Spin-Polarized Current in Spin Valves and Magnetic Tunnel Junctions

Spin-polarized currents can be generated by spin-dependent diffusive scattering in magnetic thin-film structures or by spin-dependent tunneling across ultrathin dielectrics sandwiched between magnetic electrodes. By manipulating the magnetic moments of the magnetic components of these spintronic materials, their resistance can be significantly changed, allowing the development of highly sensitive magnetic-field detectors or advanced magnetic memory storage elements. Whereas the magneto-resistance of useful devices based on spin-dependent diffusive scattering has hardly changed since its discovery nearly two decades ago, in the past five years there has been a remarkably rapid development in both the basic understanding of spin-dependent tunneling and the magnitude of useful tunnel magnetoresistance values. In particular, it is now evident that the magnitude of the spin polarization of tunneling currents in magnetic tunnel junctions not only is related to the spin-dependent electronic structure of the ferromagnetic electrodes but also is considerably influenced by the properties of the tunnel barrier and its interfaces with the magnetic electrodes.Whereas the maximum tunnel magnetoresistance of devices using amorphous alumina tunnel barriers and 3d transition-metal alloy ferromagnetic electrodes is about 70% at room temperature, using crystalline MgO tunnel barriers in otherwise the same structures gives tunnel magnetoresistance values of more than 350% at room temperature.