Emergent ferromagnetism and insulator-metal transition in δ-doped ultrathin ruthenates

Abstract

Heterostructures of complex transition metal oxides are known to induce extraordinary emergent quantum states that arise from broken symmetry and other discontinuities at interfaces. Here we report the emergence of unusual, thickness-dependent properties in ultrathin CaRuO3 films by insertion of a single isovalent SrO layer (referred to as δ-doping). While bulk CaRuO3 is metallic and nonmagnetic, films thinner than or equal to ~15-unit cells (u.c.) are insulating though still nonmagnetic. However, δ-doping to middle of such CaRuO3 films induces an insulator-to-metal transition and unusual ferromagnetism with strong magnetoresistive behavior. Atomically resolved imaging and density-functional-theory calculations reveal that the whole δ-doped film preserves the bulk-CaRuO3 orthorhombic structure, while appreciable structural and electronic changes are highly localized near the SrO layer. The results highlight delicate nature of magnetic instability in CaRuO3 and subtle effects that can alter it, especially the role of A-site cation in electronic and magnetic structure additional to lattice distortion in ruthenates. It also provides a practical approach to engineer material systems via highly localized modifications in their structure and composition that may offer new routes to the design of oxide electronics.

Enhancement of Extraordinary Size Effect on CaRuO3 Ultrathin Films

“…The structure of the RuO 6 octahedron in ruthenium oxides is dominant for physical properties. [ 13,42–45 ] This study clarifies the changes in the structure of the RuO 6 octahedron contained in CRO at the interface above the substrate. The structure and composition of the samples were investigated via atomic‐solution inverse contrast annular bright‐field (ABF)/high‐angle annular dark field (HAADF)‐STEM imaging.…”

Symmetry‐Mismatch‐Induced Ferromagnetism in the Interfacial Layers of CaRuO3/SrTiO3 Superlattices

“…As revealed in Figure S2 (Supporting Information), the nearest RuO 2 plane is sandwiched between a CaO plane and a (Ca,Sr)O plane at the CRO/STO interface, which may cause a Sr δ ‐doping effect to the nearest neighboring CRO layer. [ 33 ] However, the increase‐to‐decrease crossover of T C versus t CRO definitely excludes the Sr δ ‐doping effect as the main reason for the emergent FM state observed here. First, the highest T C always occurs in the case of t CRO = 6 uc rather than in the case of t CRO = 2 uc for which the strongest Sr δ ‐doping effect is naturally expected.…”

“…[42] However, the experimental results are controversial. [31][32][33][34][35][36]43,44] For the same CRO film on STO (with a tensile strain of 1.4%), some works reported a weak FM phase, [43,44] while others showed the absence of long-range magnetic order. [31][32][33] The robust FM order observed here in the (CRO n /STO 1 ) 10 SLs is unlikely to be caused by strain effect.…”

“…[31][32][33][34][35][36]43,44] For the same CRO film on STO (with a tensile strain of 1.4%), some works reported a weak FM phase, [43,44] while others showed the absence of long-range magnetic order. [31][32][33] The robust FM order observed here in the (CRO n /STO 1 ) 10 SLs is unlikely to be caused by strain effect. We found that a CRO bare film on LSAT did not show any features of FM ordering.…”

“…[28][29][30] However, due to the larger orthorhombic distortion of the RuO 6 octahedra, CRO maintains the nonmagnetic metal behavior to at least 2 K in either bulk or thin film form. [31][32][33][34][35][36] Here we report on symmetry-mismatch-induced FM in the interfacial layers of the CaRuO 3 /SrTiO 3 (CRO/STO) SLs on either (LaAlO 3 ) 0.3 (SrAl 0.5 Ta 0.5 O 3 ) 0.7 (LSAT) or STO substrates. By combining PM CRO and diamagnetic STO, two oxides with distinct symmetries and oxygen octahedron arrays, [28][29][30] we can effectively tune the tilting/rotation of RuO 6 octahedra at CRO/STO interface, causing an angle increase from ≈150° to ≈165° for RuORu bond.…”