Size Effects of Top-down Processed Ni-Mn-Ga

Magnetic shape memory alloy samples such as Ni-Mn-Ga elements change their shape when exposed to a variable magnetic field. Magneto-mechanical properties of Ni-Mn-Ga single crystals depend on the size of the sample. Magnetic field induced strain has been reported up to 12% in bulk samples, and can be reduced through surface effects which lead to size effects. We created small cuboids through a top-down process method from a Ni-Mn-Ga bulk single crystal, cut along the {100} and mechanically trained to lower the twinning stress. We measured the magnetic properties including the saturation field and the switching field with a vibrating sample magnetometer (VSM). The switching field indicates the ease of twin boundary motion which increases with increasing defect concentration. The lower the switching field is, the better is the quality of the sample. Mechanical testing showed a twinning stresses ranging from 5 MPa to 22 MPa, which is above the maximum magneto-stress generated by a magnetic field. Stress induced from cutting and polishing caused a higher twinning stress than those found in single crystals produced the same way with a larger volume.