Investigation of the Interaction of Magnetic Field With the Auxetic Behavior of Galfenol

Iron-Gallium alloy (Galfenol) is a magnetostrictive material (λ_sat ~ 400 ppm) with potential for robust transduction. In addition, Galfenol is auxetic in the <;110> direction with Poisson´s ratio values of as low as -0.7. In recent years, this phenomenon has been examined with several experimental protocols. The negative Poisson´s ratio of Fe-Ga (attributed to lattice softening under applied stresses) has been documented using tensile tests (with and without a dc magnetic bias) and resonant ultrasound spectroscopy. In addition, density functional theory has been used to produce simulations that explain atomic mechanisms for the observed highly auxetic behavior in this alloy. This paper examines the auxetic response of Galfenol to varying magnetic fields and zero applied stress. An energy based mathematical model of the coupled magnetoelastic properties of the alloy is derived and used to predict Poisson´s ratio at magnetic saturation. The results from this approach with no mechanical excitation shows an increase in the Poisson´s ratio with increase in the Ga content of the alloy. This is opposite to the trends observed with applied stresses where there is a decrease in the Poisson´s ratio with increase in the Ga content. Experimental results for Galfenol of compositions between 15 and 33 atomic percent Gallium will be presented and compared with values from the magnetoelastic model.