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. Treating this property using statistical mechanics yields the Brillouin (“bree-WANN”) and Langevin functions. Gadolinium is used as a contrast agent because it strongly exhibits this property. This property’s proportionality constant chi (“kye”) is inversely proportional to temperature by Curie’s law. (10[1])Materials with this property have positive (-5[1])susceptibility, and have unpaired (-5[1])electrons whose moments align with external (-5[2])fields. For 10 points, name this property of materials that are weakly attracted by magnetic fields, contrasted with diamagnetism. (10[1]-5[1])■END■ (10[5]0[2])