Microstructure and Magnetic Tuning in Oxygen-Deficient Sr(Ti,Co)O3-δ

Intellectual Merit
The magnetic and alectronic properties of perovskite-structured oxides depend critically on their microstructure, strain state and defect level, and these are affected by the growth conditions and substrate, allowing great latitude for controlling the material properties.

Here, SrTi0.7Co0.3O3-δ (STCo) films of varying thicknesses were grown on SrTiO3 (001) substrates using pulsed laser deposition. The STCo films grew with their crystal structure matching that of the stubstrate, but above a certain thickness, a new strain-relieving mechanism was observed in which STCo crystals of a new orientation form within the STCo matrix. The crystal structure, strain state and magnetic properties vary as a function of film thickness. Both the magnetic moment and the coercivity show maxima at the critical thickness just before the second orientation forms, where the strain and defect level are highest.

A transmission electron micrograph of the STCo film showing the triangular regions of the second orientation, which form once the film is about 100 nm thick. Also shown in the magnetic hysteresis, indicating that the material is magnetized out of plane. Its magnetic moment is maximum at the critical thickness, when the second orientation crystals start to form, then decreases as strain is relaxed.

Broader Impacts
Understanding how strain, defects and magnetic properties are coupled in perovskite films will allow us to design and process materials for applications in memory and logic devices, such as electrically switched magnetic memory. Recent measurements of ionic liquid gating and annealing of these magnetically-substituted perovskites shows that the magnetism can be modulated by changing the oxygen content. Therefore we may be able to design a memory cell in which oxygen is pumped in or out of the magnetic layer, and its state is measured via its magnetic properties.

The formation of the second orientation (called double epitaxy) is expected to influence other important perovskite properties including ferroelectricity, piezoelectricity, and thermal or electronic conductivity. Moreover, it provides films containing well-defined interfaces oriented at an angle to the film plane, which may be useful in studies of transport and other properties.