Chemical Control of Spin Chirality in $(N{d}_{1-x}D{y}_x{)}_{2}M{o}_2{O}_7$

Magnetization and Hall resistivity have been investigated for single crystals of Dy-doped Nd${}_2$Mo${}_2$O${}_7$. A sharp decrease of the Hall resistivity upon a metamagnetic transition, perhaps associated with magnetic-field ($H$) induced flop of Dy${}^{3+}$ moments, has been observed in the Dy-doped crystals only for $H\parallel [111]$ direction. In addition, the sign of the Hall resistivity at a high field, both for $H\parallel [100]$ and for $H \parallel [111]$, changes with the Dy-doping. These results are explained in terms of the sign change of Mo spin chirality that is controlled by the Dy${}^{3+}$ moments with a different sign of $f$-$d$ interaction from the Nd${}^{3+}$ moments.