Abstract
Spin-Torque-Transfer Magnetic Random Access Memory (STT-MRAM) has the potential to become a universal memory due to its superior properties compared with conventional memory technologies. However, the main challenge for implementing STT writing mode in high-density and high-speed memory is the substantial reduction of the intrinsic current density Jc0 required to switch the magnetization of the free layer (FL) while maintaining high thermal stability required for long-term data retention.
The intrinsic switching current reduction can be pursued by using materials with low saturation magnetization (Ms), low damping constant (α), high spin polarization (η) and high magnetic anisotropy energy density (Ku) at low dimension. Previous experiments showed that CoFeB incorporated in MgO-based MTJ is one of the most efficient materials for this purpose. In this work chromium was substituted into CoFeB alloys to further reduce their magnetization attributed to the antiferromagnetism of Cr. Films in composition of Cr at 30% showed a reduced magnetization of 300 emu/cm3 and an enhanced coercive field of 500 Oe, making this material suitable for device application.
Moreover, the switching current density Jc0 is predicted to be reduced significantly by using materials with perpendicular magnetic anisotropy (PMA), compared with the normal in-plane ones. Perpendicular Magnetic anisotropy was found in crystalline D022 Mn2.5Ga films with optimal magnetic anisotropy energy of 0.5 ×107 erg/cm3, which leads to high thermal stability. A low saturation moment (Ms = 260 emu/cc) was investigated, attributed to the strong antiferromagntic coupling between Mn atoms at different sites. With a reported high spin polarization of 88%, D022- Mn2.5Ga film has the potential to be used in spintronic devices.
Amorphous ferrimagnetic GdxFe93-xCo7 films were also found to exhibit low magnetization and perpendicular magnetic anisotropy, which is attributed to the competition between antiferromagnetic coupling of rare-earth (RE) with transition-metal (TM) ions and ferromagnetic interaction between the TM ions. Furthermore, low magnetization was achieved at room temperature in previously unreported high-Gd region of x=52-59, accompanied by a large PMA with coercivity reaching ~6.6 kOe. Our findings have broadened the composition range of transition metal-rare earth alloys for designing PMA films, making it attractive for tunable magnetic anisotropy in nanoscale devices.
In addition to Ms and η, one can also reduce FL intrinsic damping constant for lowering Jc0, but practically PMA films often have higher damping constant α due to strong spin-orbit coupling. With reported low damping constant of 0.0025, CoFeGe film is promising spintronic material. Perpendicular magnetic anisotropy was achieved in the MgO/CoFeGe/MgO structure with an optimized magnetic anisotropy energy density of 2 106 erg/cm3, which is contributed by the interfacial anisotropy between CoFeGe and MgO. The B2-ordered CoFeGe is not half metallic but has a perfectly spin-polarized ∆1 band along the [001] direction, thus the perfect spin transfer efficiency with MgO barrier is expected in CoFeGe-base MTJs.
