Basic Principles of NMR
Nuclear magnetic resonance (NMR) is the oldest nuclear method in solid state physics. It is based on the principle, that transitions between nuclear magnetic energy levels corresponding to differently oriented nuclear spins in a static magnetic field should be observable when applying a second, time-dependent magnetic field perpendicular to the static one. The second magnetic field should oscillate at the Larmor frequency of the nuclei. The first NMR measurements were performed contemporaneously by Purcell, Torrey and Pound in Cambridge and by Bloch, Hansen and Packard in Stanford. Purcell and his colleagues observed the radio frequency absorption of protons in solid paraffin at room temperature using a resonant cavity, a sweepable magnet and radio frequency power of about 10-11 W . At the same time Bloch, Hansen and Packard reported in a very short paper (~ 260 words) the observation of radio frequency absorption by protons in water at room temperature using conventional radio frequency techniques . Bloch, who was not limited to a fixed cavity frequency, could already measure at different frequencies and fields and confirmed that the ratio H/v was always the same: the gyromagnetic ratio γ of the protons. In his paper, which was published on Christmas Eve of 1945, Purcell already proposed various applications of the newly established effect, for instance precise measurements of gyromagnetic ratios, investigations of the spin-lattice coupling as well as standardizations of magnetic fields. All these applications (and many more) were realized sooner or later. This was the twofold birth of NMR, which already included the basic principle of NMR based on the isotope-specific gyromagnetic ratios as well as first indications of the broad applicability of NMR in condensed matter and beyond. For completeness and to honor the nowadays unfortunately unpopular practice to publish also negative results, it should be noted that an earlier attempt to detect nuclear magnetic resonance four years before the discoveries of Bloch and Purcell was unsuccessful . Over the years, NMR became a powerful method in condensed matter as well as in in chemistry, biology and medicine, where it is widely used for structural analysis and non-destructive diagnostic imaging.