Enhanced interface perpendicular magnetic anisotropy in Ta|CoFeB|MgO using nitrogen doped Ta underlayers

Abstract

We show that the magnetic characteristics of Ta|CoFeB|MgO magnetic heterostructures are strongly influenced by doping the Ta underlayer with nitrogen. In particular, the saturation magnetization drops upon doping the Ta underlayer, suggesting that the doped underlayer acts as a boron diffusion barrier. In addition, the thickness of the magnetic dead layer decreases with increasing nitrogen doping. Surprisingly, the interface magnetic anisotropy increases to ~1.8 erg/cm 2 when an optimum amount of nitrogen is introduced into the Ta underlayer. These results show that nitrogen doped Ta serves as a good underlayer for Spintronics applications including magnetic tunnel junctions and domain wall devices.*Email: hayashi.masamitsu@nims.go.jp Perpendicular magnetic anisotropy (PMA) is one of the key material parameters in modern spintronics devices. 1 The threshold current needed to switch the direction of magnetic moments in magnetic tunnel junctions and the current required to move magnetic domain walls in magnetic nanowires can be significantly reduced by introducing perpendicularly magnetized materials into the system. 2-5 For device application purposes, the magnetic layer needs to be thin enough such that current induced torques, including conventional spin transfer torques 6, 7 and more recently discovered spin orbit torques, 8-10 can act on the magnetic moments to switch the magnetization and/or move domain walls. An attractive step towards building suitable materials systems for such applications is to make use of the interface magnetic anisotropy, which plays an important role in thin film heterostructures, in particular, in metallic multilayers. 11,12 Interface PMA allows the system to maintain its magnetic anisotropy down to small thicknesses, whereas the bulk anisotropy, including crystalline and magneto-elastic anisotropies, are lowered as the thickness is reduced. 13 Recently, it has been reported that significant interface PMA exists in CoFeB|MgO, 2 a system that is at the heart of advanced magnetic tunnel junctions. As the CoFeB thickness is reduced, the magnetic easy axis changes from lying within the film plane to lying along the film normal owing to the large PMA at the CoFeB|MgO interface.Recent theoretical calculations suggest that the large PMA at this interface is due to an electronic effect. 14 On the other hand, it has been experimentally shown that the underlayer immediately adjacent to the opposite side of the CoFeB layer also plays a critical role in determining the magnitude of PMA. 3,15 Up to date, Ta (or, more recently, Hf 15 ) is considered to be the optimal choice for introducing large PMA for CoFeB|MgO.

Strong perpendicular magnetic anisotropy energy density at Fe alloy/HfO2 interfaces

“…The fit to the Hf dusting series (A) gives a saturation magnetization of M s ¼ 1260 emu=cm 3 and a very small apparent "dead layer" thickness t d % 0:1 nm; both are consistent with previous results from as-deposited Ta/FeCoB/MgO structures. 1,11,14 In contrast, the series B samples indicate t d % 0:8nm and a much larger M s ¼ 1800 emu=cm 3 ; the results are comparable to some previous studies of annealed ($300 C) Ta/FeCoB/MgO samples where the dead layer 15 has been attributed to the undesirable diffusion of Ta into FeCoB, perhaps to the ferromagnet/oxide interface. 16 Thus, we tentatively attribute the thick dead layer in series B samples to the intermixing of Ta and FeCoB during the deposition of the Ta dusting layer.…”

“…While Ta/FeCoB/MgO structures with a thin FM layer typically only exhibit, at most, a weak perpendicular magnetic anisotropy (PMA) in the as-deposited state, 3,14,15 we obtained robust PMA behavior in as-deposited structures with the HfO 2 dusting layer. For example, in Figure 2( …”

Influence of Ta insertions on the magnetic properties of MgO/CoFeB/MgO films probed by ferromagnetic resonance

“…K v is negative and does not vary appreciably in samples where Ta is present, in contrast to the positive value found for zero-insertion samples. The role of Ta with regard to K v is not yet understood, as previously pointed out by Sinha et al [20], and a detailed study of the amount, proximity, and profile of Ta would be necessary to clarify these effects.…”

Current-induced magnetization switching in Pt/Co/Ta with interfacial decoration by insertion of Cr to enhance perpendicular magnetic anisotropy and spin–orbit torques

“…Moreover, as the perpendicular magnetic anisotropy field (H 0 an ) is also one of the key parameters in SOT systems with perpendicular magnetic anisotropy (PMA), so far, considerable research efforts have also been devoted to obtain an enhanced H 0 an to improve the thermal stability of spintronic devices [15,16], such as engineering the interface quality [17,18,19], changing identified SOC materials [16], and utilizing different post processing methods [20]. In a word, all these reports with views are not only to increase the magnitude of θ SH , but also to enhance the PMA.…”