  The crystallographic and magnetic properties of
La{0.5+delta}A{0.5-delta}Mn{0.5+epsilon}Ru{0.5-epsilon}O{3} (A= Ca, Sr, Ba)
were investigated by means of neutron powder diffraction. All studied samples
show the orthorhombic perovskite crystal structure, space group Pnma, with
regular (Mn,Ru)O{6} octahedra and no chemical ordering of the Mn3+ and Ru4+
ions. Ferromagnetic spin structures were observed below Tc ~ 200-250 K, with an
average ordered moment of ~ 1.8-2.0 Bohr magnetons per (Mn,Ru). The observation
of long-range ferromagnetism and the absence of orbital ordering are
rationalized in terms a strong Mn-Ru hybridization, which may freeze the
orbital degree of freedom and broaden the eg valence band, leading to an
orbital-glass state with carrier-mediated ferromagnetism.
