$^{57}$Fe NMR study of layered chalcogenide Fe$_7$S$_8$

The paper presents the results of a nuclear magnetic resonance (NMR) study of the magnetically ordered phase of layered chalcogenide Fe$_7$S$_8$ (pyrrhotite) on iron nuclei $^{57}$Fe in a zero external magnetic field in the temperature range $T$ = 4.2 – 295 K. The obtained NMR spectra on the $^{57}$Fe nuclei represent several lines in the frequency range $\Delta\nu$ = 30 – 50 MHz. The absence of iron ions in the Fe$^{3+}$ state in Fe$_7$S$_8$ is proven. The presence of several magnetically nonequivalent positions of iron ions, differing in the number and location of vacancies near them, is detected. The rates of spin-spin and spin-lattice relaxation of the nuclear magnetic moments of $^{57}$Fe are measured in different frequency regions of the spectrum at different temperatures. Local magnetic fields on iron nuclei are determined and the magnetic moment of Fe ions is estimated. It is shown that the $^{57}$Fe spectra in the Fe$_7$S$_8$ chalcogenide can be interpreted in the 4$C$-type superstructure model. The results obtained in this work expand the prospects for the use of NMR spectroscopy in the mining industry, in particular in the detection, separation and real-time selection of ore fragments containing pyrrhotite Fe$_7$S$_8$.