Influence of alkali-fluoride insertion layers on the perpendicular magnetic anisotropy at the Fe/MgO interface

The Fe/MgO interface is a cornerstone of spintronics applications, where enhancing its perpendicular magnetic anisotropy (PMA) has been a formidable challenge. Recent studies have reported an increase in PMA by introducing an ultrathin LiF layer, hinting at the potential of fluorine atoms with strong electronegativity. However, the underlying cause of this enhancement, whether stemming from the strong electronegativity of fluorine atoms or improved lattice matching, remains uncertain. In this paper, to disentangle these two contributing factors, we introduce a NaF layer with suboptimal lattice matching at the Fe/MgO interface and investigate the magnetic anisotropy energy. We find that the interfacial PMA energy is enhanced by the insertion of a 0.1-nm-thick NaF layer but weakened by a thicker NaF layer insertion (0.2–1 nm). The observed PMA enhancement, even in an Fe/NaF interface with suboptimal lattice matching, highlights the essential role of Fe-F orbital hybridization. Our findings shall provide a foundational basis for dielectric layer design to strengthen the PMA of ultrathin ferromagnetic layers.

Figure 1

(a) Schematic of the multilayers. (b) RHEED patterns of the multilayers: 0.6-nm-thick Fe layer, 0.1- and 0.6-nm-thick NaF layers, and MgO overlayer on 0.1-nm-thick NaF layer. All the RHEED patterns are screened along the MgO [100] zone axis. (c) Normalized reciprocal of the distance between streaks (1/$d$) in V, Fe, and NaF epilayers obtained from pixel analysis. Here the V layer is chosen as a reference and denoted as a red dashed line. The 0-nm-thick NaF layer is denoted as a blue point, corresponding to the Fe underlayer.

Figure 2

(a) Hysteresis loops of the NaF 0.1-nm samples with various Fe thicknesses measured using the polar magneto-optic Kerr effect. (b) Fe thickness dependence of the saturation Kerr rotation angle. NaF and LiF (Ref. [19]) thickness dependence of the (c) magnetic moment and the (d) dead layer. Reprinted LiF data with permission from [19].

Figure 3

Normalized magnetization curves for the (a) Fe/MgO and (b) with 0.1-nm-thick NaF layer insertion. The magnetic anisotropy energies $K_{\mathrm{eff}}$ are estimated from the magnetization curves, as indicated in the shaded area in the inset in (b). (c) Magnetic anisotropy energy multiplied by the effective Fe thickness $K_{\mathrm{eff}}{t}_{\mathrm{eff}}$ as a function of the effective Fe thickness. (d) Interfacial ($K_{\mathrm{I}}$) and (e) volume ($K_{\mathrm{V}}$) magnetic anisotropy energies as a function of NaF and LiF thickness. Reprinted LiF data with permission from [19].