MAGNETISM OF SYNTHETIC AND NATURAL ANNITE MICA - GROUND-STATE AND NATURE OF EXCITATIONS IN AN EXCHANGE-WISE 2-DIMENSIONAL EASY-PLANE FERROMAGNET WITH DISORDER

We have studied the magnetism of annite mica (the Fe-end-member layer silicate having ideal structural formula: {K+}[Fe3(2+)] (Al3+ Si3(4+) O10(2-)(OH-)2 using SQUID magnetometry, Mossbauer spectroscopy, and ne utron diffraction on well characterized samples of both a synthetic po wder and a natural single crystal. In disagreement with previous predi ctions, we find T(c) > 10 K and as large as T(c) = 58 K in the true en d member and a metamagnetic ground state spin structure (predominantly ferromagnetic layers with in-plane moments, stacked antiferromagnetic ally 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 z ero-field magnetic domain structure and that it is entirely due to dip ole-dipole forces. The most striking features are that a paramagnetic Fe2+ temperature dependent fraction persists far below T(c) and that t he Fe3+ moments abruptly disorder significantly below T(c), with the t etrahedral Fe3+ going first as temperature is increased. Plausible cau ses for the latter intrinsic features are discussed.