From antiferromagnetism to high-Tc weak ferromagnetism manipulated by atomic rearrangement in Ba3NiOs2O9

Polycrystalline samples of $\mathrm{B}{\mathrm{a}}_{3}\mathrm{NiO}{\mathrm{s}}_{2}{\mathrm{O}}_{9}$ were synthesized at ambient pressure (AP) and high pressure (HP) conditions, respectively. Both samples are electrically semiconducting. The AP $\mathrm{B}{\mathrm{a}}_{3}\mathrm{NiO}{\mathrm{s}}_{2}{\mathrm{O}}_{9}$ crystallizes in the $6H$ perovskite structure with space group $P{6}_{3}/mmc$, consisting of face-sharing $\mathrm{O}{\mathrm{s}}_{2}{\mathrm{O}}_{9}$ dimer units and corner-sharing $\mathrm{Ni}{\mathrm{O}}_{6}$ octahedra. Magnetic measurements indicated that AP $\mathrm{B}{\mathrm{a}}_{3}\mathrm{NiO}{\mathrm{s}}_{2}{\mathrm{O}}_{9}$ is antiferromagnetically ordered below 130 K. HP $\mathrm{B}{\mathrm{a}}_{3}\mathrm{NiO}{\mathrm{s}}_{2}{\mathrm{O}}_{9}$ crystallizes in the $6H$ perovskite structure too, but the face-sharing octahedral sites appear to be occupied by both $\mathrm{N}{\mathrm{i}}^{2+}$ and $\mathrm{O}{\mathrm{s}}^{5+}$ ions, whereas the corner-sharing site is occupied exclusively by $\mathrm{O}{\mathrm{s}}^{5+}$. HP $\mathrm{B}{\mathrm{a}}_{3}\mathrm{NiO}{\mathrm{s}}_{2}{\mathrm{O}}_{9}$ undergoes a high-temperature ($\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}400\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) weak ferromagnetic transition, which is much different from the antiferromagnetism of the AP phase. The long-range magnetic order of HP $\mathrm{B}{\mathrm{a}}_{3}\mathrm{NiO}{\mathrm{s}}_{2}{\mathrm{O}}_{9}$ was confirmed by neutron powder diffraction. X-ray magnetic circular dichroism analysis supported ferromagnetic coupling between Os and Ni moments which leads to a spin arrangement, where the ferromagnetic moments mainly arise from $\mathrm{N}{\mathrm{i}}^{2+}$ ions.