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.

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

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

Phys. Rev. Lett. 118, 127203 – Published 24 March 2017

https://journals.aps.org/prl/abstract/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 $T_C$.

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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-Long-living terahertz magnons in ultrathin metallic ferromagnets

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• H. J. Qin,
• Kh. Zakeri,
• A. Ernst,
• L. M. Sandratskii,
• P. Buczek,
• A. Marmodoro,
• T. -H. Chuang,
• Y. Zhang
• & J. Kirschner
• • http://www.nature.com/ncomms/2015/150127/ncomms7126/full/ncomms7126.html

The main idea behind magnonics is to use the elementary magnetic excitations (magnons) for information transfer and processing. One of the main challenges, hindering the application of ultrafast terahertz magnons in magnonics, has been the short lifetime of these excitations in metallic ferromagnets. Here, we demonstrate that the engineering of the electronic structure of a ferromagnetic metal, by reducing its dimensionality and changing its chemical composition, opens a possibility to strongly suppress the relaxation channels of terahertz magnons and thereby enhance the magnons’ lifetime. For the first time, we report on the long-lived terahertz magnons excited in ultrathin metallic alloy films. On the basis of the first-principles calculations, we explain the microscopic nature of the long lifetime being a consequence of the peculiar electronic hybridizations of the species. We further demonstrate a way of tailoring magnon energies (frequencies) by varying the chemical composition of the film.