The Effect of Eddy Currents on Nuclear Magnetic Resonance in Metals

Abstract

A theoretical and experimental investigation of this effect is described. The theoretical treatment is developed for specimens in the form of (i) a flat plate, (ii) a long cylinder and (iii) a sphere; and is applicable to foils, wires and powders with spherical particles. In each case the power absorption per unit volume W is expressed as W = W0 + W1, where W1 determines the nuclear resonance absorption and is of the form W1 ∝ aχ' + bχ'', where (χ' - iχ'') is the complex susceptibility. Explicit expressions are derived for W0, a and b as functions of p = 2t/δ, where δ is the skin depth, and 2t is (i) the thickness of the plate, (ii) the diameter of the cylinder, (iii) the diameter of the sphere. These results indicate that both the dependence on reduced specimen dimension and the magnitude of the effect differ considerably from those previously supposed. Measurements of the nuclear magnetic resonance absorption in aluminium foils, in the temperature range 20°K, to 300°K, are described, and it is shown that the results are in satisfactory agreement with the theoretical treatment. The effect of eddy currents on the position of the centre, the shape and the amplitude of the absorption line are discussed, and methods of correcting observed lines to obtain the undistorted line are indicated. Extensions of the treatment to methods of observing nuclear resonance other than by absorption measurements are also outlined.