Influence of boron diffusion on the perpendicular magnetic anisotropy in Ta|CoFeB|MgO ultrathin films

Abstract

We have studied structural and magnetic properties of Ta|CoFeB|MgO heterostructures using cross-section transmission electron microscopy (TEM), electron energy loss spectrum (EELS) imaging, and vibrating sample magnetometry. From the TEM studies, the CoFeB layer is found to be predominantly amorphous for as deposited films, whereas small crystallites, diameter of ∼5 nm, are observed in films annealed at 300 °C. We find that the presence of such nanocrystallites is not sufficient for the occurrence of perpendicular magnetic anisotropy. Using EELS, we find that boron diffuses into the Ta underlayer upon annealing. The Ta underlayer thickness dependence of the magnetic anisotropy indicates that ∼0.2 nm of Ta underlayer is enough to absorb the boron from the CoFeB layer and induce perpendicular magnetic anisotropy. Boron diffusion upon annealing becomes limited when the CoFeB layer thickness is larger than ∼2 nm, which coincides with the thickness at which the saturation magnetization MS and the interface magnetic anisotropy KI drop by ∼20%. These results show the direct role which boron plays in determining the perpendicular magnetic anisotropy in CoFeB|MgO heterostructures.

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

“…200 C) annealing. [19][20][21] In the inset of Fig. 2(b), we also show the effective PMA energy density K eff for the series B …”

“…There are generally two important functions of this annealing process: (i) avoidance of the over-oxidation of the FeCoB surface that occurs during MgO deposition 13,22 and (ii) promotion of the out-diffusion of boron from the initially amorphous FeCoB 13,19 to obtain a more ordered, crystalline FeCo/MgO interface. Our results here indicate that the first function is quite important.…”

The effect of growth sequence on magnetization damping in Ta/CoFeB/MgO structures

“…When sputtered on top of CoFeB, the Ta layer might intermix with the CoFeB because the Ta is a heavy metal and has a higher kinetic energy emitted from the target. At the same time, the boron atoms diffuse out from the CoFeB layer and are absorbed easily by Ta layer during annealing [8,28,32,33]. Considering the equation (2), we believe that the B and Ta atoms cause to increase by the increase of the mixing conductance in the CoFeB/Ta interface.…”

“…Current-induced spin-transfer torque (STT) is very important in high density magnetic media and spintronics devices [1][2][3][4][5]. Regarding the energy consumption, it is critcal to find out the method to reduce the critical current that switches the spin direction in STT-MRAM application.…”

Tunnel Junction with Perpendicular Magnetic Anisotropy: Status and Challenges

“…As has been mentioned, with (001) texture MgO as template, the amorphous CoFeB can crystalized into bcc (001) texture at high temperature, which is critical to generate high TMR ratio, as well as interfacial PMA in CoFeB/MgO structure based MTJ. Moreover, the annealing temperature dependent tendency of magnetization M and Keff can be observed in Figure 3 [30], where the initial increase can be attributed to the B absorption by Ta, and the later decrease arises from the formation of additional magnetic dead layer. Referring to in-plane magnetic anisotropy based MTJ, this procedure should be performed in an external magnetic field around 10 kOe to fix the magnetization direction reference layer through the exchange bias with pinned layer.…”