Muon spin relaxation and neutron diffraction investigations of quadrupolar and magnetically ordered states of YbRu₂Ge₂

The magnetic- and quadrupolar-ordered states of polycrystalline YbRu ₂Ge₂ have been investigated using zero-field (ZF) muon spin relaxation (μSR) and neutron diffraction measurements. Specific heat measurements show three successive phase transitions with decreasing temperature from a paramagnetic to a quadrupolar state at T₀ ∼ 10 K, from the quadrupolar to a magnetic state at T₁ ∼ 6.5 K, and a possible change in the magnetic ground state at T₂ ∼ 5.5 K. Clear evidence for the magnetic transition below 7 K (spectrum at 8 K reveals paramagnetic state) and a likely change in the magnetic structure near 5.8 K is observed in the ZF μSR measurements. The μSR data, however, do not reveal any signature of magnetic order in the temperature range 8-45 K. This result is further supported by neutron-diffraction measurements, where clear magnetic Bragg peaks have been observed below 8 K, but not above it. Below 8 K, the magnetic Bragg peaks can be characterized by an incommensurate antiferromagnetic (AF) ordering with the propagation vector q = [0.352, 0, 0] and the magnetic moment 2.9(3)μ_B of Yb along the b-axis. These results are discussed in terms of quadrupolar-ordered and magnetically ordered states.