Temperature-dependent Mn-diffusion modes in CoFeB- and CoFe-based magnetic tunnel junctions: Electron-microscopy studies

Abstract

We show that Mn atoms diffuse with two different mechanisms at high and low temperatures in CoFeB-and CoFe-based magnetic tunnel junctions. By combining high resolution and scanning transmission electron microscopy, we reveal that below 300°C, the amorphous CoFeB and the textured CoFe are equally effective in blocking the diffusion of Mn, contradicting the conventional wisdom that the diffusion occurs primarily along grain boundaries. Below 300°C, Mn diffusion in crystalline CoFe occurs through the bulk and is assisted by oxygen atoms which only diffuse parallel to the bcc close-packed ͑110͒ plane. Above 300°C, Mn diffuses through vacancies along the grain boundaries of CoFe and in the bulk of amorphous CoFeB. A universal diffusion temperature is proposed based on an isokinetic relation. The physics of spintronics devices 1-5 is studied in terms of their electronic, magnetic, and structural properties. These properties are intimately connected. The first two have been reasonably understood. The much less understood structural property of magnetic tunnel junctions ͑MTJs͒ includes the critical question of their thermal stability. It is known that Mn atoms diffuse into the barrier layer and act as magnetic impurity, which can greatly reduce the tunneling magnetoresistance ͑TMR͒ of MTJs in a manner similar to spin valves. [6][7][8][9]11 Despite a great deal of work relating Mn diffusion to annealing temperature T a ͑Refs. 12-14͒ and to oxygen diffusion, 9 and the speculation 14 that at moderate T a amorphous CoFeB can block Mn diffusion more effectively than polycrystalline CoFe, there is not a clear physical picture of Mn-diffusion mechanism and what roles individual microstructures play.It was shown that the measured diffusivity of Mn in Co cannot account for the observed Mn profile in annealed MTJs, and oxygen-assisted diffusion was proposed 9 as the most likely Mn-diffusion mechanism. Usually, diffusion along grain boundaries is much faster than in the bulk.10 This is the assumed reason for why amorphous CoFeB can block Mn ͑or oxygen͒ diffusion. This assumption implies that diffusion mechanisms in crystalline CoFe and amorphous CoFeB are different. Then, is it a coincidence that the TMRs for both types of MTJs peak at the same annealing temperature of just below 300°C?In this paper, we first present observations from CoFeBand CoFe-based MTJs using high resolution transmission electron microscopy ͑HRTEM͒ and high angle annular dark field ͑HAADF͒ image with line scan energy dispersive spectrum ͑EDS͒ in scanning transmission electron microscopy ͑STEM͒ mode. We show that both amorphous CoFeB and textured polycrystalline CoFe can effectively block Mn diffusion at low temperature, while nontextured polycrystalline CoFe cannot. Based on this observation, we propose that two different diffusion mechanisms are at play. At low temperature, the dominant mechanism is oxygen-assisted Mn diffusion. Such diffusion occurs more easily through the bulk of the crystalline CoFe but only parallel to the close-packed ͑110͒ plane, whi...

X-ray diffraction analysis and Monte Carlo simulations of CoFeB-MgO based magnetic tunnel junctions

“…Up till now, 604% is the highest TMR obtained at room temperature in the pseudo spin valve (P-SV) after annealing at T a ¼ 475 C. 6 The TMR ratios for the exchange biased spin valve (EB-SV) with the IrMn antiferromagnetic (AF) layer start to decrease at T a > 400 C due to the diffusion of Mn and Ru into the MgO barrier. 7,8 Below this critical temperature, TMR increases in the same way as in the P-SV junctions and reaches the highest TMR ratio of 361% (Refs. 9 and 10) at T a ¼ 425 C. Crystallization and texturing of CoFeB are important in the applications of spin valves because of their influence on TMR, 11 magnetic anisotropy, 12 and interface scattering.…”

Wheatstone bridge sensor composed of linear MgO magnetic tunnel junctions

“…These values are lower ͑for TMR͒ or higher ͑for RA͒ than those usually achieved with this layer stack ͑over 60% TMR with linear response, RA around 800 ⍀ m 2 , and breakdown voltage around 1.2 V͒. 11 The slight decrease in TMR in J2 and J3 may originate from the degradation of exchange bias and sample surface oxidation caused by external heating by the hotplate. In addition, the transfer curves show around 7-9 Oe offset field due to the Neel coupling between pinned and free layer, 7 which becomes crucial in the MTJs with thinner barrier and free layer.…”

Symmetry-dependent electron-electron interaction in coherent tunnel junctions resolved by measurements of zero-bias anomaly

“…35 No Mn was found in the barrier and this scenario is not consistent with the fact that after annealing ZBA is reduced. 27,52 During the annealing process, the CoFeB layer partially converts to crystalline (001) CoFe with boron diffused away and this process reduces disorder (R sq is smaller), which leads to smaller ZBA following Eq. (1).…”