Magnetization dynamics in lanthanide metals probed by photoelectron spectroscopy with higher-order harmonic radiation

本课题拟研究镧系金属磁有序膜的超快磁化动力学。为此，我们使用来自可调谐高阶谐波光束线的极紫外辐射飞秒脉冲来映射交换分裂（5d6s）^3价带结构的瞬态，并跟踪角分辨光电发射中4f核能级的磁线性二色性（MLD）。飞秒红外脉冲将被用作泵浦脉冲来启动超快磁化动力学。我们之前的光发射工作表明，纯钆和铽金属的4f自旋系统的响应有显著差异，我们将其归因于它们不同的轨道动量（L=0 vs. L=3）。对于Gd， 5d和4f自旋系统的动力学发生在很大程度上不同的时间尺度上（~0.8 vs ~14ps），而对于Tb，两个自旋系统同步反应并且更快（~0.4ps）。这些结果不仅与x射线磁圆二色性（XMCD）测量结果形成对比，而且通常挑战了3d4f合金中超快磁开关的微观描述。我们现在计划进行三种类型的专门设计的实验，使我们能够进一步发展对稀土中4f耦合和磁化动力学的微观理解。(i)将研究轨道动量对GdTb化合物磁化动力学的影响。改变GdTb成分，我们将绘制瞬态交换分裂和Gd和Tb 4f MLD，并解决它们在合金中的反应是否不同。（ii）我们想回答Gd表面的磁化动力学是否与它的体积不同。为了将表面与体贡献分开，我们将研究一层单层铕（4f^7 (5d6s)^2）覆盖的Gd薄膜。（iii）作为第三种方法，我们将研究金属间化合物tfe，其中磁化是由不同亚晶格交换耦合之间的相互作用决定的。根据浓度的不同，tfe薄膜可以表现出非常低的矫顽力，我们预计与纯Tb薄膜相比，其4f自旋晶格耦合明显较弱。这将导致较慢的4f自旋动力学。

In the proposed project we intend to study the ultrafast magnetization dynamics in magnetically ordered films of lanthanide metals. To this end we use femtosecond pulses of extreme-ultraviolet radiation from a tunable higher-order harmonics beamline to map transients of the exchange-split (5d6s)^3 valence band structure and follow the magnetic linear dichroism (MLD) of the 4f core-levels in angle-resolved photoemission. Femtosecond IR pulses will be used as pump pulses to initiate the ultrafast magnetization dynamics.Our previous photoemission work shows a significant difference in the response of the 4f spin system of pure Gadolinium and Terbium metal, which we attribute to their different orbital momentum (L=0 vs. L=3). For Gd, the dynamics of 5d and 4f spin systems occurs on largely disparate timescales (~0.8 vs. ~14ps), while for Tb both spin systems react synchronously and faster (~0.4ps). These results are not only in contrast to X-ray magnetic circular dichroism (XMCD) measurements but generally challenge the microscopic description of ultrafast magnetic switching in 3d4f alloys.We now plan for three types of specifically designed experiments which allow us to further develop the microscopic understanding of the 4f coupling and magnetization dynamics in the rare-earths. (i) The influence of the orbital momentum on the magnetization dynamics will be studied for GdTb compounds. Varying the GdTb composition we will map transient exchange splitting and Gd and Tb 4f MLD and address whether they respond differently in the alloy. (ii) We want to answer whether there is a different magnetization dynamics at the Gd surface as compared to its bulk. To separate surface from bulk contributions we will study a Gd film covered with one monolayer of europium (4f^7 (5d6s)^2). (iii) As a third approach, we will investigate the intermetallic compound TbFe, where the magnetization is determined by the interplay between the exchange couplings of the different sublattices. Depending on the concentration, TbFe films can show very low coercivity and we expect a significant weaker 4f spin - lattice coupling as in pure Tb films. This should result in slower 4f spin dynamics.