L.B. Steren

Anisotropic magnetoresistance in manganites: experiment and theory

We present measurements of the anisotropic magnetoresistance (AMR) of La(0.75)Sr(0.25)MnO(3) films deposited on (001) SrTiO(3) substrates, and a model that describes the experimental results. The model, based on the electronic structure of manganites plus the spin-orbit coupling, correctly accounts for the dependence of the AMR on the direction of the current to the crystalline axes. We measure an AMR of the order of 10(-3) for the current I parallel to the [100] axis of the crystal and vanishing AMR for I , in agreement with the model predictions. Further, we calculate the planar Hall effect and show its connection to AMR.

Substrate effect on the magnetic behavior of manganite films

The film thickness (t) dependence of the magnetic properties of La0.6Sr0.4MnO3 (LSMO) films grown on (001) MgO and (001) SrTiO3 substrates has been studied. Hysteresis loops measured at low temperature show a smooth increase of the retentivity accompanied by a decrease of the coercitive field as the film thickness increases. The increase of coercitivity with decreasing t can be interpreted in terms of a change in the domain structure of the films mainly due to an augmentation of domain pinning defects. The magnetic anisotropy has been measured using ferromagnetic resonance (FMR). A volume (VA) and a surface (SA) anisotropy contribution have been deduced from FMR angular dependence studies for both series of samples. In the LSMO films grown on MgO a VA component that corresponds to an easy-axis perpendicular to the plane of the films has been found while in contrast, the LSMO films grown on SrTiO3 present an easy-plane anisotropy. The SA is positive for both series favoring a perpendicular magnetic anisotr...

Metal-insulator transition induced by postdeposition annealing in low doped manganite films

We studied the transport and magnetic properties of low-doped manganite films after different oxygenation processes. The oxygen content was adjusted by postdeposition annealing at different oxygen pressures and annealing times. For all the samples we observed an increase in the Curie temperature and the remnant magnetization with the oxygen content. In general, for decreasing number of oxygen vacancies, samples under expansive strain become more homogeneous and their electrical resistivity decreases. A metal-insulator transition is induced in highly oxygenated films grown on SrTiO3, probably related to a shift of the mobility edge crossing below the Fermi energy. We found that the oxygenation dynamics depend critically on the strain field induced by the substrates and also on the Sr doping concentration.

Magnetic properties of Fe/ZnSe/Fe trilayers

Abstract The magnetic properties of Fe/Zn/Fe trilayers have been studied by ferromagnetic resonance and magnetization measurements. These measurements have been used to investigate the magnetic anisotropy of the iron layers and the magnetic coupling across the semiconductor spacer. The angular dependence of the resonance spectra has been measured in-plane and out-of-plane in order to deduce magnetic anisotropy constants of the samples. Experimental data were fitted by using an energy-density expression that includes bulk cubic anisotropy, growth-induced uniaxial in-plane anisotropy and perpendicular-surface anisotropy terms. A small ferromagnetic coupling is observed in the trilayers with spacer thickness up to 50 A .

Magnetic and transport properties of Ag/Co90Fe10 granular multilayers

We report experimental results on the magnetic and transport properties of Ag∕Co90Fe10 multilayers with variable Ag and Co90Fe10 thicknesses. The magnetism samples was characterized by magnetization measurements and ferromagnetic resonance spectroscopy. These measurements show that the magnetic layers are in general composed by single-domain particles and that the shape and the size of the magnetic clusters can be changed by using an applied magnetic field (Hdep) on deposition. The resistivity of these multilayered systems is described by a mixed model that includes current-in-plane and current perpendicular plane transport contributions. The transport model proposed to explain the experimental results emphasizes the role played by the magnetic entities on the scattering processes and strongly depends on the silver layer thickness. An electrical percolative transition is observed for samples grown at Hdep>Hdep* while the magnetic properties of these samples still show a granularlike behavior.We report experimental results on the magnetic and transport properties of Ag∕Co90Fe10 multilayers with variable Ag and Co90Fe10 thicknesses. The magnetism samples was characterized by magnetization measurements and ferromagnetic resonance spectroscopy. These measurements show that the magnetic layers are in general composed by single-domain particles and that the shape and the size of the magnetic clusters can be changed by using an applied magnetic field (Hdep) on deposition. The resistivity of these multilayered systems is described by a mixed model that includes current-in-plane and current perpendicular plane transport contributions. The transport model proposed to explain the experimental results emphasizes the role played by the magnetic entities on the scattering processes and strongly depends on the silver layer thickness. An electrical percolative transition is observed for samples grown at Hdep>Hdep* while the magnetic properties of these samples still show a granularlike behavior.

Correlation between structure and magnetic properties of manganite-based multilayers

We have studied the structure, magnetic, and transport properties of manganite-based multilayers A/Bi, with A=La0.55Sr0.45MnO3 (metallic ferromagnet), and spacers (Bi) of different nature: B1=SrTiO3 (nonmagnetic insulator), B2=La0.9Sr0.1MnO3 (insulator ferromagnet), and B3=La0.67Ca0.33MnO3 (paramagnetic-insulator/ferromagnet-metal). The samples are strongly textured in the direction perpendicular to the sample surface, and present a good interface quality with small roughness and interdiffusion. We have found that in the A/B1/A trilayers, the ferromagnetic electrodes are ferromagnetically coupled for thin spacer layers and becomes decoupled for spacer thickness larger than 3 nm. Instead, the others multilayers are ferromagnetic for all spacer thicknesses and temperature range. This result was expected for temperatures well below the spacer Curie temperature. We attributed the ferromagnetic behavior of the system, found for temperatures above the ordering temperature of the spacer, to direct exchange coupl...

Calculation of transport properties of Co–Ag-based multilayered granular alloys

We calculate electronic transport properties of multilayered granular alloys, composed of discontinuous Co layers embedded in Ag alternated with complete Ag layers. We focus our attention on the conductivity dependance on the shape and size of the clusters. The electronic structure is self-consistently calculated using a tight binding hamiltonian which includes a Hubbard term within the unrestricted Hartree–Fock approximation. We obtain different transport regimes depending on the growth conditions and transport direction. r 2002 Elsevier Science B.V. All rights reserved.

Magnetic after-effect in manganite films

Abstract The time dependence of the magnetic and transport properties on La 0.6 Sr 0.4 MnO 3 films and bulk samples has been studied through magnetization and resistivity measurements. A magnetic after-effect has been observed in all samples. At low temperatures, the low-field magnetization, can be described by the function M(t)=M c +M d exp(− t/τ)+S(H,T )ln( t) . The resistivity increases logarithmically in the same temperature range, indicating the evolution of the sample to a more disordered state. Above a characteristic temperature, this behaviour is reversed and an increase of the magnetization with time is observed. The relaxation parameters depend on the bulk or films character of the samples. In the latter case, a dependence on the film thickness was found. A direct correlation between the time dependence of the resistivity and magnetization curves in manganite compounds was found.