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24 August 2010 Coherent strong-field coupling of a ferromagnetic nanomagnet with a photonic cavity
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We predict that strong coupling is feasible between photons and a ferromagnetic nanomagnet, due to exchange interactions that cause very large numbers of spins to coherently lock together with a significant increase in oscillator strength while still maintaining very long coherence times. The interaction of a ferromagnetic nanomagnet with a single photonic mode of a cavity is analyzed in a fully quantum-mechanical treatment. Exceptionally large quantum-coherent magnet-photon coupling with coupling terms in excess of several THz are predicted to be achievable in a spherical cavity of ~ 1 mm radius with a nanomagnet of ~ 100 nm radius and ferromagnet resonance frequency of ~ 200 GHz. This should substantially exceed the coupling observed in solids between orbital transitions and light. Eigenstates of the nanomagnet-photon system correspond to entangled states of spin orientation and photon number over 105 values of each quantum number. Initial coherent state of definite spin and photon number evolve dynamically to produce large coherent oscillations in the microwave power with exceptionally long dephasing times of few seconds. In addition to dephasing, several decoherence mechanisms including elementary excitation of magnons and crystalline magnetic anisotropy are investigated and shown to not substantially affect coherence upto room temperature. The optimal nanomagnet size is predicted to be just below the threshold for failure of the macrospin approximation.
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Ö. O. Soykal and M. E. Flatté "Coherent strong-field coupling of a ferromagnetic nanomagnet with a photonic cavity", Proc. SPIE 7760, Spintronics III, 77600G (24 August 2010);

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