Calculation of the volume pinning force inMgB2superconductors

Abstract

The field dependence of the volume pinning force in anisotropic polycrystalline superconductors is calculated for various well established pinning models. The anisotropy substantially changes the field dependence of the volume pinning force and shifts its maximum to significantly lower reduced fields. A scaling procedure that allows the identification of the dominant pinning mechanism from the peak position is proposed.

Development of MgB₂-Based Bulk Supermagnets

“…As a result, γ(T) changes from 6 to 2 on increasing T, yielding h 0 = 0.158 to 0.33. In contrast to this, p c is only slightly varying between 0.2 and 0.3 [15]. The large variation of h 0 obtained for the samples prepared at 775 and 800°C implies that the grain boundary character (connectivity) must change when processing the samples at higher reaction temperatures [16].…”

“…This effect was discussed previously in Ref. [15], where j c was calculated using the parameters γ = 4 and p c = 0.25. This produces a shift of h 0 from 0.33 (isotropic case, point pinning) to about 0.18.…”

Effect of temperature and pressure to pinning centers in bulk MgB2 under high pressure

“…The critical current density was obtained from magnetization measurements in an Oxford Instruments 3001 vibrating sample magnetometer (VSM) using the Bean model. The pinning mechanism was estimated using a method proposed in [9]: k = B peak /B n , where B peak is the field, where the volume pinning force is maximal (F p(max) ) and B n is the field, at which F p(max) drops to half on the high field side; k is expected to be at 0.34 and 0.47 for grain boundary (GBP) and point pinning (PP), respectively. Connectivity, B c2 and B irr were estimated based on resistivity measurements by the four-probe method (direct current flow through the materials) in magnetic fields of up to 15 T. The typical dependence of the critical current densities for the HP-synthesized materials is shown in Fig.…”

Application Prospects of ${\rm MgB}_{2}$ in View of Its Basic Properties

“…The upper critical field anisotropy, γ, changes the field dependence of the critical current density [8] and of the corresponding volume pinning force [9] drastically. This is illustrated in and is expected to be 0.2 to 0.3 [8].…”

“…In the following p c = 0.25 is assumed, leading to a reduction of B ρ=0 to 0.63B c2 (for γ = 5). All calculations are based on the model proposed in [8], but with the anisotropic scaling approach [11], as described in [9].…”