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We have observed high exchange bias for a Cr2O3 film, which was crystallized from an amorphous Cr-oxide by annealing in O2 flow. To clarify the origin of the high exchange bias, we characterized the morphology of the Cr2O3 film. From X-ray diffraction, atomic force microscopy, and transmission electron microscopy measurements, we discovered that our polycrystalline Cr2O3 film was found to grow not to be entirely random orientation, but to form large R-planes on the surface. That is, R-planes were self-organized during crystallization. Since uncompensated Cr spins exist on R-planes of Cr2O3, the origin of the high exchange bias would be the self-organized R-planes. We successfully explained the peculiar temperature-dependent exchange bias and coercivity of the Cr2O3 film. For the Cr2O3 film, perpendicular exchange bias was obtained whereas out of plane direction is hard magnetization axis. This is because of the oblique Cr spin directions from out of plane direction of the film. The results demonstrated that exchange bias higher than coercivity (\mu0Hex\gg \mu0Hc) was realized near room temperature. [ABSTRACT FROM PUBLISHER]

We investigated the effect of Pt insertion on a Cr2O3/Co exchange coupling system. The perpendicular exchange bias μ0Hex decreased with increasing Pt insertion layer thickness, and we observed positive μ0Hex for samples with relatively thick Pt insertion layers. We also examined the cooling field μ0Hfc dependence of μ0Hex for the samples. At small μ0Hfc, all samples exhibited negative μ0Hex. With increasing μ0Hfc, a shift of μ0Hex from negative to positive was observed. In the past, similar behaviors were observed for FeF2/Fe systems exhibiting positive μ0Hex. In addition, the μ0Hfc dependence of μ0Hex was well fitted by an equation taking into account the Zeeman energy at the surface of an antiferromagnet as well as an antiferromagnetic exchange coupling. The results strongly suggest that (1) Cr2O3 surface spin is affected by the external magnetic field and (2) the coupling at the Cr2O3/Pt/Co interface is antiferromagnetic. [ABSTRACT FROM AUTHOR]

In this study, we fabricated a Cr2O3 (0001) film without and with a Pt buffer layer and investigated its effect on perpendicular exchange coupling in a Cr2O3/Co3Pt interface. The results showed that the exchange bias field (μ0Hex) and blocking temperature (TB) of a Cr2O3 film without and with Pt were very different. The Cr2O3 film without Pt had a lower μ0Hex of 176 Oe and a lower TB of 75 K, whereas that with Pt had a higher μ0Hex of 436 Oe and a higher TB of 150 K. We discussed this difference in μ0Hex and TB values based on collinear/non-collinear coupling in a ferromagnetic and antiferromagnetic interface using Meiklejohn and Bean's exchange anisotropy model. [ABSTRACT FROM AUTHOR]

The linear magnetoelectric effect was measured in 500-nm Cr2O3 films grown by radio frequency sputtering on Al2O3 substrates between top and bottom thin film Pt electrodes. Magnetoelectric susceptibility was measured directly by applying an alternating current (ac) electric field and measuring the induced ac magnetic moment using superconducting quantum interference device magnetometry. A linear dependence of the induced ac magnetic moment on the ac electric field amplitude was found. The temperature dependence of the magnetoelectric susceptibility agreed qualitatively and quantitatively with prior measurements of bulk single crystals, but the characteristic temperatures of the film were lower than those of single crystals. It was also possible to reverse the sign of the magnetoelectric susceptibility by reversing the sign of the magnetic field applied during cooling through the Néel temperature. A competition between total magnetoelectric and Zeeman energies is proposed to explain the difference between film and bulk Cr2O3 regarding the cooling field dependence of the magnetoelectric effect. [ABSTRACT FROM AUTHOR]