MINERAL SOLID SOLUTIONS WITH COUPLED STRUCTURAL AND MAGNETIC PHASE TRANSITIONS: THE EXAMPLE OF TROILITE (FES) – PYRRHOTITE FE1-XS

The stability of magnetic structures can be understood in terms of pairwise interactions between individual moments but tends to be greatly enhanced if there are accompanying changes in other structural parameters. In general terms, the additional interactions might be referred to as spin-lattice coupling, spin-orbital coupling or, if there is a significant distortion of the lattice, as coupling of the magnetic order parameter with strain. When the magnetic transition occurs in a crystal which has additional instabilities with respect to other symmetry breaking mechanisms, the net result can be stabilisation due to coupling between multiple order parameters. The implications are important not only with respect to palaeomagnetism but also to the technological field of multiferroics. The pyrrhotite solid solution, Fe_1-xS (0 < x < 0.125) is a special case in which the additional instability is due to ordering of vacancies on the cation site. Instead of a single magnetic transition occurring at ~590 K, structures with different space groups develop as a consequence of different ordering schemes for the vacancies. The favoured ordering scheme is sensitive to thermal history and stoichiometry, and further magnetic transitions lead to a phenomenologically rich phase diagram. The different structure types and magnetic ordering schemes observed in natural and synthetic samples can be understood from a group theoretical perspective in terms of an order parameter with symmetry U₁, belonging to points along the line between the L and M points of the Brillouin zone, and of two magnetic order parameters with symmetry mG4+ and mG5+, belonging to the zone centre. Coupling between these is expected to be biquadratic. Practical treatments of magnetic, heat capacity, strain and elasticity data based on this approach will be illustrated for 4C pyrrhotite with composition Fe₇S₈, which undergoes a sequence of phase transitions with falling temperatures between structures with different space groups as: P6₃/mmc - C2'/c'- P-1. The low temperature transition (Besnus transition) is second order in character. By way of contrast, the sequence for 5C pyrrhotite is likely to be P6₃/mmc - Cmca - P2₁ and the low temperature magnetic spin-flop transition is first order in character.