A comprehensive microstructure study has been conducted experimentally for identifying the origin or mechanism of perpendicular magnetic anisotropy (PMA) in the ultra-thin (10 Å) CoFeB layer on the top of magnetic tunnel junction (MTJ). The high resolution transmission electron microscopy reveals that the feature of crystal structure in 10 Å-CoFeB layer is localized in nature at the CoFeB-MgO interface. On the other hand, the strain-relaxed crystalline structure is observed in the thick CoFeB (20 Å) layer at the CoFeB-MgO interface, associated with a series of dislocation formations. The electron energy loss spectroscopy further suggests that the local chemical stoichiometry of the ultra-thin 10 Å-CoFeB layer is notably changed at the CoFeB-MgO interface, compared with an atomic stoichiometry in a thick 20 Å-CoFeB layer. The origin of PMA mechanism is therefore identified experimentally as an interface effect, which can be attributed to a change of local atom bonding or lattice constant of the transition metal at the CoFeB-MgO based MTJ interface. Furthermore, such a local interfacial atom bonding change is seemly induced by the localized anisotropic strain and consistent with previous theoretical speculations and calculations. The observed experimental findings provide some perspective on microstructure and chemistry on PMA in ultra-thin CoFeB film at the MTJ interface, then deepening our understanding of the mechanism of PMA within MTJ stack and thus facilitating advancement for emerging spintronics technology.
The properties of tantalum thin film prepared by ion beam sputtering with Xe ions ͑0.6 -1.5 KV͒ have been investigated. -Ta thin films with high resistivity ͑170 ⍀ cm͒ on Si substrate were obtained without Cr underlayer. With Cr underlayer, however, a body-centered-cubic phase ͑bcc͒ ␣-Ta thin film with low resistivity ͑20 ⍀ cm͒ has been successfully obtained at room temperature. The experimental results indicate that the Cr underlayer plays an important role in ␣-Ta formation and that the critical thickness of Cr is 20 Å. Properties of ␣-Ta are also influenced by Ta thickness and ion beam deposition process conditions. ␣-Ta with lower resistivity was achieved by using approximate beam energy of 1000 eV and beam current of 150 mA. X-ray diffraction analysis indicates that changes in resistivity can be attributed to changes in microstructure that is influenced by the ion beam conditions.
FeRhN(∼50 nm)/NiFe(∼5 nm) laminated thin films were deposited by dc magnetron sputtering. Upon optimizing the sputtering conditions, FeRhN/NiFe laminated thin films showed good soft magnetic properties, and a well defined uniaxial anisotropy: Bs∼20.3 kG, Hc<1.0 Oe, Hk∼5 Oe. It was shown that a NiFe underlayer improved the soft magnetic properties of a FeRhN layer more effectively than a NiFe top layer. The data obtained by x-ray diffraction study showed that the FeRhN film with a NiFe underlayer had a much stronger (110) texture, as compared to single layer FeRhN film. Columnar grain structure with size of about 10 nm was observed in FeRhN films by high resolution transmission electron microscope. It is believed that the modification of FeRhN microstructure by NiFe underlayer plays a more important role in improving the soft magnetic properties than the exchange coupling between the FeRhN and NiFe layers.
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