Spin Pumping in Ferromagnet-Semiconductor Heterostructures

Non-Technical Abstract:

Magnetic materials such as iron form the basis of storage technologies such as computer hard disks, while semiconductors such as silicon are the foundation of a vast array of microelectronic devices. This project addresses the fundamental question of how these two very different types of materials can be combined to create electronic devices with new capabilities, including the integration of memory and processing functions on a single chip. To help accomplish this, the principal investigator and his team investigate a process in which information is transferred from the magnetic material to the semiconductor using microwaves, which are the types of waves used in applications such as cellphones. The microwaves are used as a 'pump' to generate a flow of spin (the carrier of information) from the magnetic material into the semiconductor. The project determines how efficient this process is and how the flow of spin can be detected inside the semiconductor. By using microwaves instead of an ordinary current generated by a battery, the speed of these devices operate can be enhanced. New magnetic materials for transferring spin into the semiconductor are explored, also allowing for more efficient operation at higher temperatures. In addition to advancing a technology that can be used in information processing, the project trains undergraduate and graduate students in techniques for fabricating and measuring this new class of electronic devices.

Technical Abstract:

Semiconductors provide a unique environment for controlling spin-polarized carriers using electric and magnetic fields, but the transfer of spin from ferromagnetic materials into semiconductors is a significant challenge. This project develops a means to transfer spins directly from a metallic ferromagnet into a semiconductor at microwave frequencies. This approach, known as spin pumping, has been applied effectively to metals, but it has not yet been tested quantitatively in the case of semiconductors. The effort exploits recent advances in the generation and detection of spin-polarized carriers in devices integrating highly-polarized Heusler alloy ferromagnets with III-V semiconductors. These heterostructures are optimized for operation at microwave frequencies, and the spin pumping approach is then compared quantitatively with established spin transport techniques, including non-local spin valve and spin Hall effect measurements. The important parameters governing the spin-pumping mechanism, including the interfacial mixing conductance and spin Hall angle, are to be measured independently. The spin pumping efficiency is enhanced by modifying the ferromagnet-semiconductor interface, allowing for spin injection into systems with strong spin-orbit interaction. The ultimate goal of the program is to demonstrate the conversion of a spin current to a charge current in a two-dimensional electron system that is driven by spin pumping from a ferromagnet. This requires a progression of devices starting in GaAs-based heterostructures and moving towards InAs quantum wells.