Ferromagnetism induced by chromium substitution in the CaRuO3 perovskite

Chromium has been substituted for ruthenium in CaRuO3 up to 35%. It is found that chromium induces a ferromagnetic contribution showing a maximum fraction for x=0.15 in CaRu0.85Cr0.15O3 with a Curie temperature (T-C) of similar to 115 K. This change of magnetic ground state goes with an electronic localization, the resistivity at 5 K increasing by more than 6 orders of magnitude as x increases from x=0.06 to x=0.35. Concomitantly, a negative magnetoresistance is induced by the chromium substitution. In order to explain the chromium effect, it is suggested that each trivalent chromium couples antiferromagnetically to its Ru4+/5+ nearest neighbors. When surrounded by six ruthenium cations, this creates "ferrimagnetic" polarons in which T-C is governed by the Ru4+/5+-O-Cr3+ antiferromagnetic exchange (T-C similar to 115 K). However, as the Cr3+ concentration increases, the Cr3+-O-Cr3+ antiferromagnetic exchange (energy) (T-N similar to 290 K) creates an antagonistic effect. This explains the existence of an optimum chromium content at x=0.15.