  A photon-magnon hybrid system can be realised by coupling the electron spin
resonance of a magnetic material to a microwave cavity mode. The quasiparticles
associated with the system dynamics are the cavity magnon polaritons, which
arise from the mixing of strongly coupled magnons and photons. We illustrate
how these particles can be used to probe the magnetisation of a sample with a
remarkable sensitivity, devising suitable spin-magnetometers which ultimately
can be used to directly assess oscillating magnetic fields. Specifically, the
capability of cavity magnon polaritons of converting magnetic excitations to
electromagnetic ones, allows for translating to magnetism the quantum-limited
sensitivity reached by state-of-the-art electronics. Here we employ hybrid
systems composed of microwave cavities and ferrimagnetic spheres, to
experimentally implement two types of novel spin-magnetometers.
