RESUMEN
Fabrication and extensive characterization of hard-soft nanocomposites composed of hard magnetic low-temperature phase LTP-MnBi and amorphous Fe70Si10B20 soft magnetic phase for bulk magnets are reported. Samples with compositions Mn55Bi45 + xâ (Fe70Si10B20) (x = 0, 3, 5, 10, 20 wt.%) were prepared by spark plasma sintering of powder mixtures. Characterization has been performed by X-ray diffraction, scanning and transmission electron microscopy, magnetometry and 57Fe MÓ§ssbauer spectroscopy. It was shown that samples contain crystallized and nanometric LTP-MnBi phases with various elemental compositions depending on the degree of Bi clustering. Complex correlations between starting compositions, processes during fabrication, and functional magnetic characteristics were observed. Unexpected special situations of the relation between microstructure and magnetic coupling mechanisms are discovered. Exchange spring effects of different strengths occur, being very sensitive to morpho-structural and compositional features, which in turn are controlled by processing conditions. An in-depth analysis of related microscopic characteristics is provided. Results of this work suggest that fabrication by powder metallurgy routes, such as spark plasma sintering of hard and soft magnetic powder mixtures, of MnBi-based composites with exchange spring phenomena have a high potential in designing and optimization of suitable materials with tunable magnetic properties towards rare-earth-free permanent magnet applications.
RESUMEN
The second magnetization peak (SMP) in the fourfold symmetric superconducting single crystals (such as iron pnictides and tetragonal cuprates) has been attributed to the rhombic-to-square transition (RST) of the quasi-ordered vortex solid (the Bragg vortex glass, BVG). This represents an alternative to the pinning-induced BVG disordering as the actual SMP mechanism. The analysis of the magnetic response of BaFe2(As1-xPx)2 specimens presented here shows that the SMP is not generated by the RST. However, the latter can affect the pinning-dependent SMP onset field if this is close to the (intrinsic) RST line, through the occurrence of a "shoulder" on the magnetic hysteresis curves m(H), and a maximum in the temperature variation of the DC critical current density. These features disappear in AC conditions, where the vortex system is dynamically ordered in the RST domain, emphasizing the essential role of vortex dislocations for an efficient accommodation of the vortex system to the pinning landscape and the SMP development. The m(H) shoulder is associated with a precipitous pinning-induced proliferation of dislocations at the RST, where the BVG elastic "squash" modulus softens. The DC magnetization relaxation indicates that the pinning-induced vortex system disordering continues above the RST domain, as the basic SMP mechanism.
RESUMEN
Bulk discs (20 mm diameter and 4.3 mm thickness) of MgB2 added with Ge2C6H10O7 were obtained by Spark Plasma Sintering. Six samples with composition MgB2(Ge2C6H10O7)0.0014 and one undoped sample were fabricated under similar conditions and were magnetically characterized in order to determine the scattering of properties and reproducibility. The main source of the scattering of the properties is the decomposition of the additive due to elimination of the organic part in gas form, which occurs stepwise with intensive vacuum drops at around ~ 560 and ~ 740 °C. A third drop, which is sometimes not well resolved being part of the second peak at 740 °C, occurs at ~ 820 °C. The critical temperature at the midpoint of the transition, Tc, shows only a relatively small variation between 37.4 and 38 K, and the irreversibility field at a low temperature of 5 K takes values between 8 and 10 T. The pinning force and pinning force related parameters do not correlate with the carbon substituting for boron in MgB2 and suggest a synergetic influence of the microstructural details and carbon. Overall, despite the superconducting properties scattering, the samples are of high quality. Stacked into a column of six samples, they can trap at the center and on the surface of the column a magnetic field of 6.78 and 5.19 T at 12 K, 5.20 and 3.98 T at 20 K and 2.39, and 1.96 T at 30 K. These promising values, combined with facile fabrication of the samples with relatively high quality and reproducibility, show the feasibility of their use in building complex and large compound arrangements for bulk magnets and other applications.