Strong competition between the δl and δTc flux pinning mechanisms in MgB2 doped with carbon containing compounds

Abstract

The transport and magnetic properties of 10 wt % malic acid and 5 wt % nanocarbon doped MgB2 have been studied by measuring the resistivity (ρ), critical current density (jc), connectivity factor (AF), irreversibility field (Hirr), and upper critical field (Hc2). The pinning mechanisms are studied in terms of the collective pinning model. It was found that both mean free path (δl) and critical temperature (δTc) pinning mechanisms coexist in both doped MgB2. For both the malic acid and nanocarbon doped samples, the temperature dependence of the crossover field, which separates the single vortex and the small bundle pinning regime, Bsb(T), shows that the δl pinning mechanism is dominant for temperatures up to t(T/Tc)=0.7 but the δTc pinning mechanism is dominant for t>0.7. This tendency of coexistence of the δl and the δTc pinning mechanism is in strong contrast with the pure MgB2, in which the δTc pinning mechanism is dominant over a wide temperature range below Tc. It was also observed that the connectivity factor, active cross-sectional area fraction (AF), are 0.11 and 0.14 for the nanocarbon and the malic acid doped MgB2, respectively, indicating that there are still rooms for further improving jc performance.

Flux pinning mechanism in SiC and nano-C doped MgB2: evidence for transformation fromδTctoδℓpinning

“…For δT c pinning, ∝ − + ( ) ( ) In the single-vortex-pinning region the following B sb temperature dependence can be obtained [17]: where the parameter ν = 0.67 and 2 for δT c and δℓ pinning, respectively. It was found that there is no agreement between the experimental data for the doped MgB 2 and equation (3) with ν = 2 or 0.67 [15,18], and that strongly suggests that the δT c and the δℓ pinning mechanisms coexist in our samples. The δℓ and δT c pinning mechanisms are the main pinning mechanism.…”

“…Partial doping substitution and Mg deficiency are the two main reasons for the T c fluctuation, while the mean free path fluctuation, and hence the δℓ pinning, is caused by the intergrain boundaries and interagrain inclusions. For MgB 2 superconductor, it has been shown that δT c pinning is dominant in undoped MgB 2 samples, while both δT c and δℓ pinning mainly coexist in carboncontaining doped MgB 2 bulk samples [15,18,24].…”

“…It has been reported that the δT c pinning is the main flux pinning mechanism in pure MgB 2 bulk [17]. It was found that both pinning mechanisms coexist in the nanoparticle doped-MgB 2 samples, depending on the temperature [15,18,24].…”

“…Therefore, the vortices have to be pinned by flux pinning centers. Numerous studies have been performed with the purpose of understanding the vortex-pinning mechanisms [2,3,[13][14][15][16][17][18][19][20][21][22][23][24] that are responsible for improving the critical current density, J c . Intergrain boundary pinning [19] and point defect pinning [16] are two main important pinning mechanisms.…”

Hydrostatic pressure induced transition fromδTCtoδℓpinning mechanism in MgB2

“…Similarly, the irradiation method can improve J c in MgB 2 , but it reduces T c values significantly (by more than 20 K in some cases) [23][24][25][26][27]. Correspondingly, the chemical doping and irradiation methods can also change the nature of the pinning mechanism in MgB 2 [24,[28][29][30]. The determination of J c and the flux pinning mechanism under hydrostatic pressure is also an important step to probe the mechanism of superconductivity in more detail in MgB 2 .…”