A team of NIST researchers from the Materials Science and Engineering Laboratory (MSEL) and the Center for Nanoscale Science and Technology (CNST) have measured the current spin polarization of Ni80Fe20 (Permalloy) over a range of temperatures using an innovative spin-wave Doppler technique.* Electrical currents in ferromagnetic metals tend to be magnetically polarized, meaning that a majority of the current is carried by either spin-up or spin-down electrons. Current spin polarization—the imbalance of spin-up and spin-down electrons in the current—is a key parameter in the spin-transfer torque effect, which describes the torque exerted by polarized conducting electrons on local magnetic moments. The ability to switch the magnetization by electrical current though spin-transfer torque is the basis for number of new technologies, such as magnetic random access memory (MRAM) and nanoscale microwave oscillators. Conventional methods to measure spin polarization are limited to low temperatures, but measurement of current spin polarization over a temperature range including room temperature is important to successful implementation of these new technologies. The MSEL and CNST researchers excited spin waves in ferromagnetic Ni80Fe20 wires using nanoscale microwave antennas and measured the frequency shift of the spin wave when passing currents through the wires. Similar to the Doppler Effect in a moving medium, the current-driven drift velocity of magnetization is obtained from the frequency shift and the wave vector of the spin wave. Then the current spin polarization is deduced from the drift velocity. This technique has the advantages of being immune to interfacial effects and offering the freedom to change the temperature. Results including increasing current spin polarization in Ni80Fe20 with decreasing temperature were reported in a Rapid Communication in the April 2010 issue of Physical Review B.
* Temperature Dependence of Magnetization Drift Velocity and Current Polarization in Ni80Fe20 by Spin-Wave Doppler Measurements, M. Zhu, C. L. Dennis, and R. D. McMichael, Physical Review B 81, 140407 (2010). [NIST Publication Database Entry] [Journal Web Site]