SPIRAL · Control of SPIRAL magnetic ordering via diamagnetic cation substitution in hexagonal ferrites

Hexagonal ferrites are a class of magnetic oxides containing iron as the main element. They are produced industrially in large quantities and used for band filters, refrigerator magnets etc., but some of the subclasses, most importantly the one named Y-type ferrites, show an interesting behaviour called magnetoelectric effect, an ability of a material to acquire electric polarisation when submitted to the magnetic field, and permanent magnetisation when submitted to electric field. This behaviour is a basis of function of certain non-volatile memory designs and it is fundamentally connected to the ordering of magnetic moments inside the structure of the crystal.The Y-type ferrites can contain many elements such as Ba/Sr, Co/Zn, Al or In, which leads to many possible magnetic orderings. The most desirable are the conical orderings, which are necessary if one wants to find new materials with magnetoelectric properties. Most of the current magnetoelectric materials only order in this fashion at low temperatures, and are thus not suitable for use in memory devices. In this project, I plan to study the magnetic structure of the more promising ones, in which the magnetoelectric effect was observed just below the room temperature. I propose to prepare a series of single crystals of Y-type ferrites substituted with aluminium, gallium and indium, and to explain the mechanism by which these atoms influence the magnetic order. I will systematically study the crystal structure and magnetic properties of these ferrites, and in few selected cases I will study their magnetic ordering with the neutron diffraction. As a result, I aim to produce a quantum-mechanical model capable of explaining and even predicting the magnetic order of Y-type ferrites with complex stoichiometry.