Doped YbRh2Si2: Not Only Ferromagnetic Correlations but Ferromagnetic Order

YbRh2Si2 is a prototypical system for studying unconventional antiferromagnetic quantum criticality. However, ferromagnetic correlations are present which can be enhanced via isoelectronic cobalt substitution for rhodium in Yb(Rh1-xCox)(2)Si-2. So far, the magnetic order with increasing x was believed to remain antiferromagnetic. Here, we present the discovery of ferromagnetism for x = 0: 27 below T-C = 1: 30 K in single crystalline samples. Unexpectedly, ordering occurs along the c axis, the hard crystalline electric field direction, where the g factor is an order of magnitude smaller than in the basal plane. Although the spontaneous magnetization is only 0.1 mu(B)= Yb it corresponds to the full expected saturation moment along c taking into account partial Kondo screening.