Additive manufacturing allows for the production of complex parts with minimum material waste, offering an effective technique for fabricating permanent magnets which frequently involve critical rare ...earth elements. In this report, we demonstrate a novel method - Big Area Additive Manufacturing (BAAM) - to fabricate isotropic near-net-shape NdFeB bonded magnets with magnetic and mechanical properties comparable or better than those of traditional injection molded magnets. The starting polymer magnet composite pellets consist of 65 vol% isotropic NdFeB powder and 35 vol% polyamide (Nylon-12). The density of the final BAAM magnet product reached 4.8 g/cm
, and the room temperature magnetic properties are: intrinsic coercivity H
= 688.4 kA/m, remanence B
= 0.51 T, and energy product (BH)
= 43.49 kJ/m
(5.47 MGOe). In addition, tensile tests performed on four dog-bone shaped specimens yielded an average ultimate tensile strength of 6.60 MPa and an average failure strain of 4.18%. Scanning electron microscopy images of the fracture surfaces indicate that the failure is primarily related to the debonding of the magnetic particles from the polymer binder. The present method significantly simplifies manufacturing of near-net-shape bonded magnets, enables efficient use of rare earth elements thus contributing towards enriching the supply of critical materials.
Topological insulators, a new quantum state of matter, create exciting opportunities for studying topological quantum physics and for exploring spintronic applications due to their gapless helical ...metallic surface states. Here, we report the observation of weak anti-localization and quantum oscillations originated from surface states in Bi₂Se₂Te crystals. Angle-resolved photoemission spectroscopy measurements on cleaved Bi₂Se₂Te crystals show a well-defined linear dispersion without intersection of the conduction band. The measured weak anti-localization effect agrees well with the Hikami-Larkin-Nagaoka model and the extracted phase coherent length shows a power-law dependence with temperature (l(Φ)∼T⁻⁰·⁴⁴), indicating the presence of the surface states. More importantly, the analysis of a Landau-level fan diagram of Shubnikov-de Hass oscillations yields a finite Berry phase of ∼0.42π, suggesting the Dirac nature of the surface states. Our results demonstrate that Bi₂Se₂Te can serve as a suitable topological insulator candidate for achieving intrinsic quantum transport of surface Dirac fermions.
Low-temperature variable-energy electron irradiation was used to induce non-magnetic disorder in a single crystal of a hole-doped iron-based superconductor, Ba1−xKxFe2As2, x = 0.80. To avoid ...systematic errors, the beam energy was adjusted non-consequently for five values between 1.0 and 2.5 MeV when sample resistance was measured in situ at 22 K. For all energies, the resistivity raises linearly with the irradiation fluence suggesting the creation of uncorrelated dilute point-like disorder (confirmed by simulations). The rate of the resistivity increase peaks at energies below 1.5 MeV. Comparison with calculated partial cross-sections points to the predominant creation of defects in the iron sublattice. Simultaneously, superconducting Tc, measured separately between the irradiation runs, is monotonically suppressed as expected, since it depends on the total scattering rate, hence on the total cross-section, which is a monotonically increasing function of the energy. Our work experimentally confirms an often-made assumption of the dominant role of the iron sub-lattice in iron-based superconductors.
The mechanism of unconventional superconductivity in iron-based superconductors (IBSs) is one of the most intriguing questions in current materials research. Among non-oxide IBSs, (Ba1-x K x )Fe2As2 ...has been intensively studied because of its high superconducting transition temperature and fascinating evolution of the superconducting gap structure from being fully isotropic at optimal doping (x ≈ 0.4) to becoming nodal at x > 0.8. Although this marked evolution was identified in several independent experiments, there are no details of the gap evolution to date because of the lack of high-quality single crystals covering the entire K-doping range of the superconducting dome. We conducted a systematic study of the London penetration depth, λ(T), across the full phase diagram for different concentrations of point-like defects introduced by 2.5-MeV electron irradiation. Fitting the low-temperature variation with the power law, Δλ ~ Tn , we find that the exponent n is the highest and the Tc suppression rate with disorder is the smallest at optimal doping, and they evolve with doping being away from optimal, which is consistent with increasing gap anisotropy, including an abrupt change around x ≃ 0.8, indicating the onset of nodal behavior. Our analysis using a self-consistent t-matrix approach suggests the ubiquitous and robust nature of s± pairing in IBSs and argues against a previously suggested transition to a d-wave state near x = 1 in this system.
