From antiferromagnetism to high-$T_c$ weak ferromagnetism manipulated by atomic rearrangement in ${\mathrm{Ba}}_3{\mathrm{NiOs}}_2{\mathrm{O}}_9$

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 ($\approx 400\mathrm{K}$) 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.

Figure 1

(a) XRD pattern of AP $\mathrm{B}{\mathrm{a}}_3\mathrm{NiO}{\mathrm{s}}_2{\mathrm{O}}_9$, (b) SXRD pattern of HP $\mathrm{B}{\mathrm{a}}_3\mathrm{NiO}{\mathrm{s}}_2{\mathrm{O}}_9$, (c) NPD pattern of HP $\mathrm{B}{\mathrm{a}}_3\mathrm{NiO}{\mathrm{s}}_2{\mathrm{O}}_9$ at room temperature. The calculated patterns (red) obtained by Rietveld refinement of the experimental patterns in space group $P{6}_3/mmc$ are compared with the observed data (black dots), and the difference is shown as blue patterns. The vertical black bars are indicating the diffraction positions for the title compound and minor impurity phases ($\mathrm{BaC}{\mathrm{O}}_3$ and Os in AP $\mathrm{B}{\mathrm{a}}_3\mathrm{NiO}{\mathrm{s}}_2{\mathrm{O}}_9$; Os in SXRD data of HP $\mathrm{B}{\mathrm{a}}_3\mathrm{NiO}{\mathrm{s}}_2{\mathrm{O}}_9$).

Figure 2

The crystal structures of (a) AP $\mathrm{B}{\mathrm{a}}_3\mathrm{NiO}{\mathrm{s}}_2{\mathrm{O}}_9$ and (b) HP $\mathrm{B}{\mathrm{a}}_3\mathrm{NiO}{\mathrm{s}}_2{\mathrm{O}}_9$.

Figure 3

(a) $\mathrm{Ni}\text{−}{L}_{2,3}$ XAS spectra of HP $\mathrm{B}{\mathrm{a}}_3\mathrm{NiO}{\mathrm{s}}_2{\mathrm{O}}_9$ and NiO. (b) $\mathrm{Os}\text{−}{L}_3$ XAS spectra of HP $\mathrm{B}{\mathrm{a}}_3\mathrm{NiO}{\mathrm{s}}_2{\mathrm{O}}_9$ and $\mathrm{S}{\mathrm{r}}_2\mathrm{FeOs}{\mathrm{O}}_6$ ($\mathrm{O}{\mathrm{s}}^{5+}$ reference).

Figure 4

Isothermal magnetization curves of AP and HP $\mathrm{B}{\mathrm{a}}_3\mathrm{NiO}{\mathrm{s}}_2{\mathrm{O}}_9$ at 5 K.

Figure 5

Temperature dependence of magnetic susceptibility of AP $\mathrm{B}{\mathrm{a}}_3\mathrm{NiO}{\mathrm{s}}_2{\mathrm{O}}_9$. The inset shows the temperature dependence of the specific heat of AP $\mathrm{B}{\mathrm{a}}_3\mathrm{NiO}{\mathrm{s}}_2{\mathrm{O}}_9$.

Figure 6

Temperature dependence of the magnetic susceptibility of HP $\mathrm{B}{\mathrm{a}}_3\mathrm{NiO}{\mathrm{s}}_2{\mathrm{O}}_9$. The upper inset shows the corresponding ${\chi }^{-1}$ vs $T$ data.

Figure 7

(a) XMCD spectra at Os-$L$ edges of $\mathrm{B}{\mathrm{a}}_2\mathrm{NiO}{\mathrm{s}}_2{\mathrm{O}}_9$ measured at 4.5 K and 13 kOe. (b) XMCD spectra at Ni $L$ edges of $\mathrm{B}{\mathrm{a}}_3\mathrm{NiO}{\mathrm{s}}_2{\mathrm{O}}_9$ measured at 2 K and 60 kOe.

Figure 8

NPD pattern of HP $\mathrm{B}{\mathrm{a}}_3\mathrm{NiO}{\mathrm{s}}_2{\mathrm{O}}_9$ collected on instrument E2 at 2 K ($\lambda =2.38\AA$). The calculated pattern of the nuclear structure (red) is compared with the calculated one, where the superimposed magnetic ordering is also taken into account. Further, both patterns are compared with the experimental pattern (black dots). The positions of two reflections of the impurity phase $\mathrm{BaC}{\mathrm{O}}_3$ are also marked.