Abstract
The D -value of Mn 2+ in CoCl 2 ·2H 2 O has been obtained to be -0.13 cm -1 by Date and Motokawa. The large negative D value is reasonably explained by introducing an effective exchange interaction of general form between the Mn 2+ and the ground Kramers doublet of Co 2+ , taking account of the crystal symmetry. It is shown that the interaction -2 J x z s x S z -2 J y z s y S z is responsible for the negative D -parameter. The effective exchange interaction is derived from the nondiagonal exchange between Mn 2+ and Co 2+ with respect to the cobaltous orbits by projecting the exchange to the ground Kramers doublet.

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