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.Comment: To appear in PRB 15 Mar 200

Persistent Spin Currents in Helimagnets

We demonstrate that weak external magnetic fields generate dissipationless spin currents in the ground state of systems with spiral magnetic order. Our conclusions are based on phenomenological considerations and on microscopic mean-field theory calculations for an illustrative toy model. We speculate on possible applications of this effect in spintronic devices.Comment: 9 pages, 6 figures, updated version as published, Journal referenc

Magnetic and Magnetotransport Properties In Co5cu95 Melt-spun Alloys

Giant magnetoresistance (GMR) has been observed in Co5Cu95 alloys fabricated by melt-spinning. The highest MR change of 28.0% occurs for Co5Cu95 after annealing at 450 degrees C for 30 min. Based on the superparamagnetic assumption, the average size of Co particles embedded in Cu matrix, ranging from 3.0 to 6.0 nm, has been determined by simulating the magnetization curves at 295 K which is higher than the blocking temperatures for the samples. Comparison with phenomenological theory for GMR indicates that the interfacial spin-dependent scattering is the dominant scattering mechanism underlying GMR origin in granular systems. Additionally, for the samples in as-quenched state or annealed at temperature T-A=350 degrees C, the electron hybridization and superparamagnetic behaviors of fine Co particles may be responsible for the low value of MR change