Terfenol-D (Tb0.3Dy0.7Fe1.92) is a giant-magnetostrictive alloy: an axial magnetic field reorients internal magnetic domains and the rod changes length, with no moving parts other than the lattice itself. A compressive preload along the rod sets an easy plane perpendicular to the axis, so with no field the domains split into stripes magnetised transverse to the rod (group A / group B below). Applying an axial field H rotates the domain moments toward the axis; each rotated domain elongates slightly along its own magnetisation, and summed over the rod that appears as macroscopic strain.
This sim uses the standard single-domain rotation approximation against an effective anisotropy field Hk (set by the preload slider):
sinθ(H) = |H| / √(H² + Hk²) domain tilt from the easy plane
λ(H) = λs · sin²θ(H) magnetostriction (λs ≈ 1600 ppm)
Δl = λ(H) · L0 elongation of a rod of length L0
Because λ depends on H², a symmetric AC drive alone would double the actuation frequency and swing the rod length only one way ("butterfly" strain). Real actuators bias the rod near the steepest part of the curve with a DC field Hbias (magnet or bias coil) so a small AC field produces a quasi-linear, single-sign displacement — that's exactly what the Bias field slider does here.
- Hbias / Hac — DC operating point and AC swing of the drive field H(t) = Hbias + Hacsin(2πft).
- Preload / Hk — a heavier mechanical preload raises the effective anisotropy field, flattening the λ–H curve and requiring more field to saturate (trades sensitivity for stroke stability).
- The rod's elongation is exaggerated ≈300× in the 3D view — the real physical strain (≈0.1–0.16%) is far too small to see at this scale.
Real-world use: this exact mechanism drives sonar transducers, fuel injectors, and high-force/short-stroke industrial actuators where Terfenol-D's response is far stiffer and faster than a piezo stack of comparable force.