Behavior of NiMnGa Under Dynamic Magnetic Fields Considering Magnetic Diffusion and Eddy Current Power Loss

Prior experimental measurements by the authors demonstrated large reversible strains of up to –0.41% along the [001] crystal direction of a cylindrical Ni₅₀Mn_28.7Ga_21.3 rod driven with a magnetic field along the same direction and no external restoring force. This represents an unusual configuration which can lead to solenoid transducers with enhanced energy density and bandwidth relative to standard electromagnet devices. Although a number of constitutive models have been developed which quantify magnetic field induced strain (MFIS) in these materials, models incorporating both the MFIS and transducer dynamics are scarce. This paper presents a transducer model that is built in three steps. In the first step, classic thermodynamics is used to calculate the volume fraction ξ of an ideal two-variant system as a function of magnetic fields and stresses. Bulk strains are then calculated through stochastic homogenization of the volume fraction. The transducer dynamics are quantified in the third step through calculation of magnetic diffusion in the sample, eddy current losses, and skin depth effects. The strain output is quantified at various magnetic field frequencies.