Role of boron diffusion in CoFeB/MgO magnetic tunnel junctions

Abstract

Several scientific issues concerning the latest generation read heads for magnetic storage devices, based on CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) are known to be controversial, including such fundamental questions as to the behavior and the role of B in optimizing the physical properties of these devices. Quantitatively establishing the internal structures of several such devices with different annealing conditions using hard x-ray photoelectron spectroscopy, we resolve these controversies and establish that the B diffusion is controlled by the capping Ta layer, though Ta is physically separated from the layer with B by several nanometers. While explaining this unusual phenomenon, we also provide insight into why the tunneling magnetoresistance (TMR) is optimized at an intermediate annealing temperature, relating it to B diffusion, coupled with our studies based on x-ray diffraction and magnetic studies.

Quasiparticles near domain walls in hexagonal superconductors

“…(3) is straightforward, see, e.g., Ref. 15, and we find that for each direction of semiclassical propagation satisfying the condition…”

“…The quasi-two-dimensional tetragonal case, which is applicable to Sr 2 RuO 4 and the ironbased superconductors, was previously studied in Ref. 15. The heavy-fermion superconductor UPt 3 has a complicated phase diagram, with two distinct phases (called A and B phases) even in the absence of external magnetic field, see Ref.…”

Anomalous Nernst effect and three-dimensional temperature gradients in magnetic tunnel junctions

“…The Ru capping layer is deposited by e-beam evaporation and prevents the underlying layers from oxidation. Ex-situ annealing with applied bias field is performed to crystallize the amorphous CoFeB electrodes and the MgO layer to obtain coherent interfaces and to activate the diffusion of B into the Ta layers [37][38][39]. Afterwards elliptical MTJs are patterned to a size of 6μm × 4μm with the long axis parallel to the direction of the magnetic field applied during the annealing by lithography processes.…”

Probing complex heterostructures using hard X-ray photoelectron spectroscopy (HAXPES)

“…Beni et al [37] used a similar approach to estimate layer resolved compositional variation in complex Al-Cr-Fe metallic alloy systems. Mukherjee et al [38] used a similar methodology to estimate thickness of the CoFeB and MgO layer in CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) to resolve an outstanding controversy [39], establishing that boron did not diffuse beyond one or two atomic planes at the interface in a suitably formed device structure.…”