A study of magnetic ordering and phase transitions in ferrous iodide by means of high field 57Fe Mössbauer spectroscopy

Abstract

High field 57Fe Mössbauer experiments on single crystals of ferrous iodide (FeI 2) are presented. The spectrometer setup used for these measurements operates in external fields up to 15 T and is described in a detailed experimental section. In this study of the magnetic phase diagram of FeI 2 we found that the electron spin on the Fe 2+-ion is always aligned along the crystallographic c-axis, in the several magnetic phases. These phases are characterized by the ratio of the numbers of antiparallel ( N↓) and parallel ( N↑) spins. Phases I, II, IV and V are given by N↓⧸ N↑ = 1.0, 0.60, 0.33 and 0, respectively. In phase III N↓⧸ N↑ drops continuously from 1.0 to 0.33 for T > 3 K and from 0.60 to 0.33 for T < 3 K. This phase information and the critical fields, at which phase transitions occur, give the opportunity to calculate exchange interaction parameters. This has been done assuming an eight-sublattice model. We conclude that the exchange interactions with the nearest and next nearest iron-neighbours in the ab-plane are very weak ( J 1 = J 2 ≈ 0). For the interaction with the third nearest neighbour we deduce J 3 < —0.8 K. The complex magnetic structure is further determined by exchange interactions with the nearest neighbours in subsequent ab-planes, but the corresponding parameters J′ 1 and J′ 2 cannot be determined unambiguously. The positions of the iodine-ions seem to determine the exchange mechanisms in this compound.