Emergent Nontrivial Phenomena in Quantum Heterostructures and Their Applications in Electronics
Abstract
Interplay between spin-polarized current and magnetization has led to many core phenomena and applications in spintronics. On the one hand, the spin-polarized current, generated by filtering through a ferromagnet (FM) or by spin Hall effect (SHE) in heavy metals and/or surface spin- momentum locking in topological insulators (TIs), can provide efficient means for manipulation of magnetization orientation through the spin angular momentum transfer. On the other hand, the magnetization can also significantly influence the electrical transport of spin-polarized current in these structures. The most well-known is the giant magnetoresistance (GMR) in the stacked FM layers with magnetization parallel or antiparallel to each other, which has played a major role in all modern developments of spintronics. Another nontrivial magnetoresistance (MR), the so-called spin-Hall magnetoresistance (SMR) in heavy metal/magnet bilayers, arises due to the back flow of spin- polarized current into the heavy metal when the SHE-induced spin accumulation at the interface is collinear with the orientation of the magnetic layer, which reduces the resistance of the heavy metal layer due to the inverse SHE. Recently, another intriguing unidirectional spin-dependent magnetoresistance (USMR) has been identified in the bilayers composed of high spin-orbit coupling (SOC) material and magnet, such as the heavy metal/FM and the Ga1-xMnxAs structures. The USMR depends on the relative orientation of the current-induced spin accumulation at the interface and the magnetization direction of the magnetic layer, parallel or antiparallel, in which the MR of the bilayer is different. The USMR could be understood from the current-in- plane GMR model or the spin-orbit torque (SOT) induced electron-magnon interactions. Both the SMR and USMR have potential applications in the sensing technology to detect the magnetization orientation in high-SOC material/magnet bilayers. Compared with heavy metals and semiconductors with high SOC, TIs exhibit much stronger SOC and inverted band structure in the bulk. Most importantly, TIs possess the unique spin-momentum locked Dirac fermions on the surface, which are expected to be more efficient for generating spin polarization/accumulation at the interface, and hence are more efficient for producing the USMR when coupled with magnetic materials. To further explore the USMR in TI-based structures for potential technological application, it is crucial to investigate whether the surface states or the bulk carriers contribute most to the USMR effect, and also systematically study the correlation between the USMR strength and the magnetism in the structure.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Dec 04, 2018
- Source ID
- W911NF1510246
Entities
People
- Kang L. Wang
Organizations
- Army Contracting Command
- United States Army
- University of California, Los Angeles