NUCLEAR MAGNETIC RESONANCE IN PARAMAGNETIC K2NIF4

Lineshift and linewidth of the 19F NMR of a single crystal of K2NiF4 have been measured in the paramagnetic region between 100 and 300 K. From the lineshift together with magnetic susceptibility data, the value for the hyperfine coupling between fluorine nuclear and the neighbouring nickel electron spin has been derived. An essentially different behaviour of the line width as function of temperature of the fluorines (FI) with one nickel neighbour and fluorines (FII) with two nickel neighbours is observed. This effect can be explained in terms of temperature dependent short range order effects which in this case of a crystal containing fairly isolated layers of magnetic atoms become very pronounced. The correlation function 〈S1·S2〉 for two neighbouring nickel spins has been computed from the observed data. The critical behaviour of the linewidth of FI and FII fluorines gives a transition point of 100.5 K for K2NiF4, which value agrees with the value predicted by Stanley and Kaplan for the two-dimensional Heisenberg ferromagnet. From the linewidth of the FI ions follows the fluctuation frequency of the Ni electron spins having a temperature dependence ∼{(T - TN)/TN}0.54 above TN. The transition points found for Rb2NiF4 and Tl2NiF4 are 90.4 and 100.8 K respectively. A large (0.5%) isotropic shift of the Tl nuclei has been observed in the paramagnetic region. © 1969.