Enhancement of Dzyaloshinskii-Moriya interaction in Pt/CoFe(B)/MgO structure by suppression of FePt phase according to boron addition

Here, we present the trend of the surface DMI (DS = D tCoFe(B)) according to the boron concentration in Pt/CoFe(B)/MgO structure, showing sharp increase (0.6 to 1.7 pJ/m) at 0~ 4 %. Such unexpected increase is interpreted with the FePt phase between Pt and CoFe(B) layer. Through the x-ray diffraction (XRD) analysis, we confirmed that the disordered fcc FePt phase exists in the CoFe (boron 0%) sample. It was also confirmed that the phase is remarkably decreased by small addition of boron (4 %) into the CoFeB layer. The correaltion between the fraction of FePt and DS of the system could be interpreted based on the lower work function difference between nonmagnetic and ferromagnetic metals. Comparison of DS according to the presence or absence of FePt spacer was carried out in the Pt/(FePt)/Fe/MgO structure, and a trend similar to that of the above experiment was confirmed.

Our results provide implications. Primarily, it suggests that disordered FePt phase can be formed as measurable thickness by inter-mixing at the Pt and Fe(Co) interface, and that the magnitude of DS could be significantly decreased by this phase. It is also proposed that the formation of FePt phase can be effectively suppressed by small addition of boron into CoFeB layer by glass former effect, while maintaining the crystallinity of CoFeB. By this approach, we report notably enhanced DS (up to 1.75 pJ/m) compared to other CoFeB-based systems (up to 1.29 pJ/m), which brings CoFeB one step closer to grafting into the skyrmion-based devices.