Structure-property correlations in novel multiferroic materials

Ferroic materials have been the subject of decades of intense research into their elastic, magnetic and electronic properties. Primary ferroics are mandated by the presence of one of the aforementioned properties through a single order parameter, usually below a threshold temperature, TN or TC.

Further discoveries led to the conception of multiferroic materials that allow for two or more of these properties to be present simultaneously in a single phase. This is important as it allows one to use a single material for multiple functions in a device. Thus saving energy, resources, and space. One such material is BiFeO3 which is multiferroic at room temperature demonstrating magnetic character and electrical behaviour simultaneously. Comprehensive structural analysis of 5% Dy3+ doped BiFeO3 along with structural, electronic and magnetic characterisations are reported. The material is shown to crystallise in the R3c symmetry between 5 - 773 K. A broad phase change to Pbnm was observer above this temperature range. The magnetic spin cycloid is preserved with a longer periodicity than is seen for the pure counterpart. Observed magnetic hysteresis is likely due to uncompensated spins as a result of small domain sizes. Doping at this level demonstrates a potential decrease in ferroelectric character, but further measurements are needed to state this definitively. The TN is increased showing that doping has an effect on the transition temperature. This is potentially due to the dysprosium ion becoming correlated by the iron oxide sublattice's magnetic moment.

We report magnetic properties of Rare Earth orthoferrites ReFeO3 (Re= Dy, Sm) in a solid solution of 25:75, 50:50, and 75:25 % compositions of Dysprosium:Samarium. It was found that mixing these materials with a simple "shake and bake" method did not produce solid solutions and instead produced impure phases. As a result, the characterisation of these materials gave complex data further discussed in the complementary section.

An emerging area of multiferroics is where a fourth ferroic order termed ferrotoroidicity can possess an electric and magnetic response simultaneously through a single order parameter instead of two. This new ferroic property could potentially be used in energy and data storage, limiting energy consumption. One material being investigated with this property is LiCoPO4 due to its large magnetoelectric coefficient. Structural and magnetic characterisation are reported for the parent material, as well as Sodium doped LiCoPO4 for compositions containing up to 40% Na. A 10% Na incorporation limit is reported, with compositions above this percentage showing impure phases and polymorphic nature attributed to the Na contribution. The 10% composition shows similar properties to the parent materials with little to no variation in TN.