Title: Doping-induced quantum cross-over in Er$_2$Ti$_{2-x}$Sn$_x$O$_7$,
Abstract: 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 $x = 1.7$.