Electron spin ensemble strongly coupled to a three-dimensional microwave cavity

We demonstrate the strong coupling between an electron spin ensemble and a three-dimensional cavity in a reflection geometry. We also find that an anticrossing in the cavity/spin spectrum can be observed under conditions that the collective coupling strength $gc$ is smaller than the spin linewidth $γs$ or the cavity linewidth. We identify a ratio of $gc$ to $γs$ $(gc/γs>0.64)$ as a condition to observe a splitting in the cavity frequency. Finally, we confirm that $gc$ scales with $N$⁠, where $N$ is the number of polarized spins.