Imaging and control of ferromagnetism in LaMnO ₃ /SrTiO ₃ heterostructures

Abstract

Oxide heterostructures often exhibit unusual physical properties that are absent in the constituent bulk materials. Here, we report an atomically sharp transition to a ferromagnetic phase when polar antiferromagnetic LaMnO3 (001) films are grown on SrTiO3 substrates. For a thickness of six unit cells or more, the LaMnO3 film abruptly becomes ferromagnetic over its entire area, which is visualized by scanning superconducting quantum interference device microscopy. The transition is explained in terms of electronic reconstruction originating from the polar nature of the LaMnO3 (001) films. Our results demonstrate that functionalities can be engineered in oxide films that are only a few atomic layers thick.

Interfacial Control of Magnetic Properties at LaMnO3/LaNiO3 Interfaces

“…When grown as a thin film on cubic SrTiO3 (STO) substrates (a=3.905 Å), LMO typically displays an insulating ferromagnetic behavior, possibly attributed to off-stoichiometry, cubic in-plane strain imposed by the substrate, and/or development of a specific orbital order and/or electronic reconstructions due to polar discontinuity. [15][16][17][18][19] On the other hand, the least distorted member of the perovskite nickelates, LNO (rhombohedral structure with a pseudocubic lattice parameter apc=3.84 Å), is the only member of the family that in bulk is a paramagnetic metal at all temperatures. [20][21][22] A thickness-driven metal-to-insulator transition however occurs as the LNO film thickness is reduced to just a few monolayers.…”

Electron Accumulation and Emergent Magnetism in LaMnO3/SrTiO3 Heterostructures

“…Initially, the lack of metallic conductivity was enough to discourage researchers from further study, but it does not necessarily mean that the interfacial polar mismatch does not have some influence on the charge or spin degrees of freedom. In fact, recent work on the LaMnO 3 =SrTiO 3 system has proposed that charge transfer may occur within LaMnO 3 as a result of a similar potential buildup that produces electron doping near the interface and hole doping near the surface and subsequently gives rise to ferromagnetism in LaMnO 3 [23,24]. Direct evidence for the electron reconstruction and the nature of magnetism has not, however, been reported, precluding further understanding and control of the functional properties.…”

“…XMCD, on the other hand, reveals ferromagnetism with a critical thickness of 5 UC. The decoupling of the charge and magnetic critical thicknesses and the absence of hole doping near the film surface suggest that the charge transfer picture [23,24] may not be directly applicable or is an oversimplification. Theoretical studies show that the potential buildup in the LaMnO 3 is 0.177 V=Å and this, together with the small band gap of LaMnO 3 , results in a critical thickness for the built-in potential to collapse at only 2 UC.…”

“…1(a)] [25]. Compared to previous local magnetometry studies using a scanning superconducting quantum interference device (SQUID) [23,24], the current approach is surface sensitive and provides a direct measurement of the valence state of the various elements in the films and substrates via XAS and enables the detection of subtle (∼0.005 μ B =atom), element-specific magnetic moments (thus excluding magnetic impurities) via XMCD [9]. XMCD measurements at the Mn L 2;3 edges at 25 K reveal strong dichroism [ Fig.…”

Ferromagnetism and matrix-dependent charge transfer in strained LaMnO3–LaCoO3 superlattices

“…A series of LaMnO 3 (LMO) and LaCoO 3 (LCO) SLs on LaAlO 3 substrates were fabricated in consideration of two factors hereinafter. Firstly, the compressive strain imposed by LAO predominantly suppress the ferromagnetism in LMO [9] and LCO [10]. Hence, ferromagnetic distraction from each constituent was diminished when studying the ferromagnetism of their SLs.…”