Ferromagnetic semiconductors (FS) with high Curie temperatures are of great interest for applications in spinbased, multifunctional devices. In particular, dilute magnetic semiconductors (DMS) have been created by doping semiconducting host materials with magnetic transition-metal ions, yielding magnetic ordering transitions near room temperature. [1,2] The ferromagnetism of DMS is currently thought to result from a local anti-ferromagnetic coupling between carriers (e.g., holes in (Ga,Mn)As) and the 3d (Mn) magnetic moments, leading to long-range ferromagnetic ordering. Nevertheless, concerns have been raised that clustering of randomly placed magnetic dopants leads to an inhomogeneous ferromagnetic state. Furthermore, DMS remain controversial from both experimental and theoretical points of view.
A major obstacle to the implementation of spin-polarized semiconductor devices is the current absence of suitable room-temperature, soft ferromagnetic semiconductors (FSs). Dilute magnetic semiconductors are under intense study for applications in spintronics. However, the weak solubility of randomly placed magnetic ions in the semiconductor host makes these materials unsuitable for devices. It is, therefore, crucial to develop a room-temperature FS based on a periodic array of magnetic ions. We have found that ternary ruthenium ferrites exhibit long-range ferromagnetic order well above room temperature, accompanied by narrow-gap semiconducting properties that include a large anomalous Hall conductance, low resistivity, and high carrier concentration. The physical properties can be tuned by simple chemical substitution of two elements, Fe and Co, or by varying the relative concentration of 3d and 4d elements within a homogeneity range that we have established. These promising properties—manifest within a single structural family—provide a fertile ground for fundamental studies and open up a host of potential device applications.
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