Canted ferrimagnetism and giant coercivity in the nonstoichiometric double perovskite La2Ni1.19Os0.81O6

The nonstoichiometric 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 A2B B 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 the presence of about 50 paramagnetic Os5 5d 3 , S 3 2 and 50 nonmagnetic Os4 5d 4 , J eff 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 noncollinear 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 oxide