Spin-Polarized Current in Spin Valves and Magnetic Tunnel Junctions
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Spin-Polarized
Current in Spin Valves and Magnetic Tunnel Junctions Stuart Parkin
Abstract 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 magnetoresistance 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 spindependent 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. Keywords: magnetic, memory, spintronic.
Introduction to Spin Valve and Magnetic Tunnel Junction Devices More than 70 years ago, it was realized that in simple ferromagnetic metals such as Fe, Co, and Ni, the current is carried by spin-polarized electrons. This phenomenon arises from a significant spin-dependent scattering of the majority (“up”) and minority (“down”) spin-polarized electrons.1 Many of the magnetotransport properties of these elements and their alloys can be understood within a “two-current” model in which the electrical current consists of independent up and down spin currents. It took more than half a century, however,
MRS BULLETIN • VOLUME 31 • MAY 2006
before it was appreciated that these currents can be manipulated in inhomogeneous magnetic systems composed of magnetic and nonmagnetic regions so as to modify the flow of current in these systems and thereby their resistance. Examples include magnetic multilayers comprising alternating thin magnetic and nonmagnetic layers2–4 such as Fe/Cr, Co/Ru, and Co/Cu, and granular magnetic alloys composed of immiscible magnetic and nonmagnetic metals such as Co and Cu.5,6 These systems exhibit very large changes
in resistance at room temperature in response to magnetic fields as the magnetization directions of neighboring magnetic layers or regions are changed. This phenomenon is often referred to as giant magnetoresistance (GMR).7–14 The largest GMR effects are found in Co/Cu multilayers,15 wit
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