Magnetic Interactions and Transport in (Ga,Cr)As
  The magnetic, transport, and structural properties of (Ga,Cr)As are reported. Zincblende Ga$_{1-x}$Cr$_{x}$As was grown by low-temperature molecular beam epitaxy (MBE). At low concentrations, x$\sim$0.1, the materials exhibit unusual magnetic properties associated with the random magnetism of the alloy. At low temperatures the magnetization M(B) increases rapidly with increasing field due to the alignment of ferromagnetic units (polarons or clusters) having large dipole moments of order 10-10$^2$$\mu_B$. A standard model of superparamagnetism is inadequate for describing both the field and temperature dependence of the magnetization M(B,T). In order to explain M(B) at low temperatures we employ a distributed magnetic moment (DMM) model in which polarons or clusters of ions have a distribution of moments. It is also found that the magnetic susceptibility increases for decreasing temperature but saturates below T=4 K. The inverse susceptibility follows a linear-T Curie-Weiss law and extrapolates to a magnetic transition temperature $\theta$=10 K. In magnetotransport measurements, a room temperature resistivity of $\rho$=0.1 $\Omega$cm and a hole concentration of $\sim10^{20}$ cm$^{-3}$ are found, indicating that Cr can also act as a acceptor similar to Mn. The resistivity increases rapidly for decreasing temperature below room temperature, and becomes strongly insulating at low temperatures. The conductivity follows exp[-(T$_1$/T)$^{1/2}$] over a large range of conductivity, possible evidence of tunneling between polarons or clusters. 
A. Dakhama, B. Lakshmi, D. Heiman