Strongly correlated oxides have attracted considerable attention because the interplay among their charge, spin, orbital, and lattice degrees of freedom can lead to a wide variety of novel phenomena and functional properties. One example is the high spin polarization in several transition-metal perovskites and double perovskites, which is highly favored for spintronic applications. However, the physical properties are often largely altered at oxide interfaces of interest, which are crucial for the device performance but challenging to be predicted. Thus, advanced characterizations of relevant boundary regions are generally required to uncover the underlying physics that describes the behavior of strongly correlated electrons near oxide interfaces. I will present two cases [1, 2] where interface-sensitive neutron and x-ray techniques are used to address the structure-property relationships.
The first case is the high-TC and high-spin-polarization ferrimagnetic double-perovskite Sr2CrReO6 (SCRO) films. We investigated the effect of Cr/Re antisite disorder and strain on the interfacial magnetization via x-ray reflectometry and polarized neutron reflectometry. It is found that these SCRO films show a nanoscale interfacial layer with reduced magnetization at interfaces with (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) and SrTiO3 (STO) substrates, consistent with the antisite disorders observed previously. Strain due to the lattice mismatch does not show a strong effect on the suppression of interfacial magnetization. Furthermore, when grown on SrCr0.5Nb0.5O3 (SCNO) buffered STO, the reduced-magnetization region narrows, but is clearly wider than the antisite disorder region at the SCRO/SCNO interface. Thus, the Cr/Re antisite disorder is not the solo mechanism for the reduced interfacial magnetization and more efforts are needed to optimize the SCRO films.
The second one is the emergent low dimensional magnetization that has recently been observed at several epitaxial oxide interfaces, which have triggered increasing efforts to explore its influence on macroscopic properties. We have utilized magnetic tunneling junctions (MTJs) consisting of ferromagnetic manganite La0.7Ca0.3MnO3 (LCMO) and insulating cuprate PrBa2Cu3O7 (PBCO) to explore its effects on the charge transport. For these MTJs, the tunnel magnetoresistance (TMR) exhibits an anomalous decrease at low temperatures. Polarized neutron reflectometry and x-ray magnetic circular dichroism (XMCD) studies ruled out the magentization degradation of the contacts as a cause. XMCD experiments also found a large interface-induced Cu magnetization. Calculations within the Wentzel-Kramers-Brillouin approximation show that the anomalous temperature dependence can be attributed to the competition between the positive spin polarization of the LCMO electrodes and the negative spin-filter effect from the interfacial Cu magnetization. This work shows that the interface-induced magnetization in layered oxide heterostructures can affect the transport properties thus provides a knob for enigneering oxide spintronics.
This work has benefited from the use of the SNS at ORNL, APS at ANL and LANSCE at LANL, which are funded by the US-DOE, BES, Scientific User Facilities.
[1] Yaohua Liu, J.M. Lucy et al., “Effects of Strain and Buffer Layer on the Interfacial Magnetization of Sr2CrReO6 Films Determined by Polarized Neutron Reflectometry ”, Phy. Rev. B, 90, 104416 (2014).
[2] Yaohua Liu, F.A. Cuellar et al., “Emergent Spin Filter at a Ferromagnetic and Insulating Layered Oxide Interface”, Phys. Rev. Lett. 111, 247203 (2013).
