Magnetomechanical coupling in Bridgman-grown Tb0.3Dy0.7Fe1.9 at high drive levels

Abstract

It is now possible to achieve substantial magnetomechanical transduction in modified Bridgman-grown samples of Tb0.3Dy0.7Fe1.9 (Terfenol-D) which are grain-oriented to achieve nearly complete [112] alignment. Large magnetic-field excursions can be converted into large fractional dimension changes (ΔL/L>10−3). In this paper measurements are reported of the average magnetomechanical coupling factor determined by (i) large field drives (>1000 Oe) and (ii) large pressure changes (> 20 MPa). By extending the small-signal magnetomechanical expressions to difference relationships Δε=sH Δσ+d ΔH and ΔB=d*Δσ+μσ ΔH, it is possible to determine large-signal coupling factors by k2=1−με/μσ, and (2) k2=1−sB/sH. Here με and μσ are the average magnetic permeabilities (ΔB/ΔH) at constant strain ε, and at constant stress σ, and sB and sH are the large signal elastic compliances (Δε/Δσ) at constant induction B, and constant field H. The square of the coupling factor is defined by k2=1−dd*/sHμσ. Using an apparatus which was designed to minimize demagnetizing effects, με, μσ, sB, sH, d, and d* were measured in large Bridgman-grown samples (10 cm×3.75 cm diam) for compressive stresses up to 49.1 MPa with field excursions from ±250 to ±750 Oe and pressure excursions from ±3.7 to ±19.0 MPa.