NMR study of magnetic order, metamagnetic transitions, and low-temperature spin freezing in Ca3Co2O6

We report on a Co59 NMR investigation of the trigonal cobaltate Ca 3Co 2O 6 carried out on a single crystal, providing precise determinations of the electric field gradient and chemical shift tensors, and of the internal magnetic fields at the nonmagnetic Co I sites, unavailable from former studies on powders. The magnetic-field-induced ferri- and ferromagnetic phases at intermediate temperature (e.g., 10 K) are identified by distinct internal fields, well accounted for by purely dipolar interactions. The vanishing transferred hyperfine field at the Co I site indicates that the Co3 + (I) orbitals do not participate in the intrachain superexchange, in disagreement with a previous theoretical model. The strong Ising character of the system is confirmed experimentally by the field dependence of the resonance lines, indicating that local moments are saturated even at the phase boundaries. In the vicinity of the critical fields, nuclear spin-spin relaxations detect the spin-reversal dynamics of large magnetic assemblies, either Ising chain fragments or finite-size domains, which drive the metamagnetic transitions. Such collective excitations exhibit a glassy behavior, slowing down to subacoustic frequencies and freezing at low temperature. The relevance of such slow-fluctuation modes for the low-temperature multistep behavior reported in the magnetization is discussed.