PRECISION MEASUREMENTS OF F19 NMR IN FEF2 FROM 4.2 DEGREES K TO CRITICAL REGION

A precise determination of the temperature dependence of the zero external field F19 NMR frequency in the antiferromagnet FeF2 has been made over the temperature range from 4.2°K to 0.99966TN. Ferrous fluoride, unlike the previously studied antiferromagnet MnF2, has a large single ion magnetic anisotropy. The present data, which were taken on a high-quality single crystal with one millidegree temperature control and measurement techniques, were carefully least-squares fitted to the equation ν19(T)/ν19(0)=D(1-T/TN)β over ranges θ <T/TN<0.99966. For θ=0.995, it is found that β=0.333±0.009, D=1.428±0.067, T N=78.373±0.005, while for θ=0.971, β=0. 320±0.001, D=1.341±0.005, TN=78.367±0.001. The present data, which demonstrate the need for precise measurements and analysis over small ranges near TN, are at variance with the wider-range power law fits reported recently by Wertheim et al. A Kouvel-Fisher plot of the present data shows that the apparent value of β drops rather rapidly for θ<0.95. This behavior is distinctly different from MnF2, where the apparent value of β varies more slowly. Although no analysis of the data below the critical region has yet been made, its accuracy should prove helpful for comparison with spin-wave interaction theories. © 1969 The American Institute of Physics.