Anomalous Magnetoresistance Effects in (CrO$_{2})_{1-x}$ : (MgB$_{2})_{x}$ powders

It is known that the electrical conduction mechanism in compressed powders and polycrystalline CrO$_{2}$ films is due to the intergranular tunneling. Here, we focus on the percolation effects in the CrO$_{2}$ powders intermixed with a superconductor, MgB$_{2}$. A mixture of (CrO$_{2})_{1-x}$ : (MgB$_{2})_{x}$ powders with 0$\le $x$\le $1 were cold-pressed to form circular disks. The electrical resistance measured by a four probe technique, as expected showed a lower resistance for the mixture when compared to the pure pressed CrO$_{2}$. The observed hysteresis extrinsic magnetoresistance (MR) showed peaks corresponding to the coercive fields of CrO$_{2}$ which can be associated with the alignment of magnetization of adjacent CrO$_{2}$ particles. Interestingly, the MR changed signs at around the superconducting transition temperature of MgB$_{2}$ ($\sim $40K) and also the slope of MR at high magnetic fields changed from positive to negative. We will discuss a possible origin of the observed effects, which we believe are related to the ferromagnet/superconductor interaction.