Complex magnetic phase separation induced by Li-doping in multiferroic CaBaCo4O7

Abstract

The doping of lithium at the cobalt sites in the multiferroic orthorhombic cobaltite CaBaCo4O7 has been investigated. The oxides CaBaCo4-2xLixGaxO7 and CaBaCo4-xLixO7 keep the same polar space group as the parent phase. In contrast, they show a spectacular decrease of their ferrimagnetic properties for very low doping levels (0 < x ≤ 0.10), with the appearance of antiferromagnetism below TN ∼ 80 K and magnetic frustration at around Tf ∼ 20 K, which should have a great impact upon multiferroism in this system. This behavior is different from that of the Ga-doped phases CaBaCo4-xGaxO7, but remarkably similar to that of the Zn-doped phases CaBaCo4-xZnxO7. The actual role of the valence of the diamagnetic cation, Li+ is interpreted from its ability to sit in the Co2+ zig-zag chain, breaking the ferromagnetism along those chains in both series of oxides. Importantly, the Li-doping in CaBaCo4-xLixO7 oxides, introduces an excess Co3+ with respect to the other two series, CaBaCo4-xZnxO7 and CaBaCo4-2xLixGaxO7. This additional Co3+ is thought to be at the origin of the complex magnetic phase separation and especially of new magnetic interactions that appear around 52 K in competition with magnetic frustration and antiferromagnetism.