Field-induced transitions in the highly frustrated magnet gadolinium gallium garnet - long- or short-range order?

In gadolinium gallium garnet (GGG) the antiferro- magnetically interacting Gd ions are located on two corner- sharing triangular sublattices. The magnetic system is there- fore highly frustrated and does not order in zero field. An- tiferromagnetic (AF) order can only be induced by the ap- plication of an external magnetic field. We have measured the magnetic diffraction pattern of a single crystal of GGG at temperatures between 50 mK and 0.9 K in fields of up to 4 T. These experiments reveal that the H-T phase dia- gram has a much more complicated nature than previously assumed. Three groups of magnetic Bragg peaks were found: strong ferromagnetic peaks, saturating in fields above 2.5 T, strong AF peaks with a maximum intensity at the lower-field phase boundary around 0.6 T and weak AF peaks which have a maximum intensity at the upper phase boundary around 1.5 T. The widths of the strong AF peaks are limited by the instrument resolution only in the vicinity of the lower phase boundary.