The Delta E (ΔE) effect is a magnetomechanical effect. The name refers to changes in the effective elastic moduli of a magnetostrictive material, particularly the Young's modulus E, when a magnetic field is applied to a material.

This effect is connected to the tendency of a previously bent rod, made of magnetostrictive material, to straighten when subjected to a magnetic field applied along the rod's axis.^[1] This macroscopic effect is sometimes called the Guillemin effect, attributed to Claude-Marie Guillemin who first discussed the effect in 1846.^[2][3][4] The first explanations to the ΔE effect were given in the 1930s.^[5]

The ΔE effect is often just considered in terms of a change of the effective Young modulus of a material under the application of a magnetic field. It is a consequence of the geometry and magnetic anisotropy of the magnetic domains.^[6] The Young's modulus changes from zero field to saturation.^[6] The change in theYoung's modulus ΔE is written as

${\displaystyle \Delta E={\frac {E_{\mathrm {s} }-E_{0}}{E_{0}}},}$

where ${\displaystyle E_{\mathrm {s} }}$ is the Young modulus at saturation and ${\displaystyle E_{0}}$ is the Young modulus in the unsaturated state.^[6] The effect is more general than this, and leads to changes in the values of all components of the elastic modulus tensor in a magnetic field.^[7]

The ΔE effect is used in applications to build actuators and sensors.^[6]

• Magnetomechanical effects
• Magnetostriction
• Magnetocrystalline anisotropy