Katsuyuki Nakada

Epitaxial growth of BiFeO3 films on TiN under layers by sputtering deposition

BiFeO3/TiN/MgO (001) films have been prepared by magneton sputtering, where TiN serves as a conductive under layer. X-ray diffraction profiles and cross-sectional transmission electron microscopy images reveal that not only (001)-epitaxial BiFeO3 films are obtained, but also both tetragonal and rhombohedral phases co-exist in BiFeO3 films. Their crystallographic relationship is shown as following: tetragonal-BiFeO3 (001) [100]//TiN (001) [100]//MgO (001) [100] and rhombohedral-BiFeO3 (001) [100]//TiN (001) [100]//MgO (001) [100]. Besides, an oxidized TiN layer (∼ 20 nm) has also been detected between BiFeO3 and TiN layers and its formation may originate from oxygen inter-diffusion from BiFeO3 layer. Despite of the existence of the oxidized TiN layer, it does not affect the epitaxial growth of BiFeO3 films. On the other hand, the coercivity electric field obtained in ferroelectric loop of BiFeO3 is greatly enhanced to 49 MV/cm due to the existence of oxidized TiN layer.

Influence of off-stoichiometry on magnetoresistance characteristics of Co2MnSi/Ag-based current-perpendicular-to-plane spin valves

The influence of off-stoichiometry of Co2MnSi (CMS) spin sources on giant magnetoresistance characteristics was investigated for CMS/Ag-based current-perpendicular-to-plane spin valves prepared with various Mn compositions α in Co2MnαSi0.82 electrodes. The magnetoresistance ratio of the prepared CMS/Co50Fe50 (CoFe) (1.1 nm)/Ag/CoFe (1.1)/CMS spin valves systematically increased with α from 11.4% for Mn-deficient α = 0.62 to 20.7% for Mn-rich α = 1.45 at 290 K. This result suggests that increasing α from a Mn-deficient to Mn-rich value increases the spin polarization by suppressing CoMn antisites harmful to the half-metallicity. Thus, our results demonstrate that appropriately controlling the film composition toward a Mn-rich one is highly effective for enhancing the half-metallicity of CMS in CMS-based spin valves, as it is in CMS-based magnetic tunnel junctions.