Multiferroics and Magnetoelectrics: A Comparison between Some Chromites and Cobaltites

Oxides containing magnetic 3d transition metals offer a large family of structures with frustrated magnetic networks. Some of those incommensurate antiferromagnetic structures are responsible for the local breaking of inversion symmetry so that these oxides are called spin-induced ferroelectrics. As listed in the introduction of this short review, the number of these multiferroics continues to increase. As for applications, the coupling between these ferroisms is needed; some magnetic oxides, despite a lack of ferroelectric ground state, exhibit large magnetoelectric coupling with a magnetic-field-induced polarization. Thus, they are classified as "magnetoelectrics". In the present review, we focus on recently studied systems showing ferroelectric-like behaviors or large magnetoelectric coefficients. This will be illustrated by chromites with the comparison between ceramics of CuCrO2 and AgCrS2: they exhibit different antiferromagnetic ground states, centrosymmetric and noncentrosymmetric. For the magnetoelectrics, the example of the cobaltite, CaBaCo4O7, which is a polar ferrimagnet, is taken to illustrate the existence of large magnetoelectric coefficients in crystals. Then, the existence of spin-induced ferroelectricity in centrosymmetric orthochromites (RECrO3 with RE = Lu or Er) is discussed. Through this selection of potential multiferroics or magnetoelectrics, it is clear that many magnetic materials in different forms (bulk/crystal/thin film) remain to be studied.