Changes in the position and shape of paramagnetic absorption lines of Nd³⁺ in neodymium ethyl sulfate, Ni²⁺ in nickel fluosilicate, and Cu²⁺ in copper potassium sulfate have been observed at 30 kMc/sec in the temperature range from 0.4 to 4.2^∘{}K. The observed shifts are compared to first and second line moments calculated from spin-spin interactions when the average populations of the spin levels are weighted by the proper Boltzmann factors. In this manner it is possible to determine the magnitude and sign of the exchange interaction Hᵢⱼ=ASᵢ·Sⱼ between spins. We find a ferromagnetic interaction A=-(3.9±0.4)×10^-18 erg between nearest-neighbor Ni²⁺ ions in NiSiF₆·{}6H₂O with no evidence of anisotropic exchange. Nearest-neighbor spins at similar lattice positions in K₂Cu(SO₄)₂·{}6H₂O appear to be coupled by an exchange interaction of approximately A=-9×10^-18 erg, while the exchange between the two neighboring dissimilar Cu²⁺ ions in the unit cell is A=-1.4×10^-18 erg. Evidence for a weak anisotropic and antiferromagnetic coupling in Nd(C₂H₅SO₄)₃·{}9H₂O is presented. We have observed magnetostatic modes in copper salts at low temperature, and this effect is briefly discussed in relation to the present type of experiment.
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Svare et al. (1964) studied this question.
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