How to Measure Degree of Spin Polarization

Spintronics is an emerging field of research combining two traditional branches of physics: magnetism and electronics. It is based on the ability of ferromagnetic materials to conduct spin-polarized currents. The effectiveness of spintronics depends on the extent to which a current is spin-polarized [1], which in turn depends on the spin polarization of the ferromagnetic materials. All device designs improve their performance as the spin polarization P approaches 100%. High degree of spin polarization P is especially important for spin injection experiments where one wants to inject spin polarized current from a ferromagnetic material to a nonmagnetic metal or semiconductor. The P of the source material sets an upper limit to the injection efficiency. Therefore, for both scientific and technological reasons, it is important to be able to measure the degree of spin polarization of a candidate material easily and accurately. This paper introduces four important techniques to measure P, namely spin-resolved photoemission spectroscopy, spin-polarized tunneling in a ferromagnet /insulator/superconductor (FM/I/S) tunnel junction (the Tedrow-Meservy method), measuring tunneling magnetoresistance in a ferromagnet 1/insulator/ferromagnet 2 (FM1/I/FM2) junction, and then calculating P through Julliere’s model, and finally, Andreev reflection in a superconducting point contact,. For each method, its principle, experimental setup, relative advantages and disadvantages are discussed.