Unusual high-spin ${\mathrm{Fe}}^{5+}-{\mathrm{Ni}}^{3+}$ state and strong ferromagnetism in the mixed perovskite ${\mathrm{SrFe}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{O}}_3$

The charge-spin-orbital state plays a vital role in correlated oxides with mixed transition-metal elements, and it is often a matter of debate. Here, we address this issue for the newly synthesized unusual high-valence perovskite ${\mathrm{SrFe}}_{1-x}{\mathrm{Ni}}_x{\mathrm{O}}_3$, using an analysis of crystal-field level diagrams, density functional calculations, and Monte Carlo simulations. We have identified the formal high-spin ${\mathrm{Fe}}^{5+}(t_{2g}^3, S=3/2)$ and high-spin ${\mathrm{Ni}}^{3+}(t_{2g}^5{e}_g^2, S=3/2)$ state for ${\mathrm{SrFe}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{O}}_3$, rather than the previously suggested high-spin ${\mathrm{Fe}}^{4+}(t_{2g}^3{e}_g^1, S=2)$ and low-spin ${\mathrm{Ni}}^{4+}$ $(t_{2g}^6, S=0)$ state. Moreover, our model and the present results well explain the observed small lattice variation of ${\mathrm{SrFe}}_{1-x}{\mathrm{Ni}}_x{\mathrm{O}}_3$ $(0\le x\le 0.5)$ and the above room temperature ferromagnetic order in ${\mathrm{SrFe}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{O}}_3$ regardless of a Fe-Ni atomic order or disorder.

Figure 1

Schematic $3d$ level diagrams of (a) high-spin ${\mathrm{Fe}}^{4+}$ and low-spin ${\mathrm{Ni}}^{4+}$ (the previous model after Ref. [11]) and (b) the high-spin ${\mathrm{Fe}}^{5+}$ and high-spin ${\mathrm{Ni}}^{3+}$ (our model). The $e_g$ electron hopping via oxygen would give a weak FM in (a) and a strong FM in (b). $\mathrm{O}2p$ orbitals are omitted for brevity.

Figure 2

DOS of ${\mathrm{SrFe}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{O}}_3$ by LSDA. It is a FM metal with a broad Ni-O-Fe $e_g$ conduction band. The red (blue) curves stand for the $e_g$ ($t_{2g}$) channel. The solid (dashed) curves refer to the up (down) spin channel. The Fermi level is set at zero energy.

Figure 3

Fixed-spin-moment calculations of FM ${\mathrm{SrFe}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{O}}_3$ by LSDA. The ground state has a magnetic moment of $2.2{\mu }_{\mathrm{B}}$/f.u., in good agreement with the experiment [11].

Figure 4

DOS of FM ${\mathrm{SrFe}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{O}}_3$ by LSDA+$U$. The topmost O $2p$ valence band and the delocalized $2p$ hole state are evident in this negative charge transfer oxide with an unusual high valence state (see also Fig. 2 for a comparison).

Figure 5

Specific-heat curves of ${\mathrm{SrFe}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{O}}_3$ by Monte Carlo simulations. $T_{\mathrm{C}}$ is about 600 K for an ideal Fe-Ni order (the blue curve), and it lies between 340 and 550 K for a complete Fe-Ni disorder having the AF Fe-Fe and Ni-Ni couplings with 0.2–0.8 times Fe-Ni FM coupling strength. The experimental above RT FM order is well reproduced here.