Strong Mn-Mn distance dependence of the Mn interlayer coupling inSmMn2Ge2-related compounds and its role in magnetic phase transitions

The interplay between the Mn interlayer coupling constant ${\mathit{n}}_{\mathrm{Mn}\mathrm{-}\mathrm{Mn}}$ and the thermal expansion has been investigated by means of thermal-expansion, specific-heat, and magnetization measurements. A linear relation between ${\mathit{n}}_{\mathrm{Mn}\mathrm{-}\mathrm{Mn}}$ and the thermal expansion was established. This linear relation is used in the formulation of a quantitative model for temperature-induced metamagnetic transitions in R${\mathrm{Mn}}_2$ ${\mathrm{Ge}}_2$ compounds. The observed relation between the unit-cell dimensions and the transition temperatures is interpreted in terms of this model. For polycrystalline ${\mathrm{Sm}}_{0.9}$ ${\mathrm{Y}}_{0.1}$ ${\mathrm{Mn}}_2$ ${\mathrm{Ge}}_2$, the relation between the metamagnetic transitions and anomalies in the electrical resistivity was investigated. A magnetic-field-induced change of the resistance of 10% was observed at 185 K. It appears likely that magnetovolume effects are responsible for the anomalous resistance behavior.