Yangkun He

TL;DR: In this paper, the authors reported the thermoelectric properties of n-type Mg3(Bi,Sb)2 alloyed single crystals grown by a one-step Mg-flux method using sealed tantalum tubes.

Abstract: The discovery of a tenfold increase in magnetostriction of Fe by alloying nonmagnetic Ga was a breakthrough in magnetostrictive materials. The large magnetostriction is attributed to tetragonal nanoheterogeneities dispersed in the bcc matrix. A further remarkable fivefold increase is achieved by trace rare earth doping (<1 at%) up to a value of ≈1500 ppm, more than 50 times that of pure iron. However, it remains a mystery why trace rare earth dopants can induce such giant magnetostriction. Here, it is found that interaction of rare earth dopants with the nanoheterogeneities produces the giant magnetostriction, through a combination of experimental studies, first‐principles calculation and phase field simulations. The dopants tend to enter the nanoheterogeneities, increasing their distortion thereby creating a larger tetragonal distortion of the matrix as well as increased magnetocrystalline anisotropy. A mesoscopic model is developed using phase field simulation showing that the bulk tetragonal distortion arises mainly from those nanoheterogeneities with fixed Ga‐Ga pairs parallel to the applied magnetic field. Increased tetragonal distortion of the doped nanoheterogeneities leads to further distortion of the matrix. The results deepen the understanding of heterogeneous magnetostriction, and will guide the search for new magnetic materials with giant magnetostriction.

Abstract: There is an increasing demand for permanent magnets with high‐temperature stability. Normally, the magnets are heat treated above the operating temperature range to saturate the irreversible losses, thereby producing magnets that are stable, but with significantly degraded performance. In addition, uncertainty about the losses in differently shaped magnets causes difficulties for efficient magnetic circuit design. The variation of the initial irreversible losses with magnet shape, hysteresis loop, and operating temperature is presented here. Losses are not associated with thermal demagnetization of the whole magnet, but the damage is concentrated in an outer surface “skin” at the pole surfaces, where the demagnetizing field is nonuniform. The initial irreversible remanence loss at room temperature ΔMr′/Mr can be predicted as the product of the slope of the high‐temperature magnetization curve k(T) and the effective demagnetizing factor N . Heat treatment with the vulnerable surfaces in contact with soft iron slabs moves the skin out of the permanent magnet and reduces the irreversible remanence loss by 30% or more, potentially improving the performance of permanent‐magnet motors, generators, bearings, and thrusters. The results deepen the understanding of the magnetism of irreversible losses and will guide the applications of permanent magnets at higher temperatures.

TL;DR: In this paper, the relation between magnetostriction and the nano-heterogeneities is established by tailoring the nanoinclusion size with alloy composition and annealing temperature, and a model is presented to explain how coherent inclusions with the ordered B2-like or fcc-like structure can induce tetragonal distortions of the matrix in the relevant two-phase regions.