Magnetite is a half-metal with a high Curie temperature of 858 K, making it a promising candidate for magnetic tunnel junctions (MTJs). Yet, initial efforts to exploit its half metallic nature in Fe₃O₄/MgO/Fe₃O₄MTJ structures have been far from promising. Finding suitable barrier layer materials, which keep the half metallic nature of Fe₃O₄at the interface between Fe₃O₄layers and barrier layer, is one of main challenges in this field. Two-dimensional (2D) materials may be good candidates for this purpose. Molybdenum disulfide (MoS₂) is a transition metal dichalcogenide (TMD) semiconductor with distinctive electronic, optical and catalytic properties. Here, we show based on the first principle calculations that Fe₃O₄keeps a nearly fully spin polarized electron band at the interface between MoS₂and Fe₃O₄. We also present the first attempt to fabricate the Fe₃O₄/MoS₂/Fe₃O₄MTJs. A clear tunneling magnetoresistance (TMR) signal was observed below 200 K. Thus, our experimental and theoretical studies indicate that MoS₂can be a good barrier material for Fe₃O₄based MTJs. Our calculations also indicate that junctions incorporating monolayer or bilayer MoS₂are metallic.

Monolayer transition metal dichalcogenides exhibit remarkable electronic and optical properties, making them candidates for application within flexible nano-optoelectronics, however direct experimental determination of their thermal expansion coefficients (TECs) is difficult. Here, we propose a non-destructive method to probe the TECs of monolayer materials using surface-enhanced Raman spectroscopy (SERS). A strongly coupled Ag nanoparticle over-layer is used to controllably introduce temperature dependent strain in monolayers. Changes in the first-order temperature coefficient of the Raman shift, produced by TEC mismatch, can be used to estimate relative expansion coefficient of the monolayer. As a demonstration, the linear TEC of monolayer WS₂ is probed and is found to be 10.3 × 10⁻⁶ K⁻¹, which would appear support theoretical predictions of a small TEC. This method opens a route to probe and control the TECs of monolayer materials.

This paper reports an all REBCO (REBa₂Cu₃Ox, where RE = Y, Gd) superconducting magnet that generated a direct-current magnetic field of 26.86 T at 4.2 K in self-field, which is the highest magnetic field achieved by the high temperature superconducting (HTS) magnet. The magnet consists of a stack of 10 double pancake (DP) coils wound with two different width REBCO conductors, and the no-insulation (NI) winding technology was applied to increase its engineering current density. The inner and outer winding diameters and overall height of the magnet are 30 mm, 103.98 mm, and 102.6 mm, respectively. In order to control the stress risk in HTS magnet, the over-banding made of high elastic modulus stainless steel tapes was employed to the magnet. In addition, no obvious degradation was found in the performance of the magnet after quench at 26.86 T of 295.5 A. These test results demonstrate the great potential of REBCO magnets in high field applications.

A higher available magnetic field can provide more flexibility for significant research and construction. Inserting a feasible high-temperature-superconducting (HTS) magnet inside a low-temperature-superconducting (LTS) external is becoming an efficient method to build ultra-high-field (UHF) magnets. However, HTS insert magnets exhibited a concentration of magnetic strain during the energization, significantly owing to the screening-current effect. This phenomenon affects the structural integrity of HTS magnets, posing a difficulty for UHF magnets to reach a center field larger than 30 T. In this paper, we report the recent progress of the 35 T all-superconducting UHF magnet project. The full-cycle development of this homemade UHF magnet was a strong collaboration with multiple domestic institutions in China. We used commercial REBa₂Cu₃O 7 − x (RE = rare earth element, REBCO) conductors to wind the 20 T HTS insert. Specifically, we optimized the electromagnetic design of this HTS insert with consideration of screening-current induced strain. We applied the no-insulation and metal-as-insulation winding techniques simultaneously to balance the time constant of the two nested REBCO coils. During the excitation test, the HTS insert maintained a 256 A operation current as designed for nearly 18 h. With the energization of LTS external magnet, the entire facility reached a center field larger than 20 T for 16 h, 30 T for 6.35 h and 32 T for 2 h. It finally reached a 32.4 T center field, at which point the LTS external quenched, inducing the HTS insert to quench as well. After quench, the HTS magnet can maintain 229 A operation current during the postmortem standalone test. This 32.4 T result exhibits the effectiveness of considering the screening-current induced strain to enhance the structural integrity of REBCO magnets, and verifies the high reliability of REBCO magnets by generating a long-duration high field environment without any quench accident originating from the REBCO magnet.

For practical application, stability investigation of high temperature superconductor (HTS) is of great importance. An insert set-up has been developed at the Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). Measurements are performed as a function of transport current and magnetic field for several operating temperatures between 30 and 50 K. This work focuses on the design of insert set-up and the estimation of normal zone propagation velocity (Vnzp) and minimum quench energy (MQE). The experimentally obtained temperature dependence of Vnzp is compared with a simplified adiabatic model.

REBa ₂ Cu ₃ O _7-x (REBCO) coated conductor has emerged as a promising material for the development of ultra-high-field (UHF) magnets. However, the presence of screening-current induced strain within this tape-shaped conductor exerts a huge impact on the operational stability of REBCO coils. We focused on the design of an all-superconducting 36 T / 40 mm UHF magnet, which included a 16 T insert magnet consisting of two nested REBCO coils. In this study, the screening-current induced strain in the 16 T magnet was numerically studied using the coupled electromagnetic-mechanical model, which considered the tilting angle of REBCO tapes and the strain dependency of the critical current. We calculated the screening-current induced strain of each pancake individually and compared these results with those calculated by the sequential model. According to the coupled model, the maximum hoop strain was 0.52 %, relatively lower than that calculated by sequential model. The most dangerous pancake was estimated to be the outermost pancake of each coils. Additionally, we varied the critical current value of the REBCO tapes and studied the relationship between the critical current value and screening-current induced strain. This work provides a feasible way to calculate the screening-current effect in large-scale HTS magnets.