RE Ba ₂ Cu ₃ O _y superconducting tapes are appropriate for high field magnet applications at low temperatures (i.e. below liquid nitrogen temperature). To clarify the morphology and the volume of the effective pinning center at low temperatures, we used a low-temperature growth technique to fabricate SmBa ₂ Cu ₃ O _y (SmBCO) films with various amounts of BaHfO ₃ (BHO) nanorods onto MgO-buffered metal substrates produced by ion-beam-assisted deposition; we investigated their flux pinning properties using a 25 T cryogen-free superconducting magnet that was recently developed at Tohoku University. According to the microstructural analysis using transmission electron microscopy, the BHO nanorods have a content-dependent morphology and are aligned for the higher content. The inclined and discontinuous BHO nanorods were observed in SmBCO films with BHO contents up to 3.8 vol%; they show an excellent flux pinning force density (1.5 TN m ⁻³ at 21 T and 4.2 K) even when the magnetic field is perpendicular to the films. Based on the effective mass model for the flux pinning, the random pinning centers are dominant at low temperatures. The correlated flux pinning is stronger for aligned nanorods; however, the random pinning center becomes weaker in the 4.5 vol% BHO-doped films. Therefore, the optimal BHO doping level is approximately 3.8 vol% in terms of the amplitude of the critical current density and the anisotropy from 4.2 K to 20 K because this provides the best mixture of correlated and random flux pinning centers.

A 33 T cryogen-free superconducting magnet (33 T-CSM) project is now in progress at HFLSM, IMR, Tohoku University. The 33 T-CSM consists of a φ 68 mm–19 T REBCO (HTS) insert and a φ 320 mm-14 T CuNb/Nb₃Sn and NbTi Rutherford (LTS) magnets. The 33 T-CSM system has been installed and tested without the HTS insert in March 2024. The LTS outsert magnet consists of three CuNb/Nb₃Sn Rutherford cable coils and two NbTi Rutherford cable coils with an epoxy impregnation. It can generate 14 T in a 320 mm bore with 879 A. This winding makes use of advanced high strength CuNb/ Nb₃Sn strands specifically developed for the 33 T-CSM project, enabling high stress design with about 275 MPa in the Nb₃Sn coil at 14 T. The 19 T-HTS insert is designed based on the robust REBCO coil technology we proposed in previous works. For the cooling system, a 9 W GM/JT cryocooler is used for the LTS coils, four 4 K-GM cryocoolers cool the REBCO coils (1.5 W each at 4.2 K) and two single-stage cryocoolers are used for the radiation shield and current leads. Helium circulation with compressors ensure the thermal connection between the coils and the cryocoolers. The initial cooling is about 7.3 days from room temperature. The test of the 14 T LTS magnet was successfully completed up to 839 A with the nominal maximum electromagnetic stress of 275 MPa after one training quench.

The strengthening of Yoroi-coil (Y-based oxide superconductor and reinforcing outer integrated coil) structure at stronger electromagnetic force was investigated using the Yoroicoil structured double-pancake coil with higher strength outer plates of carbon-fiber reinforced plastic (C-FRP) than those of glass-FRP. The coil achieved 1.14 kA transporting at 4.2 K in 14-T back-up magnetic field and the superconducting wire was burnt by thermal runaway after that. The calculated maximum hoop stress, based on the BJR (magnetic field × current density × coil radius) calculations, at the hoop stress test reached 2.0 GPa. This value was far exceeding the tensile strength of superconducting wire ranging from 1 to 1.3 GPa; however, the maximum strain of superconducting wire in the test coil was held about 0.5%. Yoroi-coil structure is useful for the applications, which need high magnetic field precision. The strengthening effect of C-FRP outer plates was observed at outer part of the coil, but it did not cover the whole coil body, so that arrangement of reinforcing outer plates could admit of some improvement.

We have characterized bulk and local magnetic properties of iron-based oxypnictide superconductor SmFeAsO _0.7 F _0.3 polycrystalline sample ( T _c ∼53 K) synthesized at ambient pressure and NdFeAsO _0.7 ( T _c ∼52 K) synthesized at high pressure by using SQUID magnetometer, micro-Hall probe, and magneto-optical imaging technique. Intragranular critical current densities evaluated from bulk magnetization and local magnetic profile give values of about 1×10 ⁵ A/cm ² at 10 K in both compounds. Magnetic field profiles of both samples are far from Bean-like, suggesting that intergrain connection is not strong. Low-field characteristics including flux-penetration field are measured by using a micro-Hall probe. Obtained results are compared with data on high temperature superconductors.

We measured critical currents and irreversibility fields of Ag-sheath (Sr, K)Fe ₂ As ₂ tapes at various temperatures, magnetic fields and magnetic field directions. The sample tapes were made by a powder-in-tube with a hot-press process in the final heat treatments. The inverse anisotropy in J _c in comparison with the effective mass anisotropy can be found in the hot-pressed (Sr, K)Fe ₂ As ₂ tapes. We found that the broad c -axis peak in angular dependence of J _c appears in a low field region and it decreases with increasing field and temperature. In addition, the crossover of J _c anisotropy appears around 10 T at 4.2 K and the crossover field decreases with increasing temperature. It is considered that these unusual anisotropic J _c behaviors may be originated by the strong random pinning center which is larger than the coherence length,