The magnetic anisotropy of crystalline nitrates and carbonates

Abstract

Recent work in the field of magnetism emphasises the relation between the crystal structure and magnetic properties of solids. Such relations exist in all crystals whether ferromagnetic, paramagnetic or diamagnetic. We propose in this paper to discuss the explanation of the anisotropy of diamagnetic crystals, particularly some nitrates and carbonates with regard to which we have data from the measurements of Voigt and Kinoshita and very recently of Rabi. These substances are especially simple because, as is well-known from the work of Kossel, Bragg and others, they consist of charged ions held together by electrostatic forces, so that we can attribute the magnetic anisotropy of the crystal to that of the individual ions. Thus, for instance, in the case of sodium and potassium nitrates we may reasonably look for the explanation of observed magnetic anisotropy of the crystal in the structure of the nitrate ion, since presumably the metallic ions are more or less isotropic. It is significant in this connection that nitric acid solution has been found by Cotton and Mouton to exhibit a measurable magnetic birefringence, thus indicating that the NO 3 - ion has a pronounced magnetic anisotropy even in the liquid state. It is possible to estimate this anisotropy in a purely optical way by combining the data for magnetic double-refraction with the measurements of the depolarisation of the light scattered in nitric acid. It is found that the magnetic anisotropy of the NO 3 group thus found, practically agrees with that necessary, as remarked above, to explain the magnetic properties of the crystalline nitrates.