Anomalous spin dynamics in CdCu2(BO3)(2) revealed by B-11 NMR and ZF-mu SR

Abstract

We report B-11 nuclear magnetic resonance (NMR) and zero-field muon spin relaxation (ZF-mu SR) measurements of the anisotropic spin tetramer system CdCu2(BO3)(2), which consists of strongly interacting Cu(1) dimers and weakly coupled nonfrustrated Cu(2) spins. Long-range magnetic order is observed at T-N = 9.8 K by a critical divergence of the nuclear spin-lattice relaxation rate 1/T-1 and an appearance of well-defined muon-spin precessions. In addition, we find the spectroscopic signature for a magnetic anomaly at T* = 6.5 K, a few kelvins below TN. For temperatures below T*, 1/T-1 ceases to follow a T-4.9(1) behavior. Instead, upon cooling below T*, both 1/T-1 and 1/T-2 become temperature-independent with a subsequent small drop. The muon frequency and the relative fraction of the Cu magnetic sites show anomalies mostly for Cu(2) spins at T*. This is ascribed to a Cu(2) spin reorientation. Site-specific change in magnetic structure is discussed in terms of the energy hierarchy of involved exchange interactions and an additional intertetramer interaction that controls magnetism at temperature T*.