Exchange interactions in Ca3Co2O6 probed locally by NMR

We have measured the nuclear spin-lattice relaxation of 59Co in the geometrically frustrated Ising-type spin-chain system Ca3Co2O6 as a function of temperature and applied magnetic field. In the nearly-saturated ferrimagnetic and ferromagnetic regimes considered here, the field-dependent energy scales governing the spin-lattice relaxation rates reflect the energy cost of a spin flip in the Co ions adjacent to the probed nuclei. This results in a thermally activated form for the nuclear relaxation rates, which can be exploited as a local probe of the exchange interactions. In particular, the measurements enable the values of intra- and interchain exchange constants J1, J2+J3 to be directly extracted from the data. By using a quasi-one-dimensional model, we then determine the value of J2 consistent with the spin-density-wave order observed in zero field.