Enhancement of T_N induced by magnetic dilution in the linear magnetoelectric Mn₄Nb₂O₉

Ferroelectric polarization induced by the magnetic field is one of the properties reported in honeycomb antiferromagnetic linear magnetoelectrics (LME) belonging to the A(4)M(2)O(9) P3c1 class of corundum compounds with A = Co, Fe, and Mn and M = Nb or Ta. Among them, Mn4Nb2O9 exhibits the highest T-N of 108 K. We show that it can even be increased by substituting partly Mn by Fe. The study of Mn4-xFexNb2O9 allows one to reach a TN up to 126 K at x = 1.5. This beneficial effect, confirmed also for Fe substituted Mn4Ta2O9, is compared to substitutions by Co or by Zn, both cations inducing a TN decrease. The linear decrease in the unit cell volume with x in Mn4-xFexNb2O9 shows that the cation size effect only cannot be inferred to explain the T-N(x) curve. Furthermore, the spin flop magnetic field of Mn4Nb2O9 decreases, thanks to the Fe for Mn substitution, a LME effect is evidenced at 10 K, Delta P(9T)(10K) = 11 mu C/m(2). These features, TN increase and LME properties, demonstrate the important role of substitutions to enhance the properties of such antiferrromagnets.