Buffer influence on magnetic dead layer, critical current, and thermal stability in magnetic tunnel junctions with perpendicular magnetic anisotropy

Abstract

We present a thorough research on Ta/Ru-based buffers and their influence on features crucial from the point of view of applications of MTJs, such as critical switching current and thermal stability. We investigate devices consisting of buffer/FeCoB/MgO/FeCoB/Ta/Ru multilayers for three different buffers: Ta 5 / Ru 10 / Ta 3, Ta 5 / Ru 10 / Ta 10 and Ta 5 / Ru 20 / Ta 5 (all thicknesses in nm). In addition, we study systems with a single FeCoB layer deposited above as well as below the MgO barrier. The crystallographic texture and the roughness of the buffers are determined by means of XRD and atomic force microscopy measurements. Furthermore, we examine the magnetic domain pattern, the magnetic dead layer thickness and the perpendicular magnetic anisotropy fields for each sample. Finally, we investigate the effect of the current induced magnetization switching for nanopillar junctions with lateral dimensions ranging from 1 µm down to 140 nm. Buffer Ta 5 / Ru 10 / Ta 3, which has the thickest dead layer, exhibits a large increase in the thermal stability factor while featuring a slightly lower critical current density value when compared to the buffer with the thinnest dead layer Ta 5 / Ru 20 / Ta 5.

Influence of intermixing at the Ta/CoFeB interface on spin Hall angle in Ta/CoFeB/MgO heterostructures

“…This is a result of transition to the superparamagnetic state, which probably appears due to local thickness discontinuity in the CoFeB layer resulting from a thick magnetic dead layer. The fact that strong mixing at the Ta/CoFeB interface can result in a MDL, was already reported before [31][32][33] . Actually, MDL for the sample with 5 nm of Ta is the widest (Table I).…”

“…This is a result of much smaller interface anisotropy contribution to effective anisotropy 41 for the sample with 5 nm of amorphous Ta due to a thick magnetic dead layer (MDL). The fact that strong mixing at the Ta/CoFeB interface can result in an MDL has already been reported 40,42,43 . Actually, MDL for the sample with 5 nm of Ta is the widest and reaches 0.55 nm, while for 10 nm of Ta and 15 nm MDL is 0.46 nm and 0.39 nm, respectively.…”

Enhancement of perpendicular magnetic anisotropy and Dzyaloshinskii–Moriya interaction in thin ferromagnetic films by atomic-scale modulation of interfaces

“…Since intermixing is the main reason for MDL formation 57,58 , we suggest that I d coincides with the Δ top for the CoFeSiB/Ta and CoFeSiB/Ru interfaces. The defined MDL depths for Ta and Ru are in good agreement with the data found for the CoFeB/Ta(Ru) interfaces 53,59,60 .…”

High temperature ferromagnetic resonance study on pMTJ stacks with diffusion barrier layers

“…At the core of the MRAM lies the array of magnetic tunnel junctions with perpendicular magnetic anisotropy (pMTJ), which requires stringent deposition conditions and intricate stack design. In a pMTJ utilizing CoFeB as the free layer, the film interfaces and crystallinity are key factors in promoting high perpendicular magnetic anisotropy (PMA) and high tunnel magnetoresistance (TMR) ratio [3][4][5][6][7][8][9][10][11][12]. In addition, the pMTJ stack should have the thermal robustness to withstand the 400 °C annealing temperature in order to be compatible with CMOS-BEOL processes [13][14][15][16].…”