High Density Spin Transfer Torque Research Articles

MnGa-based fully perpendicular magnetic tunnel junctions with ultrathin Co2MnSi interlayers

Because tetragonal structured MnGa alloy has intrinsic (not interface induced) giant perpendicular magnetic anisotropy (PMA), ultra-low damping constant and high spin polarization, it is predicted to be a kind of suitable magnetic electrode candidate in the perpendicular magnetic tunnel junction (p-MTJ) for high density spin transfer torque magnetic random access memory (STT-MRAM) applications. However, p-MTJs with both bottom and top MnGa electrodes have not been achieved yet, since high quality perpendicular magnetic MnGa films can hardly be obtained on the MgO barrier due to large lattice mismatch and surface energy difference between them. Here, a MnGa-based fully p-MTJ with the structure of MnGa/Co2MnSi/MgO/Co2MnSi/MnGa is investigated. As a result, the multilayer is with high crystalline quality, and both the top and bottom MnGa electrodes show well PMA. Meanwhile, a distinct tunneling magnetoresistance (TMR) ratio of 65% at 10 K is achieved. Ultrathin Co2MnSi films are used to optimize the interface quality between MnGa and MgO barrier. A strong antiferromagnetic coupling in MnGa/Co2MnSi bilayer is confirmed with the interfacial exchange coupling constant of −5erg/cm2. This work proposes a novel p-MTJ structure for the future STT-MRAM progress.

Perpendicular magnetic tunnel junction with thin CoFeB/Ta/Co/Pd/Co reference layer

Integration of high density spin transfer torque magnetoresistance random access memory requires a thin stack (less than 15 nm) of perpendicular magnetic tunnel junction (p-MTJ). We propose an innovative approach to solve this challenging problem by reducing the thickness and/or moment of the reference layer. A thin reference layer structure of CoFeB/Ta/Co/Pd/Co has 60% magnetic moment of the conventional thick structure including [Co/Pd] multilayers. We demonstrate that the perpendicular magnetization of the CoFeB/Ta/Co/Pd/Co structure can be realized by anti-ferromagnetically coupling to a pinned layer with strong perpendicular anisotropy via Ruderman-Kittel-Kasuya-Yosida exchange interaction. The pMTJ with thin CoFeB/Ta/Co/Pd/Co reference layer has a comparable TMR ratio (near 80%) as that with thick reference layer after annealing at 280 °C. The pMTJ with thin reference layer has a total thickness less than 15 nm, thereby significantly increasing the etching margin required for integration of high density pMTJ array on wafers with form factor of 300 mm and beyond.

Amorphous ferromagnetic CoSiB/Pd multilayer with perpendicular magnetic anisotropy

Perpendicular magnetic anisotropy has been studied for possible use in high-density spin transfer torque magnetic random access memories. The key issues of high-density spin transfer torque magnetic random access memories are decreasing the switching current and have the high thermal stability. In order to solve these problems, the approach of developing a new amorphous ferromagnetic material for use as a pinned layer for a multilayer with a low saturated magnetization value has been suggested. In a previous study, amorphous ferromagnetic material CoSiB was shown to have a low saturated magnetization (Ms = 470 emu/cm3) and a modest anisotropy constant (Ku = 1,500 erg/cm3). Then, we investigated the perpendicular magnetic anisotropy of a [CoSiB tCoSiB nm/Pt 1.4 nm]5 multilayer, where tCoSiB are 0.1, 0.2, 0.3, 0.4, and 0.5 nm. In this study, we present a new multilayer type based on the previous study, and this multilayer, CoSiB/Pd multilayer has the perpendicular magnetic anisotropy. In this letter, we have investigated the perpendicular magnetic anisotropies of a [CoSiB tCoSiB nm/Pd 1.3 nm]5 multilayer, where tCoSiB are 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 nm and [CoSiB 6.3 nm/Pd tPd nm]5 multilayer, where tPd are 1.1, 1.2, 1.3, 1.4, 1.5, and 1.6 nm.

Perpendicular magnetic anisotropy of amorphous ferromagnetic CoSiB/[Pt,Au] multilayer

Perpendicular magnetic anisotropy is being widely studied as a possible candidate for a high-density spin-transfer torque magnetic random access memory. The key issues of a high-density spin-transfer torque magnetic random access memory are decreasing the switching current and the high thermal stability. In order to solve these problems, two approaches are suggested: One is the development a new amorphous ferromagnetic material as a pinned layer for a multilayer with a low saturated magnetization (Ms) value because of the interface roughness between the two layers. The other is a search for the most suitable materials with high perpendicular magnetic anisotropy in order to have high thermal stability. In this work, we present an amorphous ferromagnetic Co75Si15B10 material and compare the magnetic properties of a [CoSiB (0.3, 0.4, 0.5 nm)/Pt (1.4 nm)]5 multilayer and new combinations [CoSiB (0.3, 0.4, 0.5 nm)/Au (1.5 nm)]5.