Quantum jumps in the magnetization of a magnet with antiferromagnetic impurities

The differential magnetic susceptibility of Gd_xY_3-xFe₅O₁₂ (0.01<x<0.2) polycrystalline compounds is studied in the temperature range 0.6 to 4.2 K and in pulsed magnetic fields up to 50 T. The magnetization of the compound with the lowest Gd content (x=0.01) shows instead of a classical, a quantum (stepwise) phase transition to a completely saturated ferromagnetic phase at T=0.6K, in magnetic fields around 33 T. To explain these results, a Heisenberg ferrimagnetic model with weakly coupled antiferromagnetic impurities is used. Following this model, the nonequidistance of the energy levels of the impurity ions causes the occurrence of susceptibility oscillations under a variation of external magnetic fields.