Unlike proper ferroelectrics, in improper ferroelectrics the polarization does not arises from the condensation of a polar lattice distortion associated to a zone-center instability. Understanding the interplay between magnetism and ferroelectricity in improper multiferroics is of interest in fundamental and applied research. At present, there is a good number of examples of improper ferroelectrics induced by antiferromagnetic order. This order is compatible with antitranslations that can suppress inversion centers or screw axis and favor symmetry breaks into polar symmetries.
Generally, cooperative distortions at the point k=0 (zone center) are more favorable to ferromagnetic configurations. In practice, ferromagnetic improper multiferroics are much more rare than antiferromagnets, and in most cases are simply weak-ferromagnets. However strong ferromagnetic multiferroics can have important advantages in magnetoelectric devices.
We completed a comprehensive study on four successive magnetic phases driven by zone-center modes in Tb2MnNiO6 ferromagnet. The interactions between A (Tb) and B (metals) spin subsystems stabilizes a ferromagnetic ground state with polar symmetry which makes this oxide potentially multiferroic with polarization by magnetic trilinear coupling. Its macroscopic magnetization is large and not related to a weak-ferromagnetic component from Dzyaloshinskii–Moriya terms.
Symmetry analysis of neutron data (ILL, Grenoble) unveils that the nonpolar/polar transition occurs due to the concurrence of two different non-polar magnetic distortions. The concurrence of these non-polar distortions produces an overall polar symmetry, in which the loss of inversion center splits the orbits of Tb and O sites. In addition, thanks to a severe spin reorientation of the ferromagnetic axis across the polar/non polar transition we anticipate that in this material the direction of the magnetization can be used as a lever to switch the polar/non-polar (ferroelectric/antiferroelectric) transformation. Likewise, the control of the magnetization direction would be possible by electrical fields.
Magnetic inversion symmetry breaking and spin reorientation in Tb2MnNiO6: a polar strong ferromagnet
Jose Luis García-Muñoz, Javier Blasco, Xiaodong Zhang, Oscar Fabelo
Physical Review B 99, 184444, 9pp, 2019
Figure: (left) Magnetic irreps and isotropy subgroups: successive activation of nonpolar magnetic modes at the A and B sites that produce the P21' polar phase, allowing a trilinear coupling with the polarization in the free energy. (right) Changes in the magnetic order at A and B sites during the FM3 (P21'/c') « FM4 (P21') transformation showing the ferromagnetic spin reorientation. Electric and magnetic mechanisms to switch the polar/non-polar (FE/anti-FE) transformation between P21'/c' and P21' states.