Specifically, for times shorter than the characteristic timescale for exchange coupling, the magnetization of Fe quenches more strongly than that of Ni. We ascribe this transient decoupling in the magnetic behavior to the finite strength of the fundamental quantum exchange interaction between Fe and Ni atoms in the material. The superior time resolution of our experiment allows us to observe that the magnetization dynamics of Fe and Ni are transiently delayed with respect to each other-by about 18 fs in permalloy and 76 fs in Cu-diluted permalloy ((Ni 0.8Fe 0.2) 1- xCu x). To answer this question, we rapidly excite permalloy with an ultrashort (≈25 fs) laser pulse and probe the element-specific demagnetization dynamics using < 10 fs high-harmonic pulses. This is a very important fundamental question that has not been addressed either theoretically or experimentally to date, the answer to which reveals how the exchange interaction can control the ultrafast dynamics of elemental spin subsystems in complex materials. In this work, we experimentally answer the fundamental question of whether the magnetization dynamics of individual elements in a ferromagnetic alloy can differ on ultrafast timescales.
However, the time resolution available in that initial experiment was insufficient to observe any differences in the response of the constituent elements on very short timescales. For that demonstration, we took advantage of magnetic birefringence at the M-edge in transition metals to independently follow dynamics for Ni and Fe. In an alternative approach, we recently demonstrated that coherent extreme ultraviolet (XUV) beams from a tabletop high-harmonic source ( 10, 11) can also be used to probe ultrafast element-specific magnetization dynamics in permalloy (Ni 0.8Fe 0.2) ( 12). XMCD has the inherent advantage of element-specific detection, and “sliced” synchrotron pulses are already used for ultrafast studies ( 5 – 9). One approach for addressing the experimental challenge is to use X-ray magnetic circular dichroism (XMCD) employing X-rays generated by a synchrotron light source.
Two reasons for this lack of fundamental understanding of ultrafast magnetism at the microscopic scale are the complexity of the problem itself, as well as the experimental challenge of accessing ultrafast and element-specific magnetization dynamics. However, a complete microscopic understanding of magnetization dynamics that involves the correlated interactions of spins, electrons, photons, and phonons on femtosecond timescales has yet to be developed. Next-generation devices will require that the magnetic state of materials be manipulated on fast timescales and at the nanometer level. Heat-assisted magnetic recording ( 1), bit-patterned data storage media ( 2), all-optical magnetization reversal ( 3), and giant tunneling magnetoresistive disk drive read sensors are examples of such technologies ( 4). Progress in magnetic information storage and processing technology is intimately associated with complex materials that are engineered at the nanometer scale. 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. We can further enhance this delay by lowering the exchange energy by diluting the permalloy with Cu.
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 show that for times shorter than the characteristic timescale for exchange coupling, the magnetization of Fe quenches more strongly than that of Ni. 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). The underlying physics of all ferromagnetic behavior is the cooperative interaction between individual atomic magnetic moments that results in a macroscopic magnetization.