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Nuclear orientation experiments in paramagnetic, antiferromagnetic and ferromagnetic substances LeBlanc, Marcel Armand Rene Joseph

Abstract

Through the technique of adiabatic demagnetization of paramagnetic salts substances may be cooled to temperatures of the order of 0.1° K and 0.01° K. At these low temperatures the hyperfine structure interaction can produce an appreciable degree of nuclear orientation. The orientation of an assembly of radioactive nuclei may be detected by measuring the anisotropy in the emission of gamma radiation. By this method we have investigated nuclear orientation in paramagnetic, antiferromagnetic and ferromagnetic substances. We have studied the nuclear orientation of Pr¹⁴² and Yb¹⁷⁵ introduced as impurities in paramagnetic single crystals of cerium magnesium nitrate. The anisotropy of the 1.57 Mev. γ-ray of Pr¹⁴² was measured as a function of temperature in the range of 0.003° K to 1.0° K both in zero external magnetic field and in a field of 320 gauss parallel to the trigonal crystal axis. Values for the magnetic moment of Pr¹⁴² were assigned from our results for two assumed decay schemes, these are 0.11 nuclear magnetons for the spin assignments 2→[formula omitted] and 0.15 nuclear magnetons for the spin assignments [formula omitted]. Similar experiments were carried out on Yb¹⁷⁵. Measurements were made in a variety of external magnetic fields up to 700 gauss, and at temperatures as low as 0.003⁰K. No anisotropy was observed for the 396 kev γ-ray, nor for the 282 kev γ-ray. The most likely explanation for this result is that the lifetime of the γ - emitting state is about 10⁻¹⁰ seconds. This conclusion has since received independent confirmation. We have established that nuclear orientation can be produced in antiferromagnetic single crystals. We have explored some of the possibilities and features of nuclear orientation in this class of substances by investigating salts of manganese and cobalt with transition temperatures differing by an order of magnitude. Single crystals of these antiferromagnetic salts were cooled in thermal contact with potassium chrome alum and the anisotropy of the gamma radiation emitted by Co⁶⁰ and Mn⁵⁴ introduced in the lattice was observed. These measurements show that the hyperfine structure splittings in the antiferromagnetic state are comparable to those found in the paramagnetic state and give indications that nuclear spin relaxation times may be of the order of minutes and hours in antiferromagnetic materials at temperatures below 1°K. We have attempted to detect nuclear orientation arising from a possible hyperfine structure interaction at the anion in antiferromagnetic MnBr₂4 H₂O and MnCl₂4 H₂O. To detect the nuclear orientation we observed the anisotropy of the gamma radiation emitted by Br⁸² introduced into MnBr₂4 H₂O and I¹⁸¹ in both McCl₂4 H₂O and MnBr₂4 H₂O. This attempt yielded negative results. The anisotropy of the gamma radiation of Co⁶⁰ in a ferromagnetic single crystal of cobalt metal was measured before and after heat treatment of the crystal. The results before heat treatment show a significant discrepancy with the data reported by other workers and differ from those found for the heat treated crystal. A qualitative explanation of these results in terms of crystalline stacking faults in cobalt metal is presented. Work was initiated on nuclear orientation in binary ferromagnetic alloys. The nuclear orientation may arise from the hyperfine structure interactions which may exist in one or both components of a ferromagnetic binary alloy. The inter-metallic compound chosen for special study was MnBi. Although only preliminary and inconclusive results on the orientation of Mn⁵⁴ nuclei in this substance were obtained the technique adopted is briefly described.

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