Induced ferromagnetism in LaMnO3 by Mn-site substitution:: The major role of Mn mixed valency -: art. no. 104420

In order to know if the origin of ferromagnetism induced by the substitution of Co or Ni for Mn in LaMnO3 could be due to their divalent oxidation states, a set of LaMn1-xMxO3 samples has been prepared for x less than or equal to0.2 and M=Li, Zn, Ni, Co, Ga, and Rh. By combining resistivity, thermopower, magnetization, susceptibility measurements, and room-temperature structural refinements, we show that, similarly to Ni and Co, univalent (Li+) and divalent (Zn2+) S=0 cations induce both ferromagnetism and a more conductive behavior. In contrast, the S=0 trivalent cations (Rh3+ and Ga3+) are found to preserve the orthorhombic O' structure, connected with the cooperative Jahn-Teller distortion of Mn3+ species in LaMnO3. Accordingly, Rh3+ - and-Ga3+ substituted LaMnO3 manganites are only weak ferromagnets, and the resistivity increases in comparison to LaMnO3. These results indicate that, for the investigated range of substitution (a maximum of 20%), both cobalt and nickel behave like divalent cations. The observation of ferromagnetism and the resistivity decrease, induced by univalent and divalent cations, Li+ and Zn2+, is due to the creation of Mn4+ species which favor Mn3+/Mn4+ double exchange.