Strong perpendicular exchange bias in sputter-deposited CoPt/CoO multilayers

Abstract

Strong perpendicular exchange bias was realized in sputter-deposited CoPt/CoO multilayers. The as-deposited multilayer shows strong perpendicular magnetic anisotropy (PMA) which remains up to a critical CoPt layer thickness as thick as 4 nm. After perpendicular field cooling, the multilayer with antiferromagnetic CoO and ferromagnetic CoPt interfaces exhibits large perpendicular exchange bias of 1730 Oe. Strong PMA in the CoPt/CoO multilayer is mainly attributed to the positive magnetoelastic energy due to the remarkable in-plane tensile stress originating from the local epitaxial growth. The mechanism for the exchange bias was explained in terms of the interfacial spin structure.

Magnetoelastically induced perpendicular magnetic anisotropy and perpendicular exchange bias of CoO/CoPt multilayer films

“…2(a) reveal that all the samples are well crystallized with preferred fcc CoPt (111) orientation. This can be attributed to the addition of Pt, since alloy thin film with Pt component is energetically favorable to form strong (111) texture along the growth direction [14]. Moreover, the multilayer structure can be confirmed by the satellite peaks around CoPt (111) main peak, and the continuous multilayer structure with sharp component transition at the CoO5nm/ CoPt5nm interface is also identified clearly by the cross-section TEM image shown in Fig.…”

“…More importantly, with the improvement of the top surface flatness (roughness value changes from 3.862 nm to 1.534 nm after annealing at 300°C) we can confirm that the flatness of CoO/CoPt interface is improved simultaneously, due to the removal of defects and the coalescence of grains. It is well known that the improvement of FM/AFM interface flatness is favorable to the enhancement of PMA but PEB [14].…”

“…In fact, the lattice mismatch between fcc CoPt (a CoPt ¼0.38 nm) and fcc CoO (a CoO ¼ 0.43 nm) is around 12% [14]. This will lead to CoPt layer in-plane tensile stress since CoPt is locally epitaxial growth on CoO layer in the [CoO5nm/CoPt5nm] 5 multilayer films.…”

Effect of MgO/Co interface and Co/MgO interface on the spin dependent transport in perpendicular Co/Pt multilayers

“…Fig. 4 26 It is known that the crystallographic property of CoO plays a critical role in determining the direction and strength of exchange coupling at the interface. When the temperature is lower than the Neel temperature, a shift of hysteresis loop, i.e., exchange bias, was observed due to the exchange coupling between antiferromagnet and ferromagnet.…”

“…Generally, strong exchange coupling enhances H c . 26,31 Therefore, CoO insertion at top and bottom interface have different effect on H c .…”