B. L. Sheu

Impurity Band Conduction in a High Temperature Ferromagnetic Semiconductor

The band structure of a prototypical dilute magnetic semiconductor (DMS), Ga1-xMnxAs, is studied across the phase diagram via infrared and optical spectroscopy. We prove that the Fermi energy (EF) resides in a Mn-induced impurity band (IB). Specifically the changes in the frequency dependent optical conductivity [sigma1(omega)] with carrier density are only consistent with EF lying in an IB. Furthermore, the large effective mass (m*) of the carriers inferred from our analysis of sigma1(omega) supports this conclusion. Our findings demonstrate that the metal to insulator transition in this DMS is qualitatively different from other III-V semiconductors doped with nonmagnetic impurities. We also provide insights into the anomalous transport properties of Ga1-xMnxAs.

Epitaxial growth and magnetic properties of the first five members of the layered Srn+1RunO3n+1 oxide series

Epitaxial thin films of the n=1–5 members of the layered Srn+1RunO3n+1 oxide series were produced by reactive molecular-beam epitaxy. X-ray diffraction and high-resolution transmission electron microscopy confirm that these films are epitaxially oriented and nearly phase pure (>98%). The Sr2RuO4 (n=1) and Sr3Ru2O7 (n=2) samples show no ferromagnetic transition in the range from 5to300K, while the Sr4Ru3O10 (n=3), Sr5Ru4O13 (n=4), and Sr6Ru5O16 (n=5) samples show ferromagnetic transitions at 85, 95, and 130K, respectively.

Capping-induced suppression of annealing effects on Ga1−xMnxAs epilayers

We have studied the effects of capping ferromagnetic Ga1−xMnxAs epilayers with a thin layer of undoped GaAs, and we find that even a few monolayers of GaAs have a significant effect on the ferromagnetic properties. In particular, the presence of a capping layer only 10 monolayers thick completely suppresses the enhancement of the ferromagnetism associated with low temperature annealing. This result, which demonstrates that the surface of a Ga1−xMnxAs epilayer strongly affects the defect structure, has important implications for the incorporation of Ga1−xMnxAs into device heterostructures.

Noncollinear spin valve effect in ferromagnetic semiconductor trilayers

We report the observation of the spin valve effect in (Ga,Mn)As/p-GaAs/(Ga,Mn)As trilayer devices. Magnetoresistance measurements carried out in the current in plane geometry reveal positive magnetoresistance peaks when the two ferromagnetic layers are magnetized orthogonal to each other. Measurements carried out for different post-growth annealing conditions and spacer layer thickness suggest that the positive magnetoresistance peaks originate in a noncollinear spin valve effect due to spin-dependent scattering that is believed to occur primarily at interfaces.

Nanoengineered Curie temperature in laterally patterned ferromagnetic semiconductor heterostructures

We demonstrate the manipulation of the Curie temperature of buried layers of the ferromagnetic semiconductor (Ga,Mn)As using nanolithography to enhance the effect of annealing. Patterning the GaAs-capped ferromagnetic layers into nanowires exposes free surfaces at the sidewalls of the patterned (Ga,Mn)As layers and thus allows the removal of Mn interstitials using annealing. This leads to an enhanced Curie temperature and reduced resistivity compared to unpatterned samples. For a fixed annealing time, the enhancement of the Curie temperature is larger for narrower nanowires.

Spin valve effect in self-exchange biased ferromagnetic metal/semiconductor bilayers

We report magnetization and magetoresistance measurements in hybrid ferromagnetic metal/semiconductor heterostructures comprised of MnAs∕(Ga,Mn)As bilayers. Our measurements show that the (metallic) MnAs and (semiconducting) (Ga,Mn)As layers are exchange coupled, resulting in an exchange biasing of the magnetically softer (Ga,Mn)As layer that weakens with layer thickness. Magnetoresistance measurements in the current-perpendicular-to-the-plane geometry show a spin valve effect in these self-exchange biased bilayers. Similar measurements in MnAs∕p-GaAs∕(Ga,Mn)As trilayers show that the exchange coupling diminishes with spatial separation between the layers.

Flux through a hole from a shaken granular medium

We have measured the flux of grains from a hole in the bottom of a shaken container of grains. We find that the peak velocity of the vibration, v max, controls the flux, i.e., the flux is nearly independent of the frequency and acceleration amplitude for a given value of v max. The flux decreases with increasing peak velocity and then becomes almost constant for the largest values of v max. The data at low peak velocity can be quantitatively described by a simple model, but the crossover to nearly constant flux at larger peak velocity suggests a regime in which the granular density near the container bottom is independent of the energy input to the system.

Substrate orientation dependence of ferromagnetism in (Ga,Mn)As

We describe a comprehensive study of the properties of both ordered and random alloy Ga1−xMnxAs grown on (001), (311), (201), and (110) GaAs substrates. Magnetization measurements show a systematic variation in the Curie temperature with epitaxial orientation in both types of samples, even for the same average Mn concentrations. Electrical characterization and thermal annealing of the random alloy Ga1−xMnxAs samples suggest that the density of As antisite defects depends on the growth surface, providing an extrinsic origin for the observed dependence of Curie temperature on substrate orientation.