•PrCo2 single crystals grown with a Boron Nitride (BN) crucible.•Reaction between the Pr-Co liquid melt and alumina crucible.•The experimentally observed saturation moment matches theoretically ...calculated value.
Single crystals of PrCo2 have been grown from flux starting from Pr0.44Co0.56 that was synthesized by arc melting of stoichiometric mixtures of the elements. The Pr0.44Co0.56 ingot was loaded in a Boron Nitride (BN) crucible and sealed inside a quartz tube under vacuum. The quartz ampoule was heated to 1203 K, and then slowly cooled down to 923 K at a rate of 2 K/h. The obtained crystals have octahedral shapes and are of cubic C15 Laves phase structure confirmed by room temperature powder and single-crystal X-ray diffraction measurements. Magnetic susceptibility data show the Curie temperature TC = 46.7 K. The experimental value of saturation magnetization Ms = 3.2 μB/f.u is consistent with our theoretical calculations.
Magnetically isotropic bonded magnets with a high loading fraction of 70 vol.% Nd-Fe-B are fabricated via an extrusion-based additive manufacturing, or 3D printing system that enables rapid ...production of large parts. The density of the printed magnet is ∼ 5.2 g/cm3. The room temperature magnetic properties are: intrinsic coercivity Hci = 8.9 kOe (708.2 kA/m), remanence Br = 5.8 kG (0.58 T), and energy product (BH)max = 7.3 MGOe (58.1 kJ/m3). The as-printed magnets are then coated with two types of polymers, both of which improve the thermal stability as revealed by flux aging loss measurements. Tensile tests performed at 25 °C and 100 °C show that the ultimate tensile stress (UTS) increases with increasing loading fraction of the magnet powder, and decreases with increasing temperature. AC magnetic susceptibility and resistivity measurements show that the 3D printed Nd-Fe-B bonded magnets exhibit extremely low eddy current loss and high resistivity. Finally, we demonstrate the performance of the 3D printed magnets in a DC motor configuration via back electromotive force measurements.
The magnetic and magnetostrictive properties of Fe/sub 1-x/Ga/sub x/ and Fe/sub 1-x-y/Ga/sub x/Al/sub y/ (a+y<0.3) single crystals were measured under compressive stresses up to 120 MPa and in ...magnetic fields up to 1 kOe. Values of /spl lambda//sub 100/ exceeding 200 ppm were observed in samples of Fe/sub 83/Ga/sub 17/ from 180 K to room temperature. Only a small monotonic decrease in magnetostriction with temperature was found. On the other hand, /spl lambda//sub 111/ maintained small negative values close to those of b.c.c. Fe. In all cases, no higher order magnetostrictive terms were detectable. With 15 to 20 atomic percent replacement of Fe by Ga, the magnetizations remain high (M, /spl cong/1.8 T) and the magnetic anisotropies reduced (K/sub 1//spl cong/10/sup 4/ J/m/sup 3/). Values of the 'stiff' (high magnetic field) Young's modulus, Bulk modulus, Poison's ratio, and elastic constant C/sub 11/-C/sub 12/ for Fe/sub 85/Ga/sub 15/ at room temperature are, respectively, 77 GPa, 111 GPa, 0.38, and 56 GPa.
We present a thorough study of doping dependent magnetic hysteresis and relaxation characteristics in single crystals of (Ba1−xKx)Fe2As2 (0.18≤x≤1). The critical current density Jc reaches maximum in ...the underdoped sample x=0.26 and then decreases in the optimally doped and overdoped samples. Meanwhile, the magnetic relaxation rate S rapidly increases and the flux creep activation barrier U0 sharply decreases in the overdoped sample x=0.70. These results suggest that vortex pinning is very strong in the underdoped regime, but it is greatly reduced in the optimally doped and overdoped regime. Transmission electron microscope (TEM) measurements reveal the existence of dislocations and inclusions in all three studied samples x=0.38, 0.46, and 0.65. An investigation of the paramagnetic Meissner effect (PME) suggests that spatial variations in Tc become small in the samples x=0.43 and 0.46, slightly above the optimal doping levels. Our results support that two types of pinning sources dominate the (Ba1−xKx)Fe2As2 crystals: (i) strong δl pinning, which results from the fluctuations in the mean free path l and δTc pinning from the spatial variations in Tc in the underdoped regime, and (ii) weak δTc pinning in the optimally doped and overdoped regime.
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