Abstract
FePt alloy thin films with cap-layers of MgO or C are prepared on MgO(001) single-crystal substrates by using a two-step method consisting of low-temperature deposition at 200 °C followed by high-temperature annealing at 600 °C. The FePt film thickness is fixed at 10 nm, whereas the cap-layer thickness is varied from 1 to 10 nm. The influences of cap-layer material and cap-layer thickness on the variant structure and the L10 ordering are investigated. Single-crystal FePt(001) films with disordered fcc structure (A1) grow epitaxially on the substrates at 200 °C. Single-crystal MgO(001) cap-layers grow epitaxially on the FePt films, whereas the structure of C cap-layers is amorphous. The phase transformation from A1 to L10 occurs when the films are annealed at 600 °C. The FePt films with MgO cap-layers thicker than 2 nm consist of L10(001) variant with the c-axis perpendicular to the substrate surface, whereas those with C cap-layers involve small volumes of L10(100) and (010) variants with the c-axis lying in the film plane. The in-plane and the out-of-plane lattices are respectively more expanded and contracted in the continuous-lattice MgO/FePt/MgO structure due to accommodations of misfits of FePt film with respect to not only the MgO substrate but also the MgO cap-layer. The lattice deformation promotes phase transformation along the perpendicular direction and L10 ordering. The FePt films consisting of only L10(001) variant show strong perpendicular magnetic anisotropies and low in-plane coercivities. The present study shows that an introduction of epitaxial cap-layer is effective in controlling the c-axis perpendicular to the substrate surface.
Highlights
Magnetic thin films with high magnetocrystalline anisotropies and with the easy magnetization axis perpendicular to the substrate surface have been investigated for high-density magnetic recording media, magnetoresistive random access memory devices, etc
In order to enhance L10 ordering with the c-axis perpendicular to the substrate surface, an application of in-plane lattice expansion is considered to be effective, since the L10 structure is based on a tetragonal lattice with the c/a ratio less than 1 (a = 0.3842 nm, c = 0.3702 nm, aE-mail: ohtake@futamoto.elect.chuo-u.ac.jp 2158-3226/2017/7(5)/056320/5
In our previous study,10 FePt films were prepared on (001) single-crystal substrates by using a two-step method consisting of low-temperature deposition followed by high-temperature annealing, where disordered phase (A1) in FePt single-crystal films transformed into L10-ordered phase
Summary
Magnetic thin films with high magnetocrystalline anisotropies and with the easy magnetization axis perpendicular to the substrate surface have been investigated for high-density magnetic recording media, magnetoresistive random access memory devices, etc. In order to enhance L10 ordering with the c-axis perpendicular to the substrate surface, an application of in-plane lattice expansion is considered to be effective, since the L10 structure is based on a tetragonal lattice with the c/a ratio less than 1 (a = 0.3842 nm, c = 0.3702 nm, 056320-2 Ohtake et al. AIP Advances 7, 056320 (2017). In order to further enhance the in-plane lattice expansion, 2-nm-thick MgO cap-layers were introduced on the FePt films formed on MgO(001) substrates, where the FePt film thickness was varied between 2 and 40 nm.. The variant structure is considered to be delicately influenced by the cap-layer thickness and the material. The influences of cap-layer thickness and material on the variant structure and the L10 ordering are investigated
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