Abstract-Group Velocity Engineering of Confined Ultrafast Magnons

Y.-J. Chen, Kh. Zakeri, A. Ernst, H. J. Qin, Y. Meng, and J. Kirschner

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.267201

Quantum confinement permits the existence of multiple terahertz magnon modes in atomically engineered ultrathin magnetic films and multilayers. By means of spin-polarized high-resolution electron energy-loss spectroscopy, we report on the direct experimental detection of all exchange-dominated terahertz confined magnon modes in a 3 ML Co film. We demonstrate that, by tuning the structural and magnetic properties of the Co film, through its epitaxial growth on different surfaces, e.g., Ir(001), Cu(001), and Pt(111), one can achieve entirely different in-plane magnon dispersions, characterized by positive and negative group velocities. Our first-principles calculations show that spin-dependent many-body correlation effects in Co films play an important role in the determination of the energies of confined magnon modes. Our results suggest a pathway towards the engineering of the group velocity of confined ultrafast magnons.

• 
• 
• 

Abstract-Temperature Dependence of Magnetic Excitations: Terahertz Magnons above the Curie Temperature

H.J. Qin, Kh. Zakeri, A. Ernst, J. Kirschner

(Submitted on 20 Feb 2017)

https://arxiv.org/abs/1702.06104v1

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 TC, 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., multi-magnon scattering. A careful investigation of physical quantities determining the magnons' propagation indicates that terahertz magnons sustain their propagating character even at temperatures far above TC.

Abstract-Relaxation Time of Terahertz Magnons Excited at Ferromagnetic Surfaces

Y. Zhang (張雨)^*, T.-H. Chuang (莊子弘), Kh. Zakeri^†, and J. Kirschner 
Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, 06120 Halle, Germany

Received 13 March 2012; published 24 August 2012

The temporal and spatial properties of terahertz magnons excited at ferromagnetic fcc Co(100) and bcc Fe(110) surfaces are investigated experimentally. The magnon lifetime is found to be a few tens of femtoseconds at low wave vectors, which reduces significantly as the wave vector approaches the Brillouin zone boundary. Surprisingly, the lifetime is very similar in both systems, in spite of the fact that the excitation energy in the Co(100) film is by a factor of two larger than in the Fe(110) film. The magnon wave packets propagate only a few nanometers within their lifetime. In addition to the fact that our results describe the damping mechanism in ultrafast time scales, they may provide a way to predict the ultimate time scale of magnetic switching in nanostructures.

© 2012 American Physical Society

URL:

http://link.aps.org/doi/10.1103/PhysRevLett.109.087203

DOI:

10.1103/PhysRevLett.109.087203

PACS:

75.30.Ds, 75.70.Ak, 75.70.Rf, 75.78.Jp