Abstract-THz-frequency cavity magnon-phonon-polaritons in the strong coupling regime

Prasahnt Sivarajah, Andreas Steinbacher, Blake Dastrup, Keith Nelson

(Submitted on 12 Jul 2017)

https://arxiv.org/abs/1707.03503

We demonstrate the strong coupling of both magnons and phonons to terahertz (THz) frequency electromagnetic (EM) waves confined to a photonic crystal (PhC) cavity. Our cavity consists of a two-dimensional array of air-holes cut into a hybrid slab of ferroelectric lithium niobate (LiNbO3) and erbium orthoferrite (ErFeO3), a canted antiferromagnetic crystal. The phonons in LiNbO3 and the magnons in ErFeO3 are strongly coupled to the electric and magnetic field components of the confined EM wave, respectively. This leads to the formation of new cavity magnon-phonon-polariton modes, which we experimentally observe as a normal-mode splitting in the frequency spectrum and an avoided crossing in the temperature-frequency plot. The cavity also has a mode volume of V=3.4×10−3λ3≃0.5(λ/n)3 μm3 and can achieve a Q-factor as high as 1000. These factors facilitate the pursuit of the fields of THz cavity spintronics and quantum electrodynamics.

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-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.