Ferromagnetism and spin-dependent transport at a complex oxide interface

Abstract

Complex oxide interfaces are a promising platform for studying a wide array of correlated electron phenomena in low-dimensions, including magnetism and superconductivity. The microscopic origin of these phenomena in complex oxide interfaces remains an open question. Here we investigate for the first time the magnetic properties of semi-insulating NdTiO 3 /SrTiO 3 (NTO/STO) interfaces and present the first milli-Kelvin study of NTO/STO. The magnetoresistance (MR) reveals signatures of local ferromagnetic order and of spin-dependent thermally-activated transport, which are described quantitatively by a simple phenomenological model. We discuss possible origins of the interfacial ferromagnetism. In addition, the MR also shows transient hysteretic features on a timescale of ~10-100 seconds. We demonstrate that these are consistent with an extrinsic magneto-thermal origin, which may have been misinterpreted in previous reports of magnetism in STO-based oxide interfaces. The existence of these two MR regimes (steady-state and transient) highlights the importance of timedependent measurements for distinguishing signatures of ferromagnetism from other effects that can produce hysteresis at low temperatures.The interface between two complex oxides can host a high-mobility conducting electron gas, even though the constituent materials are insulators. Since the initial discovery of this phenomenon in LaAlO 3 /SrTiO 3 (LAO/STO) heterostructures [1], several other materials systems with this property have been identified [2][3][4]. These systems host a range of highly-correlated phases, including superconductivity [5,6] and ferromagnetism [7][8][9][10][11][12]. While superconductivity has been conclusively detected using transport [5,6,8,13] and scanning-probe measurements [9], reports of magnetism have been more difficult to reconcile across experiments. Numerous studies have investigated the magnetic properties of this class of oxide interfaces, with the majority of work to date focusing on the canonical LAO/STO system [7][8][9][10]14]. However, a unified picture of ferromagnetism is still lacking [7][8][9][15][16][17][18].Magneto-transport studies [7,10,19], have reported pronounced magnetic hysteresis, suggestive of

Disentangling spin-orbit coupling and local magnetism in a quasi-two-dimensional electron system

“…The extra STO capping layer is grown to protect NTO from degradation due to oxygen absorption in the air [6]. We note that the electrons which accumulate at the STO-on-NTO type interfaces tend to have very low electron mobility and exhibit insulating behavior at low temperatures [5,24]. Our measurement and data analysis in this work treat the sample as a single quasi-2D electron system consistently and show no effects arising from parallel conduction at the top interface.…”

Probing surfaces and interfaces in complex oxide films via in situ X-ray photoelectron spectroscopy

“…Here, delta-doping of electrons produces the 2DEG, with the rare-earth A-site ion providing an additional electron for the system. Emergent phenomena have also been reported in these interfacial materials, including superconductivity [28] and ferromagnetism [29,30]. As with the LaAlO 3 /SrTiO 3 system, however, defects can still play a significant role, albeit in a different fashion.…”

From weak antilocalization to Kondo scattering in a magnetic complex oxide interface

“…Conducting interfaces between SrTiO 3 (STO) and other complex oxides are an ideal system for investigating twodimensional electron systems in the high-density regime [1][2][3][4]. Our experimental system consists of epitaxial layers of SrTiO 3 and NdTiO 3 (NTO) [4][5][6], which host a highdensity quasi-two-dimensional electron gas coupled to local ferromagnetic or superparamagnetic regions [7,8]. The ferromagnetic order is thought to originate from spatially inhomogeneous canting of the antiferromagnetically-alligned spins in NTO [7,9], which is a Mott-Hubbard insulator with a Néel temperature of ∼ 90 K [10,11].…”

“…1(a)). The top STO layer protects the underlying heterostructure from degradation [6], however only the bottom NTO-on-STO interface is expected to contribute to transport, as STO-on-NTO interfaces are typically insulating [5,7]. In this system, itinerant electrons, which reside primarily on the STO side of the interface [5], experience the combined effect of k-cubic Rashba spin-orbit coupling and a magnetic exchange interaction of several 10's of Tesla due to the local, inhomogeneous magnetic order [8].…”