|
The recent discovery of Galfenol as a "large" magnetostrictive material (as high as 400με) offers a particularly
promising transducer material that combines largely desirable mechanical attributes with superior magnetic properties.
The high permeability of this material makes it easy to magnetize, however it also causes a relatively low cutoff
frequency in dynamic applications, above which eddy currents form and introduce significant power losses. To reduce
the eddy current losses, magnetostrictive drivers used in dynamic applications are commonly laminated. A second
transducer design consideration is that in materials which exhibit positive magnetostriction, it is common to impose an
initial compressive "prestress" to the material that is sufficient to align the orientation of the magnetic moments within
the material to a direction perpendicular to the stress axis. This is done to maximize the magnetostriction realized when a
magnetic field applied along the stress axis rotates the moments parallel to the stress axis. An alternative to the
application of a compressive prestress is to build-in a uniaxial magnetic anisotropy through stress annealing. Stress
annealing is a high temperature process with simultaneous application of a compressive load and subsequent cooling
under load in which the magnetic moment alignment developed at temperature is retained upon removal from the stress
anneal fixture. The compressive load needed to build in a useful uniaxial magnetic anisotropy in Galfenol is greater than
the buckling load for Galfenol laminae sized for use in high frequency dynamic applications. In this study, prior work on
stress annealing of solid rods of single and polycrystalline samples of Galfenol is successfully extended to thin laminae
of Galfenol by introducing fixtures needed to avoid buckling. The standard stress annealing device uses a hydraulic
actuator to apply compressive stress to the sample. Two linear guides have been added to ensure a normal compression
load path to reduce the potential for buckling of thin laminations. In addition, a mechanical holding fixture was used to
maintain proper alignment of the thin laminations during stress annealing. Data are presented that demonstrate the
magnetic uniaxial anisotropy developed by stress annealing of laminated Galfenol rods.
|
