More evidence for magnetic ordering inCeCu6at mK temperatures

Abstract

Dc-magnetization measurements in various magnetic fields on two samples from the same single crystal of ${\mathrm{CeCu}}_6$ in a superconducting quantum interference device system show a plateau of the magnetic susceptibility between 400 and 50 mK, followed by a Curie-like increase towards lower temperatures. At very low temperatures the magnetization starts deviating from a 1/T behavior, saturating around 900 μK in 2.7 mT, the highest B/T ratio obtained in this experiment. We show that this Brillouin behavior must be due to a tiny amount of the magnetic ion ${\mathrm{Gd}}^{3+}$, present in a concentration of 1.5(3) ppm. After subtracting this contribution from the magnetization curves in various lower fields, a field-dependent structure with a drop of the magnetic susceptibility around 3 mK is left. This decrease is reduced with increasing field and has vanished in 2.7 mT. In the lowest fields the structure is very similar to one observed in ac-susceptibility measurements. The fact that this drop of χ occurs in different samples and that it can be quenched by the magnetic field points to an intrinsic antiferromagnetic ordering in ${\mathrm{CeCu}}_6$ around 3 mK. We cooled one sample to a lowest temperature of ${450}_{\mathrm{-}10}^{+40\mathrm{}}$ μK in a field of 0.3 mT. The susceptibility further decreases with decreasing temperature and shows a small shoulder at 650 μK. In addition, we measured the specific-heat capacity of the same single crystal of ${\mathrm{CeCu}}_6$ in zero magnetic field down to a temperature of 11 mK. Between 40 and 11 mK c/T increases from 1.55 to 2.8 J/mole ${\mathrm{K}}^2$, most likely an indication of the high-temperature end of the magnetic transition.