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X-Ray Magnetic Circular Dichroism Study of Enhanced Interfacial Perpendicular Magnetic Anisotropy in LiF-Inserted Fe/MgO Interface

Miwa Group

Fe(CoB)/MgO interfaces are crucial for spintronics applications such as magnetoresistive random access memories (MRAMs) due to their giant tunneling magnetoresistance (TMR) effect and strong interfacial perpendicular magnetic anisotropy (PMA). Strong PMA is key to shrinking the size of magnetic cells while keeping thermal stability intact, and improving PMA is one of the most significant challenges in MRAM development.

Recently, Nozaki et al. demonstrated that inserting an ultrathin LiF layer between the MgO and Fe layers significantly boosts interfacial PMA while preserving or even enhancing TMR ratio [1]. Although this discovery is promising, the cause of the enhancement remains elusive, and clarifying its origin is of great importance for further improvements in PMA. For such purpose, we conduct x-ray magnetic circular dichroism (XMCD) measurements on Fe/LiF/MgO multilayers [2].

The Fe/LiF/MgO structures were grown on single-crystalline MgO (001) substrates using molecular beam epitaxy. The sample structure is illustrated in Fig. 1(a). XMCD measurements were conducted at the BL-16A beamline in the Photon Factory.

Figure 1(b) presents an out-of-plane magnetic hysteresis loop measured with the magneto-optical Kerr effect. The loops are perfectly square, indicating that the Fe/LiF/MgO multilayers exhibit PMA. The coercive field becomes larger with LiF thickness up to 0.4 nm but slightly declines when the LiF layer reaches 0.6 nm, in agreement with the previous study [1]. This suggests that PMA energy increases with LiF insertion.

To uncover the origin of the enhanced PMA, we measured XMCD spectra with both out-of-plane and nearly in-plane (70°) magnetic fields. Figure 2(a) compares the XMCD spectra for samples without LiF and with a 0.4-nm-thick LiF layer. The XMCD spectra are normalized to the Fe L2-edge maxima. The XMCD spectra consists of broad single peak for each L3 and L2 edge, confirming the absence of Fe oxides or fluorides at the interface. The intensity of the XMCD is stronger for the out-of-plane magnetic fields than for the in-plane magnetic fields. This intensity anisotropy becomes more pronounced with LiF insertion, indicating that the orbital magnetic moment becomes more anisotropic.

To be more quantitative, we estimated spin and orbital magnetic moments using XMCD sum rules. Figure 2(b) shows the obtained anisotropy of the orbital to spin magnetic moment ratio, defined as Δ(morb/mspin) = (morb/mspin)θ=0° − (morb/mspin)θ=70°. This anisotropy increases with LiF thickness. This strengthened orbital moment anisotropy is consistent with the PMA enhancement because the PMA energy is proportional to the orbital moment anisotropy in the simplest approximation. Indeed, the coercive fields behave similarly to the orbital moment anisotropy, as displayed in Fig. 2(b). The Δ(morb/mspin) values seem saturated at the LiF thickness of 0.2 nm, the origin of which may be attributed to the fact that the Fe layer is almost fully covered by a monolayer of LiF (~0.2 nm).

We infer that the enhancement of the orbital moment anisotropy arises from the more robust interfacial electron localization and electron-electron correlation, due to the highly ionic nature of LiF and weak Fe-F hybridization, or from improved interface quality with fewer defects.


References
  • [1] T. Nozaki, T. Nozaki, T. Yamamoto, M. Konoto, A. Sugihara, K. Yakushiji, H. Kubota, A. Fukushima, and S. Yuasa, NPG Asia Materials 14, 5 (2022).
  • [2] S. Sakamoto, T. Nozaki, S. Yuasa, K. Amemiya, and S.Miwa, Phys. Rev. B 106, 174410 (2022).
Authors
  • S. Sakamoto, T. Nozakia, S. Yuasaa, K. Amemiyab, and S. Miwa
  • aAIST
  • bKEK