Uncompensated antiferromagnetism and phase boundary with variable curvature in the mixed magnet Mn1-xNixCl2•4H2O -: art. no. 184413

The magnetic properties of Mn1-xNixCl2.4H(2)O are examined by dc magnetization and susceptibility measurements on a wide range of compositions. The pure components are isomorphous three-dimensional Heisenberg antiferromagnets, ordering at 1.62 K (MnCl2.4H(2)O) and 2.99 K (NiCl2.4H(2)O). So far as is known, exchange interactions in either material are antiferromagnetic only. The Curie and Weiss constants, in chi(M)=C/(T-theta) fits to high temperature data, vary with composition in somewhat unexpected ways. C(x) is not as linear as often seen in mixed magnets; theta(x) exhibits a high degree of scatter. Remarkably, for compositions from x=0.10 through x=0.95, the susceptibility shows no standard antiferromagnetic maximum; rather, sharp upturns appear, at temperatures which vary systematically with composition. Only very near the composition extremes does the susceptibility exhibit somewhat oddly shaped maxima. Ordering temperatures are estimated, and a T-c(x) phase boundary deduced. It is monotonic but displays several curvature changes, more than anticipated from certain theoretical treatments. Low temperature magnetization isotherms display inflections, the location of which varies with composition. There is also an evolution with composition of the extrapolated zero-field magnetization at sufficiently low temperatures. It is likely that incomplete cancellation of antiferromagnetic sublattice moments occurs in mixtures.