Stress controlled antiferromagnetism for low energy electronics

In the challenge to develop high-density logic and memory devices, the concept of employing electron spin transport as the data vector (i.e. spintronics) has great potential for very low energy devices. Antiferromagnetic materials play a pivotal role in many spintronic devices through the phenomenon of magnetic exchange bias at the interface between a ferromagnet and an antiferromagnet. This phenomenon allows such heterostuctures to act as spin-valves to control the conduction of electrons according to their spin. The ability to modulate the exchange bias, for example by switching the antiferromagnetism on and off, would be a valuable new concept in spintronic device design.
We have previously demonstrated that in the perovskite oxide solid solution (1-x)BiFeO3-xPbTiO3, the antiferromagnetic Neel temperature drops from above room temperature to below room temperature on crossing the phase boundary between rhombohedral and tetragonal symmetry at x = 0.3. We have also shown that in particulate samples, the symmetry can be switched from tetragonal to rhombohedral with an isostatic pressure of 0.6 GPa, with the result that at room temperature the pressure changes the phase from antiferromagnetic to paramagnetic, i.e. stress turns the antiferromagnetism on or off.
The aim of the Bragg PhD project is to further investigate the phenomenon of pressure modulated antiferromagnetism, focusing on thin films in order to assess its viability for spintronic devices. Piezoelectricity will be employed, both in selected substrates and in the BiFeO3-PbTiO3 itself, to provide the required stress/strain modulation in the material. The project will comprise the following objectives:
(i) deposition and characterization of epitaxial BiFeO3-PbTiO3 on a single crystal piezoelectric substrate;
(ii) study of the antiferromagnetism in the BiFeO3-PbTiO3 films and its modulation by stress from the piezoelectric actuation of the substrate;
(iii) deposition of epitaxial BiFeO3-PbTiO3 on substrates selected to provide favourable lattice matching for characterization of the ferroelectric & piezoelectric properties of the BiFeO3-PbTiO3 films;
(iv) investigation of direct electrical modulation of antiferromagnetism by ferroelectric phase switching in BiFeO3-PbTiO3 films;
(v) demonstration and characterization of the modulation of exchange bias using the above effects;
(vi) evaluation of the outlook for devices based on the observed phenomena.
The results will be discussed with potential industrial partners for future device demonstrator projects.