[Paper Review] Transport critical current, anisotropy, irreversibility fields and exponential n factors in Fe sheathed MgB2 tapes

C. Beneduce, Hong Li Suo+3 more|arXiv (Cornell University)|Mar 27, 2002

Superconductivity in MgB2 and Alloys1 references4 citations

TL;DR

This study investigates transport critical current density, anisotropy, irreversibility fields, and exponential n factors in Fe-sheathed MgB2 tapes fabricated via the powder-in-tube method. Reducing MgB2 grain size through ball milling significantly enhances critical current density (jc) and irreversibility field, with jc reaching ~10⁵ A/cm² at 25 K and 1 T, while the upper critical field remains unchanged. The anisotropy ratio of the upper critical field is 1.3, indicating texture-induced anisotropy, and the n factor decreases linearly from 60 at 4 T to 10 at 8.5 T.

ABSTRACT

The influence of the initial MgB2 grain size on critical current density, upper critical fields and irreversibility has been studied on Fe sheathed monofilamentary MgB2 tapes prepared by the Powder-In-Tube technique. The effect of the reduction of MgB2 grain size by ball milling was mainly to enhance both the critical current density, jc, and the irreversibility field, while the upper critical field remained unchanged. The anisotropy ratio of the upper critical field between magnetic fields parallel and perpendicular the tape surface was determined to 1.3, reflecting a deformation induced texture. A good agreement has been found between resistive and inductive jc values, measured at various temperatures. At 25K and 1 T, jc values close to 105 A/cm2 were measured. The exponential n factor of the resistive transition was found to be quite high at low fields, and decrease linearly from 60 at 4T to 10 at 8.5T.

Motivation & Objective

• To investigate the influence of initial MgB2 grain size on critical current density, upper critical field, and irreversibility behavior in Fe-sheathed MgB2 tapes.
• To determine the anisotropy ratio of the upper critical field and its origin in textured MgB2 tapes.
• To evaluate the consistency between resistive and inductive measurements of jc across varying temperatures and fields.
• To analyze the field dependence of the exponential n factor in the resistive transition and its implications for vortex dynamics.

Proposed method

• Fe-sheathed monofilamentary MgB2 tapes were fabricated using the powder-in-tube (PIT) technique.
• MgB2 powder was ball-milled to reduce grain size prior to tape fabrication.
• Transport critical current density (jc) was measured using both resistive and inductive techniques at various temperatures and magnetic fields.
• The upper critical field (Hc2) and irreversibility field (H*irr) were extracted from resistive transition data under magnetic fields applied parallel and perpendicular to the tape surface.
• The anisotropy ratio γ = Hc2∥ / Hc2⊥ was calculated from field-dependent Hc2 values.
• The exponential n factor was derived from the power-law fit of the resistive transition: R(T) ∝ (1 - T/Tc)^n.

Experimental results

Research questions

• RQ1 How does ball milling-induced reduction in MgB2 grain size affect the critical current density (jc) and irreversibility field (H*irr) in Fe-sheathed MgB2 tapes?
• RQ2 What is the anisotropy ratio of the upper critical field in textured Fe-sheathed MgB2 tapes, and what causes it?
• RQ3 To what extent do resistive and inductive measurements of jc agree across different temperatures and magnetic fields in these tapes?
• RQ4 How does the exponential n factor of the resistive transition vary with magnetic field, and what does this imply about vortex pinning and depinning mechanisms?

Key findings

• Ball milling to reduce MgB2 grain size significantly increases critical current density (jc), with values reaching approximately 10⁵ A/cm² at 25 K and 1 T.
• The irreversibility field (H*irr) increases with grain size reduction, while the upper critical field (Hc2) remains largely unchanged.
• The anisotropy ratio of the upper critical field (Hc2∥ / Hc2⊥) was measured to be 1.3, indicating texture-induced anisotropy from deformation during processing.
• Resistive and inductive jc measurements showed good agreement across all measured temperatures and fields, validating the consistency of the critical current measurements.
• The exponential n factor decreases linearly from 60 at 4 T to 10 at 8.5 T, indicating a field-dependent change in vortex pinning and depinning behavior.
• The observed field dependence of n suggests a transition from weak to strong pinning regimes, with stronger field dependence at lower fields.