Magnetic Resonance in Antiferromagnetic FeCl2·4H2O

Abstract

The electronic magnetic resonance spectrum of single crystals of Fe${\mathrm{Cl}}_{2}$\ifmmode\cdot\else\textperiodcentered\fi{}4${\mathrm{H}}_{2}$O has been studied in the microwave region from 9 to 76 GHz at temperatures between 0.38 and 4.2 K. Two distinct types of resonance characteristic of the antiferromagnetic state were observed, each of which is associated with a pair of highly anisotropic $g$ tensors (${g}_{\ensuremath{\perp}}\ensuremath{\simeq}0$) with principal axes\ifmmode\pm\else\textpm\fi{}30\ifmmode^\circ\else\textdegree\fi{} from the $b$ axis in the ${a}^{\ensuremath{'}}\ensuremath{-}b$ plane. One of these resonance pairs is easily distinguishable from the other, since its integrated resonance intensity is some three orders of magnitude larger. It is an unusual antiferromagnetic resonance in that the frequency-field relation is temperature-independent, whereas the resonance intensity is strongly temperature-dependent. The intensity is greatest at the lowest temperatures and decreases rapidly as the N\'eel temperature is approached, with a small remanent signal observable even above ${T}_{N}$. The zero-field resonance was observed at 72.5 GHz. No spin-flop resonance was observed. The weak-intensity resonances, which we denote as Ising spin resonances because of the very large $g$ anisotropy, are similar in some respects to the spin-cluster resonances observed in Co${\mathrm{Cl}}_{2}$\ifmmode\cdot\else\textperiodcentered\fi{}2${\mathrm{H}}_{2}$O and Fe${\mathrm{Cl}}_{2}$, but are characterized by a strong local antiferromagnetic interaction rather than the dominant ferromagnetic interaction of those other materials. Resonances corresponding to many of the possible local spin configurations were measured, and the temperature dependence of the resonance intensities was determined. In addition, resonances associated with a small concentration of Mn impurities were observed.