Charge transfer ferromagnetism in defect-ridden oxide films

Ferromagnetic magnetization curve of certain oxide thin films, whether undoped or doped 3$d$ ions, shows a characteristic anhysteretic approach to saturation of the form M $\approx $ M$_{s}$tanh(H/H$_{0})$, which is \textit{independent of temperature} below RT. There is no magnetic ordering of the dopant ions, but ferromagnetism is associated with Stoner splitting of a defect-related impurity band. In a model of ferromagnetic grain boundaries, H$_{0}$ = 0.16 M$_{0}$, where M$_{0}$ is the magnetization of the ferromagnetic regions. Data mining on six oxide systems shows that \textit{no more than 1 -- 2 {\%} of the volume of the films is magnetically ordered}. Charge transfer ferromagnetism arises when the impurity band can be populated from a proximate charge reservoir. Changing electron concentration leads to fulfillment of the Stomer criterion. There is a rich phase diagram as a function of bandwidth $W$, Stoner integral $I$ and band occupancy (which is related to the cost of electron transfer $U)$ with metallic and insulating regions which may be ferromagnetic, half metallic or nonmagnetic, consistent with behavior as a function of electron concentration observed experimentally. Our model of sparse Stoner ferromagnetism with variable electron concentration accounts consistently for the main experimental features of these puzzling materials.