Excitation of Whispering Gallery Magnons in a Magnetic Vortex

Abstract

One of the most fascinating topics in current quantum physics are hybridised systems, in which different quantum resonators are strongly coupled. Prominent examples are circular resonators with high quality factors that allow the coupling of optical whispering gallery modes 1-5 to microwave cavities 6 or magnon resonances in optomagnonics 7-9 . Whispering gallery modes play a special role in this endeavour because of their high quality factor and strong localisation, which ultimately increases the overlap of the wavefunctions of quantum particles in hybridised systems. The hybridisation with magnons, the collective quantum excitations of the electron spins in a magnetically ordered material, is of particular interest because magnons can take over two functionalities: due to their collective nature they are robust and can serve as a quantum memory 10 and, moreover, they can act as a wavelength converter between microwave and THz photons 9 . However, the observation of whispering gallery magnons has not yet been achieved due to the lack of efficient excitation schemes for magnons with large wave vectors in a circular geometry. To tackle this problem, we studied nonlinear 3-magnon scattering 11-14 as a means to generate whispering gallery magnons. This Letter discusses the basics of this non-linear mechanism in a confined, circular geometry from experimental and theoretical point of view.Whispering gallery magnons can only live in systems with rotational symmetry. This not only applies to the geometry of the magnetic element but also to the magnetisation texture therein. For that reason, we study a Ni 81 Fe 19 disc that inherently exhibits a magnetic vortex structure [15][16][17][18][19] . The arrows in Fig. 1a depict the generic features of such a vortex state: the magnetic moments curl in plane along circular lines around the vortex core, a nanoscopic region in the center of the disc where the magnetisation tilts out of plane. According to this rotational symmetry, the magnon eigenmodes in a vortex are characterised by mode numbers (n, m), with n = 0, 1, 2, ... counting the number of nodes across the disc radius and m = 0, ±1, ±2, ... counting the number of nodes in azimuthal direction over half the disc 20,21 .Other than commonly known waves, like sound, water or electromagnetic waves, magnons exhibit a strongly anisotropic dispersion relation in in-plane magnetised thin films 20 . In a vortex, this results in increasing (decreasing) mode energies for increasing n (m) as shown by the analytic calculations in Fig. 1b. The four exemplary intensity profiles for the eigenmodes (0, 0), (0, 10), (0, 20), and (0, 30), that are shown in Fig. 1c, reveal the character of whispering gallery magnons: the larger m, the more the magnon intensity is pushed toward the perimeter of the disc which can be understood intuitively by the reduction of exchange energy: Leaving an extended area around the vortex core with zero amplitude avoids a strong tilt of neighbouring spins close to the vortex core and, therefore, reduces the total ...

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