Laser Technique Aids Spintronics

Hank Hogan

A research group at Eindhoven University of Technology in Eindhoven, the Netherlands, and at IMEC vzw in Louvain, Belgium, has demonstrated an optical technique that could provide needed materials data for the development of spintronics, leading to the production of devices based on magnetic spin. Such devices promise advantages over their electronic analogs.

An optical technique promises to aid the development of spintronic devices. The graph indicates the periodic changes in the magnetization (ΔM_z/M_z) of one element on a lithographically defined permalloy array, seen in an optical microscope image (inset). Courtesy of Maarten van Kampen.

For example, spintronics memory could store data without power, such as on a hard disk, but retrieve it rapidly, such as computer RAM. Before such gizmos appear, however, basic research needs to be done, including identifying methods to detect microscopic magnetic spin. The optical technique may satisfy this requirement.

It relies on a one-two laser punch. First, the researchers focused a 70-fs pulse of 780-nm radiation onto a 10-µm-wide area of the film being characterized. The pulse, which had a power of about a nanojoule, induced rapid, localized heating and a corresponding change in magnetization. A second laser pulse, 100 times weaker than the first, then enabled the researchers to probe the magnetization by measuring the change in the polarization of the reflected radiation.

In their experiments, the scientists discovered that the magnetization circled around its undisturbed state like a top as the material cooled. The results indicate that this magnetic precession may be probed from 10 ps to several nanoseconds after the initial pump pulse. Continued probing provided additional information.

Maarten van Kampen, a member of the Dutch team, noted that the wavelength of the technique is not critical and that greater spatial resolution, even submicron resolution, is possible. Because the technique is optical, there is no contact, and local characteristics can be probed. On the other hand, the material must be reflective and reachable by the pulse. It also is not possible to switch the material's magnetic state using this method.

The team sees several directions for the research, including investigating means to improve both the optical and magnetic techniques. Van Kampen said that the group might seek to combine magnetic and optical excitation into a thermally assisted switching technique.