Magnetic properties and hyperfine interactions of iron-borate single crystals and nanoparticles

We discuss the development of a correct theory of Mössbauer spectra of an iron borate single crystal, starting with a simple, but desperately needed technique of correction of any Mössbauer spectrum for the absorber (crystal) thickness, which is followed by the treatment of the Mössbauer spectra in the framework of the Hamiltonian of combined hyperfine magnetic and quadrupole interaction of 57Fe nuclei in ground and excited states. The latter results in appearance of two additional spectral lines to a conventional magnetic sextet and describes the specific asymmetry of pairs of lines observed in low-temperature experimental spectra of FeBO3 crystals. We demonstrate and explain also the characteristic difference in the shape of Mössbauer spectra of the FeBO3 single crystal and nanoparticles. A drastic difference in the shapes of magnetization curves of the FeBO3 single crystal and nanoparticles is demonstrated and evaluated in the framework of the generalized Stoner-Wohlfarth model which is extended for antiferromagnetic nanoparticles.