School Colloquia Series - Clemens Ulrich - Multiferroics: from bulk to thin films
Multiferroics: from bulk to thin films
(a comprehensive neutron and Raman light scattering investigation)
A/Prof. Clemens Ulrich
School of Physics, University of New South Wales, Sydney, Australia
Multiferroic materials demonstrate excellent potential for next-generation multifunctional devices, as they exhibit coexisting ferroelectric and magnetic orders. At present, the underlying physics of the magnetoelectric coupling is not fully understood, and competing theories exist with partly conflicting predictions. Therefore we have investigated isotopically substituted TbMn16/18O3 and DyMn16/18O3, both powder and single crystal samples, by Raman light scattering and neutron diffraction, to elucidate the spin-phonon coupling as well as the crystallographic and magnetic phase transitions, in order to shine light on the multiferroic coupling mechanism in both compounds.
Artificially grown thin film heterostructures of transition metal oxides by far exceed the capabilities of current semiconducting technology as they offer further functionalities such as metal-insulator transitions, magnetism, superconductivity, or multiferroicity. Bismuth ferrite (BiFeO3) is the rare case of a room temperature multiferroic material and offers as such the most promising pathway for spintronics applications. The existence of a spin cycloid is a mandatory requirement to establish a direct magnetoelectric coupling. Thus far, internal strain in epitaxial grown films has limited the stability of the spin cycloid for BiFeO3 films with less than 300 nm thickness when grown on SrTiO3. Our neutron diffraction experiments have demonstrated that we were able to realize a spin cycloid in films of just 100 nm thickness through improved electrostatic and epitaxial constraints. Further fascinating examples are SrCoO3 thin films. Theoretical calculations have predicted ferromagnetic to antiferromagnetic phase transitions induced by epitaxial strain. With the proper choice of substrate material we were able to confirm the FM-AFM transition by neutron diffraction. As such, SrCoO3 would constitute a new class of multiferroic material where magnetic and electric transitions can be driven through external strain. This opens new avenues for fundamental research and technical applications in spintronic or magnonic devices.