Ultrafast Magnetics

Capturing dynamics in magnetic materials

Figure: Probing layer-selective spin dynamics in magnetic multilayers driven by ultrafast light pulses. After transiently heating the top Nickel layer (blue layer) with an infrared femtosecond laser pulse (red pulse), spin-polarized electrons can rapidly move into the buried Iron layer (yellow layer). Remarkably, this can cause the magnetization of the Iron layer to increase. These new spin dynamics in magnetic multilayers can be uncovered using ultrafast and coherent high harmonic extreme ultraviolet (XUV) radiation, that can capture magnetization dynamics in the Ni and Fe layers simultaneously with element-selectivity at the 3p (M) absorption edges of Ni (66 eV) and Fe (52 eV), respectively. This novel capability is leading to many new insights about the fastest motions at the femto- and nanoscale.

Magnetism has been the subject of scientific inquiry for more than 2000 years; however, it is still an incompletely understood phenomenon. The fundamental length and time scales for magnetic phenomena are nanometers (nm) and femtoseconds (fs). Furthermore, a detailed understanding of nanoscale magnetism has become much more critical in the 21st century with dramatic recent advances in magnetic data storage applications, as bits on a hard disk are already packed  at scales of about 20nm. However, a comprehensive microscopic model of how spins, electrons, photons and phonons interact does not yet exist. This understanding is fundamentally constrained in large part by our current very-limited ability to directly observe magnetism on all relevant time and length scales. Further advances in storage capacity and energy efficiency depend critically on a detailed understanding of the limits of magnetic switching speed and storage density.

Until recently, measuring magnetic material dynamics used either ultrafast lasers and visible-wavelength light, or x-rays from large-scale electron storage facilities, such as synchrotrons and free electron lasers. Our recent work has shown that the fastest dynamics in magnetic materials can be captured using extreme ultraviolet (XUV) harmonics – with elemental resolution and at multiple atomic sites simultaneously. We first probed how fast the magnetic state can be destroyed in an Fe-Ni alloy, with elemental sensitivity for the first time [1,2]. Then, by increasing the time resolution to 10 fs, we were able to see that different elements in the alloy responded on different timescales, due to the finite exchange interaction energy, also for the first time [3]. This is a very important fundamental question that has not been addressed either in theory or experiment to-date, the answer to which reveals how the exchange interaction can control ultrafast magnetic dynamics. In our latest work, we uncovered new understanding of laser-generated superdiffusive spin currents in magnetic multilayers, which undergo a laser driven ultrafast demagnetization [4, 5]. By exciting magnetic multilayers with a laser pulse and probing the magnetization response simultaneously but separately in Ni and Fe, we surprisingly find that optically induced demagnetization of the Ni layer transiently enhances the magnetization of the buried Fe layer, when the two layer magnetizations are initially aligned parallel. Our findings provide crucial new answers for open questions in femtosecond magnetization dynamics in the case of metallic, multi-species, exchange-coupled systems [6].

Looking to the future, it will be possible to extend these measurements to full time-resolved dynamic magnetic imaging as well as to the L absorption edges of magnetic media in the soft x-ray region of the spectrum. Recent advances in generating bright, coherent, x-rays from femtosecond lasers in the keV region of the spectrum make this feasible for the first time. Imaging of buried magnetic structures, domain interactions, strongly coupled nano layers, and spin dynamics will all be accessible with femtosecond time and nanometer spatial resolution.

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