Magnetic interactions and transport in (Ga, Cr)As

The magnetic, transport, and structural properties of (Ga, Cr)As are reported. Zincblende (formula presented) was grown by low-temperature molecular-beam epitaxy. At low concentrations, (formula presented) the materials exhibit unusual magnetic properties associated with the random magnetism of the alloy. At low temperatures the magnetization (formula presented) increases rapidly with increasing field due to the alignment of ferromagnetic units (polarons or clusters) having large dipole moments of order (formula presented) A standard model of superparamagnetism is inadequate for describing both the field and temperature dependence of the magnetization (formula presented) In order to explain (formula presented) at low temperatures we employ a distributed magnetic-moment 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 (formula presented) The inverse susceptibility follows a linear-(formula presented) Curie-Weiss law and extrapolates to a magnetic transition temperature (formula presented) In magnetotransport measurements, a room-temperature resistivity of (formula presented) and a hole concentration of (formula presented) are found, indicating that Cr can also act as an acceptor similar to Mn. The resistivity increases rapidly for decreasing temperature below room temperature, and becomes strongly insulating at low temperatures. The conductivity follows (formula presented) over a large range of conductivity, possible evidence of tunneling between polarons or clusters. © 2003 The American Physical Society.