We are studying special semiconductor heterostructures that act as magnets.  We grow these materials in collaboration with Prof. Art Gossard's group at UCSB, using low-temperature molecular beam epitaxy to substitute Mn atoms onto Ga sites in the host GaAs crystal.  The indirect coupling of the Mn spins by the charge carriers in the semiconductor produces the exchange interactions responsible for the onset of spontaneous magnetization below the Curie temperature, Tc.  Tc is typically 10 - 100 K, depending on growth parameters.

These magnetic semiconductor materials are promising technologically because they offer new routes to control and manipulate the spins responsible for magnetism using the "knobs" of semiconductor device physics, such as photoexcitation and gate-control in FET devices.  Semiconductor ferromagnets are also very interesting from a physics standpoint.  Due to the carrier-mediated coupling of the Mn spins, the magnetization state strongly effects electrical properties and vice versa.   The high degree of disorder in these materials, due to randomness in the placement of Mn and to other defects, also strongly affects their behavior.

We are using a combination of magnetometry and low temperature magnetotransport measurements to address
•Transmission of spin information between magnetic planes in layered materials
•Tuning the magnetic switching properties through choice of the distribution of Mn within the heterostructure (in collaboration with Prof. David Awschalom's group)
• Understanding the unusual behavior of the large anomalous Hall effect in these materials (in collaboration with Prof. Leon Balents and Dr. Anton Burkov).

Semiconductor devices that exploit electron spin information are important candidates for next-generation technology in information processing. Such "spintronic" devices are expected to speed processing and increase integration densities through low-dissipation switching of spins, and to contribute new spin-derived functionalities to device physics.

The ferromagnetic semiconductors we study are potentially of key importance to this emerging technology for their use as spin injectors and for providing new ways to control spin interactions.  However, the low Tc values in most semiconductor alloys and their weak response to applied electric fields limit what can be achieved with these materials.

To overcome these limitations, we are investigating a new chemical approach to controlling magnetism in semiconductor ferromagnets.  We have found that adsorption of monolayers of selected organic molecules onto the surface of ferromagnetic semiconductors produces large, anisotropic changes in magnetism.  Thus such  hybrid organic/semiconductor structures may open new routes to manipulate spin populations chemically by exploiting the huge range of organic molecular structures and the ease of fabrication by molecular self-assembly. (The chemical "know-how" for this work comes through collaboration with Prof. Ron Naaman's group in the Chemical Physics Dept. at the Weizmann Institute.)

We investigate electrical transport phenomena that arise from tunneling between "edge states", which are special one-dimensional states that lie at the perimeter of two-dimensional electron gases in the regime of the quantum Hall effect.  A unique feature of edge states is their uni-directional flow, in contrast to conventional quantum wires in which states of opposite momenta are occupied. New transport phenomena result when different puddles of electron gas are brought close enough together for electrons to tunnel between edge states.  In the geometry we study, electrons tunnel vertically between the edge states that encircle a stack of electron gas layers in GaAs/AlGaAs multi-quantum well structures.  These special materials are grown by molecular-beam epitaxy in Prof. Art Gossard’s group.

These studies provide information on both the network of coupled edge-states and on the localized states in the “bulk” quantum Hall system that the edge states surround.  Due to its chiral nature, the edge state system is less sensitive to the effects of disorder than conventional quasi-one-dimensional systems, and thus provides a different angle on the interplay of interactions and disorder in condensed matter systems.