Magnetizing Oxides by Substituting Nitrogen for Oxygen

We describe a possible pathway to new magnetic materials with no conventional magnetic elements present. The substitution of nitrogen for oxygen in simple nonmagnetic oxides leads to holes in N $2p$ states which form local magnetic moments. Because of the very large Hund’s rule coupling of Nitrogen and O $2p$ electrons and the rather extended spatial extent of the wave functions these materials are predicted to be ferromagnetic metals or small band gap insulators. Experimental studies support the theoretical calculations with regard to the basic electronic structure and the formation of local magnetic moments. It remains to be seen if these materials are magnetically ordered and, if so, below what temperature.

Figure 1

Ferromagnetic total and N $2p$ partial densities of states calculated in LSDA (left panel) and $\mathrm{LSDA}+U$ (right panel) for 25% N substitution. The $\mathrm{LSDA}+U$ density of states is calculated with $U^{\mathrm{N}2p}=U^{\mathrm{O}2p}=4.6\mathrm{eV}$ and $J_H^{\mathrm{N}2p}=J_H^{\mathrm{O}2p}=1.2\mathrm{eV}$. The zero of energy is at Fermi energy.

Figure 2

$\mathrm{LSDA}+U$ spin density calculated for 0.9% Mn (a) and nitrogen (b) substitution in SrO with $U^{\mathrm{N}2p}=U^{\mathrm{O}2p}=4.6\mathrm{eV}$, $J_H^{\mathrm{N}2p}=J_H^{\mathrm{O}2p}=1.2\mathrm{eV}$, $U^{\mathrm{Mn}3d}=2.34\mathrm{eV}$, and $J_H^{\mathrm{Mn}3d}=0.89\mathrm{eV}$. Shown is a cut in the (100) plane of $6\times 6$ super cell. Note that the charge-compensating hole resides in the N $p_z$ orbital. Lattice sites are not shown for clarity.

Figure 3

Room temperature XPS valence band spectra of ${\mathrm{SrO}}_{1-x}\mathrm{Nx}$ films. The zero of binding energy is at the Fermi level of copper. The SrO spectrum is shifted towards the low binding energy by 1.4 eV to match the energy of the O $2s$ core level in SrO and ${\mathrm{SrO}}_{1-x}{\mathrm{N}}_x$.

Figure 4

Room temperature x-ray absorption spectra with $E\perp c$ of ${\mathrm{SrO}}_{1-x}{\mathrm{N}}_x$ films, taken in the total electron yield mode at the nitrogen (a) and oxygen (b) $K$ edges. Note the preedge structure at 394.5 eV in (a) and 529.25 eV in (b).

Figure 5

Room temperature N $1s$ core-level XPS spectra of ${\mathrm{SrO}}_{1-x}{\mathrm{N}}_x$ film with $x0.25$. The three peak structure is due to a local spin $1/2$ moment.