Oxide interface displaying electronically controllable ferromagnetism

#The present invention is generally directed to oxide interfaces and more specifically to oxide interfaces displaying electronically controlled ferromagnetism.

#The two-dimensional electron liquid that forms at the interface between the two insulating non-magnetic oxides LaAlO3 (“LAO”) and SrTiO3 (“STO”) has drawn widespread attention due to its possession of a remarkable variety of emergent behavior including superconductivity, strong Rashba-like spin-orbit coupling, and ferromagnetism. See Ohtomo, A. & Hwang, H. Y., “A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface,” Nature, 427: 423-426 (2004); Reyren et al., “Superconducting interfaces between insulating oxides.” Science, 317: 1196-1199 (2007); Ben Shalom et al., “Tuning Spin-Orbit Coupling and Superconductivity at the SrTiO3/LaAlO3 Interface: A Magnetotransport Study,” Phys. Rev. Lett., 104 (2010); Brinkman, A. et al., “Magnetic effects at the interface between non-magnetic oxides.” Nature Materials, 6: 493-496 (2007).

#Despite this interest, the existence and nature of magnetism in structures comprising a LAO/STO interface has remained controversial. Neutron reflectometry measurements by Fitzsimmons et al. on LAO/STO superlattices found no magnetic signatures; their measurements established a bulk upper limit thirty times lower than what was reported by Li et al. See Fitzsimmons et al., “Upper Limit to Magnetism in LaAlO3/SrTiO3 Heterostructures,” Phys. Rev. Lett., 107: 217201 (2011). Salman et al. reported relatively small moments from LAO/STO superlattices (˜2×10−3 μB/unit cell) using β-detected nuclear magnetic resonance. See Salman et al., “Nature of Weak Magnetism in SrTiO3/LaAlO3 Multilayers,” Phys. Rev. Lett., 109: 257207 (2012). As a result, control of such magnetism has remained elusive.

#FIGS. 1A-1B depicts a top gated oxide interface. They show the effects of applying different voltages on the in-plane ferromagnetic state of an oxide interface.

#FIG. 2 depicts an LAO/STO oxide interface with a top circular electrode and an annular interfacial contact. It shows a voltage being applied between the interface and the LAO layer via those contacts.

#FIG. 3 depicts a phase diagram showing charge carrier density and temperature dependence of a ferromagnetic state at an oxide interface. The phase diagram is a very rough approximation, and any numbers shown therein are not necessarily accurate.

#The present invention provides for electronic control of ferromagnetism at oxide interfaces by making use of the fact that ferromagnetism at such interfaces depends strongly on charge carrier density at the interface. The manuscript titled “Room-Temperature Electronically-Controlled Ferromagnetism at the LaAlO3/SrTiO3 Interface,” authored by Bi et al., available at http://arxiv.org/ftp/arxiv/papers/1307/1307.5557.pdf, is hereby incorporated by reference in its entirety. The charge carrier density at an oxide interface can be controlled using a number of techniques, including but not limited to metallic gating, reorientable ferroelectric materials, electrolytes, polar adsorbates, self-assembled monolayers, and nanoscale control using conductive atomic force microscopy lithography.

#Under certain conditions described herein, depleting charge carriers (for example, electrons) from an oxide interface results in the formation of a ferromagnetic phase with defined domain walls, the ferromagnetic phase being substantially in the plane of the interface, wherein the plane is not necessarily flat. Increasing charge carrier density at the interface by introducing additional charge carriers via, for example, electrical gating, results in the weakening and/or removal of the ferromagnetic state. The added charge carriers align antiferromagnetically with the existing magnetic state, weakening it and potentially removing it. Subsequent depletion of charge carriers results in a new, uncorrelated magnetic pattern at the interface. FIG. 1A shows an oxide interface displaying a non-ferromagnetic phase, and FIG. 1B shows an oxide interface displaying a ferromagnetic phase, where V1 caused an increase in charge carrier density at the interface, and V2 caused a decrease in charge carrier density at the interface.

#An “interface,” as referred to herein, includes a plane between two layers in contact with each other, and further includes a thickness of about 4 unit cells, extending from the plane, into each layer. The interface also includes any passivation layer, such as TiO2, which terminates one or both surfaces of the two layers in or around the plane of contact.