Abstract
Resistivity of coldโpressed 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 needleโshaped with an aspect ratio of 7:1 and have an average length of 200 nm, and are covered with an ultraโthin (โผ2.5 nm) native Cr~2~O~3~ layer. Temperature dependent conductivity can be well described by the expression below 55 K, a feature of electron tunneling, and by the higherโorder hopping for temperature above 55 K. The magnetoresistance, , reaches โผ33.5% at 2 K and decreases to nearly zero at room temperature. Temperature dependent saturation magnetization follows well the relation below 240 K. The spinโwave stiffness constant, D, is estimated to be โผ91 meV ร
^2^. By fitting the specific heat measured in the temperature range of 1.8โ34 K by the relation , we obtained the electronic specific heat coefficient ฮณ = 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.