New magnetically ordered phases in YbRh2Si2

YbRh2Si2 is a widely studied heavy Fermion system with a variety of interesting properties, among them a quantum critical point at 60 mT for B // a,b-plane 1. At low temperatures we previously found a new magnetically ordered phase below 2.2 mK 2 taking place in the well known antiferromagnetic phase which starts at 70 mK 3. In addition to this “A” phase, we now observe a second, weaker phase transition below 14 mK and 5 mT which we named B phase. To study these phases of YbRh2Si2 at the lowest possible temperatures, we measured the DC magnetization in magnetic fields up to 60 mT using a home made rf SQUID magnetometer. The A phase is very robust and extends at least up to a magnetic field of 23 mT. Above this field, it is suppressed to below our detection limit of 1 mK. The A transition is characterized by a reduced magnetic susceptibility in the ordered state and there is zero magnetization in the limit B  0. From these findings, we expect it to be an antiferromagnetic ordered state. The experiments were performed in our low temperature cryostat at the Walther Meissner Institute in Garching which includes a 0.9-mole PrNi5 nuclear demagnetization stage with a final temperature of 400 µK. The temperatures of the nuclear stage were measured by pulsed NMR on a Cu rod thermally anchored to it. YbRh2Si2 single crystals of very high quality have been grown in an Indium flux at the MPI CPfS Dresden. The samples were clamped in a 5 N Ag rod screwed to the nuclear stage and extending into the pick-up coil of a conventional flux transformer. Their temperatures were always close to that of the nuclear stage. In addition, we performed specific heat measurements with the semiadiabatic heat-pulse method.