Doping-induced quantum cross-over in Er${}_2$Ti${}_{2-x}$Sn${}_x$O${}_7$

We present the results of the investigation of magnetic properties of the Er${}_2$Ti${}_{2-x}$Sn${}_x$O${}_7$ series. For small doping values the ordering temperature decreases linearly with $x$ while the moment configuration remains the same as in the $x=0$ parent compound. Around $x=1.7$ doping level we observe a change in the behavior, where the ordering temperature starts to increase and new magnetic Bragg peaks appear. For the first time we present evidence of a long-range order (LRO) in Er${}_2$Sn${}_2$O${}_7$ ($x=2.0$) below $T_N=130$ mK. It is revealed that the moment configuration corresponds to a Palmer-Chalker type with a value of the magnetic moment significantly renormalized compared to $x=0$. We discuss our results in the framework of a possible quantum phase transition occurring close to $x=1.7$.