Lee Taekhyeon, Park Min Tae, Ko Hye-Won, Oh Jung Hyun, Ko San, Hwang Seongmun, Jang Jae Gwang, Baek Geon-Woo, Kim Se Kwon, Lee Hyun-Woo, Jung Myung-Hwa, Kim Kab-Jin, Lee Kyung-Jin
Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea.
Department of Physics, Sogang University, Seoul, Korea.
Nature. 2025 Feb;638(8049):106-111. doi: 10.1038/s41586-024-08367-z. Epub 2025 Jan 29.
A particle current generated by pumping in the absence of gradients in potential energy, density or temperature is associated with non-trivial dynamics. A representative example is charge pumping that is associated with the quantum Hall effect and the quantum anomalous Hall effect. Spin pumping, the spin equivalent of charge pumping, refers to the emission of a spin current by magnetization dynamics. Previous studies have focused solely on transversal spin pumping arising from classical dynamics, which corresponds to precessing atomic moments with constant magnitude. However, longitudinal spin pumping arising from quantum fluctuations, which correspond to a temporal change in the atomic moment's magnitude, remains unexplored. Here we experimentally investigate longitudinal spin pumping using iron-rhodium (FeRh), which undergoes a first-order antiferromagnet-to-ferromagnet phase transition during which the atomic moment's magnitude varies over time. By injecting a charge current into a FeRh/platinum bilayer, we induce a rapid phase transition of FeRh in nanoseconds, leading to the emission of a spin current to the platinum layer. The observed inverse spin Hall signal is about one order of magnitude larger than expected for transversal spin pumping, suggesting the presence of longitudinal spin pumping driven by quantum fluctuations and indicating its superiority over classical transversal spin pumping. Our result highlights the significance of quantum fluctuations in spin pumping and holds broad applicability in diverse angular momentum dynamics, such as laser-induced ultrafast demagnetization, orbital pumping and quantum spin transfer.
在不存在势能、密度或温度梯度的情况下，通过泵浦产生的粒子流与非平凡动力学相关。一个典型的例子是与量子霍尔效应和量子反常霍尔效应相关的电荷泵浦。自旋泵浦是电荷泵浦的自旋等效物，指的是由磁化动力学产生的自旋电流发射。以往的研究仅关注由经典动力学产生的横向自旋泵浦，这对应于具有恒定大小的进动原子磁矩。然而，由量子涨落产生的纵向自旋泵浦，即对应于原子磁矩大小的时间变化，仍未得到探索。在这里，我们使用铁铑（FeRh）对纵向自旋泵浦进行了实验研究，铁铑在一阶反铁磁到铁磁相变过程中，原子磁矩的大小随时间变化。通过向FeRh/铂双层注入电荷电流，我们在纳秒内诱导FeRh发生快速相变，从而导致向铂层发射自旋电流。观察到的逆自旋霍尔信号比横向自旋泵浦预期的大一个数量级，这表明存在由量子涨落驱动的纵向自旋泵浦，并表明其优于经典横向自旋泵浦。我们的结果突出了量子涨落在自旋泵浦中的重要性，并在多种角动量动力学中具有广泛的适用性，如激光诱导超快退磁、轨道泵浦和量子自旋转移。
