  Microwaves couple to magnetic moments in both ferromagnets and
antiferromagnets. Although the magnons in ferromagnets and antiferromagnets
radically differ, they can become entangled via strong coupling to the same
microwave mode in a cavity. The equilibrium configuration of the magnetic
moments crucially governs the coupling between the different magnons because
the antiferromagnetic and ferromagnetic magnons have the opposite spins when
their dispersion relations cross. We derive analytical expressions for the
coupling strengths and find that the coupling between antiferromagnets and
ferromagnets is comparable to the coupling between two ferromagnets. Our
findings reveal a robust link between cavity spintronics with ferromagnets and
antiferromagnets.
