Low-temperature magnetic order and spin dynamics in YbRh2Si2

Muon spin rotation and relaxation experiments have been carried out in single crystals of YbRh2Si2, a compound that exhibits non-Fermi-liquid (NFL) behavior associated with a quantum critical point (QCP) at T=0. The zero-field muon relaxation rate is found to be independent of temperature down to 100 mK but to increase below similar to70 mK, which suggests magnetic order at low temperatures. From the relation between the internal field at the mu(+) stopping site and the hyperfine coupling constant the ordered Yb3+ moment is very small, similar to2x10(-3)mu(B). Muon spin rotation linewidths in a transverse field of 6 kOe indicate a homogeneous susceptibility down to 2 K, which is an order of magnitude lower than the characteristic (Kondo) temperature T(K)approximate to25 K. This is evidence against the importance of disorder-driven NFL mechanisms in YbRh2Si2. In longitudinal magnetic fields the muon spin-lattice relaxation function G(t) is exponential, again indicative of a homogeneous system. The relaxation obeys the time-field scaling relation G(t,H)=G(t/H), which suggests long-lived spin correlations at low temperatures. The Yb3+ spin dynamics derived from muon spin relaxation appear to be intimately related to critical magnetic fluctuations near the QCP.