Influence of Alkali-Fluoride Insertion Layers on the Perpendicular Magnetic Anisotropy at the Fe/MgO Interface
Miwa Group
Fe/MgO-based systems have attracted significant attention because of their strong perpendicular magnetic anisotropy (PMA) and giant tunneling magnetoresistance (TMR). Recently, it was reported that an ultrathin LiF layer insertion at the Fe/MgO interface could enhance the interfacial PMA while maintaining the TMR ratio [1, 2], and the following study showed that inserting other alkali-halide layers, such as NaCl and CsI, degrades the interfacial PMA [3]. Such findings suggest the importance of the strong electronegativity of fluorine atoms. However, since LiF has better lattice matching with Fe than MgO (aLiF = 0.403 nm, aMgO = 0.421 nm, √2 aFe = 0.405 nm), it remains unclear whether the presence of fluorine atoms on the Fe atoms or the improved lattice matching between Fe and LiF layers contributes more significantly to the PMA enhancement. In this study, we insert an ultrathin NaF layer with suboptimal lattice matching to Fe at the Fe/MgO interface and characterize the PMA energy to disentangle the effects of strong electronegativity and lattice matching [4]. NaF, LiF, and MgO share the same NaCl-type crystal structure with lattice constants of 0.462, 0.403, and 0.421 nm, respectively.
The schematic of the multilayer structure is shown in Fig. 1(a). The multilayers consist of single-crystalline MgO (001) substrate/MgO (5 nm)/V (30 nm)/Fe (tFe = 0.3−0.9 nm)/NaF (0−1 nm)/MgO (5 nm)/SiO2 (5 nm). We performed reflection high-energy electron diffraction (RHEED) measurements to examine the surface crystallinity. The RHEED images of the 0.6-nm-thick Fe layer, 0.1- and 0.6-nm-thick NaF layers, and the corresponding MgO cap layer on the 0.1-nm-thick NaF layer, are shown in Fig. 1(b), respectively. The sharp streaks observed in the RHEED pattern indicate the well-epitaxial deposition of each layer.

Fig. 1. (a) Schematic of the multilayers. (b) RHEED patterns of the multilayers: 0.66-nm-thick Fe layer, 0.1- and 0.6-nm-thick NaF layers, and MgO overlayer on 0.1-nm-thick NaF layer. (c) Normalized reciprocal of the distance between streaks (1/d) in V, Fe, and NaF epilayers obtained from pixel analysis.
To evaluate the lattice matching properties, we estimated the in-plane lattice constants of the NaF with various thicknesses by measuring the distance between the streaks in the RHEED patterns [represented as d in Fig. 1(b)]. As the in-plane lattice constant is inversely proportional to d, we plotted 1/d values for the Fe and NaF layers normalized to that for the V layer in Fig. 1(c). The lattice constant of the Fe underlayer is plotted in blue at a NaF thickness of 0 nm. The 1/d value remained constant with a 0.1-nm-thick NaF insertion but drastically increased when the NaF thickness exceeded 0.1 nm. The in-plane lattice constant of the 0.6-nm-thick NaF layer is estimated as ~0.323 nm, approaching its unconstrained bulk lattice constant (aNaF/√2 = 0.327 nm). These results indicate that a NaF layer epitaxially forms islands on the Fe layer when the NaF layer is thinner than a monolayer. However, for thicker NaF layer insertions, accumulated internal stress overcomes the epitaxial stress and creates interfacial defects, and therefore, the lattice constant approaches the bulk lattice constant.
The magnetic properties were characterized by polar magneto-optical Kerr effect (polar-MOKE) measurement. The magnetic dead layer thickness of the NaF sample and the compared LiF sample are shown in Fig. 2(a). The dead layer exhibits robustness after LiF insertion and remains unchanged with a 0.1-nm-thick NaF insertion. However, it drastically increases as the NaF thickness becomes thicker, suggesting interlayer mixing between NaF and Fe layers. We estimated the magnetic anisotropy energies from magnetization curves and extracted the interfacial magnetic anisotropy from the fitting. The interfacial magnetic anisotropy energies (KI) of NaF and the compared LiF samples are shown in Fig. 2(b). The 0.1-nm-thick NaF insertion shows a slight enhancement of KI which is similar to the LiF case, except for the critical thickness difference probably originating from the difference in lattice matching conditions. Despite the suboptimal lattice matching, the KI enhancement in the Fe/NaF interface underscores the importance of fluorine atoms on the Fe atoms.
In summary, we have investigated the influence of NaF insertion on magnetic anisotropy at the Fe/MgO interface to disentangle the effects of fluorine electron negativity and lattice matching. Our result deepens the understanding of the effects of fluorine insertion on magnetic anisotropy at the Fe/MgO interface.
References
- [1] T. Nozaki, T. Nozaki, T. Yamamoto, M. Konoto, A. Sugihara, K. Yakushiji, H. Kubota, A. Fukushima, and S. Yuasa, NPG Asia Mater. 14, 5 (2022).
- [2] S. Sakamoto, T. Nozaki, S. Yuasa, K. Amemiya, and S. Miwa, Phys. Rev. B 106, 174410 (2022).
- [3] J. Chen, S. Sakamoto, H. Kosaki, and S. Miwa, Phys. Rev. B 107, 094420 (2023).
- [4] J. Chen, S. Sakamoto, and S. Miwa, Phys. Rev. B 109, 064413 (2024).