Using a light source that creates X-ray pulses only one quadrillionth of a second in duration, a Boulder, Colorado team was able to observe how magnetism in nickel and iron atoms works, and they found that each metal behaves differently. One quadrillionth of a second is a million times faster than one billionth of a second.
Many technology experts believe that next-generation computer disk drives will use optically-assisted magnetic recording to achieve much higher drive capacities, according to NIST scientist Tom Silva, who worked with CU-Boulder physics professors Margaret Murnane and Henry Kapteyn on the research. However, many questions remain about how the delivery of optical energy to the magnetic system can be optimized for maximum drive performance. And this finding could help researchers answer some of their questions.
“The discovery that iron and nickel are fundamentally different in their interaction with light at ultrafast time scales suggests that the magnetic alloys in hard drives could be engineered to enhance the delivery of the optical energy to the spin system,” according to NIST scientist Tom Silva, who worked with CU-Boulder physics professors Margaret Murnane and Henry Kapteyn on the research.
The underlying physics of all ferromagnetic behavior is the cooperative interaction between individual atomic magnetic moments that results in a macroscopic magnetization. In this work, we use extreme ultraviolet pulses from high-harmonic generation as an element-specific probe of ultrafast, optically driven, demagnetization in a ferromagnetic Fe-Ni alloy (permalloy). We show that for times shorter than the characteristic timescale for exchange coupling, the magnetization of Fe quenches more strongly than that of Ni. Then as the Fe moments start to randomize, the strong ferromagnetic exchange interaction induces further demagnetization in Ni, with a characteristic delay determined by the strength of the exchange interaction. We can further enhance this delay by lowering the exchange energy by diluting the permalloy with Cu. This measurement probes how the fundamental quantum mechanical exchange coupling between Fe and Ni in magnetic materials influences magnetic switching dynamics in ferromagnetic materials relevant to next-generation data storage technologies.
“What we have seen for the first time is that the iron spins and the nickel spins react to light in different ways, with the iron spins being mixed up by light much more readily than the nickel spins,” said Silva. “In the end, the exchange interaction still pulls the two spin systems back into synchronization after a few quadrillionths of a second. Seeing such a difference was only possible by taking advantage of the extremely fast X-ray technology developed at the University of Colorado and elsewhere.”
The laser technology used in the experiment, known as “high harmonic generation,” can generate laser-like beams of X-rays that span a wide portion of the electromagnetic spectrum, including the spectral region where nickel and iron interact very strongly with X-rays.