  We report the observation of a field-induced transformation from the
easy-plane antiferromagnetic structure to the easy-axis ferrimagnetic structure
in a single crystal of TbFe_4.4Al_7.6 (tetragonal ThMn_12 structure) at 5 K.
Such a field-induced, irreversible transition has been identified for the first
time. This transition is accompanied by a giant orthorhombic distortion:
epsilon_aa = - epsilon_bb ~ 3.5x10^-4 that is associated with a magnetic
hardness (mu_0H_C ~ 3 T) that is unprecedented in this category of materials.
