We study ultrafast magnetization quenching of ferromagnetic iron following excitation by an optical versus a terahertz pump pulse. While the optical pump (photon energy of 3.1 eV) induces a strongly nonthermal electron distribution, terahertz excitation (4.1 meV) results in a quasithermal perturbation of the electron population. The pump-induced spin and electron dynamics are interrogated by the magneto-optic Kerr effect (MOKE). A deconvolution procedure allows us to push the time resolution down to 130 fs, even though the driving terahertz pulse is about 500 fs long. Remarkably, the MOKE signals exhibit an almost identical time evolution for both optical and terahertz pump pulses, despite the 3 orders of magnitude different number of excited electrons. We are able to quantitatively explain our results using a nonthermal model based on quasielastic spin-flip scattering. It shows that, in the small-perturbation limit, the rate of demagnetization of a metallic ferromagnet is proportional to the excess energy of the electrons, independent of the precise shape of their distribution. Our results reveal that, for simple metallic ferromagnets, the dynamics of ultrafast demagnetization and of the closely related terahertz spin transport do not depend on the pump photon energy.

中文翻译：

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光脉冲与太赫兹脉冲引起的铁超快退磁

我们研究了由光学与太赫兹泵浦脉冲激发后铁磁铁的超快磁化淬灭。虽然光泵（3.1 eV 的光子能量）会引起强烈的非热电子分布，但太赫兹激发（4.1 meV）会导致电子群的准热扰动。磁光克尔效应 (MOKE) 询问泵诱导的自旋和电子动力学。尽管驱动太赫兹脉冲的长度约为 500 fs，但解卷积程序允许我们将时间分辨率降低到 130 fs。值得注意的是，尽管激发电子的数量有 3 个数量级的不同，但 MOKE 信号对于光学和太赫兹泵浦脉冲表现出几乎相同的时间演变。我们能够使用基于准弹性自旋翻转散射的非热模型定量解释我们的结果。它表明，在小扰动极限内，金属铁磁体的退磁率与电子的过剩能量成正比，与电子分布的精确形状无关。我们的结果表明，对于简单的金属铁磁体，超快退磁和密切相关的太赫兹自旋输运的动力学不依赖于泵浦光子能量。