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 (-5[1])to temperature by Curie’s law. Materials with this property have positive susceptibility, and have unpaired electrons whose moments (-5[1])align (-5[1])with external fields. For 10 points, name this property of materials that are weakly attracted by magnetic fields, contrasted with diamagnetism. ■END■ (10[2])