Multi-layer magnetism and thermal stability in perpendicular magnetic tunnel junctions
Thermal stability is one of the critical issues for applications of nano-magnets for spin-logic applications. Our work is focused on the thermal stability in perpendicular magnetic tunnel junctions (p-MTJs) for MRAM and STT-RAM purposes. Most of the research so far has been focused on p-MTJs based on CoFeB/MgO interfacial anisotropy as the sandwich structures with bcc ferromagnetic electrodes and MgO spacer have large magnetoresistance. It has been demonstrated that this interfacial anisotropy by itself is not sufficient to reduce the p-MTJ diameter down to 20 nm. To overcome this problem, we have proposed and investigated hard-soft composite structures: [Co/Pt multilayers]/ (non-magnetic element) /CoFeB/MgO, to control the perpendicular magnetic anisotropy (PMA) of the CoFeB soft layer by exchange coupling with Co/Pt multilayers having bulk anisotropy. Ta has been studied as the first non-magnetic insertion element, since it helps in crystallization of CoFeB by absorbing the B. The other elements studied as insertions are V and Zr, since they have low damping constants. A micromagnetic model has been set up on the basis of experimental results and ab-initio calculations to study the effect of insertion thickness and damping parameter on switching current density and switching time. To understand the mechanism of CoFeB/MgO interfacial anisotropy, low temperature scaling of interface anisotropy (Ks(T)) and saturation magnetization (Ms(T)) is measured, since at low temperature (T), Ks(T) vs Ms(T) scaling is sensitive to the details of the anisotropy mechanism. For the first time, we experimentally show that for CoFeB/MgO, Ks(T) scales as Ms(T) 2.2, hence indicating the two-ion type anisotropy as the dominant mechanism.