Abstract
We describe an apparatus allowing the observation of the NMR static longitudinal component of nuclear magnetizationM z by a SQUID at low magnetic fields and atT=4.2 K in Plexiglas, Teflon, and CaF2. The higher order NMR Larmor lines at 2f 0, 3f 0, and 4f 0, wheref 0 is the Larmor frequency of the allowed NMR line, were measured. These lines correspond to a flip of two, three, and four spins, respectively, by one photon. The amplitudes of 2f 0 and 3f 0 lines and their field dependences show that the common assertion that the forbidden lines at 2f 0 and 3f 0 appear in the same order of perturbation theory and that their intensity decreases with static magnetic fieldB 0 asB −20 is wrong. In fact, the third Larmor line 3f 0 appears at higher order than the second Larmor line 2f 0 and its intensity decreases asB −40 . This is also shown by a theoretical calculation of its intensity. For the first time the NMR SQUID technique allowed the detection of the 4f 0 line, corresponding to the simultaneous flip of four spins by one photon.
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Kohl, M., Odehnal, M., Petříěek, V. et al. Observation of higher order NMR larmor lines by SQUID in solids at low magnetic field. J Low Temp Phys 72, 319–343 (1988). https://doi.org/10.1007/BF00682103
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DOI: https://doi.org/10.1007/BF00682103