Induced orbital disordering and ferromagnetism in LaMnO/sub 3/ by Mn site substitution: evidence for valency effect

Summary form only given. Ferromagnetism can be induced in LaMn/sub 1-x/M/sub x/O/sub 3/ when M=Co or Ni. There is a lot of controversy about the origin of the ferromagnetism in these compounds. To clarify the origin of ferromagnetism, we have investigated the influence of several cations, magnetic and amagnetic, to separate the valency effect from the magnetic effect coming from the substituting cation. As LaMnO/sub 3/ properties are very sensitive to oxygen stoichiometry, all the samples LaMn/sub 1-x/M/sub x/O/sub 3/ have been prepared in evacuated silica ampoula to ensure a controlled oxygen content. Several non magnetic cations have been used (M=Li, Zn, Ga, Rh) as well as magnetic cations (M=Co, Ni). Using room temperature X ray diffraction, a detailed investigation of the crystallographic parameters of these compounds was carried out to probe the evolution of orbital ordering as the foreign cation is introduced. Magnetic (SQUID magnetometry, high temperature susceptibility) and transport measurements (resistivity, thermopower) were performed on the different samples.