Eschenlohr A, Persichetti L, Kachel T, Gabureac M, Gambardella P, Stamm C
Fakultät für Physik, Universität Duisburg-Essen, Lotharstr. 1, 47057 Duisburg, Germany.
J Phys Condens Matter. 2017 Sep 27;29(38):384002. doi: 10.1088/1361-648X/aa7dd3. Epub 2017 Jul 5.
Ultrafast spin currents induced by femtosecond laser excitation of ferromagnetic metals have been found to contribute to sub-picosecond demagnetization, and to cause a transient enhancement of the magnetization of the bottom Fe layer in a Ni/Ru/Fe layered structure. We analyze the ultrafast magnetization dynamics in such layered structures by element- and femtosecond time-resolved x-ray magnetic circular dichroism, for different Ni and Fe layer thicknesses, Ru and Ta interlayers, and by varying the pump laser fluence. While we do not observe the transient enhancement of the magnetization in Ni/Ru/Fe discovered previously, we do find a reduced demagnetization of the Fe layer compared to a Ni/Ta/Fe layered structure. In the latter, the spin-scattering Ta layer suppresses spin currents from the Ni layer into Fe, consistent with previous results. Any spin current arriving in the lower Fe layer will counteract other, local demagnetization mechanisms such as phonon-mediated spin-flip scattering. We find by increasing the Ni and Fe layer thicknesses in Ni/Ru/Fe a decreasing effect of spin currents on the buried Fe layer, consistent with a mean free path of the laser-induced spin currents of just a few nm. Our results suggest that in order to utilize ultrafast spin currents in an efficient manner, the sample design has to be optimized with these considerations in mind, and further studies clarifying the role of interfaces in the employed layered structures are needed.
人们发现，飞秒激光对铁磁金属的激发所诱导的超快自旋电流有助于亚皮秒级的退磁，并导致镍/钌/铁层状结构中底部铁层的磁化强度出现瞬态增强。我们通过元素分辨和飞秒时间分辨的X射线磁圆二色性，针对不同的镍和铁层厚度、钌和钽中间层，并通过改变泵浦激光通量，来分析此类层状结构中的超快磁化动力学。虽然我们没有观察到先前在镍/钌/铁中发现的磁化强度的瞬态增强，但我们确实发现，与镍/钽/铁层状结构相比，铁层的退磁有所减弱。在后者中，自旋散射钽层抑制了从镍层到铁层的自旋电流，这与先前的结果一致。任何到达下层铁层的自旋电流都会抵消其他局部退磁机制，比如声子介导的自旋翻转散射。我们发现，通过增加镍/钌/铁中镍和铁层的厚度，自旋电流对埋入铁层的影响会减小，这与激光诱导自旋电流的平均自由程仅为几纳米相一致。我们的结果表明，为了有效地利用超快自旋电流，必须在考虑这些因素的情况下优化样品设计，并且需要进一步研究来阐明所采用的层状结构中界面的作用。
