Magnetic properties of Co-ferrite/FeCo bilayers

Co-ferrite with spinel structure is one of the attractive materials for spintronics and artificial multiferroics. Due to the high resistivity and high-Curie temperature properties, Co-ferrite thins films have potential applications in such so-called spin filters, where a combination of insulating ferrite with a non-magnetic electrode is believed capable of creating pure spin polarized currents. Co-ferrite also shows large magnetostrictive effect, that can be used for enhancing magnetoelectric coupling between ferroelectric phase and ferromagnetic phase of multiferroic nanostructures. Films with (00l) orientation and perpendicular magnetic anisotropy are essential to realize the interesting properties of Co-ferrite thin films. So far, Single crystal substrate such as SrTiO
3
, MgO have been reported for the heteroepitaxial growth of (00l) orientated Co ferrite films. However, from the practical application point of view, it is strongly recommended to prepare orientated Co ferrite films onto thermally oxidized silicon wafers (SiO
2
/Si) Here, a unique process has been developed to deposit Co-ferrite thin films onto thermally oxidized silicon wafer with perpendicular magnetic anisotropy by facing target sputtering. Metallic FeCo was used as underlayers. Co-ferrite films with preferential (001) orientation have been successfully prepared. In this experiment, all the films are deposited without substrate heating. The as-deposited films show nanocrystalline structure with the X-ray diffractometry (XRD) characterization. We have also checked the depth profile of X-ray photoelectron spectroscopy. It is confirmed that the metallic FeCo underlayer got fully oxidized after annealing. Fig. 1 shows hysteresis loops of as-deposited Co-Fe-O films with thickness of 60 nm. The films are deposited without FeCo underlayer (a) and with 3 nm thick of FeCo underlayer. Films deposited without FeCo underlayer show saturation magnetization of around 110 emu/cc. The value is almost one fourth of bulk Co-ferrite. Which indicates that non-magnetic amorphous phase in the nanocrystalline films. However, saturation magnetization dramatically increases with FeCo underlayer of only 3 nm. X-ray diffractometry results clearly exhibit the formation of crystallized spinel structure even without substrate heating with FeCo underlayer. These results suggested that FeCo underlayers can dramatically decrease the crystallization temperature for spinel structures. For all the films, a dramatically increases of coercivity have been achieved by annealing the samples at $800 ^{\circ}\mathrm {C}$ for 2 hours. Fig. 1 shows hysteresis loops of (c) annealed films without FeCo underlayer and (d) annealed films with FeCo underlayer. Both films show preferential perpendicular magnetic anisotropy. However, films deposited onto FeCo underlayer show smaller in-plane coercivity. Which indicate that such a film have better perpendicular magnetic anisotropy as compared with films without FeCo underlayer. Fig. 2 shows XRD results of (a) annealed films without FeCo underlayer and (b) annealed films with FeCo underlayer. It is clearly that films deposited without FeCo underlayer show (311) plane as the strongest diffraction but films deposited onto FeCo underlayer show (004) plane as the strongest diffraction. Further investigation shows three-dimensional lattice parameters determined by in-plane X-ray diffractometry to be $\mathrm {a}= \mathrm {b}=8.43$Å, $\mathrm {c}=8.35$Å. This indicating a compressive c-axis of for the cubic spinel structure.