Séminaire exceptionnel SPCSI (Attention lieu inhabituel) et CAPMAG Abstract: 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 over three decades ago. This idea 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 non-volatile memory, logic elements etc). In a latest development with symmetry matched epitaxial MTJs, it is possible to achieve over 800% change in the tunnel current. Uniquely, one can also utilize extreme sensitivity of tunneling probability to tunnel barrier properties to create near 100% spin polarized current, an ideal spin polarized source for spin injection into semiconductors. In this regard, 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. This has been adopted also to determine the exchange splitting in ultra thin ferromagnetic films that are otherwise extremely sensitive. Some of the other recent areas of study will be addressed in this seminar. One of these is the discovery of coexistence of ferromagnetism and superconductivity at the same time in Ni. The spin transport properties of organic semiconductors are one of the least explored areas likely pertinent for future spin-based electronics: electron spin polarized tunneling through ultrathin layers of the molecular organic semiconductors such as Alq3 and Rubrene will be discussed. In another study we show that 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. This research is supported by ONR, NSF and KIST-MIT grants.
Francis Bitter Magnet Lab, Massachusetts Institute of Technology, Cambridge, MA 02139