A chemical CrO3 -> CrO2 transformation reaction occurs in the presence of Ag+ species forming CrO2 core-shell magnetic nanoparticles. The process involves heating CrO3 with AgNO3 in an aqueous solution in ambient air at a low temperature of 330-350 K. Shell thickness (diamagnetic amorphous silver) is controlled to be similar to 2 nm tunable to the magnetic and impedance properties. Improved coercivity H-c=75.7 mT and the Curie temperature T-C=415 K are found with saturation magnetization M-s=80 Am-2/kg after annealing the sample at 573 K in air for 2 h. On heating, the impedance (or electrical resistance) varies steadily through a maximum in the T-C point in a typical half-metallic ceramic behavior. In a dynamic response to frequency f, the T-C increases from 418 K at 1 kHz to as high a value as 505 K at 1 MHz. A nearly f independent and low value of dissipation factor phi as 0.05 is promising for low power loss high-frequency applications of such CrO2 particles.