NMR of Co⁵⁹ in an Approximate Ising System : CoCl₂·2H₂O

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Nuclear Magnetic Resonance (NMR) of Co⁵⁹ has been investigated in the three metamagnetic phases of CoCl₂·2H₂O at liquid helium temperatures. The hyperfine field in zero external field at 0 K was found to be +421.1±0.4 kOe, which agrees well with the calculated value where the orbital contribution is dominant. When the external field H₀ was applied parallel to b-axis, the interaction of the nucleus with the electronic moment induced by H₀ was observed to be about 0.29 times the direct coupling with the external field. The jumps of the hyperfine field were also observed as the manifestation of the changes of the exchange interactions of Co²⁺ spin with adjacent linear chains. The most striking feature of the data is the effects of the spin reduction of 0.1∼1% depending on H₀ caused by the magnon bound states in an approximate Ising system to CoCl₂·2H₂O.