Perpendicular Tunnel Junctions Research Articles

Perpendicular magnetic tunnel junction with W seed and capping layers

We present a study on perpendicular magnetic tunnel junctions with W as buffer and capping layers. A tunneling magnetoresistance of 138% and an interfacial magnetic anisotropy of 1.67 erg/cm2 were obtained in optimally annealed samples. However, after extended annealing at 420 °C, junctions with W layers showed extremely small resistance due to interdiffusion of W into the MgO barrier. In contrast, in Ta-based junctions, the MgO barrier remained structurally stable despite disappearance of magnetoresistance after extended annealing due to loss of perpendicular magnetic anisotropy. Compared with conventional tunnel junctions with in-plane magnetic anisotropy, the evolution of tunneling conductance suggests that the relatively low magnetoresistance in perpendicular tunnel junctions is related to the lack of highly polarized Δ1 conducting channel developed in the initial stage of annealing.

Enhanced tunnel magnetoresistance and electric-field effect in CoFeB/MgO/CoFeB perpendicular tunnel junctions with W underlayer

We investigated the dependence of tunnel magnetoresistance (TMR) and its electric-field effect in CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJ) on different underlayer (UL) materials. We observed enhancements in the TMR ratio and its temperature dependence as well as electric-field effect in MTJs with W UL, as compared to those in MTJs with a conventional Ta UL. This is attributed to better thermal stability of perpendicular magnetic anisotropy (PMA) in W/CoFeB/MgO, which sustains up to 380 °C, compared to that of Ta/CoFeB/MgO which starts to degrade at 270 °C. This demonstrates that the PMA in a CoFeB/MgO structure and its electric-field dependence can be enhanced by the careful selection of underlayer, opening the way for the realization of electric-field effect-driven spintronic devices.

TEM studies of microstructure, interfaces, and intermixing of FePt/MgO/FePt/Pt/Cr(Ru) films

We have investigated the microstructure, interfaces, and intermixing in sputter-deposited FePt/MgO/FePt/Pt/Cr(Ru)/SiN/Si magnetic tunnel junctions by high-resolution transmission electron microscopy (HRTEM) and correlated these results to the magnetic properties. Cr90Ru10 is used as a texturing layer for the growth of perpendicular magnetic anisotropy (PMA) films. HRTEM confirmed the formation of the desired [001] texture in cubic Cr90Ru10 as well as in the adjacent L10-FePt layer using our growth methods. A 1.5 nm layer of MgO was deposited as a tunnel barrier for magnetic tunnel junction in an attempt to fabricate fully perpendicular tunnel junctions of these materials. The HRTEM observations, however, show that the MgO layer was partially crystalline but randomly oriented. The top FePt layer was also found to grow as discrete islands with random orientations and is mostly fcc crystal structure reducing the utility of this multilayer in PMA tunnel junction devices. The fine probe energy-dispersive X-ray spectroscopy showed the interdiffusion of Cr to Pt/FePt layers. The magnetic properties show out-of-plane coercivity larger than that of the in-plane values but with a loss of squareness, consistent with the loss of [001] texture and of L10 structure in the top magnetic layer.

Hard layer demagnetization by soft layer cycling in a MgO-based perpendicular magnetic tunnel junction

We report here that in perpendicular tunnel junction the hard layer demagnetizes when the soft layer is cycled. This happens faster when the cycling field is closer to the reversal field of the hard layer. Magnetic force microscopy imaging done at different stages of the cycle after several loops show compact demagnetized areas surrounded by large saturated zones in the hard layer. A mechanism based on interlayer magnetostatic coupling induced by the stray field created by domain wall in the soft layer is presented.