The longitudinal components of the zero-field-cooled (MZFC) and field-cooled (MFC) magnetization of dilute Cu:Mn have been measured in magnetic fields between 2 G and 30 kG. The difference, AMi rr =MFc -AfzFC, is taken to be the irreversible magnetization. Above « 500 G, a weak irreversibility and a strong irreversibility are observed. The onset temperature of the former, or weak irreversibility, follows the Gabay-Toulouse H-T relation with a coefficient close to the mean-field value for a Heisenberg system. The onset temperature of the latter is found to agree quantitatively with the mean-field d'Almeida-Thouless-like H-T crossover relation for longitudinal freezing in a Heisenberg system.

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