Magneto-optical study of the magnetic phase diagram of the dilute metamagnet FepMg1-pCl₂

The mixed phase region of the magnetic phase diagram of Fe p Mg 1-p Cl 2 (p=1.00-0.71) has been mapped by a direct optical method. The tricritical temperature falls with increasing dilution by magnesium and is extrapolated to zero at p approximately 0.6, close to the percolation limit for a plane triangular lattice rather than the three-dimensional analogue, despite the essential three-dimensional nature of the metamagnetic transition. In contrast, the tricritical field tends to a finite value as p decreases.