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The intensity of the exciton-magnon combination band at 631 nm in the ionic ferromagnet Rb 2 CrCl 4 has been measured as a function of temperature from 0.85 to 3.6K and as a function of magnetic field (up to 5 T in the (100) direction) at temperatures from 2.1 to 4.2K. The zone-centre magnon gap is estimated as 1.4+or-0.1K from the zero-field measurements and 0.9+or-0.4K from the field-dependent ones. From the latter the authors also find g=2.1+or-0.1.
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Day et al. (1979) studied this question.
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