Strain Effects in Epitaxial VO₂ Thin Films on Columnar Buffer-Layer TiO₂/Al₂O₃ Virtual Substrates

Epitaxial VO₂/TiO₂ thin film heterostructures were grown on (100) (m-cut) Al₂O₃ substrates via pulsed laser deposition. We have demonstrated the ability to reduce the semiconductor–metal transition (SMT) temperature of VO₂ to ∼44 °C while retaining a 4 order of magnitude SMT using the TiO₂ buffer layer. A combination of electrical transport and X-ray diffraction reciprocal space mapping studies help examine the specific strain states of VO₂/TiO₂/Al₂O₃ heterostructures as a function of TiO₂ film growth temperatures. Atomic force microscopy and transmission electron microscopy analyses show that the columnar microstructure present in TiO₂ buffer films is responsible for the partially strained VO₂ film behavior and subsequently favorable transport characteristics with a lower SMT temperature. Such findings are of crucial importance for both the technological implementation of the VO₂ system, where reduction of its SMT temperature is widely sought, as well as the broader complex oxide community, where greater understanding of the evolution of microstructure, strain, and functional properties is a high priority

Room-Temperature Spin Filtering in Metallic Ferromagnet–Multilayer Graphene–Ferromagnet Junctions

We report room-temperature negative magnetoresistance in ferromagnet–graphene–ferromagnet (FM|Gr|FM) junctions with minority spin polarization exceeding 80%, consistent with predictions of strong minority spin filtering. We fabricated arrays of such junctions <i>via</i> chemical vapor deposition of multilayer graphene on lattice-matched single-crystal NiFe(111) films and standard photolithographic patterning and etching techniques. The junctions exhibit metallic transport behavior, low resistance, and the negative magnetoresistance characteristic of a minority spin filter interface throughout the temperature range 10 to 300 K. We develop a device model to incorporate the predicted spin filtering by explicitly treating a metallic minority spin channel with spin current conversion and a tunnel barrier majority spin channel and extract spin polarization of at least 80% in the graphene layer in our structures. The junctions also show antiferromagnetic coupling, consistent with several recent predictions. The methods and findings are relevant to fast-readout low-power magnetic random access memory technology, spin logic devices, and low-power magnetic field sensors