High-field phase diagram of the heavy-fermion metal YbRh2Si2

The tetragonal heavy-fermion (HF) metal YbRh2Si2 ( Kondo temperature T-K approximate to 25 K) exhibits a magnetic field-induced quantum critical point related to the suppression of very weak antiferromagnetic (AF) ordering (T-N = 70 mK) at a critical field of B-c = 0.06 T (B perpendicular to c). To understand the influence of magnetic fields on quantum criticality and the Kondo effect, we study the evolution of various thermodynamic and magnetic properties upon tuning the system by magnetic field. At B > B-c, the AF component of the quantum critical fluctuations becomes suppressed, and FM fluctuations dominate. Their polarization with magnetic field gives rise to a large increase of the magnetization. At B* = 10 T, the Zeeman energy becomes comparable to k(B)T(K), and a steplike decrease of the quasi-particle mass deduced from the specific-heat coefficient indicates the suppression of HF behaviour. The magnetization M(B) shows a pronounced decrease in slope at B* without any signature of metamagnetism. The field dependence of the linear magnetostriction coefficient suggests an increase of the Yb-valency with field, reaching 3+ at high fields. A negative hydrostatic pressure dependence of B* is found, similar to that of the Kondo temperature. We also compare the magnetization behaviour in pulsed fields up to 50 T with that of the isoelectronic HF system YbIr(2)S(i)2, which, due to a larger unit-cell volume, has an enhanced T-K of about 40 K.