Magneto-optical study of the magnetic H-T phase diagram of single crystal Ca3Mn2Ge3O12

Magneto-optical studies have been carried out in order to determine the H-T phase diagram of the antiferromagnetic garnet Ca3Mn2Ge3O12 in magnetic fie lds H parallel to[1 0 0] up to 14 T and temperatures T below 40 K. Below th e Neel temperature T-N = 13.5 K, the magnetic field induces a first-order p hase transition from the antiferromagnetic to a metamagnetic state. Above T -N, the magnetic held induces a second-order phase transition to the metama gnetic state at temperatures up to 20 K. T-N can be considered as a multicr itical point in the H-T phase diagram of the garnet Ca3Mn2Ge3O12 for H para llel to[1 0 0]. The existence of the magnetic-field induced ordered state i n a wide temperature range above T-N can be related to the crossover from a 3D to a XY-symmetry of the spin system or to the presence of quadrupolar o rdering. (C) 1999 Elsevier Science B.V. All rights reserved.