Nano Focus: Superdiffusive electron transport mediates laser-induced demagnetization
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Nano Focus Superdiffusive electron transport mediates laser-induced demagnetization
T
he mechanics of laser-induced demagnetization has attracted considerable attention in efforts to develop fastswitching optomagnetic logic devices. A femtosecond laser pulse for instance can demagnetize a ferromagnet within a few hundred femtoseconds. Various theories have been proposed to explain how an ultrafast laser pulse can demagnetize a
under the coldest conditions, becoming briefer as the temperatures rose. These waves then vanished entirely at about 100 K, actually surviving at much higher temperatures than superconductivity. The researchers then hunted for those same signatures in cuprate films with slightly different chemical compositions and a greater density of mobile electrons. “Interestingly, the superconducting sample with the highest critical temperature,
about 39 K, showed no CDW signatures at all,” Gedik said. The consistent emergence of CDWs would have bolstered the conjecture that they play an essential role in HTS. Instead, the research team’s detection of such electron waves in one sample but not in another (with even higher critical temperature) demonstrates that another mechanism may be driving the emergence of HTS in cuprate superconductors.
magnetic thin film, ranging from indirect spin-flip scattering to direct laserinduced spin flips. Now, A. Eschenlohr, M. Battiato, and colleagues at Helmholtz Zentrum Berlin and Uppsala University propose that a novel mechanism of superdiffusive electron transport drives the demagnetization. In the January 27 online edition of Nature Materials (DOI: 10.1038/ NMAT3546), the researchers describe a unique pump-probe study to explore magnetization dynamics in Ni films. They first deposited Au/Ni/Pt/Al and
Pt/Ni/Pt/Al multilayers using magnetron sputtering. The former film was grown with a Au capping layer thick enough to absorb almost all the laser light, while the latter features a very thin Pt capping layer. The top layer of the samples was irradiated with a 50 fs laser pulse and the Ni magnetization was measured in a pump-probe fashion using x-ray magnetic circular dichroism (XMCD) to probe the Ni response element-selectively. The results were modeled using superdiffusive transport theory, which allowed the researchers to extract the spatial and temporal dependence of magnetization in each sample. The results show that in the Au / Ni sample, the spinmajority electrons are mostly conducted from the Ni to the substrate and the Au cap, while spin-minority electrons remain largely trapped in the Ni layer. This leads to a very efficient and ultrafast demagnetization of the Ni layer, which is almost as fast as the response of the Pt/ Ni sample measured in parallel. From these results, the researchers conclude that direct optical excitation does not need to occur for the demagnetization process to take place. This also precludes spin flips as the dominant mechanism for demagnetization, at least for the Au/Ni structure. The researchers conclude that the model of laser-induc
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