$\mathrm{Na}\mathrm{Os}{\mathrm{O}}_3$: A high Neel temperature 5$d$ oxide

The origin of a high Neel temperature in a 5$d$ oxide, $\mathrm{Na}\mathrm{Os}{\mathrm{O}}_3$, has been analyzed within the mean-field limit of a multiband Hubbard model and compared with the analogous 4$d$ oxide, $\mathrm{Sr}\mathrm{Tc}{\mathrm{O}}_3$. Our analysis shows that there are a lot of similarities in both of these oxides on the dependence of the effective exchange interaction strength ($J_0$) on the electron-electron interaction strength ($U$). However, the relevant value of $U$ in each system puts them in different portions of the parameter space. Although the Neel temperature for $\mathrm{Na}\mathrm{Os}{\mathrm{O}}_3$ is less than that for $\mathrm{Sr}\mathrm{Tc}{\mathrm{O}}_3$, our results suggest that there could be examples among other 5$d$ oxides that have a higher Neel temperature. We have also examined the stability of the G-type antiferromagnetic state found in $\mathrm{Na}\mathrm{Os}{\mathrm{O}}_3$ as a function of electron doping within GGA + $U$ calculations and find a robust G-type antiferromagnetic metallic state stabilized. The most surprising aspect of the doped results is the rigid bandlike evolution of the electronic structure, which indicates that the magnetism in $\mathrm{Na}\mathrm{Os}{\mathrm{O}}_3$ is not driven by Fermi surface nesting.

Figure 1

Density of states of $\mathrm{Na}\mathrm{Os}{\mathrm{O}}_3$ obtained from GGA calculation ($U$ = 0) (a) The total as well as partial density of states for nonmagnetic configuration. (b) The up (top) and the down spin (bottom) Os $d$ projected partial density of states for G-AFM magnetic structure. The zero of energy corresponds to the Fermi energy.

Figure 2

The up (utop) and down spin (bottom) projected Os $d$ and O $p$ partial density of states for G-AFM spin configuration for $\Delta =2$ eV, $J_h=0.1$ eV as a function of $U$ from mean-field multiband Hubbard calculations for $\mathrm{Na}\mathrm{Os}{\mathrm{O}}_3$. The zero of energy corresponds to the Fermi energy.

Figure 3

The variation of the band gap (left axis) and the magnitude of magnetic moment of each Os atom (right axis) as a function of $U$ are plotted for $\Delta =2.0$ eV.

Figure 4

Variation of effective exchange interaction strength ($J_0$) with $U$ for $J_h=0.1$ eV from mean-field multiband calculations for $\Delta =2$ and 0 eV, respectively. The hopping interaction strengths for the tight-binding part of the Hamiltonian are taken for the system shown in parentheses.

Figure 5

(a) Variation of magnetic stabilization energy of G-AFM configuration with Mg doping ($U=1$ eV) from GGA + U calculations. Up (top) and down spin (bottom) Os $d$ projected partial density of states for (b) $\mathrm{Na}\mathrm{Os}{\mathrm{O}}_3$ and (c) Na${}_{0.75}$Mg${}_{0.25}$OsO${}_3$ at $U=1$ eV in the G-AFM configuration.

Figure 6

Up (top) and down spin (bottom) Os $d$ projected partial density of states for Na${}_{0.50}$Mg${}_{0.50}$OsO${}_3$ in various magnetic configurations from GGA + $U$ calculations. The zero of energy represents the Fermi energy.

Figure 7

Distortion in Mg-O bond lengths for 3.125% Mg doped case.

Figure 8

Spin wave dispersion along different symmetry directions for $\mathrm{Na}\mathrm{Os}{\mathrm{O}}_3$ and 25% Mg doped case. The zero of energy corresponds to energy of G-AFM state for both cases.