Single crystal M�ssbauer spectroscopy on the three principal sections of a synthetic fayalite sample in the antiferromagnetic state

Application of a Difference Electron Nanoscope (DEN) – a computer hard‐ and software to display 3D electron densities and magnetical structures based on spectroscopic and diffractometric data

“…Here, the orientation and size of the internal magnetic field H(0) with respect to the main axes of the EFG can be detected in favourable cases. This field direction is strongly correlated with the spatial orientation of the magnetic moments μ from magnetometry or neutron diffractometry, as has been proved , e.g., in the case of synthetic fayalite . We even managed to give an – up to this time – unknown explanation for the second magnetic transition at low temperatures which had been escaped the neutron diffraction data interpretation .…”

“…Through these two quantities ΔE Q and η it is possible to derive the size and direction of the EFG within the crystallographic axes system by refining two angle parameters β and α from the Mössbauer spectra of single crystal thin sections. The arduous procedure to obtain reliable data has been described elsewhere – in the end we were able to present a table with all 3 EFG components with respect to the 3 crystallographic axes (9 elements) for both sites M1 and M2 . This experimental EFG should be valid not only for ambient and moderate temperatures but for the low‐temperature case (in the antiferromagnetic state) as well since only a small shift of the EFG axes has been observed there .…”

A low-temperature calorimetric study of synthetic (forsterite+fayalite) {(Mg2SiO4+Fe2SiO4)} solid solutions: An analysis of vibrational, magnetic, and electronic contributions to the molar heat capacity and entropy of mixing

“…Robie et al [31], using the work of Santoro et al [14], assigned a low temperature feature around 16 K in their heat capacity data to the same effect. In later work Lottermoser et al [17] concluded, based largely on single-crystal Mö ssbauer measurements and neutron diffraction results on fayalite [58], that the physical explanation for the transition as proposed by [31] is not correct, but instead a discontinuity in the temperature dependent canting along the b-and c-axes should cause both the low temperature features in the heat capacity and magnetic susceptibility. The susceptibility measurements were interpreted as indicating that for olivine compositions between Fo 40 Fa 60 and Fo 60 Fa 40 the magnetic phase transition occurs directly from the paramagnetic to the canted spin state [21].…”

“…The higher temperature feature they proposed was related to the paramagnetic-antiferromagnetic transition and the lower temperature feature was presumably related to a transition from a collinear to a canted-spin structure [21] as suggested for fayalite [14]. It was subsequently proposed [17] that a discontinuity in temperature dependent canting angle along the b-and c-axes could account for the lower temperature feature. Hoye and O'Reilly [21] reported magnetic order for all compositions more iron-rich than Fo 60 Fa 40 .…”

“…The canting angle between the magnetic moment of Fe 2+ at the M1 site and the b axis increases with decreasing temperature from about 16°at 50 K to 40°at 10 K [15,16] (figure 2). Further Mö ssbauer spectroscopic investigations conducted on single-crystals at low temperatures, focused on the nature of the electric field gradient around Fe 2+ in fayalite [17,18]. Above T N short-range magnetic order has experimentally been observed at least up to 80 K [19] or 130 K [11] and could potentially exist at temperatures as high as roughly 500 K based on Raman spectra recorded at different temperatures [20].…”