Project title: Controlling structural change in multiferroic materials by ultrafast laser excitation
Supervisor: Dr Eamonn Murray and Dr Paul Tangney
Multiferroic materials possess domains of more than one ferroic order parameter, and so demonstrate more than one switchable physical (electric, elastic or magnetic) property. Recently multiferroics have gained attention as their coupling between different order parameters leads to many possible applications such as sensors, oscillators and memory devices.
This project uses electronic structure methods and atomistic simulations to understand how intense ultrafast laser pulses affect the domain structure and dynamics in multiferroics. We begin by studying prototypical perovskite multiferroics, such as (Ba,Sr)TiO3, which have coupled ferroelectric and ferroelastic order. A constrained density functional method is used to calculate the effects on the lattice dynamics of above-band-gap electronic excitation. The initial focus is on timescales of a few picoseconds and length scales of several perovskite unit cells.
Preliminary calculations indicate laser-induced softening of bonds and that, in the photoexcited system, there is a marked reduction of polarization and of the energy barriers to domain switching, suggesting photo-induced lowering of the coercive field and the phase transition temperature.
The information gained from our first principles calculations will be used to make atomistic potentials suitable for the electronically-excited state, allowing studies of effects on significantly longer length- and time-scales such as photo-induced domain nucleation and ferroelectric domain reversal. This will help interpret results of recent pump-probe experiments, guide novel ultrafast X-ray experiments and open new possibilities in the design and optimisation of optically controlled devices.