Ferromagnetism Observed in Transition Metal Doped High K Oxides

Polycrystalline Co doped La₂O₃ and CeO₂ bulk samples with Co content of 0 ~ 8% were fabricated by conventional ceramic method. XRD and XPS studies indicate no Co metal clusters or secondary magnetic phases in any samples in this study. For both systems it is found that the undoped or slightly doped samples show only diamagnetic signal, while samples with relatively higher Co content exhibit distinct room temperature ferromagnetism and its saturation magnetization (M_s) exponentially increase with Co concentration to a critical value (e.g. 2%). After this critical value, the increase of magnetic cations will induce an antiferromagnetic superexchange (AFE), leading to a quick decrease of M_s. Our results hint that a universal magnetic behavior in the TM doped high-k oxides, which can be well explained by the oxygen-vacancy (F-Center) mediated mechanism under the framework of bound magnetic polaron model.