Name of the Phenomenon: Dimensional Change on Magnetization When a ferromagnetic specimen is magnetized, very small changes in its dimensions are observed. What is this phenomenon called in materials science and magnetics?

Introduction / Context:
Ferromagnets exhibit several magneto-mechanical effects due to coupling between magnetic domain structure and the lattice. One important effect is a minute change in dimensions when magnetization changes, which influences precision devices, transformers, and sonar transducers. Identifying and understanding this effect helps in both exploiting and mitigating it in engineering designs.

Given Data / Assumptions:

• Ferromagnetic material under changing magnetization.
• Elastic, reversible strains for small fields; larger fields may involve irreversible domain processes.
• No external mechanical load required for basic observation.

Concept / Approach:

Magnetostriction is the change in shape or dimensions of a ferromagnet when its magnetization changes. Domain wall motion and rotation alter magnetoelastic energy, causing strains aligned with magnetization. Materials with large magnetostriction (e.g., Terfenol-D) are used in actuators; low-magnetostriction steels are preferred in transformer cores to reduce audible hum and energy loss.

Step-by-Step Solution:

Verification / Alternative check:

Measurement techniques (strain gauges, interferometry) confirm microstrain-level changes synchronized with magnetization cycles, demonstrating magnetostriction curves vs field.

Why Other Options Are Wrong:

• Hysteresis: concerns B–H loop area (loss), not dimensional change.
• Diamagnetism: universal weak negative response, unrelated to length change in ferromagnets.
• Dipolar relaxation: pertains to dielectric polarization dynamics, not magnetoelasticity.

Common Pitfalls:

Confusing audible transformer hum (a symptom) with the underlying magnetostriction mechanism; mixing magnetic and dielectric relaxation terms.

Final Answer:

Magnetostriction