The elastic moduli of the highly magnetostrictive TbxDy1−x alloys (x=0.5, 0.6, and 0.67) were measured at 77 K under conditions of constant magnetic field and constant magnetic induction. From these values the magnetic contribution to the moduli (intrinsic ΔE effect) and magnetoelastic coupling factor k were calculated. For Young’s moduli measured under constant flux density (magnetically blocked conditions), it was found that EB∼20 to 50 GPa. For measurements made while maintaining a constant magnetic field (magnetically free conditions), it was found that Young’s moduli EH minima range from ∼3 to 5 GPa. Such large differences between EB and EH yield magnetoelastic coupling factors in excess of 0.9. Theoretical expressions of the magnetic contribution to the elastic compliance, (1/EH−1/EB), were derived using the single vector magnetization rotation model.
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.
A combined magnetostrictivdpiezoelectric hybrid sonar tonpilz transducer which produces enhanced motion at one end and cancelled motion at the opposite end has been developed. The transducer combines the high strain magnetostrictive material Terfenol-D with that of Lead Zirconate Titanate (PZT-4) piezoelectric ceramic. The transducer has the ability to electrically self-tune and shows an improved effective coupling coefficient 25% greater than a conventional tonpilz type transducer. The in-water mechanical resonance and electrical self-tuning occur in the vicinity of 4.25 kHz, where a 15 dB front-toback pressure ratio was obtained under a 12 element array loading condition. The high coupling coefficient and double resonant system makes this transducer attractive for wide bandwidth operation. With simple manipulation of the design parameters analytical results predict a vastly increased bandwidth of the transducer. An increase in bandwidth from the traditional tonpilz value of 20% to over 100% can be achieved.
The magnetostriction, magnetization, and Young's moduli were measured on laminated rods of thin hot rolled sheets of T b , , D y , , at 77 K. The results were compared to those obtained on single crystals. Approximately 65% of the single crystal saturation magnetostriction was achieved in the rolled samples. While the hot rolling produces the desired planar texture, the magnetostriction and magnetomechanical coupling strongly depend upon the final heat treatment.
The field dependencies of the magnetization and magnetostriction of the Laves phase pseudobinary TbxDyyHozFe1.95 (x+y+z=1) compounds were measured as a function of compressive stress T (10 MPa<|T|<70 MPa) and applied magnetic field H (0<H<135 kA/m). Values of x, y, and z were chosen to obtain minimum magnetic anisotropy and easy magnetization axis rotation near room temperature. At a compressive stress of 34 MPa, the addition of Ho to the binary Tb1−xDyxFe1.95 compound reduced the width of the strain versus magnetic field hysteresis curves in Tb0.28Dy0.57Ho0.15Fe1.95 and Tb0.26Dy0.54Ho0.2Fe1.95 by 23% and 54%, respectively, compared to the original alloy, while the strains were reduced by only 7% and 10%.
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