We fabricated CaKFe4As4 round wires through a powder-in-tube method followed by hot isostatic pressing up to 175 MPa. The transport critical current density, Jc, at 4.2 K almost reached the level for practical applications (100 kA cm−2) under self-field and reached 7.6 kA cm−2 at 100 kOe. Magneto-optical images confirmed that the grains were well-connected at low temperatures. Although high-pressure sintering at high temperatures enhanced the Jc and density of the core of the wires, X-ray diffraction and scanning electron microscopy–energy-dispersive X-ray spectroscopy analyses indicated that impurity phases were present in both the polycrystalline powder and the core of the wire.
(Nd,Eu,Gd)Ba2Cu3Ox (NEG123) bulk shows high irreversibility fields over 14 T at 77.3 K. In order to study the origin of the high irreversibility field, transport measurements for NEG123 bulk were performed as functions of temperature, high magnetic fields (up to 27 T) and field angle. We found that c-axis correlated disorder exists for samples with 3 and 40 mol% NEG211, although their irreversibility fields are different. The enhancements of the irreversibility fields at 77.3 K due to the c-axis correlated disorder are almost the same (about 1 T) for both samples and are in agreement with that of Y123 bulk with a twin boundary. The partially entangled vortex liquid and the Bose glass phases, which are induced by the c-axis correlated disorder, play an important role in the improvement of the irreversibility field. However, in order to explain the much higher irreversibility field, we should take account not only of the c-axis correlated disorder but also of the other pinning centres such as the nanolamella.
Processing of materials under the magnetic levitation condition is an important new technique for synthesis of novel materials, because containerless melting can be performed in a quasi-microgravity environment. For materials with anisotropic magnetic susceptibility, the magnetic orientation effect can be expected in addition to the microgravity effect. We performed containerless melting of paraffin under the magnetic levitation condition, using a newly developed YAG laser furnace, in order to examine the possibility of synthesizing oriented spherical materials. By magnetization measurement and X-ray diffraction measurement, we confirmed the orientation of paraffin solidified under the magnetic levitation condition. This containerless melting method is expected to provide opportunities for obtaining materials with new functions.
Cu-NMR study under high-field up to 17 T has been performed on the three-dimensional spin dimer system NH4CuCl3, which shows the two-stepped magnetization plateaus. The local magnetization of Cu-site measured by NMR showed a significant deviation from the macroscopic magnetization at low temperatures, suggesting the localization of the field-induced triplons. In the high field region around 16 T, two inequivalent Cu sites are observed. Only one of the two sites shows a split in the resonance line below TN ≃ 3.2 K, indicating the existence of the field-induced magnetic order, to which a part of spins in the system participate.
Monofilament MgB2/Fe wires or tapes were fabricated with two different purity crystalline boron powders, using internal magnesium diffusion (IMD) and in situ powder-in-tube (PIT) processes. To evaluate which method was more insensitive to the purity of the boron powders, a sensitivity factor was used. It was found that the IMD process was less sensitive to the purity of the boron powders, compared to the PIT method. Furthermore, Jc values of the IMD-processed wires were higher than those of the PIT-processed samples. The reduced porosity and hence the increased density in the IMD-processed samples was thought to be the main reason. Although a number of B-rich particles remained in the MgB2 region, a layer Jc as high as 4.8 × 104 A cm−2 at 4.2 K and 10 T was still obtained in the IMD-processed wire, which was comparable to that of IMD-processed wires fabricated using amorphous boron powders.
We succeeded in demonstrating a 15.1 T cryocooled Nb 3 Sn superconducting magnet with a 52 mm room temperature bore. Two operating currents of 157 A and 90 A for the divided section coils were utilized for the first time in a cryocooled superconducting magnet system. It is found that the current leads are no longer the dominant heat loads because of the use of high-temperature superconductors. In order to realize a high-field cryocooled superconducting magnet comparable to the usual superconducting magnet immersed in liquid helium at 4.2 K, we maintained the coil temperature at a value below 5.0 K during the field sweep in a high magnetic field region.
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