  The non-stoichiometric double perovskite oxide La2Ni1.19Os0.81O6 was
synthesized by solid state reaction and its crystal and magnetic structures
were investigated by powder x-ray and neutron diffraction. La2Ni1.19Os0.81O6
crystallizes in the monoclinic double perovskite structure (general formula
A2BB'O6) with space group P21/n, where the B site is fully occupied by Ni and
the B' site by 19 % Ni and 81 % Os atoms. Using x-ray absorption spectroscopy
an Os4.5+ oxidation state was established, suggesting presence of about 50 %
paramagnetic Os5+ (5d3, S = 3/2) and 50 % non-magnetic Os4+ (5d4, Jeff = 0)
ions at the B' sites. Magnetization and neutron diffraction measurements on
La2Ni1.19Os0.81O6 provide evidence for a ferrimagnetic transition at 125 K. The
analysis of the neutron data suggests a canted ferrimagnetic spin structure
with collinear Ni2+ spin chains extending along the c axis but a non-collinear
spin alignment within the ab plane. The magnetization curve of
La2Ni1.19Os0.81O6 features a hysteresis with a very high coercive field, HC =
41 kOe, at T = 5 K, which is explained in terms of large magnetocrystalline
anisotropy due to the presence of Os ions together with atomic disorder. Our
results are encouraging to search for rare earth free hard magnets in the class
of double perovskite oxides.
