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Why does electron-spin resonance usually use microwaves instead of radio waves?

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magnetic resonance Figure 1: Precession of a magnetic dipole moment μ in the presence of a constant field H and a rotating field H′ (see text)

Electron-spin resonance (ESR) utilizes microwaves rather than radio waves because of the significantly higher frequencies required to induce transitions in electronic spin states compared to nuclear spin states.1

In magnetic resonance experiments, the frequency at which resonance occurs is determined by the interaction between the particle's magnetic moment and the applied magnetic field. For a given magnetic field strength, the resonance frequency for an electron is much higher than that of a nucleus due to the electron's much larger magnetic moment.1

For example, in a magnetic field of 10 kilogauss, the proton nuclear magnetic resonance (NMR) frequency is approximately 42.58 megahertz, which falls within the radio-frequency range. In that same field, the ESR frequency for a free spin is 28,000 megahertz, placing it squarely in the microwave or radar range. Consequently, while NMR typically operates using radio waves, ESR requires the higher-energy photons found in the microwave portion of the electromagnetic spectrum to drive the resonance of unpaired electrons.12