Fast times in ferromagnetic alloys

As magnetic materials for storage and other applications get pushed ever smaller and faster, a solid understanding of their behavior—including the correlated interactions among electrons, photons, and phonons—at those scales will be critical. X rays from synchrotron light sources are one way to obtain element-specific information at ultrafast time scales. Now, scientists from JILA and their colleagues from NIST and Germany have used tabletop techniques to probe magnetic dynamics on the fastest time scales; in particular, they studied the role of the exchange coupling between components in a ferromagnetic alloy. Working with magnetic diffraction gratings of permalloy (Ni_0.8Fe_0.2), as sketched here, the researchers first use a short, strong IR laser pulse to excite the electrons, which causes the alloy to demagnetize. Then, to trace the evolution of the magnetization, they illuminate the gratings with 10-femtosecond bursts of extreme UV light, obtained through high-harmonic generation (see Physics Today, March 2005, page 39). At the wavelengths corresponding to the M-shell absorption edge of each element, the reflected light intensity depends on the degree of that element's magnetization, so the researchers can independently and simultaneously monitor the dynamic magnetic response of Fe and Ni, even when alloyed together. In a surprising finding, the Ni demagnetization lags that of Fe by a time on the order of 10–100 fs, depending on the spins' exchange-coupling strength (which can be varied by adding copper); after that delay, Fe and Ni demagnetize at the same rate. Such results, claim the researchers, will provide crucial information for addressing open questions in ultrafast magnetization dynamics. (S. Mathias et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.1201371109.)—Richard J. Fitzgerald