Exchange-biased oxide-based core–shell nanoparticles produced by seed-mediated growth in polyol

We report, for the first time, the synthesis of magnetically contrasted oxide-based core–shell nanoparticles using the polyol process. Iron oxide nanoparticles were first prepared by forced hydrolysis of iron acetate salt in a polyol and were then recovered by centrifugation and re-dispersed in a fresh cobalt acetate polyol solution. The microstructure of the resulting powder was finely characterized combining X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. Interestingly, the produced particles appeared to be of a very high crystalline quality with a perfect epitaxy between the spinel-like iron oxide core and the rock-salt-like cobalt monoxide shell, leading to a substantial exchange bias at low temperatures. As a consequence, a net blocking temperature, T B, increase was measured on the superparamagnetic iron oxide cores when they were coated by CoO, reaching a T B value of 298 K, very close to room temperature.