If a material contains unpaired electrons, it is possible to use ESR (electron spin resonance) to study the interaction of the electrons with an external magnetic field and with other spins. These experiments rely on microwave photons; in our lab, we cover the frequency range from 2-35 GHz. ESR experiments allow one, e.g., to investigate the structure and dynamics of biologically important molecules. In addition, electron spins are very interesting quantum bits (qubits), for applications in quantum information processing.
Left: Structure of the active center of the metalloprotein Azurin. Right: ESR Microresonator with a single permalloy stripe.