Ferromagnetic exchange, spin–orbit coupling and spiral magnetism at the LaAlO3/SrTiO3 interface

Abstract

The electronic properties of the polar interface between insulating oxides is a subject of great interest 1-3 . An exciting development is the observation of robust magnetism 4-8 at the interface of two non-magnetic materials, LaAlO 3 (LAO) and SrTiO 3 (STO). Here we present a microscopic theory for the formation and interaction of local moments that depends on essential features of the LAO/STO interface. We show that correlation-induced moments arise owing to interfacial splitting of orbital degeneracy. We find that conduction electrons with a gate-tunable Rashba spin-orbit coupling mediate ferromagnetic exchange with a twist. We predict that the zero-field ground state is a long-wavelength spiral. Its evolution in an external field accounts semi-quantitatively for torque magnetometry data 5 and describes qualitative aspects of the scanning superconducting quantum interference device measurements 6 . We make several testable predictions for future experiments.Recent experiments on the LAO/STO interface have seen tantalizing magnetic signals 4-8 , often persisting up to high temperatures ∼100 K. A large magnetization of 0.3-0.4µ B per interface Ti was observed by torque measurements 5 in an external field. In contrast, scanning superconducting quantum interference device (SQUID) experiments 6 found an inhomogeneous state with a dense set of local moments with no net magnetization, only isolated micron-scale ferromagnetic patches. Our goal is to reconcile these seemingly contradictory observations and to gain insight into the itinerant versus local moment nature of the magnetism, the exchange mechanism and the ordered state.LAO and STO are both band insulators, but the TiO 2 layers at the interface are n-doped when LAO is terminated by a LaO + layer. The polar catastrophe 1 arising from a stack of charged LaO + and AlO − 2 layers grown on STO is averted by the transfer of 0.5 electrons per interface Ti. Oxygen vacancies are also known to provide additional electrons at the interface 1 .What is the fate of these electrons at the interface? Transport data suggest that only a small fraction of the electrons (5-10% of the 0.5 electrons per Ti) are mobile 2,9-11 . Interestingly, magnetotransport studies show a large, gate-tunable Rashba spin-orbit coupling (SOC) for these conduction electrons, arising from broken inversion at the interface 12 . Most of the electrons (comparable to 0.5 electrons per Ti) seem to behave like local moments in the magnetic measurements 5,6 discussed above.We propose a microscopic model of electrons in Ti t 2g states at the LAO/STO interface that leads to the following results: local moments form in the top TiO 2 layer owing to correlations, with interfacial splitting of t 2g degeneracy playing a critical role; conduction electrons mediate ferromagnetic double-exchange interactions between the moments; Rashba SOC for the conduction Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA. *e-mail: randeria@mps.ohio-state.edu electrons leads to a Dzyaloshinskii-Moriya inte...

Room-temperature electronically-controlled ferromagnetism at the LaAlO3/SrTiO3 interface

“…The mechanism depends on the relative density of localized versus itinerant electrons, as well as their orbital character 20,31,32 . Fidkowski et al 32 describe a ferromagnetic Kondo model in which local d xy moments couple ferromagnetically to delocalized d xz /d yz carriers; models with ferromagnetic exchange are also described by Joshua et al 20 and Bannerjee et al 33 Michaeli et al 34 describe a model based on Zener (antiferromagnetic) exchange between localized and delocalized d xy carriers 35 .…”

Electronic Reconstruction at the Interface Between Band Insulating Oxides: The LaAlO3/SrTiO3 System

“…a spin polarization of the q2DES, but not necessarily to a magnetic order. This is an intriguing result, which requires further investigations, also in view of recent theoretical proposal of spiral magnetism in q2DES [82].…”

Giant spin splitting of the two-dimensional electron gas at the surface of SrTiO3

“…Interestingly, the-7 oretical calculations have found that clusters of oxygen vacancies could induce an orbital reconstruction, generating an interfacial magnetic state with a dominant contribution from the d xy states to the magnetic moment and an exchange energy splitting as large as a fraction of eV [29,30]. Alternative models, based the interfacial splitting of t 2g orbital degeneracy in combination with electron correlations, have also been proposed [2,[31][32][33][34].It is interesting to contrast our findings with the case of spin-polarized quantum well states in ultra-thin films of non-magnetic materials grown on ferromagnetic surfaces, like e.g. Cu/Co(001) [35,36].…”