanced MR could be due to the ferroelectric spacer layer and the associated magnetoelectric coupling.

Current-induced metallic behavior in Pr 0.5 Ca 0.5 MnO 3 thin films: Competition between Joule heating and nonlinear conduction mechanisms

Thin films of

Magnetodielectric effect in Bi 2 NiMnO 6 -La 2 NiMnO 6 superlattices

{\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_{3}

Electronic structure and optical band gap of CoFe2O4 thin films

manganites exhibiting charge∕orbital-ordered properties with colossal magnetoresistance have been synthesized by the pulsed laser deposition technique on both