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Tuning the laser-induced ultrafast demagnetization of transition metals

机译：调整过渡金属的激光诱导超快退磁

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The ultrafast demagnetization (UFD) dynamics of itinerant ferromagnets is theoretically investigated as a function of the characteristics of the initial laser excitation. A many-body pd-band Hamiltonian is considered which takes into account hybridizations, Coulomb interactions, spin-orbit interactions, and the coupling to the laser field on the same electronic level. In this way, a fruitful connection is established between the nonadiabatic quantum dynamics and the well-known equilibrium statistical mechanics of itinerant-electron ferromagnets. The time evolution during and after the pulse absorption is determined exactly by performing numerical Lanczos propagations on a small cluster model with parameters appropriate for Ni. The most relevant laser parameters, namely, the fluence F, wave length lambda, polarization (epsilon) over cap, and pulse duration tau(p) are varied systematically. The results show how F, (epsilon) over cap and tau(p) allow one to control the total absorbed energy, the spectral distribution of the initial excitation, and the subsequent magnetization dynamics. The calculations show that reasonable changes in these parameters do not affect the UFD dynamics qualitatively and have only a minor influence on the timescale tau(dm) which characterizes the initial demagnetization. In contrast, our model predicts that the degree of demagnetization Delta S-z/S-z(0) achieved for t greater than or similar to tau(dm) correlates well with the average number of electrons excited by the laser or average number of absorbed photons n(ph), which can be tuned by varying the fluence, spectral distribution and polarization of the laser pulse. The theoretical results are discussed by comparing them with available experiments. From a fundamental perspective, the robustness of the ultrafast demagnetization effect is theoretically demonstrated, as a phenomenon reflecting the intrinsic dynamics of the metallic 3d valence electrons. A wide variety of well-focused possibilities of tailoring the efficacy of the ultrafast demagnetization process is thereby opened.
机译：理论上研究了流动铁磁体的超快退磁（UFD）动力学，该动力学是初始激光激发特性的函数。考虑了多体p波段哈密顿量，其中考虑了杂交，库仑相互作用，自旋轨道相互作用以及在同一电子水平上与激光场的耦合。这样，在非绝热量子动力学与流动电子电子铁磁体的众所周知的平衡统计力学之间建立了卓有成效的联系。通过在具有适合Ni的参数的小型簇模型上执行数值Lanczos传播，可以精确确定脉冲吸收期间和之后的时间演变。最相关的激光参数，即注量F，波长λ，帽上的极化（ε）和脉冲持续时间tau（p）都会有系统地变化。结果显示帽和tau（p）上的F（ε）如何允许人们控制总吸收能量，初始激发的光谱分布以及随后的磁化动力学。计算表明，这些参数的合理变化不会定性地影响UFD动力学，而对表征初始退磁的时间标度tau（dm）的影响很小。相反，我们的模型预测，大于或等于tau（dm）的t所达到的去磁度Delta Sz / Sz（0）与激光激发的平均电子数或吸收的光子的平均数n（ ph），可以通过改变激光脉冲的注量，光谱分布和偏振来进行调整。通过将理论结果与可用实验进行比较来讨论理论结果。从基本的角度看，超快退磁效应的鲁棒性在理论上得到了证明，它是反映金属3d价电子固有动力学的现象。因此，提供了多种用于调整超快退磁过程功效的广泛关注的可能性。

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  • 来源
    《Physical review》 |2019年第2期|024402.1-024402.16|共16页
  • 作者

    Toews W.; Pastor G. M.;

  • 作者单位

    Univ Kassel, Inst Theoret Phys, Heinrich Plett Str 40, D-34132 Kassel, Germany;

    Univ Kassel, Inst Theoret Phys, Heinrich Plett Str 40, D-34132 Kassel, Germany;

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