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The electrical transport properties of grain boundaries in the epitaxial La 2/3 Ca 1/3 MnO 3−δ thin films have been studied as a function of temperature and applied magnetic field. Below the Curie temperature T C an additional grain boundary resistance, highly non-linear current-voltage curves, and a large magnetoresistive effect in the whole temperature regime below T C are found. The results can be explained consistently by the presence of a disordered, a few nm wide paramagnetic grain boundary layer that is depleted below T C due to an increase of the work function of the ferromagnetic grain material adjacent to this layer. The related band bending and space charge effects are important for the physics of grain boundaries in the manganites.

Abstract. -We have performed a systematic analysis of the voltage and temperature dependence of the tunneling magnetoresistance (TMR) of grain boundaries (GB) in the manganites. We find a strong decrease of the TMR with increasing voltage and temperature. The decrease of the TMR with increasing voltage scales with an increase of the inelastic tunneling current due to multi-step inelastic tunneling via localized defect states in the tunneling barrier. This behavior can be described within a three-current model for magnetic tunnel junctions that extends the two-current Jullière model by adding an inelastic, spin-independent tunneling contribution. Our analysis gives strong evidence that the observed drastic decrease of the GB-TMR in manganites is caused by an imperfect tunneling barrier.The tunneling resistance between two ferromagnetic metal layers separated by a thin insulating barrier depends on the relative orientation of the magnetization and the electron spin polarization in each layer [1]. Since for materials with large spin polarization a large tunneling magnetoresistance (TMR) can be achieved, magnetic tunnel junctions have recently attracted much attention and their use in memory devices in envisaged [2,3]. Due to their half-metallic ferromagnetic state with only a single spin band crossing the Fermi level, the spin polarization of the perovskite manganites of composition La 2/3 D 1/3 MnO 3 with D = Ca, Sr, and Ba is close to 100%. According to the Jullière model[1], a high TMR ratio ∆R/R = (R ↑↓ − R ↑↑ )/R ↑↑ = (2P 1 P 2 )/(1 − P 1 P 2 ) is expected making these materials attractive for magnetic tunnel junctions. Here, R ↑↑ and R ↑↓ is the tunneling resistance for parallel and anti-parallel magnetization orientation and P i = 2a i − 1 is the spin polarization where a i is the fraction of majority spin electrons in the density of states of layer i. Indeed a high TMR ratio well above 100% has been achieved at low temperatures and low applied fields H of the order of 10 mT [4,5]. In addition to planar type tunnel junctions, a large low field magnetoresistance was found for grain boundaries (GBs) in the perovskite manganites Typeset using EURO-T E X

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We report on the preparation and characterization of the double-perovskite compound Sr2CrWO6 with a Curie temperature of 390 K. We have fabricated both Sr2CrWO6 bulk sintered polycrystalline bars and high-quality epitaxial thin films on SrTiO3 substrates by pulsed-laser deposition. The samples were characterized by thermogravimetric analysis, x-ray diffraction, electrical transport, and magnetization measurements. Polycrystalline samples containing a large number of grain boundaries show a large low-field magnetoresistance of up to 100% at 5 K. At room temperature, this effect is reduced to a few percent. Our results show that Sr2CrWO6 is an interesting candidate for room-temperature magnetoelectronic materials.