Conductivity of pressed powders of chromium dioxide with spin-dependent electron tunneling: The effect of thickness and composition of dielectric layers

Abstract

The resistive, magnetoresistive, and magnetic properties for nine compacted CrO2 powder samples synthesized by hydrothermal method from chromic anhydride were studied. The proposed new synthesis method allows adjusting the thickness of the dielectric shells on the surface of CrO2 nanoparticles. The powders consisted of either rounded nanopartides (with an average diameter of ≈ 120 nm) or needle-like crystals (≈22.9 nm in diameter and 302 nm long). In all cases, nanoparticles were covered by dielectric shells of varied thickness and composition (for example, chromium oxide Cr2O3 or chromium oxyhydroxide β-CrOOH). The effect of material properties and thickness of the intergranular dielectric layers, as well as the shape of CrO2 nanoparticles, on the magnitude of the tunnel resistance and magnetoresistance (MR) of compacted powder samples was investigated. For all the samples studied, the nonmetallic temperature behavior of the resistance and the giant negative tunneling MR were detected at low temperatures. The maximum values of MR at T ≈ 5 K and relatively small magnetic field (H = 0.5 T) were approximately 37%. With increasing temperature, the MR rapidly decreased (down to ≈1% at H = 1 T, T ≈ 200 K).