Magnetic hysteresis behavior and magnetic pinning in a d ferromagnet/superconductor nanostructure

Abstract

We investigate the interaction between superconductivity and defect-induced d0 ferromagnetism using a composite consisting of MgB2 and MgO nanocrystals. The composite exhibits a ferromagnetic hysteresis behavior in the temperature region from 40 to 300 K. Defective MgO nanocrystals (∼20 nm) embedded in the composite are considered to be responsible for the observed ferromagnetism. The zero field cool and field cool magnetization curves show that the superconducting transition occurs at Tc = 38.6 K, in agreement with Tc of pure MgB2. In the temperature region from Tc to 0.9Tc (∼35 K), the magnetization hysteresis curves show a superposition of ferromagnetic (F) and superconducting (S) signals. When the temperature of the system is decreased below 0.65Tc (∼25 K), the S signals dominate over the F signals. The resulting magnetic hysteresis loops are highly asymmetric and the descending filed branch is nearly flat, as predicted in the case of surface pinning. At temperatures below 0.5Tc (∼20 K), a sharp peak is developed near zero field in the magnetization hysteresis curves, implying an enhancement of superconducting vortex pinning. The observed pinning enhancement most likely results from magnetic pinning due to randomly distributed magnetic MgO grains, which yield the magnetic inhomogeneity and the related pinning potential in a length scale of ∼100 nm. Thus, the present ferromagnetic/superconducting composite provides an ideal model system that demonstrates the availability of d0 ferromagnetism as a source of magnetic potential for effective vortex pinning.

Effects of magnetic impurities on upper critical fields in the high-Tcsuperconductor La-doped CaFe2As2

“…The fielddependent M value at 50 K for LaCa122-B rapidly increases with magnetic field up to ~1 kOe and then is almost saturated. This FM-like behavior is probably responsible for the small hump developed at 1 kOe in the M − H curve at 2 K, as indicated by the arrows [31]. These results suggest that the significant upward curvature in μ0Hc2(T) near Tc is mainly caused by the FM ic impurities related to Fe or the undoped Ca122 phase, because ferromagnetic ordering can act as a strong pair-breaking source in superconductors [10,[32][33][34].…”

Proximity coupling of superconducting nanograins with fractal distributions

“…The method had been originally developed by the present authors to synthesize highly defective MgO microcrystals with a large number of oxygen vacancies [26][27][28]. The resulting black powders were then processed using a spark plasma sintering (SPS) system.…”

Coherent interface structures and intergrain Josephson coupling in dense MgO/Mg2Si/MgB2 nanocomposites

“…11 In this work, we propose a method to synthesize fully dense nanocomposites using solid phase reaction between metallic magnesium and bulk borosilicate glass. Since borosilicate glass includes SiO 2 and B 2 O 3 as glass forming oxides, the reactions are expected to proceed as follows: 12,13 4Mg þ SiO 2 ! 2MgO þ Mg 2 Si;…”