We study photon-mediated interaction between two macrospins subjected to a static magnetic induction and undergoing Larmor precession. In free space, the macrospin-macrospin interaction is reciprocal. The cooperative frequency shift and decay rate are oscillatory decaying functions of the distance of macrospins, when the retardation effect is significant. This sign-changing photon-mediated interaction of macrospins is reminiscent of the electron-mediated Ruderman-Kittel-Kasuya-Yosida interaction between magnetic impurities in metals. Inside a metallic waveguide, however, the photon-mediated interaction can become nonreciprocal. This phenomenon arises from the chiral coupling between the transverse electric (TE) modes of the waveguide and the precessing macrospins. By tuning the Larmor frequency and thereby controlling the impact of the lowest TE modes, the external magnetic induction determines the reciprocal or nonreciprocal nature of the interaction. Furthermore, optimally positioning of the macrospins within the waveguide enhances the nonreciprocal effect.

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• Cavity quantum electrodynamics
• Spin dynamics
• Spintronics
• Waveguides