Abstract The temperature of the annealing treatment applied to the MnBi alloys with the intention to stabilize the α-MnBi “low-temperature” phase – a promising material for the rare-earth-free permanent magnets – is usually kept below the range of the β-Mn1.08Bi “high-temperature” phase. This work is aimed at the investigation of the effects of partial and complete α → β phase transformation in bulk magnetic-field-annealed magnets on their structure and magnetic properties. MnBi alloys modified with In, Mg and Sb were melt-spun, compacted to their full density and then heated in a magnetic field of 30 kOe to temperatures sufficiently high for the beginning of the α → β transformation. A heating of 15 °C/min was used which was immediately followed by cooling at the rate of 7.5 °C/min; the degree of the phase transformation was measured by in situ monitoring of changes in the sample magnetization. In all the studied alloys, the α → β transformation was found to improve the [0 0 1] fiber texture induced by the applied magnetic field, although it simultaneously decreased the coercivity. A partial transformation has resulted in a 11% increase of the maximum energy product for the Mn50Bi49Sb0.5In0.5 magnet, to 8.3 MGOe, and a 3% increase for the Mn50Bi46Mg3Sb0.5In0.5 magnet, to 12.8 MGOe. In the latter alloy, the α → β and reverse transformations were found to initially occur in well-defined pockets leading to clusters of large, highly textured α crystallites. In the Mn50Bi48.5Sb1.5 magnet, only part of the α phase could be recovered after the transformation, which instead promoted the paramagnetic MnBi0.9Sb0.1 phase.

中文翻译：

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由熔纺合金制备的 MnBi 基磁体：场退火过程中 α → β 相变的影响

摘要 用于稳定 α-MnBi“低温”相的 MnBi 合金的退火处理温度通常保持在 β -Mn1.08Bi“高温”相。这项工作旨在研究块状磁场退火磁体中部分和完全 α → β 相变对其结构和磁性能的影响。用 In、Mg 和 Sb 改性的 MnBi 合金进行熔纺，压实到它们的全密度，然后在 30 kOe 的磁场中加热到足够高的温度以开始 α → β 转变。使用 15°C/min 的加热速度，然后立即以 7.5°C/min 的速度冷却；通过原位监测样品磁化强度的变化来测量相变的程度。在所有研究的合金中，发现 α → β 转变改善了由外加磁场引起的 [0 0 1] 纤维织构，尽管它同时降低了矫顽力。部分转变导致 Mn50Bi49Sb0.5In0.5 磁体的最大能量积增加了 11%，达到 8.3 MGOe，Mn50Bi46Mg3Sb0.5In0.5 磁体的最大能量积增加了 3%，达到了 12.8 MGOe。在后一种合金中，发现 α → β 和反向转变最初发生在明确的口袋中，导致大的、高度织构的 α 微晶簇。在 Mn50Bi48.5Sb1.5 磁体中，转变后只能恢复部分 α 相，反而促进了顺磁性 MnBi0.9Sb0.1 相。