Mechanical and electrical properties of low temperature phase MnBi

Abstract

Low temperature phase (LTP) manganese bismuth (MnBi) is a promising rare-earth-free permanent magnet material due to its high intrinsic coercivity and large positive temperature coefficient. While scientists are making progress on fabricating bulk MnBi magnets, engineers have begun considering MnBi magnets for motor applications. Physical properties other than magnetic ones could significantly affect motor design. Here, we report results of our investigation on the mechanical and electrical properties of bulk LTP MnBi and their temperature dependence. A MnBi ingot was prepared using an arc melting technique and subsequently underwent grinding, sieving, heat treatment, and cryomilling. The resultant powders with a particle size of ∼5 μm were magnetically aligned, cold pressed, and sintered at a predefined temperature. Micro-hardness testing was performed on a part of original ingot and we found that the hardness of MnBi was 109 ± 15 HV. The sintered magnets were subjected to compressive testing at different temperatures and it was observed that a sintered MnBi magnet fractured when the compressive stress exceeded 193 MPa at room temperature. Impedance spectra were obtained using electrochemical impedance spectroscopy at various temperatures and we found that the electrical resistance of MnBi at room temperature was about 6.85 μΩ m.

Synthesis of Hard Magnetic α-MnBi Phase by High Pressure Torsion and Field Assisted Annealing

“…3) and hardness measurements. The measured hardness values or the α-MnBi phase (66 ± 3 HV) are lower as previously reported 109 ± 15 HV [44], however, the microstructure is not comparable. A hardness value similar to the literature is expected when the ratio o α-MnBi phase increases or nanoindentation would have been perormed directly at the existing α-MnBi phase.…”

Atomically thin oxide layer on the elemental superconductor Ta(001) surface

“…The way we obtained this state points to the presence of oxygen vacancy defects during formation of the surface. The enhanced oxygen vacancy diffusion was shown to be the source of the adaptive crystalline structure in amorphous and crystalline forms of Ta 2 O 5 [32], an experimental prototype for the resistive random access memory. Recently [33], also stable polarons were predicted to exist in charged vacancy sites that should affect the carriers mobility.…”

“…3)). The parental bulk material, especially for the local structure containing the sp 3 -hybridized trimer, is tantalum pentoxide Ta 2 O 5 [32,41]. It is an oxide with a high dielectric constant, and with a reported band gap of ∼ 1 -4 eV.…”

Preparation and magnetic properties of MnBi alloy and MnBi /Fe hybrid magnets

“…It was found that the temperature stability of the magnetic properties of the NdFeB and Sm 2 Fe 17 N x magnet could be improved by compounding MnBi [6,7].There are many phases in MnBi materials, but only lowtemperature-phase MnBi shows ferromagnetism and its coercivity temperature coefficient is positive. The ferromagnetic low-temperature phase (LTP) MnBi exhibits a hexagonal structure and has uniaxial magnetocrystalline anisotropy at room temperature [8][9][10]. The magnetic properties of MnBi alloy are extremely sensitive to the content of low-temperature MnBi phase.…”