Transport and magnetotransport properties of cold-pressed CrO ₂ powder

Resistivity of cold-pressed CrO ₂ powder has been measured over the temperature range of 2 to 350 K with a magnetic field up of to 50 kOe. High resolution transmission electron microscopy (HRTEM) images show that the CrO ₂ particles are needle-shaped with an aspect ratio of 7:1 and have an average length of 200 nm, and are covered with an ultra-thin (∼2.5 nm) native Cr ₂O ₃ layer. Temperature dependent conductivity can be well described by the expression exp[-(Δ/T) ^1/2] below 55 K, a feature of electron tunneling, and by the higher-order hopping for temperature above 55 K. The magnetoresistance, MR = [ρ(max) - ρ(H)]/ρ(max), reaches ∼33.5% at 2 K and decreases to nearly zero at room temperature. Temperature dependent saturation magnetization follows well the relation M(T) = M _s(1-BT ^3/2) below 240 K. The spin-wave stiffness constant, D, is estimated to be ∼91 meV Å ². By fitting the specific heat measured in the temperature range of 1.8-34 K by the relation C = γT + βT ³ + β ₁T ⁵ + αT ^3/2, we obtained the electronic specific heat coefficient γ=1.7 mJ/mol K ², this corresponds to a density of states at the Fermi level of 0.73 states/eV, which is in good accordance with the theoretical value of 0.69 states/eV.