Evolution of ferromagnetic order in Yb(Rh1-xCox)2Si2 with x ≤ 0.27

Antiferromagnetic (AFM) order appears in YbRh2Si2 at TN = 70 mK although strong ferromagnetic fluctuations are observed. The application of chemical pressure x in the series Yb(Rh1-xCox)2Si2 stabilizes the AFM phase, i.e. TN increases with increasing x. A second phase transition at TL < TN emerges with increasing x and the strength of the ferromagnetic fluctuations raises up to a maximum for x ∼ 0.27 Eventually, in pure YbCo2Si2 both phase transitions TN and TL are of AFM nature with 4f local moments aligned along the crystallographic ab-plane. However, the sample with x = 0.27 reveals only one phase transition at 1.30 K which is surprisingly ferromagnetic with moments parallel to the c-axis. This motivates a deeper investigation of the magnetic properties in Yb(Rh1-xCox)2Si2 with the magnetic field along the c-axis. Here, we present ac-susceptibilty measurements of single crystals with x ≤ 0.27 down to a temperature of 25 mK.