Blümich, Bernhard, Christian Rehorn, and Wasif Zia. “Magnets for Small-Scale and Portable NMR.” In Micro and Nano Scale NMR, by Jens Anders and Jan G. Korvink, 1–20. Advanced Micro and Nanosystems. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA, 2018.
Nuclear magnetic resonance (NMR) exploits the resonance of the precessing motion of nuclear magnetization in magnetic fields. From the measurement methodology, three groups of common techniques of probing resonance can be assigned: those employing forced oscillations, free oscillations, and interferometric principles. In either case, the sensitivity depends on the strength of the nuclear magnetic polarization, which, in thermodynamic equilibrium at temperatures higher than few degrees above absolute zero, is in good approximation proportional to the strength of the magnetic field. In recognition of this fact, one guideline in the development of NMR magnets has always been to reach high field strength.The highest field strength of temporally stable magnetic fields today is achieved with superconducting electromagnets. This is why most standard NMR instruments used for NMR spectroscopy in chemical analysis and magnetic resonance imaging (MRI) in medical diagnostics employ superconducting magnets cooled to the low temperature of boiling helium with cryogenic technology.