From spin induced ferroelectricity to dipolar glasses: Spinel chromites and mixed delafossites

Magnetoelectric multiferroics showing coupling between polarization and magnetic order are attracting much attention. For instance, they could be used in memory devices. Metal-transition oxides are provided several examples of inorganic magnetoelectric multiferroics. In the present short review, spinel and delafossite chromites are described. For the former, an electric polarization is evidenced in the ferrimagnetic state for ACr2O4 polycrystalline samples (A=Ni, Fe, Co). The presence of a Jahn–Teller cation such as Ni2+ at the A site is shown to yield larger polarization values. In the delafossites, substitution by V3+ at the Cr or Fe site in CuCrO2 (CuFeO2) suppresses the complex antiferromagnetic structure at the benefit of a spin glass state. The presence of cation disorder, probed by transmission electron microscopy, favors relaxor-like ferroelectricity. The results on the ferroelectricity of ferrimagnets and insulating spin glasses demonstrate that, in this research field, transition-metal oxides are worth to be studied. - Graphical abstract: Electric polarization as a function of temperature is measured up to TC in three chromite ferrimagnetic spinels. Largest values are reached for spinels with Jahn–Teller cations at the A site (Ni or Fe). Highlights: ► Electric polarization is evidenced in the ferrimagnetic state of the chromite spinels. ► Jahn–Teller cations at the A site of these spinels lead to larger polarization values. ► Vanadium substituted at the Cr (or Fe) site of delafossites changes the antiferromagnetic state to spin glass. ► Electric polarization is not the result of magnetic ordering but magnetic disordering in Cr or Fe delafossites. ► Relaxor-type ferroelectricity or spin induced ferroelectricity can be observed in the delafossites.