Crystal and magnetic structure of Cr- and Ni-substituted (La0.50Ca0.50)MnO3

The crystal and magnetic structure of (La0.50Ca0.50)(Mn1−xBx )O3 (x = 0.00, 0.03, 0.08; B = Cr, Ni) has been investigated between 5 and 300 K by means of dc magnetic measurements and neutron powder diffraction followed by Rietveld refinement. In the pristine compound an orthorhombic to monoclinic phase transition is detected on cooling, accompanied by a CE-type antiferromagnetic (AFM) ordering arising. Ni2+ and Cr3+ substitutions have similar effects on the structural and magnetic properties of (La0.50Ca0.50)MnO3, despite the fact that these ions are characterized by different external electronic configurations. After substitution, the orthorhombic to monoclinic phase transition is hindered. As a consequence, charge and orbital orderings are suppressed, as is the superexchange; double exchange takes place inducing ferromagnetic (FM) interactions. No evidence for stable magnetic interaction between Cr3+ or Ni2+ and the neighbouring Mn ions was detected. Nevertheless, in the Ni-substituted samples a detectable quantity of monoclinic phase forms during cooling, inside which AFM interactions take place. The amount of this secondary monoclinic phase decreases on increasing Ni substitution; the global FM magnetic moment decreases as well, due to a spin-cluster glass-type state arising. As a result a FM state is found to coexist with a spin-cluster glass-type state.