Relaxation (NMR)

About: Relaxation (NMR) is a research topic. Over the lifetime, 29342 publications have been published within this topic receiving 689851 citations.

Nuclear Double Resonance in the Rotating Frame

Abstract: A double nuclear resonance spectroscopy method is introduced which depends upon effects of magnetic dipole-dipole coupling between two different nuclear species. In solids a minimum detectability of the order of ${10}^{14}$ to ${10}^{16}$ nuclear Bohr magnetons/cc of a rare $b$ nuclear species is predicted, to be measured in terms of the change in a strong signal displayed by an abundant $a$ nuclear species. The $a$ magnetization is first oriented by a strong radio-frequency field in the frame of reference rotating at its Larmor frequency. The $b$ nuclear resonance is obtained simultaneously with a second radio-frequency field; and with the condition that the $a$ and $b$ spins have the same Larmor frequencies in their respective rotating frames, a cross relaxation will occur between the two spin systems. The cross-relaxation interaction, which lasts for the order of a long spin-lattice relaxation time of the $a$ magnetization, is arranged to produce a cumulative demagnetization of the $a$ system when maximum sensitivity is desired. Final observation of the reduced $a$ magnetization indicates the nuclear resonance of the $b$ system. The concepts of uniform spin temperature, when it is valid, and of nonuniform spin temperature where spin diffusion is important, are applied. The density matrix method formulates the double resonance interaction rate in second order. Preliminary tests of the double resonance effect are carried out with a nuclear quadrupole system.

Theory of the Effect of Spin-Orbit Coupling on Magnetic Resonance in Some Semiconductors

Abstract: The effect of spin-orbit coupling on the usual band theory of electrons in a lattice is considered. Particular attention is given to the bands in impurity semiconductors with diamond-type structure. $g$-values are calculated for electron states typical of various possible cases and it is found that different values are obtained according as to whether the Fermi level is near or distant from a band degeneracy. The spin-lattice relaxation time is calculated so that the effect of spin-orbit coupling on the wave functions is included, and times in fair agreement with those observed in silicon and alkali metals are obtained.

Macroscopic quantum tunnelling of magnetization in a single crystal of nanomagnets

Abstract: THE precise manner in which quantum-mechanical behaviour at the microscopic level underlies classical behaviour at the macroscopic level remains unclear, despite seventy years of theoretical investigation. Experimentally, the crossover between these regimes can be explored by looking for signatures of quantum-mechanical behaviour—such as tunneling—in macroscopic systems1. Magnetic systems (such as small grains, spin glasses and thin films) are often investigated in this way2–12 because transitions between different magnetic states can be closely monitored. But transitions between states can be induced by thermal fluctuations, as well as by tunnelling, and definitive identification of macroscopic tunnelling events in these complex systems is therefore difficult13. Here we report the results of low-temperature experiments on a single crystal composed of super-paramagnetic manganese clusters (Mn12-ac), which clearly demonstrate the existence of quantum-mechanical tunnelling of the bulk magnetization. In an applied magnetic field, the magnetization shows hysteresis loops with a distinct 'staircase' structure: the steps occur at values of the applied field where the energies of different collective spin states of the manganese clusters coincide. At these special values of the field, relaxation from one spin state to another is enhanced above the thermally activated rate by the action of resonant quantum-mechanical tunnelling. These observations corroborate the results of similar experiments performed recently on a system of oriented crystallites made from a powdered sample4.