Spin Functionality through Complex Oxide Heteroepitaxy

Y. Suzuki, Principle Investigator

The main objective of the program is to develop novel complex oxide thin films and heterostructures that will lead to an understanding of the role of surfaces and interfaces in spin polarized transport.  Control of interface strain, roughness, chemical mixing, and magnetism will not only enable control of spin transport across interfaces, but also can be used to devise entirely new materials with properties distinct from their bulk counterparts.  In particular, complex spin polarized oxide materials provide a rich playground for the exploration of novel magnetic properties not found in the bulk constituents and enable the development of functional interfaces to be incorporated into energy efficient technological applications.

We study two novel classes of spin-polarized materials where interfaces play a key role:

• complex oxide heterostructures in which the interface gives rise to spin polarized properties that are not found in the constituent materials and that are due to discontinuities in the valence, bandwidth and interaction lengths
• ordered perovskite based oxide thin film materials with high or complete spin polarization that have been otherwise unattainable using equilibrium routes.

CURRENT PROJECTS      

New Interface and Thin Film Materials

We are studying both (i) novel complex oxide and chalcogenide thin films with a range of functional properties and (ii) functional interfaces created from materials that do not exhibit these functional properties.

Magnetic Junction Devices

We are developing magnetic junction devices in order (i) to understand the role of interfaces and conduction mechanisms from a fundamental perspective and (ii) to develop new classes of devices from a technological perspective.

Magnetic Nanostructures

We are interested in the behavior of highly spin polarized oxide materials when geometrically confined to the nanometer length scale. Such studies are of fundamental interest but also have potential applications as future magnetic media or magnetic random access memory.

Highlights of recent work include (i) spin polarization at the surfaces of half metallic oxides La0:7Sr0:3MnO3 and Fe3O4, (ii) the development of a new type of hybrid magnetic tunnel junction/ spin filter devices that exploits oxide interfaces, (iii) double barrier LaAlO3/SrTiO3 junctions and (iv) tunable conductivity in LaTiO3 thin films.

We find that spin polarization depends on the crystal orientation of the surface. When juxtaposed with paramagnets or ferromagnets, we are able to demonstrate strong magnetic coupling for isostructural and no magnetic coupling for non-isostructural interfaces. We have exploited these interfacial phenomena in a new type of hybrid spin filter/ magnetic tunnel junction. Our hybrid spin-filter/magnetic-tunnel junction devices are epitaxial oxide junctions of highly spin polarized La0:7Sr0:3MnO3 and Fe3O4 electrodes with magnetic NiMn2O4 insulating barrier layers. These devices are composed of two kinds of interfaces: a non-isostructural perovskite-spinel interface with very little magnetic coupling and an isostructural spinel-spinel interface with strong magnetic coupling. These contrasting behaviors are confirmed by surface sensitive, element specific X-ray magnetic circular dichroism combined with X-ray absorption spectroscopy. Depending on whether the barrier is in a paramagnetic or ferromagnetic state, the junction exhibits magnetic tunnel junction behavior where the spin polarized conduction is dominated by the electrode-barrier interface or spin filter behavior where conduction is dominated by barrier layer magnetism.

Figure: Magnetic nanostructures composed of highly spin polarized La0.7Sr0.3MnO3 show flux closure domains.