Unusual magnetism in Cu x Co 3 − x O 4 nanoparticles

In Cu-doped ${\mathrm{Co}}_{3}{\mathrm{O}}_{4}$ nanoparticles $({\mathrm{Cu}}_{x}{\mathrm{Co}}_{3\text{\ensuremath{-}}x}{\mathrm{O}}_{4};\phantom{\rule{0.16em}{0ex}}0\ensuremath{\le}x\ensuremath{\le}0.5)$ Cu occupies both octahedral and tetrahedral sites with a $2+$ oxidation state. As the Cu doping increases, we observe changes in the crystal structure corresponding to a Jahn-Teller distortion of the ${\mathrm{Cu}}^{2+}$ sites. To mediate charge balance with ${\mathrm{Cu}}^{2+}$ entering the octahedral sites, a hole forms in the O $2p$ orbitals bonded to the ${\mathrm{Cu}}^{2+}({O}_{h})$. ${\mathrm{Cu}}^{2+}({T}_{d})$ is noninteracting and disrupts the existing antiferromagnetic interactions between the ${\mathrm{Co}}^{2+}({T}_{d})$ ions, while ${\mathrm{Cu}}^{2+}({O}_{h})$ exhibits a ferromagnetic response as a result of a hybrid form of exchange occurring between ${\mathrm{Cu}}^{2+}({O}_{h})$ and ${\mathrm{Co}}^{2+}({T}_{d})$. Emergence of the $3{d}^{9}$ ligand hole is directly responsible for the origin of ferromagnetic Cu in the octahedral sites and this results in the unusual magnetism in ${\mathrm{Cu}}_{x}{\mathrm{Co}}_{3\text{\ensuremath{-}}x}{\mathrm{O}}_{4}$.