  We report on a 59Co NMR investigation of the trigonal cobaltate Ca3Co2O6
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 non-magnetic 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 intra-chain 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 multi-step behavior reported in
the magnetization is discussed.
