19F NMR studies in ABF4-type layered antiferromagnets.

Abstract

$^{19}\mathrm{F}$ NMR studies of polycrystalline ${\mathrm{NH}}_{4}$${\mathrm{FeF}}_{4}$, ${\mathrm{RbFeF}}_{4}$, and ${\mathrm{KFeF}}_{4}$ in the paramagnetic phase reveal the existence of two distinct types of fluorine sites. From the line-shift measurements together with the available magnetic susceptibility data, the value for the transferred hyperfine coupling between the fluorine nuclear spin and the neighboring iron electron spin has been derived. The values of the hyperfine tensor components indicate the almost isotropic nature of the Fe-F bond in the three compounds. Furthermore, the hyperfine fields at both types of fluorine sites are found to be almost temperature independent in the paramagnetic phase, which is in agreement with the susceptibility behavior of these compounds. Thus apparently no difference is reflected in the behavior of the time-averaged local properties as a function of temperature among the three compounds. However, an essentially different behavior of the linewidth with temperature of the fluorine atoms [(F(1)] bonded with one iron neighbor and fluorine atoms [F(2)] bonded with two iron neighbors is observed. In all the compounds the resonance lines for both types of fluorines are broadened anomalously below certain temperatures and finally both the lines disappear at ${\mathit{T}}_{\mathit{N}}$. Although the transition temperatures (${\mathit{T}}_{\mathit{N}}$) of the three compounds are very close among themselves (${\mathit{T}}_{\mathit{N}}$ lie in the range 133.5--136 K), the anomalous line broadening appears in widely different temperature regions.These results are compared with those reported earlier for ${\mathrm{CsFeF}}_{4}$. Moreover, an attempt has been made to correlate the temperature-dependent linewidth of the F(1) line with the effect of a critical slowing down of the fluctuations of the electronic spins of the ${\mathrm{Fe}}^{3+}$ ion with a lowering of the temperature. The temperature-dependent linewidth \ensuremath{\delta}${\mathit{H}}_{1}$(T) is found to fit well with the relation \ensuremath{\delta}${\mathit{H}}_{1}$(T)\ensuremath{\propto}${\mathrm{\ensuremath{\epsilon}}}^{\mathrm{\ensuremath{-}}\mathit{w}}$ in the critical regions, where \ensuremath{\epsilon}=(T-${\mathit{T}}_{\mathit{N}}$)/${\mathit{T}}_{\mathit{N}}$ is the reduced temperature. The exponent w involves, besides two static exponents \ensuremath{\gamma} and \ensuremath{\nu}, the dynamic exponent z and is related by w=-\ensuremath{\gamma}+\ensuremath{\nu}(d-z). A comparison of the values of w with that predicted by the current theory of critical dynamics indicates that the magnetic interactions in ${\mathrm{NH}}_{4}$${\mathrm{FeF}}_{4}$, ${\mathrm{RbFeF}}_{4}$, and ${\mathrm{CsFeF}}_{4}$ are three dimensional in nature, whereas in ${\mathrm{KFeF}}_{4}$ the two-dimensional (2D) interactions are predominant. Nevertheless, the presence of a broad critical region even in each of the first three systems as observed from the linewidth data indicates that the 2D interactions are still present there.