If you like fast cars, you may be all too familiar with the Doppler shift. It forms the basis of the police radar gun, which can work out your speed by measuring the shift in frequency of microwaves fired from the gun and reflected off your car.

Now, however, physicists in France have used a different take on the Doppler shift to measure the rate at which currents of spin-polarized electrons flow through a conductor. The technique could help in the development of spintronic devices, which use both the spin and current of electrons to store and process information more efficiently than conventional electronics.

Spintronics relies on the manipulation of spin currents — in which most electrons have spin magnetic moments pointing in a particular direction. Such currents could be used, for example, to reverse the direction of magnetization of a magnetic data bit.

However, physicists have struggled to come up with a simple way of making direct measurements of spin current. Most techniques involve inferring the spin current from measurements of magnetization or electrical current, which would be expensive and difficult to implement in practical spintronic devices

Spin-wave speed gun

The new technique, developed by Vincent Vlaminck and Mattieu Bailleul from the CNRS Institut de Physique et Chimie des Matériaux de Strasbourg and the Louis Pasteur University, involves measuring changes in the frequency of "spin waves" — magnetic fluctuations that can propagate through a material (Science 322 410).

They did this by placing two antennas on either side of a tiny, 2 µm wide strip of permalloy, which is a magnetic alloy of nickel and iron. A high frequency signal was passed through one antenna, causing spin waves with a wavelength of about 800 nm to propagate across the permalloy, where they were detected by the second antenna.

The researchers then sent a spin current through the permalloy along the same direction as the spin waves. This current “pulls” the spin waves along, causing its wave fronts to pile up at the second antenna. As a result, the spin-wave frequency is shifted upwards by an amount proportional to the spin current.

Spintronics expert David Awshalom of the University of California at Santa Barbara described the scheme as an “important tool for the field of spintronics — an elegant technique aimed at probing current-driven spin transport in ferromagnetic systems”.

Vlaminck told physicsworld.com that the technique could be extended to shorter wavelengths where the short-range quantum “exchange” interactions come into play. This could help physicists understand how spin currents are affected by short-range changes in the magnetization of a material.

For example, the spin of the flowing electrons could be misaligned with the local magnetization and understanding this misalignment could be of great importance for the development of current-controlled magnetic devices.