The magnetization and the volume magnetostriction were measured up to 39 T at low temperatures in the mixed valence ${\mathrm{Yb}}_{0.38}$${\mathrm{In}}_{0.62}$${\mathrm{Cu}}_2$ and the dense Kondo ${\mathrm{Yb}}_{0.50}$${\mathrm{Ag}}_{0.50}$${\mathrm{Cu}}_2$ compounds. ${\mathrm{Yb}}_{0.38}$${\mathrm{In}}_{0.62}$${\mathrm{Cu}}_2$ shows a distinct metamagnetic transition at 33 T. A remarkable negative volume magnetostriction was observed at the transition, giving evidence of an abrupt increase in Yb valence from the mixed-valence state to the trivalent state. ${\mathrm{Yb}}_{0.50}$${\mathrm{Ag}}_{0.50}$${\mathrm{Cu}}_2$ shows an appreciable upturn of magnetization and a gradual volume shrinkage, corresponding to a gradual Yb valence change toward the trivalent state also in the dense Kondo system.

• Received 3 November 1987

DOI:https://doi.org/10.1103/PhysRevLett.60.851