Mössbauer and magnetization studies of Fe3BO5

We report on magnetometry, resistivity and Mössbauer spectroscopy measurements on single-phase Fe 3 BO 5 in a wide temperature range from 4.2 to 620 K. Magnetically split Mössbauer spectra were observed below T N = 114 K, although a proportion of the Fe 2+ ions in the basal plane (crystallographic site Fe1) remain paramagnetic down to the ferrimagnetic transition at T w = 74 K. This unusual magnetic behaviour known as `idle-spin behaviour' can be explained by the competitive magnetic interactions with its neighbouring ordered ions and should be related to the topology of the octahedral Fe1 sites in the crystal structure of Fe 3 BO 5 . The behaviour change recorded in the dc magnetization and Mössbauer data indicates that the magnetic structure of Fe 3 BO 5 should change at 74 K, where the spin frustration gradually ceases to exist. Electron delocalization in the Fe3-Fe2-Fe3 triads between pairs of Fe 2+ and Fe 3+ ions was observed at all temperatures between 4.2 and 180 K and coexists with localized ferric and ferrous (Fe1 and Fe4 crystallographic sites) states. Above 180 K the delocalization in the Fe3-Fe2-Fe3 triads includes the rest of the Fe 3+ ions and some evidence can be seen in the resistivity data as well.