Enhanced magnetoresistance and surface state of CrO2 particles improved by chemical process

The paper presents an approach to enhance a magnetoresistance (MR) effect in CrO2 powder compact by an oxidization reaction process. An aqueous potassium permanganate (KMnO4) was used to react with the CrO2 particles coated naturally with Cr2O3 layer. The experiment indicates that the strong oxidant can effectively adjust thickness of the natural Cr2O3 layer, and thereby change the surface state of the CrO2 particles. Structural and magnetic properties for the improved CrO2 particles have been characterized by X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS) and SQUID magnetometer. The results exhibit that the magnetotransport behavior of CrO2 particles depends sensitively on the chemical reaction time. An optimal reaction process yields an obvious increase up to −33% in magnetoresistance at a temperature of 5 K for the chemical treated CrO2 powder, compared to MR=-27% for the original CrO2 powder. The mechanism of magnetotransport is assumed to originate from the spin-dependent tunneling in the granular system, which is consistent with our experimental results. The simple chemical approach has a potential to achieve an enhanced magnetoresistance in a metallic particle system by adjusting the surface state of the magnetic nanoparticle.