Enhanced nuclear magnetism: some novel features and prospective experiments

Abstract

This review of enhanced nuclear magnetism discusses a number of features not previously considered, with special reference to new experiments that use dynamic methods to produce high nuclear polarization, followed by adiabatic demagnetization in the rotating frame (a. d. r. f.) to produce nuclear ordered states that may be investigated by the scattering of beams of neutrons. Section 2. The ‘enhancement’ of the nuclear moment arises from the electronic magnetization M I induced through the hyperfine interaction. It is shown that the spatial distribution of M I is the same as that of M H , the Van Vleck magnetization induced by an external field, provided that J is a good quantum number. The spatial distributions are not in general the same in Russell-Saunders coupling, e. g. in the 3d group. Section 3. The Bloch equations are extended to include anisotropic nuclear moments. Section 4. The ‘truncated’ spin Hamiltonian is derived for spin-spin interaction between enhanced moments. Section 5. A general cancellation theorem for second-order processes in spin-lattice relaxation is derived, showing that the intrinsic direct process must be of third order. The relaxation rate obeys an equation similar to that for Kramers electronic ions, but reduced as the fifth power of the resonance frequencies. The relaxation rates observed experimentally (except in very high fields) are ascribed to paramagnetic impurities, so that these can be used to produce dynamic nuclear polarization (d. n. p.). Section 6. The interactions of neutrons with the true nuclear moment μ I the Van Vleck moment M H , the ‘pseudonuclear’ moment M I and the ‘pseudomagnetic’ nuclear moment μ * I are discussed. It is shown that the four contributions can be observed separately by measurement of the form factor for neutron scattering as a function of temperature and direction of the applied magnetic field. Precession of the neutron spin in the ‘pseudomagnetic’ field H * is discussed with reference to the case of HoVO 4 , and an experiment on ‘pseudomagnetic resonance’ is proposed, in which the neutron spin is ‘flipped’ by resonance with the precessing H * of 169 Tm in TmPO 4 polarized by d. n. p. Section 7. Ordered states of enhanced nuclear moment systems are considered, together with the conditions under which they might be produced by a. d. r. f. following d. n. p. On the assumption that a transverse helical ordered structure can be created in TmPO 4 , a pseudomagnetic resonance experiment is suggested making use of 169 Tm, in which the neutron spin is flipped without the intervention of an external radio-frequency field.