Spin statistics, magnetic resonance, spin motion and echoes

Abstract

Spin temperature and spin thermodynamics Magnetic resonance and magnetic dipole transitions NMR and EPR Spin echo Cooling at extremely low temperatures This Chapter, dealing with nuclear and electronic angular momenta in magnetic fields, further develops topics already discussed in Chapters 4 and 5. The new arguments involve some aspects of spin statistics and of magnetic resonance (namely how to drive the angular and magnetic moments and to change their components along a magnetic field). The magnetic resonance experiment in most cases is equivalent to drive magnetic dipole transitions among Zeeman-like levels. 6.1 Spin statistics, spin-temperature and fluctuations Let us refer to a number N (of the order of the Avogadro number) weakly interacting spins S = 1/2, each carrying magnetic moment µ = −2µ B S, in static and homogeneous magnetic field H along the z-axis. At the thermal equilibrium the statistical distribution depicted in Fig. 6.1 occurs. The number of spins on the two energy levels (statistical populations) are N − = N e