Anomalous high-field magnetization and negative forced volume magnetostriction in ${\mathrm{Yb}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{M}}_{\mathrm{x}}$${\mathrm{Cu}}_2$ (M=In and Ag)—evidence for valence change in high magnetic fields

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.