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The magnetic properties of ThMn12-type Fe-rich compounds were investigated by producing Sm(Fe0.8Co0.2)10Si2 ribbons. The produced ribbons, with different conditions by varying the melt-spinning conditions, were characterized to investigate physical and magnetic properties. Weight fraction of ThMn12-type phase decreased from 94.5 to 57.1 wt. % as the melt-spinning wheel speed increased from 6.5 to 39 m/s, and corresponding magnetizations and coercivities were substantially varied; the coercivity increased up to 0.175 T from 0.058 T by increasing the wheel speed from 6.5 to 26 m/s, and their magnetization also increased from 89.81 Am2/kg to 105.58 Am2/kg, even though the content of ThMn12-type phase decreased. Morphologies of the ribbons were also observed to verify the melt-spinning effects on the surface conditions and grain sizes. It was found that the particle and grain sizes in the ribbons became smaller and striped patterns appeared as the wheel speed increased. The grain size decreased from about 1 μm to 250 nm by increasing the wheel speed from 6.5 to 39 m/s. [ABSTRACT FROM AUTHOR]

The low coercivity in MnAl alloys has become a barrier to develop high performance MnAl‐based magnets for more than five decades. Herein, a high room‐temperature coercivity of 0.59 T in MnAl disc is achieved by severe plastic deformation. The grain size of the MnAl decreases significantly when ε‐phase is transformed into τ‐phase and when the τ‐phase is severely deformed. The aging process enhances the magnetization of the deformed τ‐phase significantly and reduces the coercivity of the MnAl disc to 0.396 T. The magnetic domain structure of the severely deformed τ‐MnAl exhibits many pinning sites in the intergranular regions and within the domains, resulting in the high coercivity of the bulk samples. The deformation process produces locally oriented texture of τ‐phase. This study opens a window for development of MnAl‐based bulk magnet with high coercivity and enhanced remanence. [ABSTRACT FROM AUTHOR]

High coercivity B-N-doped MnAl alloys were prepared by the traditional induction melting, high temperature homogenization, water-quenching, post-annealing, and severe plastic deformation process. The massive phase transformation of the ε→τ in the B-N-doped ε-MnAl occurs at the temperatures above 680 K and accomplishes at 760 K, both of which are slightly higher than those of the ε-MnAl without elemental doping. The Currie point of the B-N-doped τ-MnAl is measured to be 620 K, which is lower than that of the doping-free τ-MnAl. High-pressure torsion was employed to decrease the crystal size and increase the coercivity of samples. A coercivity up to 0.54 T was achieved in the B/N doped MnAl bulk sample after severe plastic deformation under 6 GPa. The aging processing on the severely deformed samples enhances the remanent magnetization significantly and decreases the coercivity slightly. Our work shows that the high-pressure torsion process is effective in developing coercivity in MnAl-based bulk magnet. [ABSTRACT FROM AUTHOR]

The ferromagnetic MnAl-C powders were prepared by using a one-step solid-state reaction method starting from Mn micro-/nano-particles and Al/C micropowders. The bulk MnAl-C with enhanced coercivity was prepared by high-pressure compaction of these MnAl-C powders. The grain size of the τ-phase was significantly reduced during high-pressure compaction, which may also result in a decomposition of the τ-MnAl. Carbon element stabilizes the τ-phase under both ambient and high pressures. The annealing temperature and time intervals are crucial for the preparation of high purity τ-phase samples. The MnAl-C with smaller particle size were produced from Mn nanoparticles. In comparison with the samples prepared from Mn micropowders, the product prepared from Mn nanoparticles shows lower purity, owing to the surface oxidation of the precursor nanoparticles. After high pressure compaction, the coercivities of the bulk MnAl and MnAl-C were increased from 0.05 T and 0.08 T to 0.39 T and 0.22 T, respectively. The room temperature magnetization of the MnAl-C sample at 4 T reached up to 95 Am2/kg. [ABSTRACT FROM AUTHOR]

