Magnetic field dependence of the antiferromagnetic order in YbCo2Si2

One of the model compounds to study quantum critical behaviour is YbRh2Si2, which orders antiferromagnetically at a very low TN∼0.07 K. Due to the small ordered moment and the high neutron absorption of Rh the magnetic structure of YbRh2Si2 is still unknown. YbCo2Si2 with its body-centered tetragonal crystal structure is isoelectronic to YbRh2Si2 and serves as a reference system. YbCo2Si2 orders antiferromagnetically below TN=1.7 K with an additional first order transition at TL=0.9 K. This compound exhibits a complex magnetic (B,T) phase diagram for applied magnetic field and shows a pronounced basal plane anisotropy observed in magnetisation and magnetoresistance measurements. While at lowest temperature the magnetic order is commensurate with τ1= (0.25 0.25 1), the magnetic structure becomes incommensurate above TL with τ2= (0.08 0.25 1). Using neutron diffraction we studied the magnetic structure in magnetic fields applied along [100], [110] and [001]. While in the commensurate phase no hysteresis was observed for all three directions of applied magnetic fields, a pronounced hysteresis of the intensity was detected in the incommensurate phase for fields along [100], which most likely can be attributed to different domain population.