Magnetic oxides show a variety of extrinsic magnetotransport phenomena: grain-boundary, tunnelling and domain-wall magnetoresistance. In view of these phenomena the role of some magnetic oxides is outstanding: these are believed to be half-metallic having only one spin-subband at the Fermi level. These fully spin-polarized oxides have great potential for applications in spin-electronic devices and have, accordingly, attracted intense research activity in recent years. This review is focused on extrinsic magnetotransport effects in ferromagnetic oxides. It consists of two parts; the second part is devoted to an overview of experimental data and theoretical models for extrinsic magnetotransport phenomena. Here a critical discussion of domain-wall scattering is given. Results on surface and interfacial magnetism in oxides are presented. Spin-polarized tunnelling in ferromagnetic junctions is reviewed and grain-boundary magnetoresistance is interpreted within a model of spin-polarized tunnelling through natural oxide barriers. The situation in ferromagnetic oxides is compared with data and models for conventional ferromagnets. The first part of the review summarizes basic material properties, especially data on the spin polarization and evidence for half-metallicity. Furthermore, intrinsic conduction mechanisms are discussed. An outlook on the further development of oxide spin-electronics concludes this review.

Extrinsic magnetotransport phenomena in ferromagnetic oxides

We review the structural, magnetic and transport properties of double perovskites (A2BB'O6) with ferromagnetism above room temperature. Ferromagnetism in these compounds is explained by an indirect B?O?B'?O?B exchange interaction mediated by itinerant electrons. We first focus on the BB' =?FeMo-based double perovskites, with Sr2FeMoO6 (TC = 420?K) being the most studied compound. These compounds show metallic behaviour and low magnetic coercivity. Afterwards, we will focus on B' = Re compounds, where the significant orbital moment of Re plays a crucial role in the magnetic properties, for example in the large magnetic coercivity and magnetostructural coupling. More specifically, we first discuss the A2FeReO6 series, with maximum TC = 520?K for Ca2FeReO6, which shows a tendency to semiconducting behaviour. Finally, we describe the Sr2(Fe1?xCrx)ReO6 series, with maximum TC = 625?K for Sr2CrReO6, which is the highest TC in an oxide compound without Fe. This compound is metallic. We discuss the impact of these materials for spin electronics in the light of their high spin polarization at the Fermi level and metallicity. In particular, we focus on the large intergrain magnetoresistance effect observed in polycrystalline samples and the possible implementation of these materials as electrodes in magnetic tunnel junctions.

https://iopscience.iop.org/article/10.1088/0953-8984/19/2/023201/pdf

Double perovskites with ferromagnetism above room temperature

The structural, transport, magnetic, and optical properties of the double perovskite A2CrWO6 with A = Sr,Ba,Ca have been studied. By varying the alkaline earth ion on the A site, the influence of steric effects on the Curie temperature TC and the saturation magnetization has been determined. A maximum TC = 458 K was found for Sr2CrWO6 having an almost undistorted perovskite structure with a tolerance factor f~1. For Ca2CrWO6 and Ba2CrWO6 structural changes result in a strong reduction of TC. Our study strongly suggests that for the double perovskites in general an optimum TC is achieved only for f~1, that is, for an undistorted perovskite structure. Electron doping in Sr2CrWO6 by a partial substitution of Sr2+ by La3+ was found to reduce both TC and the saturation magnetization Ms. The reduction of Ms could be attributed both to band structure effects and the Cr/W antisites induced by doping. Band structure calculations for Sr2CrWO6 predict an energy gap in the spin-up band, but a finite density of states for the spin-down band. The predictions of the band structure calculation are consistent with our optical measurements. Our experimental results support the presence of a kinetic energy driven mechanism in A2CrWO6, where ferromagnetism is stabilized by a hybridization of states of the nonmagnetic W site positioned in between the high spin Cr sites.

/pdf/structural-and-doping-effects-in-the-half-metallic-double-1k1y2bj10b.pdf

Structural and doping effects in the half-metallic double perovskite A 2 CrWO 6 (A=Sr, Ba, and Ca)

Concomitant with the development of metal-based spintronics in the late 1980's and 1990's, important advances were made on the growth of high-quality oxide thin films and heterostructures While this was at first motivated by the discovery of high-temperature superconductivity in perovskite Cu oxides, this technological breakthrough was soon applied to other transition metal oxides, and notably mixed-valence manganites The discovery of colossal magnetoresistance in manganite films triggered an intense research activity on these materials, but the first notable impact of magnetic oxides in the field of spintronics was the use of such manganites as electrodes in magnetic tunnel junctions, yielding tunnel magnetoresistance ratios one order of magnitude larger than what had been obtained with transition metal electrodes Since then, the research on oxide spintronics has been intense with the latest developments focused on diluted magnetic oxides and more recently on multiferroics In this paper, we will review the most important results on oxide spintronics, emphasizing materials physics as well as spin-dependent transport phenomena, and finally give some perspectives on how the flurry of new magnetic oxides could be useful for next-generation spintronics devices

Oxide spintronics

In the search for new spintronic materials with high spin polarization at room temperature, we have synthesized an osmium-based double perovskite with a Curie temperature of 725 K. Our combined exp ...

Sr2CrOsO6 : End point of a spin-polarized metal-insulator transition by 5d band filling

Electron doping in the ferromagnetic and metallic double perovskites ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6}$ is achieved via the partial substitution of ${\mathrm{Sr}}^{2+}$ by ${\mathrm{La}}^{3+}.$ We show that this doping promotes an extraordinary rising of the Curie temperature of about 70 K above that of the pristine compound. This finding reveals that the ferromagnetic coupling is mediated by itinerant carriers, thus providing solid experimental support to the double-exchange picture for the magnetic interactions. The observation that ${T}_{C}$ can be substantially enhanced may be of relevance for technological applications of these materials in advanced spin devices.

J. Navarro

Papers

Raising the Curie temperature in Sr 2 FeMoO 6 double perovskites by electron doping

Aging of Sr2FeMoO6 and related oxides

Magnetoresistive oxides: new developments and applications