How NMR magnetometers work
A quantum effect
If a nucleus has spin, it tends to align itself to an external magnetic field. However, by giving it exactly the right additional amount of energy, the nucleus can be induced to flip into the opposite spin state. Nuclear Magnetic Resonance (NMR) occurs when a radio-frequency field applied to a sample is just the right frequency – called the Larmor frequency – to induce this spin-flip. Electron Paramagnetic Resonance (EPR) is a similar effect, with an electron rather than a nucleus.
It turns out that the energy difference between the aligned and counter-aligned nuclear states depends linearly on the field strength. Thus the ratio of the resonant frequency to field strength is a physical constant, called the gyromagnetic ratio (gamma). It is approximately 42.5 MHz/T for protons (hydrogen nuclei).
Other nuclei also exhibit NMR, but with a different gamma, for example, 6.5 MHz/T for deuterium and 40 MHz/T for fluorine. EPR has a much higher gamma, approximately 28 GHz/T.
Limitations: measurement rate
After a resonance, the sample is usually allowed to shed the absorbed energy and regain its original, spin-aligned state. This limits the measurement rate to around 10-100 Hz. Also, to reduce the variability, many such measurements are often averaged together, yielding effective measurement rates approaching a second. Therefore, NMR is usually only useful for slowly changing fields.
If the field is not uniform, one edge of the sample will resonate at a different frequency from the other side. The resonance peak broadens and flattens until it completely disappears in the noise. This phenomenon determines the field homogeneity limit of NMR.
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