An apparent, size-dependent form of this property in metal nanoparticle assemblies arises when the temperature exceeds the blocking temperature. A temperature-independent form of this property exhibited by trivalent europium oxides is named for J. H. Van Vleck. (10[1])Treating this property using statistical mechanics yields the Brillouin (“bree-WANN”) and Langevin functions. Gadolinium is used as a contrast agent because it (10[1])strongly exhibits this property. (-5[1])This property’s proportionality constant chi (“kye”) is inversely proportional (-5[1])to temperature by Curie’s law. (10[3]-5[2])Materials (10[1])with (-5[1])this property have (10[1]-5[1])positive (-5[1])susceptibility, (10[2]-5[1])and have unpaired (-5[1])electrons (10[1])whose moments (-5[1])align (10[2]-5[2])with external (-5[2])fields. (-5[2])For 10 points, name this (10[1]-5[1])property of materials that are weakly attracted by magnetic fields, (-5[1])contrasted with diamagnetism. (10[5]-5[1])■END■ (10[18]0[4])