Applying a magnetic field to ferromagnetic systems in the temperature vicinity of a first order structural transition can lead to magnetic field induced structural modifications and large magnetocaloric effects. Many Ni-Mn-based ferromagnetic Heusler alloys undergo first order martensitic transformations. Applying a magnetic field at temperatures close to the transition causes large field-induced strains and large magnetocaloric effects. In many of these systems, the field-induced strain is due to a field-induced reverse martensitic transformation to the austenitic state. The magnetocaloric effect around the transition is ‘inverse’, in which case the sample cools on applying a field as opposed to the ‘conventional’ case where it warms. In order to gain more insight into the mechanism of the large field-induced strains and the inverse magnetocaloric effect, we have performed diffuse polarized neutron diffraction experiments using the XYZ-polarization analysis technique. These experiments reveal the development of strong antiferromagnetic correlations just below the martensitic transition temperature. We discuss the relationship of these correlations with the inverse magnetocaloric effect and the field-induced strains.

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