spin temperature[′spin ‚tem·prə·chər]
a quantity that characterizes the distribution of paramagnetic particles (particles with spin) among the magnetic sublevéis formed when the particles’ levels are split in a magnetic field (seeZEEMAN EFFECT). In equilibrium, the distribution can be described by the equation (seeBOLTZMANN STATISTICS)
n(ℰ) = Cexp(– ℰ/kTs)
Here, n(ℰ) is the number of particles with energy ℰ, C is a con stant, k is the Boltzmann constant, and Ts is the spin temperature. A state of internal equilibrium in a system of paramagnetic particles and, consequently, a spin temperature differing from the lattice temperature are established only if the energy exchange within the system of particles (spin-spin relaxation) occurs more rapidly than the energy exchange between the paramagnetic particles and the crystal lattice (spin-lattice relaxation). The spin temperature may be not only positive but negative. A negative spin temperature corresponds to an inversion of the populations of the energy levels (seeQUANTUM MECHANICALAMPLIFIER).
A. V. FRANTSESSON