Influence of disorder on H _{c 2} ‐anisotropy and flux pinning in MgB ₂

Abstract

The upper critical field and flux pinning in MgB 2 single crystals were investigated. The implications of these properties for technical applications are discussed and compared with transport properties of polycrystalline bulk samples and wires. In these untextured materials current percolation is important, especially at high magnetic fields. It is shown that the anisotropy of the upper critical field influences the "irreversibility line" and that the application range of MgB 2 is limited by the smallest upper critical field (i.e., for the field direction perpendicular to the boron planes). Disorder, introduced by irradiation with neutrons, enhances the upper critical field, reduces the anisotropy and drastically changes flux pinning. While the enhanced H c2 and the reduced anisotropy generally improve the transport properties of the polycrystalline samples, the contribution of the radiation-induced defects to flux pinning is small compared to the as-grown defect structure (grain boundary pinning).

Influence of the upper critical-field anisotropy on the transport properties of polycrystalline MgB2

“…The upper critical field increases from 21.4 T and 15.4 T to 30 T and 18.6 T, respectively, the anisotropy γ (obtained from fitting) decreased from about 4.4 in both samples to 2.8 and 2.6 in the bulk and in the copper sheathed wire, respectively. This decrease of γ is consistent with direct measurements of the upper critical field anisotropy in single crystals [23,24]. The influence on pinning is rather small.…”

“…This can be demonstrated by neutron irradiation experiments. Neutron irradiation increases the upper critical field of MgB 2 and decreases its anisotropy [23,24]. The resistive transition of a sintered MgB 2 sample is plotted in Fig.…”

Enhanced flux pinning in neutron irradiatedMgB2

“…The total current is then calculated as the sum of the grain boundary contribution and a point defect contribution, with two free parameters A P and γ. In the main panel of reasonably, A P increases with the fluence, while the anisotropy γ decreases, in agreement with literature values 15,11 . Instead, for the samples P4, P5 and P6 the rescaled grain boundary contribution is even larger than the experimental data points, indicating a failure of the scaling procedure, which will be discussed later.…”

“…10,11 In ref. 11 the rise of J c with irradiation was ascribed to the increase of B c2 , without invoking additional pinning centers.…”

Abrikosov flux-lines in two-band superconductors with mixed dimensionality

“…Although H c2 is sensitive to the amount of impurities in the sample, in MgB 2 single crys-tals, H c2 for field parallel to the c axis has been measured to be H c2 (T = T c /2) ≈ 2 Tesla in several experiments [50,51,52,53,54,55,56,57,58,59,60]. This value roughly corresponds to Λ somewhere between -0.1 and 0.1 for ξ π /ξ σ = 3, ρ 0 = 0.3, and n π = n σ in our model.…”