In contrast to the interaction between two magnets with opposite magnetization directions, the interaction between a permanent magnet and a superconductor can be stable and result in magnetic levitation. This property can be exploited for the development of high velocity rotating bearings with no mechanical contacts and for the development of levitated trains. In this review, we focus on this latter application. After a brief description of the other techniques developed for levitating trains and the resulting achievements, we describe the magnet–superconductor interaction and recall the achievements in this field. We then give insights into the properties of the employed magnets and arrangement of magnets and we detail the characteristics and the fabrication processes of the most frequently used superconductors. Focusing on physics, we detail the procedures generally used for measuring the vertical (levitation) and the lateral (guidance) forces in magnetic levitation and the results obtained from experiments. We detail and give a critical review of the various models proposed for reproducing the force measurements. In the conclusion we discuss the possible future developments of the technology.

We report levitation force cycles resulting from measurements carried out on a YBCO and a MgB ₂ disk cooled down in the field of a permanent magnet. In both cases the amplitude of the levitation force tends toward maximum values as the temperature decreases. Otherwise, the cycles are almost closed at low temperature and strongly hysteretic in the high temperature range. The hysteresis of the force cycles is attributed to the distribution of the currents induced in the sample by the field of the magnet. The saturation of the levitation forces at low temperature is related to that of the magnetic moment of the disks. We show that this type of measurement allows for the determination of the critical current density of superconductors in a restricted domain of temperatures.

In situ MgB₂ superconducting samples were prepared by using the spark plasma sintering method. The density of the obtained bulks was up to 95% of the theoretical value predicted for the material. The structural and microstructural characterizations of the samples were investigated using X-ray diffraction and SEM and correlated to their superconducting properties, in particular their critical current densities, J_c, which was measured at 20 K. Extremely high critical current densities of up to 6.75 × 10⁵ A/cm² in the self-field and above 10⁴ A/cm² at 4 T were measured at 20 K, indicating that vortex pinning is very strong. This property is mainly attributed to the sample density and MgB₂ nanograins in connection to the presence of MgO precipitates and areas rich in boron.

From the applications' point of view, the advantage of low density given by MgB ₂ material must be taken into consideration, and the generation of strong magnetic flux densities using MgB ₂ should be investigated. In this paper, we have studied the magnetic properties of samples processed by a fast spark plasma sintering machine that is able to produce dense and high-quality MgB ₂ samples. Experiments were carried out both on small pieces of the samples and on large-sized samples performing trapped field measurements in a field-cooling process. For temperatures between 10 and 30 K, the results show a strong dependence of the magnetic behavior of the large-sized samples to the applied magnetic field sweep rate, whereas nothing particular appeared on the small samples. As the magnetic flux density produced by MgB ₂ bulks is directly linked to the potential of the applications, we report on the field produced at the surface of one single MgB ₂ sample of 30-mm diameter and the field produced inside a stack of two MgB ₂ samples of 20-mm diameter. A generation of magnetic flux density (magnetic polarization μ ₀ M) up to 4.80 T at 10 K and 3.92 T at 20 K inside the stack of the two MgB ₂ samples was observed under a negative supporting field of -1.47 and -1.95 T, respectively. According to these values to their very low density and to their ease of manufacturing, MgB ₂ bulks are promising materials for the applications of superconductors.

We present preliminary results suggesting that MgB ₂ is a superconductor that could be used in Maglev vehicles. For this purpose, we compare the levitation capability of MgB ₂ samples to those of YBCO bulks.

This study aims to investigate the sintering mechanisms of magnesium diborides using spark plasma sintering and optimize the sintering process conditions. Three different precursor materials were used: (i) commercial powder MgB₂, (ii) mixtures of magnesium and nano boron powder (Mg + 2B), and (iii) mixtures of Mg and MgB₄ powder for an ‘in-situ’ reactive synthesis. The density of the obtained bulks exceeds 90% of the theoretical density of the material. The structure and microstructure of the samples were analyzed using x-ray diffraction and scanning electron microscope, which were correlated with their superconducting properties. The critical current density (J _c), pinning force (f _p), and electrical resistivity (ρ) of the samples were also investigated. The results show that the in-situ reactive synthesis of MgB₂ using magnesium and nano boron powder was the most effective method for producing high-quality samples with improved superconducting properties. The findings could lead to the development of more efficient methods for producing high-performance of MgB₂ bulks.