Denis Rancourt
D.G. Rancourt, I.A.D. Christie, G. Lamarche, I. Swainson, S. Flandrois
Journal of Magnetism and Magnetic Materials
Volume 138, Issues 1–2, 2 November 1994, Pages 31-44
https://doi.org/10.1016/0304-8853(94)90396-4
Abstract
We have studied the magnetism of annite mica (the Fe-end-member layer silicate having ideal structural formula: {K+}[Fe32+] <Al3+Si34+〉 O102-(OH-)2) using SQUID magnetometry, Mössbauer spectroscopy, and neutron d iffraction on well characterized samples of both a synthetic powder and a natural single crystal. In disagreement with previous predictions, we find Tc > 10 K and as large as Tc = 58 K in the true end member and a metamagnetic ground state spin structure (predominantly ferromagnetic layers with in-plane moments, stacked antiferromagnetically along the c∗-axis) that is stable despite significant amounts of Fe3+, diamagnetic octahedral ions, and Mn2+ in the natural sample. We propose that the antiferromagnetic stacking is part of the intrinsic zero-field magnetic domain structure and that it is entirely due to dipole-dipole forces. The most striking features are that a paramagnetic Fe2+ temperature dependent fraction persists far below Tc and that the Fe3+ moments abruptly disorder significantly below Tc, with the tetrahedral Fe3+ going first as temperature is increased. Plausible causes for the latter intrinsic features are discussed.