X-ray magnetic circular dichroism and anomalous Hall effect in proton-doped spinel NiCo2O4 thin films

Element-specified x-ray magnetic circular dichroism (XMCD) and the anomalous Hall effect (AHE) have been investigated to provide deep insights into the magnetic and electronic structural evolutions of different proton-doping spinel ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ (NCO) films. Our results show that the magnetic dichroism at the Co ${L}_{23}$-edge is mainly contributed by the spin magnetic moment at the tetrahedral (${T}_{d}$)-site ${\mathrm{Co}}^{2+}$ in pristine NCO. The spin magnetic moments at Co and Ni sites have antiparallel alignments. The magnetic anisotropy is related to the considerable orbital magnetic moments therein. Upon proton-doping in ${\mathrm{NCO}}_{\text{int}}$ films (the intermediate state with the mixed phases of ${\mathrm{H}}_{x}\mathrm{NCO}$ and NCO), the magnetic dichroism at the Co ${L}_{23}$-edge still exists. The doping of one extra electron resides at the octahedral (${O}_{h}$)-site ${\mathrm{Co}}^{3+}$ partially and changes ${\mathrm{Co}}^{3+}$ ($S=0)$ to ${\mathrm{Co}}^{2+}$ ($S=3/2$). This ${O}_{h}$-site ${\mathrm{Co}}^{2+}$ aligns in an antiparallel manner with ${T}_{d}$-site ${\mathrm{Co}}^{2+}$ through O-ions mediated antiferromagnetic superexchange interaction, resulting in a reduced spin magnetic moment in ${\mathrm{NCO}}_{\text{int}}$ compared to pristine NCO films. Moreover, the reduced magnetic anisotropy in ${\mathrm{NCO}}_{\text{int}}$ is due to the decreasing of the orbital magnetic moment, which can be related to the charge redistribution at Co $3d$ energy levels and the crystal-field effect. In the two-proton doped ${\mathrm{H}}_{2}\mathrm{NCO}$ films, the magnetic dichroism is absent where the antiferromagnetism arises from the antiparallel spin alignments of ${\mathrm{Co}}^{2+}$ in ${T}_{d}$ and ${O}_{h}$ sites. The intrinsic ferromagnetism is also verified by the AHE, which shows a square-shaped hysteresis as a function of the magnetic fields for pristine NCO and ${\mathrm{NCO}}_{\text{int}}$. Interestingly, the proton-doped ${\mathrm{NCO}}_{\text{int}}$ films exhibit an enhanced coercive field and the exchange bias effect with an asymmetric hysteresis loop at low temperatures, which can be ascribed to the antiferromagnetic phase stemming from partial Co valence state reduction by proton doping.