"Spin Polarized Tunneling: Before and Now "

Jagadeesh S. Moodera

National High Magnetic Field Laboratory

Francis Bitter Magnet Lab Massachusetts Institute of Technology
Cambridge, MA 02139

Electron tunneling phenomenon has contributed enormously for our understanding of various branches of physics over the years. Spin conserved tunneling from a ferromagnetic (FM) electrode through an insulating barrier with a superconductor as the spin detector, was discovered by Meservey and Tedrow three decades ago. This phenomenon of spin polarized tunneling (SPT) has been successfully utilized over the years to understand many aspects of magnetism and superconductivity. In the field of spintronics, electrical spin injection/detection in a semiconductor is strongly believed to succeed through such an approach. The successful observation of a large change in tunnel current in magnetic junctions (MTJ) in the last several years has brought extreme activity in this field both from basic study as well as extensive application in mind (as sensors, nonvolatile memory elements, logic elements etc). In a latest development with symmetry matched epitaxial MTJs, it is possible to achieve over 800% change in the tunnel current. Eu chalcogenide semiconductor compounds when used as a tunnel barrier shows a remarkable property of spin filtering resulting from the large exchange splitting of the conduction band below their Curie temperature. It is possible to achieve total spin filtering of the tunnel current starting from unpolarized electrons by control and tailoring of the interfaces down to atomic level, serving as an ideal spin polarized source for spin injection into semiconductors. This phenomenon also allows one to determine the exchange splitting in ultra thin ferromagnetic films by tunneling. Some of the other recent areas of study will be addressed in this seminar. For example, the spin transport properties of organic semiconductors are a least explored area and are pertinent for future spin-based electronics. Recent developments in electron spin polarized tunneling through ultrathin layers of the molecular organic semiconductors such as Alq3 and Rubrene will be discussed. In another fundamental study, the superconducting state is influenced by injecting spin polarized current in a controlled manner by tailoring the interfacial transmittivity between a ferromagnet (F) and a superconductor (S), resulting in a large magnetoresistance (MR) of over 1000% and even infinite MR for an F/S/F multilayer system.

*Work done in collaboration with Robert Meservey, Paul Tedrow, Guo-Xing Miao, Tiffany S. Santos, Taro Nagahama Jenny Shim, Joosang Lee and Karthik Venkataraman at MIT. This research is supported by ONR, NSF and KIST-MIT grants.