Multiferroics: hidden functionalities beyond magnetoelectric coupling

Requirements to "good multiferroics" are tough. They are supposed to have a spontaneous magnetization and polarization, preferably parallel to each other, with a strong magnetoelectric coupling between them. Inevitably, this leads to a multiferroic state that is described by a very complex set of order parameters – complex enough to provide the symmetry degrees of freedom to fulfil so many requirements at once [1]. With the focus on electric-field-controlled magnetic order, it goes unnoticed that these degrees of freedom will permit many functionalities other than a refined magnetoelectric coupling. In my talk, I will describe the quest for such functionalities in our group. I will discuss up to four cases. (i) For the multiferroic hexagonal manganites I will show that amplitude and phase of the order parameter may exhibit different coherence length. Taking this into account, we resolve the long-standing controversial question of how exactly the topological ferroelectric state in this system arises [2]. (ii) The emergence of a magnetic bulk phase transition out of the spin structure in the domain walls is shown for (Tb,Dy)FeO₃. (iii) Inversion of a ferroelectric and a ferromagnetic multi-domain state in homogeneous external fields is demonstrated: In each domain, the direction of the order parameter is reversed but the domain pattern as such is left untouched [3].

[1] M. Fiebig et al., Nature Rev. Mater 1, 16046 (2016)
[2] M. Lilienblum et al., Nature Phys. 11, 1070 (2015)
[3] N. Leo et al., Nature 560, 466 (2018)