Valence states and magnetic properties of LaNi_1−xMn_xO₃ (for 0≤x≤0.2 and x=0.5)

Abstract

Single-phase LaNi_1−xMn_xO₃ samples in the compositional range 0<or=x<or=0.2 were prepared by the decomposition of precursor hydroxide solid solutions, and La₂NiMnO₆ was prepared from a mixture of carbonates. All the samples contained a disordered array of nickel and manganese ions. X-ray absorption and XPS measurements indicate the presence of Mn(III) and Ni(III) valence states at room temperature for x=0.1, 0.2 and 0.5. Magnetic susceptibility data suggest Mn⁴⁺ solute ions and Stoner-enhanced Pauli paramagnetism of the metallic solvent for x=0.01 with a smooth transition to Mn³⁺ solute ions and a spontaneously magnetised solvent conduction band at x=0.05. Below 200K, the x=0.05 sample forms superparamagnetic clusters, and below 40K there is evidence for an antiferromagnet spin-density wave. Comparisons with LaCo_0.95Mn_0.05O₃ and La_0.98Sr_0.02CoO₃ confirm that the long-range magnetic coupling occurs via solvent electrons in a narrow conduction band. The conductivity changes from that of a narrow-band metal for x<0.01 to that more characteristic of diffusive motion for x>0.05, but any motional enthalpy appears to remain small ( Delta H_m approximately=0). The x=0.1 sample exhibits ferrimagnetic spin glass behaviour below 40K, and the ferromagnetic interactions increase with manganese concentration. The oxide with x=0.50 is ferromagnetic with a well defined Curie temperature.