Behavior of magnetic field–annealed Galfenol steel

Abstract

An attractive feature of the Galfenol alloy system is the ability to impart enough uniaxial anisotropy into the alloy such that no pre-load mechanism is required to attain saturation magnetostriction. Stress annealing is typically employed to generate this anisotropy. Recent published results indicate that magnetic field annealing could generate sufficient anisotropy to provide full magnetostriction with no prestress, which will suffice for the majority of applications. The magnetic field annealing apparatus used in these experiments applies a constant 1 T magnetic field at a temperature between 300 °C and 700 °C in a 7.6 cm square × 15.2-cm-long cavity. The magnetic behavior of field-annealed Galfenol is compared to stress-annealed samples. Field-annealed Galfenol rods behave differently than stress-annealed rods. While the saturation strain values are equivalent, the magnetic field required to saturate the Galfenol is much less, 60 Oe versus 200 Oe, in the field-annealed state. This behavior is also exhibited in the flux density versus field. Both curves have a significant impact on device design. The type (mechanical or magnetic) and magnitude of stored energy largely determines the optimum magnetic bias point and impacts operational efficiency.