- Chemical properties of individual elements and compositions of the MTJ sensor
With the TEM and STEM imaging seen before, during and after annealing, the composition of CoFeB – MgO – CoFeB differed. During the magnetron sputtering, a preferential BOx forms in the CoFeB layer. During annealing, Mg-B-O, which is an interfacial layer, develops due to simultaneous diffusion of B. In the 1nm oxide layer, a misaligned Mg – B – O polycrystalline other than the expected Mg-O layer. However, an increase in TMR was still observed. An image from the TEM showed an amorphous layer formed between MgO and CoFeB layer of thickness of about 0.88nm shown by an arrow as displayed in the image below.
An EELS data obtained from the aberration-corrected Nikon UltraSTEM also affirmed the formation of BO. For the as-grown instance, the boron is found in the CoFeB layer (Char et al., 2009). Additionally, BOx is seen in the interface between MgO and CoFeB layers. The preceding annealed occurrence, B, is not observed in the bottom CoFeB layer. However, BOx is seen in the amorphous MgO interfacial layer. The results seen suggested the diffusion of B element into MgO during the latter process forming the observed configuration. The uniformity of the BOx is seen as the yellow line in the above image (Char et al., 2009). Additionally, the boron element is not observed in the interface layer after annealing, suggesting that the unoxidized B element diffused from the CoFeB electrodes, leaving the CuFe compound. Conforming to this, since B is an element that forms glass when it diffuses out of the CoFeB electrodes, crystallization into a bcc lattice structure is enabled. This improves the TMR of the MTJ sensor, related to tunnelling coherence enhancement. Additionally, an affirmation of the Mg – B – O formation is the different chemical and electron properties observed during and after annealing. The chemical properties had sharp layer interfaces.