University of Kaiserslautern and Research Center OPTIMAS, 67663 Kaiserslautern, Germany.
Forschungszentrum Jülich GmbH, Peter Grünberg Institut (PGI 6), 52425 Jülich, Germany.; Experimentalphysik Universität Duisburg-Essen, Lotharstraße 1, 47057 Duisburg,Germany.
Sci Adv. 2017 Mar 24;3(3):e1602094. doi: 10.1126/sciadv.1602094. eCollection 2017 Mar.
The evolution of the electronic band structure of the simple ferromagnets Fe, Co, and Ni during their well-known ferromagnetic-paramagnetic phase transition has been under debate for decades, with no clear and even contradicting experimental observations so far. Using time- and spin-resolved photoelectron spectroscopy, we can make a movie on how the electronic properties change in real time after excitation with an ultrashort laser pulse. This allows us to monitor large transient changes in the spin-resolved electronic band structure of cobalt for the first time. We show that the loss of magnetization is not only found around the Fermi level, where the states are affected by the laser excitation, but also reaches much deeper into the electronic bands. We find that the ferromagnetic-paramagnetic phase transition cannot be explained by a loss of the exchange splitting of the spin-polarized bands but instead shows rapid band mirroring after the excitation, which is a clear signature of extremely efficient ultrafast magnon generation. Our result helps to understand band structure formation in these seemingly simple ferromagnetic systems and gives first clear evidence of the transient processes relevant to femtosecond demagnetization.
简单铁磁体 Fe、Co 和 Ni 的电子能带结构在其众所周知的铁磁-顺磁相变过程中的演化已经争论了几十年，到目前为止，还没有明确的甚至相互矛盾的实验观察结果。使用时间和自旋分辨光电子能谱，我们可以实时制作一部电影，展示在超短激光脉冲激发后电子性质如何实时变化。这使我们能够首次监测到钴的自旋分辨电子能带结构的大瞬态变化。我们表明，磁矩的损失不仅发生在费米能级附近，在那里状态受到激光激发的影响，而且还延伸到电子能带的更深层次。我们发现，铁磁-顺磁相变不能用自旋极化能带的交换劈裂的损失来解释，而是在激发后表现出快速的能带反转，这是极高效超快磁子产生的明显特征。我们的结果有助于理解这些看似简单的铁磁体系统中的能带结构形成，并首次提供了与飞秒退磁相关的瞬态过程的明确证据。
