Observation of magnetic-field-induced transformation in MnCo0.78Fe0.22Ge alloys with colossal strain output and large magnetocaloric effect

•First-order magnetostructural transformation from paramagnetic parent phase to ferromagnetic martensite was achieved.•Large magnetocaloric effect was obtained owing to the magnetostructural coupling.•Magnetic field-induced martensitic transformation was directly evidenced at 250 K through in-situ X-ray diffraction experiments.

The thermal, structural and magnetic properties were studied for the hexagonal MnCo0.78Fe0.22Ge alloys, which undergoes a first-order phase transformation from paramagnetic hexagonal phase into ferromagnetic orthorhombic martensite on cooling. Owing to the magnetostructural coupling, large magnetocaloric effect (∆SM=−10.97 J kg−1 K−1) was obtained at 254 K. In-situ synchrotron high-energy X-ray diffraction experiments were conducted to reveal the detailed change in crystallographic structure of phases and the effect of applied magnetic field on phase transformation behaviors. An anomalously huge strain of 11.89% and volume expansion of 4.35% in unit-cell were obtained between martensite and parent phase across the transformation. Furthermore, the magnetic field-induced martensitic transformation was directly evidenced at 250 K, which eventually demonstrates the possibility to achieve magnetic-field-induced strain and large magnetocaloric effect simultaneously.