Observations on Antiferromagnetic CuCl2·2H2O Crystals

Abstract

Four series of observations carried out in the Kamerlingh Onnes Laboratory on Cu${\mathrm{Cl}}_{2}$\ifmmode\cdot\else\textperiodcentered\fi{}2${\mathrm{H}}_{2}$O crystals are reviewed. The magnetization curves confirm the general picture given by the theoretical analysis of Gorter and Haantjes. At the lower temperature a sharp threshold field is found in the crystallographic $\mathrm{ac}$ plane with a minimum value of 6500 \ensuremath{\varphi}; at higher temperature its value increases somewhat.Specific heat measurements reveal the existence of a short-range order tail above the sharp N\'eel temperature at which antiferromagnetism vanishes.In the investigations on proton magnetic resonance a sharp transition is found between a nonsymmetrical paramagnetic resonance diagram and a symmetrical resonance diagram characteristic for antiferromagnetism. In this way it is found that the N\'eel temperature depends on the direction of the magnetizations.It is possible to follow the turn-over of the magnetizations near the threshold field. The position of the protons and the nature of the sublattices have been determined. The sublattices do not exchange position within ${10}^{\ensuremath{-}4}$ sec. Microwave resonances have been observed at 9400 and at 4000 megacycles/sec. The absorption bands are confined to the neighborhood of the turn-over fields. General agreement with Ubbink's theoretical treatment is found.