4d and 5d magnetic oxides developed by machine-learning-assisted molecular beam epitaxy

日時　2024年10月28日　（月）　14：00～15：10
場所　広島大学放射光科学研究所　2階　セミナー室
講師　若林　勇希
(NTT物性科学基礎研究所)

We have explored the deliberate manipulation of spin-orbit coupling (SOC) in novel magnetic oxides using molecular beam epitaxy (MBE) as a growth technique. Traditionally, magnetic oxides have relied on 3d transition metals, where SOC is minimal. Here, we present the realization of Sr₃OsO₆ (SOO), a 5d perovskite exhibiting a ferromagnetic insulating state, and ultrahigh-quality SrRuO₃ (SRO), a 4d perovskite displaying a magnetic Weyl semimetal state. Notably, both materials derive their exotic properties from the robust SOC experienced by their 4d and 5d electrons. We introduce the synthesis of B-site ordered cubic double-perovskite SOO, achieving a Curie temperature (TC) of ~1060 K, the highest reported for any insulator or oxide [1]. Resistivity and optical characterization confirm SOO’s insulating nature, with a band gap of 2.65 eV. Density functional theory calculations elucidate that the strong SOC of Os⁶⁺ 5d² orbitals drives SOO towards a ferromagnetic jeff = 3/2 insulating state. For obtaining ultrahigh-quality SRO films, we implemented a Bayesian optimization (BO) algorithm for MBE growth optimization [2]. This approach yielded SRO films with exceptional crystalline quality, evidenced by a residual resistivity ratio exceeding 80. Transport measurements conclusively demonstrate the presence of both bulk 3D and surface 2D Weyl fermions [3]. These findings establish the effectiveness of MBE in engineering novel magnetic oxides with precisely tailored properties achievable through the exploitation of enhanced SOC in 4d and 5d electron systems. The developed materials, SOO and SRO, present tremendous possibilities for applications in high-temperature spintronics and the burgeoning field of topological electronics.

[1] Y. K. Wakabayashi* et al., Nat. Commun 10, 505 (2019).
[2] Y. K. Wakabayashi*, T. Otsuka et al., APL Mater. 7, 101114 (2019).
[3] K. Takiguchi, Y. K. Wakabayashi* et al., Nat. Commun 11, 4969 (2020).

問合せ先　黒田健太、武田崇仁(先進理工系科学研究科)、出田真一郎(放射光科学研究所）