An antiferromagnetic to ferromagnetic transition for Happlied > Hc and/or T > T t where Hc and T t refer to the critical magnetic field and transition temperature, respectively.
Source:
PAC, 1994, 66, 577. 'Definitions of terms relating to phase transitions of the solid state (IUPAC Recommendations 1994)' on page 586 (https://doi.org/10.1351/pac199466030577)