Structural and magnetic phase transitions inCeCu6−xTx(T=Ag,Pd)

Abstract

The structural and the magnetic properties of ${\mathrm{CeCu}}_{6\ensuremath{-}x}{\mathrm{Ag}}_{x}$ $(0\ensuremath{\le}x\ensuremath{\le}0.85)$ and ${\mathrm{CeCu}}_{6\ensuremath{-}x}{\mathrm{Pd}}_{x}$ $(0\ensuremath{\le}x\ensuremath{\le}0.4)$ have been studied using neutron diffraction, resonant ultrasound spectroscopy (RUS), x-ray diffraction measurements, and first principles calculations. The structural and magnetic phase diagrams of ${\mathrm{CeCu}}_{6\ensuremath{-}x}{\mathrm{Ag}}_{x}$ and ${\mathrm{CeCu}}_{6\ensuremath{-}x}{\mathrm{Pd}}_{x}$ as a function of Ag/Pd composition are reported. The end member, ${\mathrm{CeCu}}_{6}$, undergoes a structural phase transition from an orthorhombic $(Pnma)$ to a monoclinic $(P{2}_{1}/c)$ phase at 240 K. In ${\mathrm{CeCu}}_{6\ensuremath{-}x}{\mathrm{Ag}}_{x}$, the structural phase transition temperature $({T}_{s})$ decreases linearly with Ag concentration and extrapolates to zero at ${x}_{S}$ $\ensuremath{\approx}0.1$. The structural transition in ${\mathrm{CeCu}}_{6\ensuremath{-}x}{\mathrm{Pd}}_{x}$ remains unperturbed with Pd substitution within the range of our study. The lattice constant $b$ slightly decreases with Ag/Pd doping, whereas $a$ and $c$ increase with an overall increase in the unit cell volume. Both systems, ${\mathrm{CeCu}}_{6\ensuremath{-}x}{\mathrm{Ag}}_{x}$ and ${\mathrm{CeCu}}_{6\ensuremath{-}x}{\mathrm{Pd}}_{x}$, exhibit a magnetic quantum critical point (QCP), at $x\ensuremath{\approx}0.2$ and $x\ensuremath{\approx}0.05$, respectively. Near the QCP, long range antiferromagnetic ordering takes place at an incommensurate wave vector $({\ensuremath{\delta}}_{1}0$ ${\ensuremath{\delta}}_{2})$, where ${\ensuremath{\delta}}_{1}\ensuremath{\sim}0.62,{\ensuremath{\delta}}_{2}\ensuremath{\sim}0.25,x=0.125$ for ${\mathrm{CeCu}}_{6\ensuremath{-}x}{\mathrm{Pd}}_{x}$ and ${\ensuremath{\delta}}_{1}\ensuremath{\sim}0.64,{\ensuremath{\delta}}_{2}\ensuremath{\sim}0.3,x=0.3$ for ${\mathrm{CeCu}}_{6\ensuremath{-}x}{\mathrm{Ag}}_{x}$. The magnetic structure consists of an amplitude modulation of the Ce moments which are aligned along the $c$ axis of the orthorhombic unit cell.