The Wiedemann Effect: Magnetization Inducing Torsion?

[vin300]

Recently saw something unusual that says a helical magnetization could alternatively induce torsion within a rod. Know that electromagnetic effects can induce stresses( hall effect, I guess), temperature can, sound waves can, but I fail to understand what they mean by a helical field, is it even possible and what causes the field to bend the material itself, because magnetization is nothing different from looping currents and wires do not seem to bend due to something like "electic pressure". Curious.

 

[mfb]

because magnetization is nothing different from looping currents

Or electron orbit or spin orientations. Distances between atoms can depend on those, and the macroscopic shape depends on those distances.
Overall, this is called Magnetostriction.
Wires rarely show this effect because it is way too small.

 

[drvrm]

The twisting of a ferromagnetic rod through which an electric current is flowing when the rod is placed in a longitudinal magnetic field. It was discovered by the German physicist Gustav Wiedemann in 1858 [1] . The Wiedemann effect is one of the manifestations of magnetostriction in a field formed by the combination of a longitudinal magnetic field and a circular magnetic field that is created by an electric current. If the electric current (or the magnetic field) is alternating, the rod will begin torsional oscillation.

In linear approach angle of rod torsion α does not depend on its cross-section form and is defined only by current density and magnetoelastic properties of the rod:[2]

,
where

• 
  is current density;
• 
  is magnetoelastic parameter, proportional to longitudinal magnetic field value;
• 
  is the shear modulus.
• 
  
• se e https://en.wikipedia.org/wiki/Wiedemann_effect for details

 

[vin300]

The hysterisis shows magnetization in response to the field in like direction non linearly, but magnetostrictive strain direction as always negative though increasing with increasing field, so is this supposed to be showing that only compressive effect is possible?

 

[vin300]

Another question is whether these deformations are plastic or elastic.

 

[drvrm]

Another question is whether these deformations are plastic or elastic.

It appears that these deformations are elastic in nature-though in certain materials its Youngs modulus gets reduced i.e. the strain is larger.
common magnetostrictive composite is the amorphous alloy its trade name Metglas
Favourable properties of this material are its high saturation-magnetostriction constant, λ, of about 20 microstrains and more, coupled with a low magnetic-anisotropy field strength, HA, of less than 1 kA/m (to reach magnetic saturation). It also exhibits a very strong ΔE-effect with reductions in the effective Young's modulus up to about 80% in bulk. This helps build energy-efficient magnetic MEMS.[
see <https://en.wikipedia.org/wiki/Magnetostriction>