Bulk MnAl-C magnets were prepared via high-pressure compaction of melt-spun ribbons. The ε → τ phase transformation (PT) in the melt-spun MnAl and MnAlC ribbons occurs mainly via the massive mode, whereas the displacive mode is not prominent. The massive PT temperatures of the melt-spun MnAl and MnAlC ribbons were measured to be in the range 660-739 K and 700-758 K, respectively. The PT temperatures of the melt-spun MnAlC ribbons are lower than those of water-quenched MnAlC. The annealing process for producing τ-phase from ε-phase was optimized based on the measured PT temperature range. As a result, no decomposition of the metastable ε/τ-phase MnAl and MnAlC were observed during the optimized high temperature annealing. High-purity τ-phase MnAlC ribbons with high magnetization (122 Am2/kg at 8 T) and low coercivity (0.13 T) were obtained. The coercivity of the bulk MnAlC obtained by high-pressure compaction was significantly enhanced and reached 0.38 T partially due to the grain refinement of the sample during high-pressure deformation. [ABSTRACT FROM AUTHOR]

Mn54Al46C2.44 alloy as new permanent magnetic materials was produced by induction melting, and its powders were produced by low energy ball milling. The milled powders were annealed to study the effect on their phase transformation and magnetic properties. The powder undergoes massive phase transformation at an annealing temperature of 450 °C, and reaches the maximum saturation magnetization ( \sigma s) when annealed at 480 °C for 5 min, while its intrinsic coercivity ( H\mathrm {ci}) continually increases with the temperature. The initial increment of the H\mathrm {ci} is due to the phase transformation from paramagnetic \varepsilon -phase to ferromagnetic \tau -phase and remaining \varepsilon -phase and formation of secondary phases, such as nonmagnetic - and \beta -phases, that acts as pinning points for the domain walls. The next one is caused by the increasing percentage of the secondary phases that decreases \sigma s . Based on the delta M analysis, it can be verified that the increased H\mathrm {ci} with increasing secondary phases when annealed at temperatures above 480 °C is due to the decreasing magnetization. [ABSTRACT FROM AUTHOR]

• The Sn added MnBi bulk magnets were prepared by hot compaction process, and it was found that it has superior coercivity compared to pure MnBi bulk magnets. • The coercivity (Hc) of Sn added MnBi bulk magnets could be effectively enhanced due to the construction of intergranular phases. • The (BH)max of 1 wt.% Sn added MnBi bulk magnet is recorded of 7.84 MGO, which shows a 7% increase compared to the pure MnBi bulk magnet. • The Sn added MnBi bulk magnet with enhanced coercivity, making it a promising candidate for the next-generation rare-earth-free permanent bulk magnets. The Sn addition MnBi bulk magnets with high density and superior coercivity (H c) were prepared by hot compaction. It was found that the added Sn was dispersed between the MnBi particles as an intergranular phase. The appropriate intergranular phase content could be beneficial in improving the H c due to the strong magnetic isolation effects. The H c of the MnBi/Sn bulk magnet was increased from 8.6 kOe to 11.5 kOe as the Sn addition contents were increased in the range of 0 wt.% to 5 wt.%. A relatively high squareness (M r / M s) of 0.91 was obtained for the 1 wt.% Sn addition Mn 56 Bi 44 bulk magnet exhibiting the maximum energy product (BH) max of 7.8 MGOe with high H c of 10 kOe at room temperature. [ABSTRACT FROM AUTHOR]

The temperature for phase transformation of ε → τ in Mn 54 Al 46 C x (hereafter denoted by MnAlC x , x  = 0–5) alloys was systematically studied by using the traditional thermal analysis method and a more precise magnetic measurement method developed in this work. The effect of carbon content and cold-rolling on the magnetic properties of the MnAlC x alloys was studied. The onset temperature of massive growth of τ- from ε-phase increases with increasing carbon content in MnAlC x . The carbon content has little effect on the onset temperature of displacive growth of ε → τ but has substantial effect on the displacive growth rate at certain temperature. Pure τ-MnAlC x with varied x was obtained by annealing the ε-phase at optimum temperatures determined by our method. The Curie temperature of the MnAlC x decreases significantly first and then increases slightly with increasing carbon content. The coercivity and the remanent magnetization of the as-prepared τ-MnAlC x increase with increasing x first and become stable at x  > 3 and x  > 4, respectively. A cold-rolling method was introduced in this work to enhance the magnetic properties of τ-MnAlC x . The coercivity and remanent magnetization of the τ-MnAl after cold-rolling were enhanced up to 4.3 times and 3.3 times, respectively. The coercivity of the cold-rolled τ-MnAlC x decreases with increasing x and becomes stable when x  > 2. The remanent magnetization and saturation magnetization of the cold-rolled τ-MnAlC x vary little with x . [ABSTRACT FROM AUTHOR]