which exhibit a paramagnetic to antiferromagnetic transition in the austenitic state, has been examined by means of differential scanning calorimetry, magnetic field susceptibility and magnetization measurements and optical microscopy, applying a pulsed ultra high magnetic field. As a result, it was found that the martensitic transformation was not thermally induced in all the alloys even at liquid helium temperature, but it occurred under magnetic fields higher than a critical one only in the

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... lloy with 4.5 at%C. The critical magnetic field vs temperature relation in the 4.5 at%C alloy showed a C-curve, being largely different from that for all the ferrous alloys previously examined. Irrespective of the formation temperature, the amount of magnetic field-induced martensites was dependent on the strength of magnetic field near the critical one, but was independent on a little higher magnetic fields. Morphology of the magnetic field-induced martensites was the same irrespective of the formation temperature. Thermodynamic analysis suggested that an antiferromagnetic short range order which exists in the paramagnetic austenite suppressed the martensitic transformation.