  The new field of spin cavitronics focuses on the interaction between the
magnon excitation of a magnetic element and the electromagnetic wave in a
microwave cavity. In strong interaction regime, such interaction usually gives
rise to the level anti-crossing for the magnonic and the electromagnetic mode.
Recently, the attractive level crossing has been observed, and is explained by
a non-Hermitian model Hamiltonian. However, the mechanism of the such
attractive coupling is still unclear. Here we reveal the secret by using a
simple model with two harmonic oscillators coupled to a third oscillator with
large dissipation. We further identify this dissipative third-party as the
invisible cavity mode with large leakage in the cavity-magnon experiments. This
understanding enables designing dissipative coupling in all sorts of coupled
systems.
