Mara Granada

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.

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.

Magnetoresistance effect in (La, Sr)MnO3 bicrystalline films

The angular dependence of the magnetoresistance effect has been measured on bicrystalline La(0.75)Sr(0.25)MnO(3) films. The measurements have been performed on an electronically lithographed Wheatstone bridge. The study of the angular dependence of both the magnetoresistance and the resistance of single-crystalline and grain-boundary regions of the samples allowed us to isolate two contributions of low-field magnetoresistance in manganites. One of them is associated with the spin-orbit effect, i.e. the anisotropic magnetoresistance of ferromagnetic compounds, and the other one is related to spin-disorder regions at the grain boundary. Complementary x-ray diffraction, ferromagnetic resonance and low temperature magnetization experiments contribute to the characterization of the magnetic anisotropy of the samples and the general comprehension of the problem.

Giant magnetoresistance in oxide-based metallic multilayers

The authors report on the first measurement of low-field giant magnetoresistance in metallic multilayers of perovskite oxides. The authors performed in-plane measurements of the magnetoelectric transport properties in La0.75Sr0.25MnO3∕LaNiO3 trilayers and succeeded in distinguishing the giant magnetoresistance effect from other contributions to the total magnetoresistance. The samples were grown on single-crystalline SrTiO3 substrates by dc sputtering.

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...