Pressure Dependent Magnetism in Magnetically Ordered Interlanthanide Chalcogenides*

Several new interlanthanide chalcogenide compounds, Ln$'$/Ln$''$/Q (Ln$’$=light and Ln$''$=heavy lanthanide, Q=S or Se) have been synthesized using a novel flux-growth technique, their complex structures determined, and their magnetic properties measured. The majority, with general formula Ln$'$Ln$''$Q$_3$ are paramagnetic for T$>$ 2K, with effective moments consistent with the magnetic Ln constituents. EuLn$_2$Q$_4$ (Ln=Tb - Lu), which crystallize in the CaFe$_2$O$_4$- type three-dimensional channel structure, are all antiferromagnetic with T$_N$ $\sim$ 3-5 K. The Ln constituent is geometrically frustrated and has secondary effects on the magnetic properties,which are dominated by the Eu-Eu superexchange coupling. The sharply defined Neel temperature increases with hydrostatic pressure to P$\sim$ 7 kbar for all EuLn$_2$Q$_4$. (For example, for EuLu2Se4, dT$_N$/dP = +0.03 K/kbar at low pressures.)