Electrical Control Of Magnetism From Group Theoretical And Quantum Mechanical Calculations

Abstract

Volatile memory devices, with their constant power requirement, are a source of energy inefficiency in today's digitised world. Using ferroically ordered materials, which retain a ferroic state in the absence of any applied field, is one route towards alleviating this issue. For example, ferroelectric materials are those with spontaneous and switchable macroscopic polarisations. One direction of polarisation could represent a "1" and the other a "0". Alternatively, ferromagnetic materials have a switchable macroscopic magnetisation. It is typically cheap to switch the polarisation in ferroelectrics but the read operation is destructive and requires a rewrite stage. In contrast, reversing the magnetisation in a ferromagnet is energetically costly because of the large external fields needed, but reading is cheap as non-destructive magnetoresistive effects can be used. Allowing for both of these ferroic order parameters in a single-phase material might allow for cross couplings permitting writing with electric fields but reading using magnetoresistive effects. In order for this to be true, the information written in the polarisation must be transmitted to the magnetic degrees of freedom.

This thesis approaches this problem from a theoretical point of view. I analysed the symmetry of multiferroic materials and constructed Landau expansions to determine how order parameters are coupled. Through this process, I determined whether the reversal of the polarisation necessitates the reversal of magnetisation. After the symmetry was analysed, I investigated candidate materials in more detail through the application of quantum mechanical simulation. I find that certain perovskite materials do show the necessary couplings to enable electric field control of magnetism. Single perovskites under mild epitaxial strains are shown to possess a universal polar instability which, when coupled to an external electric field, induces a transition from an antiferromagnetic state to a ferromagnetic one. Additionally, the symmetry analysis identifies that improper ferroelectrics are most likely to host the desired couplings and that cation ordering, such as in CeBaMnO, is the easiest route to achieve them. Future work would focus on stabilising the important cation orderings and investigating the detailed switching dynamics of candidate materials.