Sublattice Magnetization of Quadratic-Layer Antiferromagnets

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Abstract
Both the temperature dependence and zero-temperature value of the sublattice magnetization have been investigated for the quadratic-layer antiferromagnets K2NiF4, K2MnF4, and Rb2MnF4 using magnetic resonance techniques. The temperature dependence has been determined by measuring the NMR frequency of out-of-layer 19F nuclei in these compounds. The results are found to agree with a simple two-dimensional spin-wave theory for T⪝TN/2, with TN = Néel temperature. For K2NiF4, the zero-point spin reduction Δ0 has been estimated by combining these results with the exchange constant obtained from neutron experiments, giving Δ0 = 0.20±0.03. For the Mn2+ compounds Δ0 has been determined by measuring the 55Mn NMR at helium temperatures with an ENDOR technique, giving Δ0 = 0.17±0.03. The zero-point spin reduction values are in excellent agreement with spin-wave theory. These as well as related results from the literature on other compounds will be discussed.