Magnetization measurements on YbRh₂Si₂ at very low temperatures

YbRh₂Si₂, a heavy fermion compound, is in the center of interest for its unconventional behavior around a quantum critical point (QCP) which can be tuned by a magnetic field. To study the system at the lowest possible temperatures, we have measured the dc magnetization in magnetic fields up to 60 mT and down to 1 mK using an rf SQUID magnetometer. Our fields were high enough to reach the QCP in the B (a, b) crystal orientation. Both field cooled (fc) and zero field cooled (zfc) data were taken. We found a sharp transition to a low magnetization ground state at 2.2 mK and differences between the fc - zfc traces below 11 mK. In this temperature range down to 2.2 mK an additional increase of the magnetization is seen, just before the final drop sets in. These results will be discussed with respect to the nature of the ground state in low magnetic fields and its relation to the QCP.