Abstract : Among the numerous exotic phenomena exhibited by honeycomb antiferromagnets, linear magnetoelectricity in niobiates and tantalates of an A(4)M(2)O(9) formula has been attracting a lot of attention. The magnetic frustration, first exploited in alpha-Cr2O3 in the 1960s, can be tuned in the "429" compounds of which structures derive from corundum, by varying the 3d orbitals filling (from 3d(5) to 3d(7) as A goes from Mn2+ to Co2+) and/or the size of the d(0)M cation (Nb5+ or Ta5+). Fe4Ta2O9 illustrates the physics richness of these compounds, as it exhibits a rare combination of multiferroism and linear magnetoelectricity associated to different magnetic field dependences of the electrical polarization.
