Magnetic susceptibilities, their temperature variation, and exchange constants of NiO

The temperature variation of the magnetic susceptibilities of NiO for a powder sample and a single crystal has been measured from 4.2 to 700 K. The single crystal was stressed along a $〈111〉$ direction on cooling through TN (=524 K) with successive stress magnitudes of 7, 30, and 42 bars, followed in each case by measurements of χ_∥* [→H parallel to a (111) plane with $H=200$ Oe] from 4.2 to 700 K. From the behavior of χ_∥* with stress, a stress of 30 bars was found to be sufficient to completely remove the T domains. With the use of the data of initial powder susceptibility χₚ=(χ_∥+2χ_⊥)3 and χ_∥*=(χ_∥+χ_⊥)2 at various temperatures, the temperature variation of χ_∥ and χ_⊥, the principal susceptibilities, is determined. After correcting for χᵥᵥ and χd (the Van Vleck and diamagnetic susceptibilities) with χᵥᵥ+χd=3.2×10^-6 cm³/g, χ(TN)=8.8×10^-6 cm³/g, χ_⊥(0)=6.8×10^-6 cm³/g, and χ_∥(0)=0 are determined. It is shown that the spin-wave theory correctly gives the ratio χ(TN)χ_⊥(0) and the temperature dependence of χ_∥ to about 200 K if the experimental value of the spin reduction ΔS=0.19 is used for Ni²⁺. From the experimental values of χ(TN) and TN and the use of random-phase-approximation Green's-function theory, the magnitudes of exchange constants for NiO, viz. J₁=34 K and J₂=202 K, are determined.