Qin H J, Zakeri Kh, Ernst A, Kirschner J
Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D-06120 Halle, Germany.
Heisenberg Spin-dynamics Group, Physikalisches Institut, Karlsruhe Institute of Technology, Wolfgang-Gaede-Strasse 1, D-76131 Karlsruhe, Germany.
Phys Rev Lett. 2017 Mar 24;118(12):127203. doi: 10.1103/PhysRevLett.118.127203.
When an ordered spin system of a given dimensionality undergoes a second order phase transition, the dependence of the order parameter, i.e., magnetization on temperature, can be well described by thermal excitations of elementary collective spin excitations (magnons). However, the behavior of magnons themselves, as a function of temperature and across the transition temperature T_{C}, is an unknown issue. Utilizing spin-polarized high resolution electron energy loss spectroscopy, we monitor the high-energy (terahertz) magnons, excited in an ultrathin ferromagnet, as a function of temperature. We show that the magnons' energy and lifetime decrease with temperature. The temperature-induced renormalization of the magnons' energy and lifetime depends on the wave vector. We provide quantitative results on the temperature-induced damping and discuss the possible mechanism, e.g., multimagnon scattering. A careful investigation of physical quantities determining the magnons' propagation indicates that terahertz magnons sustain their propagating character even at temperatures far above T_{C}.
当给定维度的有序自旋系统经历二级相变时，序参量（即磁化强度）对温度的依赖关系可以通过基本集体自旋激发（磁振子）的热激发很好地描述。然而，磁振子本身作为温度的函数以及在转变温度(T_{C})附近的行为仍是一个未知问题。利用自旋极化高分辨率电子能量损失谱，我们监测了在超薄铁磁体中激发的高能（太赫兹）磁振子随温度的变化。我们发现磁振子的能量和寿命随温度降低。温度引起的磁振子能量和寿命的重整化取决于波矢。我们给出了温度诱导阻尼的定量结果，并讨论了可能的机制，例如多磁振子散射。对决定磁振子传播的物理量的仔细研究表明，太赫兹磁振子即使在远高于(T_{C})的温度下仍保持其传播特性。
