Low temperature investigations of YbRh2Si2 doped with Ir and Co

The tetragonal heavy fermion system YbRh2Si2 shows interesting properties due to its vicinity to an antiferromagnetic quantum critical point (QCP). At zero magnetic field the system becomes antiferromagnetic below TN=70 mK. The system can be tuned through a field-induced QCP at Bc=0.05 T into a Landau-Fermi liquid (LFL) state at B>Bc. This state is restricted to temperatures below a scale TLFL(B). Recently, an additional energy scale T*(B) was found that vanishes at the QCP and was related to the disintegration of the heavy quasiparticles. We studied the effect of chemical pressure on the phase diagram, using high quality Yb(Rh1-xMx)2Si2 single crystals with M=Ir,Co. Here, we present recent results focusing on measurements of the magnetic susceptibility