  Perpendicular magnetic anisotropy (PMA) is a key property of magnetoresistive
random access memory (MRAM). To increase areal density of MRAM it is important
to find a way to enhance the PMA. Recently a strong enhancement of the PMA by
inserting an ultrathin LiF layer at an Fe/MgO interface was reported [T. Nozaki
et al., NPG Asia Materials (2022) 14: 5]. To understand the origin of the
observed enhancement of the PMA we perform first-principles calculations of
magetocrystalline anisotropy energy (MAE) of the following four kind of
multilayer structures: Fe/MgO, Fe/LiF/MgO, Fe/FeO/MgO, and Fe/FeF/LiF/MgO. We
find that the MAEs of the Fe/LiF/MgO and the Fe/FeF/LiF/MgO structures are
almost the same as that of the Fe/MgO structure, while the MAE of the
Fe/FeO/MgO structure is less than a half of that of the Fe/MgO structure. The
results show that the major origin of the enhancement of the PMA obtained by
inserting an ultrathin LiF layer at an Fe/MgO interface is the suppression of
the mixing of Fe and O atoms at the interface. We also find that the in-plane
Fe-F coupling gives a positive contribution to the MAE while the in-plane Fe-O
coupling gives a negative contribution. The results are useful for designing of
high-PMA materials.
