We performed a full series of DC and AC magnetic measurements, as well as X-ray absorption spectroscopy (XAS) study, on a set of mica samples with variable total iron content in order to characterize their electronic, magnetic and structural properties. We found a magnetic phase transition at T-c similar to 110 K only for a specific Fe2+/Fe3+ occupancy ratio (1.65) of the tetrahedral and octahedral sheets and showed that the magnetic behavior for such a material is strongly anisotropic if measured along the mica planes or orthogonally to them. Moreover, the AC magnetic measurements gave a definite evidence of the transition as a history-dependent phenomenon. Finally, we tried to give a theoretical explanation of our observations in terms of a Hubbard Hamiltonian in an octahedral crystal field. Our conclusions definitively rule out a high-spin to low-spin iron transition. An order-to-disorder phase transition can be a possible alternative mechanism, to be confirmed by future experiments.
Octahedral low spin symmetric configurations vs. high spin octahedral distorted configurations: the case of Fe in natural layered silicates / A., Marcelli; G., Cibin; S., Di Matteo; J., Chaboy; D., Di Gioacchino; P., Tripodi; Brigatti, Maria Franca; A., Mottana; M. L., Marco. - In: JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS. - ISSN 0022-3697. - STAMPA. - 65:(2004), pp. 1491-1500. [10.1016/j.jpcs.2003.11.041]