A potential oxide for magnetic refrigeration application:: CrO2 particles

The magnetocaloric effect (MCE) of half-metallic CrO2 particles has been studied with respect to particle size on the nanometre scale. Results from superconducting quantum interference device (SQUID) measurements indicate that acicular CrO2 particles with a length of 400 nm yield a large magnetic entropy change Delta S-M of 5.1 J kg(-1) K-1 at an applied field of 15 kOe and an adiabatic temperature change Delta T-ad of 2.0 K near the Curie temperature (similar to 390 K). These results are among the highest for magnetic oxides, and are comparable to that for pure Gd. However, smaller CrO2 particles with a length of 260 nm only exhibit Delta S-M = 2.25 J kg(-1) K-1 and Delta T-ad = 0.95 K. The difference in MCE between these two sizes of CrO2 particles results primarily from disordered spins on the surface of the particles. In addition, measurements and calculations of the specific heat capacity for the CrO2 particles are presented. These results indicate that the total specific heat capacity is dominated by the magnetic specific heat contribution. Therefore, we believe that these CrO2 nanoparticles may hold future promise in the development of new magnetic refrigerants.