Magneto-transport properties of nanocomposite cobalt/carbon systems

In producing diluted magnetic semiconductors (DMS), it remains challenging to obtain a uniform distribution of magnetic ions. Composite ferromagnet/semiconductor systems are easily formed when the concentration of magnetic ions is larger than its solubility at given preparation conditions. This is usually considered as a failure for spintronics applications. However, as long as the embedded (nano)ferromagnets can polarize charge-carriers in the semiconducting matrix, the composite systems can also fullfill spintronics requirements. For example, nanocomposite systems with ferromagnetic MnAs nanocrystals being epitaxially embedded inside GaAs matrix reveal a giant magnetoresistance [1]. Recently, the phase separation in Mn doped Ge [2] and Si [3] has been indentified by high resolution characterization techniques. Therefore, the research interest in composite ferromagnet/semiconductor systems arises in parallel with that in DMS materials. Here we present the magnetic and magneto-transport properties of cobalt nanocrystals embedded inside carbon. Co(40%)/C nanocomposite films were prepared by ion beam co-sputtering method using silicon substrates with a 500 nm thick oxide layer. The phase separation was controlled by varing substrate temperatures from room temperature to 500 °C. We have measured their magnetic and magneto-transport properties. Two significant observations will be discussed: (i) a giant anomalous Hall effect (AHE) amounting to 2 μohm cm compared with pure Co metal [4], and (ii) a negative magnetoresistance. This encourages future applications in Hall sensors and spintronic-devices.