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 (10[1])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 susceptibility, and have unpaired electrons whose moments align with external fields. For 10 points, name this property of materials that are weakly attracted by magnetic fields, contrasted with diamagnetism. ■END■