Control of crystallization and magnetic properties of CoFeB by boron concentration

Abstract

Controlling the crystallinity of CoFeB is the most essential issue for designing various spintronics devices. Here we show the microstructure and magnetic properties of MgO/CoFeB/MgO structures for various boron concentration. We present the effect of boron on the crystallinity of CoFeB into two categories: the critical boron concentration (5 ~ 6%) at which CoFeB crystallizes and the effect of remaining boron (0 ~ 5%) in the crystallized CoFeB. And the trends of the saturation magnetization, exchange stiffness, exchange length, domain wall energy and Gilbert damping constant according to the boron concentration are provided. Abrupt variation of properties near the critical boron concentration (5 ~ 6%) and a noticeable change in the crystallized CoFeB (0 ~ 5%) are confirmed, revealing a clear causal relationship with the structural analysis. These results propose that the crystallization, microstructure, and major magnetic properties of CoFeB are governed by the amount of boron, and emphasize the need for delicate control of boron concentration.

Enhancement in Interfacial Dzyaloshinskii–Moriya Interaction in Pt/CoFe(B)/MgO Structures by Suppression of FePt Interface Phases with the Addition of Boron

“…With the addition of 4% boron to the CoFe sample, the D S abruptly increases to −1.75 pJ/m, which is almost three times larger than that of the 0% sample and one of the largest values in the CoFeB-based systems (the maximum iDMI values of the systems including CoFeB are listed in Table , and the highest values in similar Pt/CoFeB systems deposited by sputtering appear as filled squares in Figure f for comparison). ,,,– Notably, this extraordinary trend is reproduced in both methods evaluating D H and D k . The reason for this abrupt increase in iDMI is ascribed to the notable decrease in the fraction of the interfacial FePt phase (Figure ) and the mere disturbance of the crystallinity of CoFeB (Figures and ), as confirmed in the microstructure analysis. ,,, …”

“…With more boron (above 12%), the CoFeB layer turns into an amorphous-dominant structure. Based on the literature showing a proportional relationship between the magnitude of iDMI and microstructural homogeneity, ,, the decrease in D S between 4 and 12% boron concentration is attributed to the crystallinity degradation of CoFeB by increasing the boron concentration . The D S hardly changes between 12 and 20% boron concentrations, where the CoFeB layer is amorphous, and this result is a basis for the above analysis.…”

“…30 The stack used in this study (Pt/CoFeB (1.5 nm)/MgO, consisting of an HM underlayer and a relatively thin CoFe(B) thickness) is expected to be an easier system for boron to diffuse into the underlayer than the previous stack (MgO/ CoFeB (22 nm)/MgO). 30,43 Thus, the boron-induced crystalline degradation and amorphization of the CoFeB layer occurs at higher concentrations (0−8 and 8−12%, respectively) than in previous studies. Between 0 and 4% of boron concentration, the CoFe (111) peaks are formed as shoulder peaks (43.7°± 0.1°), next to the MgO (002) peaks.…”

“…Herein, we focus on the effect of the glass-former boron on the iDMI since the correlation has not been highlighted, despite the clear causal effect of boron on the crystallinity of CoFeB. 30 We investigate the trend of iDMI and the microstructural characteristics according to the boron concentration (x, 0−20%) in the Pt (4 nm)/(Co 10 Fe 90 ) 100−x B x (1.5 nm)/MgO structures. As a result, it is experimentally observed that the magnitude of D S is significantly influenced by two microstructural features: the crystallinity of the CoFe(B) layer 24,31,32 and the presence of ultrathin FePt at the interface between the Pt and CoFe(B) layers.…”

“…23,24,33,34 The two features are sensitively controlled by the fraction of boron in CoFe(B). 30,35 Interestingly, the magnitude of iDMI changes more sensitively (from −0.63 to −1.75 pJ/m in the region of 0−4% boron concentration) with the changing degree of formation of the FePt phase than with the changing crystallinity of the CoFeB layer (from −1.75 to −0.97 pJ/m in the region of 4−12% boron concentration). We attribute this unexpected significant effect of FePt to the reduced spin− orbit coupling energy difference (ΔE SOC ) and the degree of scattering potential between FePt and the CoFe layer, which is known to have a proportional relationship with the work function difference (ΔW NM−FM ).…”