engleski

Observation of Spin Polarization in a Diluted System: Co-doped La_0.55 Sr_0.5 TiO_3

Spintronics applications require the use of highly spin-polarized current sources. With this purpose in mind, half-metallic ferromagnets, such as various magnetic oxides, have been integrated in devices and high magnetoresistance values have been recorded. However, the Curie temperatures of the involved materials are too close to room temperature for technological applications. New approaches to achieve high spin-polarized sources at high temperatures involve doping host materials with magnetic ions, as in diluted magnetic semiconductors (DMS). We have followed this approach by doping metallic non-magnetic La_0.5Sr_0.5TiO_3 with Co (LSTO(Co)) in the range up to 3%. The samples were grown by pulsed laser deposition with a frequency-tripled Nd:YAG laser (355 nm) on SrTiO_3 (STO) (001) and LaAlO_3 (LAO) (001) substrates. X-ray diffraction structural characterization shows that films are epitaxial and without parasitic phases. RHEED and AFM measurements show that the growth is 3D for samples grown on LAO, while it is 2D on STO substrates, with very low roughness values. No Co-clusters are observed neither by means of high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS) or Auger spectroscopy. The dependence of the resistivity and the magnetization on growth parameters such as oxygen growth pressure, film thickness and Co-content have been carefully analyzed. The films show magnetic hysteresis cycles at room temperature and the observed magnetic anisotropy supports an intrinsic origin for ferromagnetism. The largest ferromagnetic signals are measured in films grown at low oxygen pressures (< 10^-5 mtorr). The main result of the present work is the determination of a spin polarization in a diluted magnetic system. LSTO(Co 1, 5%)/STO/Co and LSTO(Co 1, 5%)/LAO/Co junctions have been fabricated either by optical lithography or by real-time resistance-controlled nanoindentation with a conductive AFM tip. Tunnel magnetoresistance values (TMR = (Rp-Rap)/Rap, Rp and Rap are the resistances in parallel and anti-parallel configurations, respectively) up to 20% are measured at low temperature. Applying Julliere's model, this implies a spin polarization of at least 50 % calculated from the highest TMR value. The analysis of the temperature dependence of the TMR reveals a fast decrease with temperature and the TMR becomes vanishingly small at T > 200 K. The origin of this TMR suppression remains to be clarified. The bias dependence of the TMR will be discussed. The present work represents an important step towards the understanding and application of the DMS approach to spintronic devices using complex oxide materials.

spintronics; tunnel magnetoresistance; spin polarization; half-metallic ferromagnets

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