Critical State Theory for the Magnetic Coupling between Soft Ferromagnetic Materials and Type-II Superconductors

Area of Science:

• Condensed Matter Physics
• Electromagnetism
• Materials Science

Background:

• Understanding the physical coupling between type-II superconductors (SC) and soft ferromagnetic materials (SFM) is crucial for applications like magnetic cloaking and shielding.
• Unexplained phenomena, including increased SC energy losses and magnetic flux density deformation, have persisted for 20 years, challenging the critical state theory (CST).

Purpose of the Study:

• To resolve long-standing issues in SC-SFM heterostructures by extending the critical state theory.
• To provide a theoretical framework that explains observed phenomena without invoking overcritical current densities.

Main Methods:

• Developed a semi-analytical model for cylindrical monocore SC-SFM heterostructures.
• Validated the model using a variational approach of multipole functionals for magnetic coupling.
• Conducted comprehensive numerical simulations for SFM sheaths with varying dimensions and magnetic relative permeabilities (μr).

Main Results:

• Demonstrated that AC-losses in SC-SFM metastructures significantly change with SC and SFM radii for μr ≥ 100.
• Numerical simulations showed good qualitative agreement with magneto-optical imaging observations.
• The study confirms that reported phenomena can be understood without the ansatz of overcritical currents.

Conclusions:

• The extended CST provides a valid explanation for previously unexplained phenomena in SC-SFM heterostructures.
• This work offers a more accurate theoretical basis for designing and applying SC-SFM metastructures.
• Eliminates the need for the controversial concept of overcritical current densities in these systems.