Tatsuo Shibata

Isothermal electric switching of magnetization in Cr2O3/Co thin film system

We investigated the magnetoelectric properties of Cr2O3/Co all-thin-film exchange coupling system with Cr spacer layer. In this system, significantly small coercivity (Hcu2009<u200950u2009Oe) was obtained by the Cr spacer layer insertion between Cr2O3 and Co layers. Owing to the small Hc, exchange bias field, Hex, larger than Hc was achieved. It enabled us to observe magnetization switching at a zero magnetic field, when Hex was reversed by magnetoelectric effect of Cr2O3 layer. Finally, we demonstrated the isothermal magnetoelectric switching of magnetization in the Cr2O3/Cr/Co all-thin-film system. By changing the direction of the electric field during the isothermal magnetoelectric switching process, both Hex and magnetization at a zero magnetic field were reversed back and forth, i.e., isothermal magnetization switching by an electric field was achieved.

Observation of magnetoelectric effect in Cr2O3/Pt/Co thin film system

We investigate the magnetic, electric, and magnetoelectric properties of a Cr2O3 thin film system that was deposited by radiofrequency magnetron sputtering. The temperature dependence of the magnetic susceptibility of our Cr2O3 film is similar to that of bulk Cr2O3, which indicates a small oxygen non-stoichiometry in the film. The Cr2O3 film exhibits high AC resistivity (∼109 Ωu2009cm) and low leakage current (4.0u2009×u200910−5u2009A/cm2 at Eu2009=u200980u2009kV/cm). Finally, we demonstrate magnetoelectric switching of the exchange bias using the Cr2O3/Pt/Co all-thin-film system. By changing the direction of the electric field during the magnetoelectric field cooling process, the exchange bias field was changed symmetrically from −160u2009Oe to +160u2009Oe, which represents the switching of the antiferromagnetic domain of Cr2O3.

Positive exchange bias observed in Pt-inserted Cr2O3/Co exchange coupled bilayers

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.

Morin transition temperature in (0001)-oriented α-Fe2O3 thin film and effect of Ir doping

The structural properties and Morin transition in c-plane-oriented α-Fe2O3 and Ir-doped α-Fe2O3 thin films have been investigated. The enhancement of the Morin transition temperature (TM) in α-Fe2O3 film by Ir doping has been demonstrated. The TM in the c-plane-oriented α-Fe2O3 thin film was determined from the temperature-dependent in-plane magnetization and change of coercivity (Hc); this TM value was found close to that of bulk α-Fe2O3. The spin directions of non-doped and Ir-doped α-Fe2O3 at room temperature were also estimated from conversion electron Mossbauer spectroscopy measurements. We confirmed that Ir doping dramatically enhances the TM of α-Fe2O3 thin film.

Néel temperature of Cr2O3 in Cr2O3/Co exchange-coupled system: Effect of buffer layer

The lattice parameter dependence of the Neel temperature TN of thin Cr2O3 in a Cr2O3/Co exchange-coupled system is investigated. Lattice-mismatch-induced strain is generated in Cr2O3 by using different buffer layers. The lattice parameters are determined from out-of-plane and in-plane X-ray diffraction measurements. The Neel temperature is detected by direct temperature-dependent magnetization measurement as well as the temperature-dependent interface exchange coupling energy. It is observed that in-plane lattice contraction can enhance TN in Cr2O3, which is consistent with theoretical calculations.

Enhancing the blocking temperature of perpendicular-exchange biased Cr2O3 thin films using buffer layers

In this study, we investigated the effect of spacer and buffer layers on the blocking temperature TB of the perpendicular exchange bias of thin Cr2O3 films, and revealed a high TB of 260 K for 20-nm-thick Cr2O3 thin films. By inserting a Ru spacer layer between the Cr2O3 and Co films and changing the spacer thickness, we controlled the magnitude of the exchange bias and TB. By comparing the TB values of the 20-nm-thick Cr2O3 films on Pt and alpha-Fe2O3 buffers, we investigated the lattice strain effect on the TB. We show that higher TB value can be obtained using an alpha-Fe2O3 buffer, which is likely because of the lattice-strain-induced increase of Cr2O3 magnetic anisotropy.

Large perpendicular exchange bias and high blocking temperature in Al-doped Cr2O3/Co thin film systems

We investigated the effect of Al doping on the perpendicular exchange bias in Cr2O3/Co thin film systems. By Al doping, a large unidirectional anisotropy energy (J K ~ 0.74 erg/cm2) and a high blocking temperature (T B) were obtained simultaneously. The variations in J K and T B induced by Al doping were discussed on the basis of Mauris model and explained from an increase in magnetic anisotropy and a decrease in the exchange stiffness constant of Cr2O3. Our first-principles calculations suggest a factor to improve the magnetic anisotropy of Cr2O3 markedly, which is different from the previously established lattice strain effect.

Inserted metals for low-energy magnetoelectric switching in a Cr2O3/ferromagnet interfacial exchange-biased thin film system

An inserted metal (IM) layer (such as Pt, Ru, or Cr) is typically fabricated between Cr2O3 and the ferromagnet in magnetoelectric switched antiferromagnet/ferromagnet exchange-biased thin film systems. The IM layer is used as a “spacer layer” to prevent oxidization of Co and enhance the blocking temperature. In this work, we found that the magnetic properties of these thin film systems vary depending on the type of IM, and they could be reproduced by bilayers or multilayers of Co and IM thin films. We also found that the IM layer was spin-polarized by the Co ferromagnetic layer based on X-ray magnetic circular dichroism data. These results suggest that the IM does not work separately as a “spacer layer”, but rather it is an integral component of Co-based stacked ferromagnetic films in magnetoelectric switched Cr2O3/ferromagnet exchange-biased systems. Furthermore, we found that some stacked ferromagnetic films easily exhibit a low unidirectional anisotropy energy (Jk), which may be a good way to decrease the magnetoelectric switching energy of antiferromagnets for practical device applications.