Magnetic properties ofR2Mn2O7pyrochlore rare-earth solid solutions

Abstract

Three ${(R,{R}^{\ensuremath{'}})}_{2}{\text{Mn}}_{2}{\text{O}}_{7}$ ferromagnetic pyrochlore systems were studied to investigate the role of the ${R}^{3+}$ ionic size versus $4f$ moment on the magnetic properties of the ${\text{Mn}}_{2}{\text{O}}_{7}$ sublattice. The Curie temperature ${T}_{\text{C}}=18\ifmmode\pm\else\textpm\fi{}1\text{ }\text{K}$ for $R=\text{Y}$ and Lu remained nearly constant for ${({\text{Y}}_{1\ensuremath{-}x}{\text{Lu}}_{x})}_{2}{\text{Mn}}_{2}{\text{O}}_{7}$ but the magnetization data show characteristics of spin-glass behavior in low magnetic fields, and at 5 T, the magnetization fails to reach the expected $3\text{ }{\ensuremath{\mu}}_{\text{B}}/{\text{Mn}}^{4+}$ found by 0.5 T in ${\text{Tl}}_{2}{\text{Mn}}_{2}{\text{O}}_{7}$ and ${\text{In}}_{2}{\text{Mn}}_{2}{\text{O}}_{7}$. A frustrated, minor antiferromagnetic component apparently competes with the ferromagnetic component of the ${\text{Mn}}^{4+}{\text{-O-Mn}}^{4+}$ interactions to give a minor antiferromagnetic component to the major ferromagnetic spin alignment of the ${\text{Mn}}^{4+}$ ions. A ${T}_{\text{C}}=42\ifmmode\pm\else\textpm\fi{}1\text{ }\text{K}$ for $R=\text{Dy}$ and Yb remains nearly constant in the ${({\text{Dy}}_{1\ensuremath{-}y}{\text{Yb}}_{y})}_{2}{\text{Mn}}_{2}{\text{O}}_{7}$ system and ${T}_{\text{C}}$ increases systematically from 19 to 42 K with Dy concentration in ${({\text{Dy}}_{1\ensuremath{-}z}{\text{Lu}}_{z})}_{2}{\text{Mn}}_{2}{\text{O}}_{7}$, which clearly shows that a ferromagnetic interaction between the ${R}^{3+}$-ion and ${\text{Mn}}^{4+}$-ion spins favors ferromagnetic alignment of the ${\text{Mn}}^{4+}$-ion spins to double the magnitude of ${T}_{\text{C}}$ by relieving the frustration of the ${\text{Mn}}_{2}{\text{O}}_{7}$ sublattice. The ${R}^{3+}$-ion size has little effect as the Mn-O-Mn bond angle changes by no more than $\ensuremath{\sim}1\ifmmode^\circ\else\textdegree\fi{}$ with changing ionic radius of the ${R}^{3+}$ ion.