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.