Tunable Perpendicular Magnetic Anisotropy in Amorphous Gd_xCo_1-x Alloys

Amorphous magnetic alloys have been at the forefront in the development of magnetic random-access memory due to their favorable perpendicular magnetic anisotropies (PMA) and deterministic read/write mechanisms with magnetoresistance and spin-orbit torques. While amorphous ferromagnets are well researched, they are limited by low switching speeds (in ∼ns) and constrained in their read/write energy-efficiency by the large angular momentum, which requires a large critical current for switching. Unlike ferromagnets, ferrimagnetically coupled systems offer faster switching dynamics due to their stiffer exchange interactions, minimal stray field due to their lower net saturation magnetization, and tunable anisotropy and net magnetization due to the existence of two counteracting sublattices. Here, we report on amorphous thin films of GdCo that have a ferrimagnetic order (antiparallel spin orientation) with bulk-like PMA and stoichiometries near its magnetic-compensation that coincide with lower total angular momentum compared to ferromagnets and the potential for much higher switching speeds on the order of tens of picoseconds. Sputter-deposited heterostructures of Ta(3 nm)/Pt(3 nm)/Gd_xCo_1-x(t)/Pt(5 nm) on Si/SiO₂(3 nm) substrates exhibit bulk-like PMA for Gd_xCo_1-x thicknesses of 5-12 nm and stoichiometries where x = 18-40%. A strong uniaxial anisotropy for out-of-plane magnetization with an anisotropy field (H_k) of 0.24 T was observed when the Gd:Co ratio was 35:65, which is near the composition that results in a magnetic compensation at room temperature. Interestingly, it was found that strong PMA in GdCo is possible only with a non-negligible amount of oxygen in the thin film. Oxygen flow rates were adjusted between 0-0.8 sccm during deposition and a maximum effective anisotropy energy density of 1 x 10⁵ erg cc^-1 was obtained for a metal cation to oxygen (R:O) ratio of 50:50, which corresponds to a flow rate of 0.5 sccm. Furthermore, X-ray photoelectron spectroscopy (XPS) revealed that with an increase in oxygen concentration, Co oxidizes preferentially over Gd, resulting in Co-deficient stoichiometries from lower sputter yields at higher oxygen flow rates. Strong PMA is observed in a stoichiometry of Gd₂₁Co₂₈O₅₁ with a Gd:Co ratio of 3:4 and an R:O ratio of 1:1, which suggests that the strong ferrimagnetic order likely arises through a superexchange-like coupling between the magnetic cations (R: Gd,Co) via the non-magnetic anion (O) in this amorphous alloy. With the single layer optimized, super-lattice heterostructures could be developed. Even greater PMA is achieved in a heterostructure with 10 repetitions of the Gd₃₅Co₆₅ alloy. These films exhibit an anisotropy field on the order of 11.6 kOe, which corresponds with switching speeds as low as 30 ps. The combination of ferrimagnetic ordering (lower total angular momentum) in amorphous GdCo (reduced thermal budget) and well-defined pathways for a strong PMA (low-energy magnetization switching due to the very short picosecond-range write pulses required) make these thin films a suitable choice for ultrafast energy-efficient memory devices.