Magnetic structures in the Ca2Fe2−xAlxO5 system-II

Recent structure analyses of crystals with different compositions in the Ca₂Fe_2?xAl_xO₅ system have led to a reinterpretation of the magnetic ‘phase diagram’ of this system. In the composition range 0 ? x ? 1, there are at least three and possibly four different magnetic structures. At room temperature, and between x = 0·00 and x less than but near 0·60 (the exact boundary is not known), the magnetic structure belongs to magnetic space group Pcm′n′; in a narrower range, approximately 0·57 ? x ? 0·65, the magnetic space group is I_pb′m′2; for x greater than about 0·80, the magnetic space group is I_pbm′2′. There is a substantial region of x and T (at room temperature 0·65 ? x ? 0·80), for which our experimental results cannot distinguish between two possibilities: either both the I_pb′m′2 and I_pbm′2′ structures coexist with spin-flipping as x or T changes or there is a continuous rotation of the spins with changing x or T. In the latter case, the most probable single magnetic space group in this region is C_pm′; with increasing x or T the transitions occur by way of I_pb′m′2 → C_pm′ → I_pbm′2′.