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Magnetic Anisotropy of Fe/MgO Interfaces Inserted with Alkali Halide Layers

Miwa Group

Fe/MgO-based systems have attracted attention for magnetic tunnel junction applications because of the simultaneous existence of robust perpendicular magnetic anisotropy (PMA) and giant tunnel magnetoresistance (TMR). Recently, it was demonstrated that a few monolayers of LiF insertion can enhance the interfacial PMA at the Fe/MgO interface while TMR and the voltage-controlled magnetic anisotropy effect are maintained [1]. It was also shown the PMA enhancement originates from increased Fe orbital magnetic moment anisotropy, and a higher electronegativity of F compared with O seems to contribute to the enhancement [2]. Motivated by these findings, in this study, we investigate the influence of alkali halide layers with different anion electronegativity and spin-orbit interaction on the magnetic anisotropy at the Fe/MgO interface [3]. Here we chose LiF, CsI, and NaCl as a fluoride, an iodide, and a chloride since they could be grown epitaxially on Fe, considering their similar lattice constants (aFe = 0.286 nm, aLiF/√2 = 0.285 nm, aCsI/√2 = 0.318 nm, aNaCl/2 = 0.282 nm).

The schematic of the samples is shown in Fig. 1(a). Epitaxial multilayers consisting of MgO (5 nm)/V (30 nm)/Fe (tFe = 0.3 − 0.9 nm)/Alkali halide (0, 0.1, 0.2, 0.4, 0.6, and 1 nm)/MgO (5 nm)/SiO2 (5 nm) were grown on single-crystalline MgO (001) substrates by molecular beam epitaxy, where LiF, CsI and NaCl were introduced as alkali halide layers. The crystallinity of each surface was evaluated by performing reflection high-energy electron diffraction (RHEED) measurements. The RHEED patterns of the 0.2-nm-thick LiF, CsI, and NaCl layers are shown in Fig. 1(b), 1(c), and 1(d), respectively. The streaky pattern in LiF indicates the epitaxial growth because of the good lattice matching. The spotty pattern in CsI represents a rough surface, and the additional 2×2 streaks may originate from only half Cs/I atoms in the CsI monolayer compared to the Fe layer. The pattern of NaCl shows a superlattice because of two models of pseudo epitaxial growth of NaCl: either NaCl[110] or [100] direction is parallel to the Fe[100] direction.

The magnetic properties were characterized by using the polar magneto-optical Kerr effect (polar-MOKE). The LiF thickness dependence of the normalized magnetization curves for the Fe 0.7-nm region is shown in Fig. 2(a). The shape of the magnetization curves is modified by the inserted LiF layers, revealing that the LiF insertion strengthens PMA, and the PMA becomes the strongest at a LiF thickness of 0.4 nm.

We estimated the total PMA energy (Keff) from the magnetization curves, and we characterized the contribution from the interfacial magnetic anisotropy (KI) to the PMA energy by employing the linear fitting: Ksffteff = (KV - 12μ0M2eff)teff + KI. The interfacial magnetic anisotropy energies as a function of alkali halide thickness for the LiF, CsI, and NaCl samples are shown in Fig. 2(b). For the LiF sample, the KI slightly increases in the 0−0.4-nm regions but starts to decrease in the 0.4−1.0-nm regions. It is suggested that the high electronegativity of F is beneficial for interfacial PMA probably because of the weaker Fe-F hybridization and stronger electron localization at the interface. For the CsI and NaCl cases, the interfacial PMA decreases monotonically with CsI or NaCl thickness. Despite their strong spin-orbit interactions, the finite magnetic dead layers suggest an intermixing of the alkali halide and Fe layers, which contributes to the interfacial PMA degradation.

In summary, we studied the effect of alkali halide insertions on magnetic anisotropy at the Fe/MgO interface, and our study shall serve as a guiding principle for designing a new dielectric layer to achieve stronger PMA in ultrathin Fe films.


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).
Authors
  • J. Chen, S. Sakamoto, H. Kosaki, and S. Miwa