MnAl-C magnets, based on the tau-phase, contain no critical elements and have the potential to achieve energy products which would enable them to replace isotropic Nd-Fe-B bonded magnets, thus contributing to the long-term sustainability of rare earth magnet production. The tau-phase is thermodynamically metastable and is formed from the hexagonal precursor phase, epsilon. The phase transformation mechanism is of interest because it determines the population of defects in the product tau-phase. The various proposed transformation mechanisms will be reviewed here. The stability of the tau-phase can be greatly improved by the addition of C. The solubility of C in the relevant phases, along with the phase equilibria will be discussed. Detailed knowledge of the intrinsic magnetic properties of the tau-phase are useful both for estimating the upper limits of the extrinsic properties and as input for first principles calculations and micromagnetic models. Recent measurements of the intrinsic properties will be summarised and typical values will be given. The best extrinsic magnetic properties in MnAl-C to date were achieved more than 40 years ago in commercially produced, extruded materials. Since then, a number of alternative processing routes have been explored and these will be compared and contrasted here. The reported energy products of all MnAl-C materials fall short of the upper limits, estimated from the intrinsic properties of the tau-phase and therefore further improvements are possible. The extruded MnAl-C materials contain a range of defects, which potentially interact with magnetic domain walls. Understanding both this interaction and the formation mechanisms of the defects is the key to developing enhanced processing routes which deliver materials with higher energy product. Various insights from the characterisation of these defects on different length scales will be discussed. 


Recent Progress in the Development of MnAl-C Magnets

Microstructural Defects in Hot-Deformed and As-Transformed τ-MnAl-C using TEM

Rare earth free MnAl-C permanent magnets have the potential to replace polymer bonded Nd-Fe-B magnets in applications where energy products of 40-100 kJm-3 are required. In addition to further improvements in magnetic properties, successful commercialisation of MnAl-C magnets will require detailed studies of their stability in various environments. MnAl-C permanent magnets consist mostly of the hard magnetic τ-phase, which has its origin in the high temperature ε-phase. In samples consisting of the ε-phase, it is possible to precipitate the carbide phase, Mn3AlC. The orientation relationship between the ε-phase and Mn3AlC is of interest as the presence of the carbide precipitates may influence the subsequent formation and magnetic properties of the τ-phase. Other phases which may be present in small volume fractions are βMn and γ2, which are the decomposition products of the metastable τ-phase, and the carbide Al4C3. Both Mn3AlC and Al4C3 are known to react with water vapour in the ambient air. In certain circumstances, the expansion caused by this reaction can lead to a decrepitation of the material. In the current work, Mn-Al-C alloys of various compositions have been produced and have been heat treated in different ways in order to influence the formation of the carbides. After characterisation in the electron microscope, the resulting materials were exposed to laboratory air for long periods of time at various temperatures and levels of humidity in order to assess the effect of composition on the environmental stability of the material.



Formation of Carbide Phases in MnAl-C Magnets

The L10 structured τ-phase of the Mn-Al-C system gained interest for the potential use as a rare earth free hard magnetic material [1]. It has been reported that the formation of a small fraction of Mn3AlC precipitates leads to an increase in coercivity of the τ-phase [2]. Mn3AlC is also of interest because it is ferromagnetic, has a wide phase field in the Mn-Al-C equilibrium diagram and shares a two-phase region with the ε-phase of MnAl, which is the origin of τ-MnAl-C. Despite its broad homogeneity range, the Curie temperature (TC) of Mn3AlC is often reported to be at around 290 K [3,4]. There are only few reports of the influence of composition on the TC [5,6,7] and one has demonstrated a dramatic increase in TC up to 880 K with increasing Mn-content [5]. Furthermore, it has been reported that a full substitution of Al with Mn is possible, while retaining the ferromagnetic properties, resulting in Mn4C with a TC of 870 K [8]. The aim of the current work is to carry out a systematic investigation of the intrinsic magnetic properties of Mn3AlC in order to confirm the dramatic results presented in the literature and to assess the possible influence of Mn3AlC precipitates on the properties of MnAl-C permanent magnets. Two series of Mn-Al-C alloys have been prepared: in one series the Mn-content was constant while Al was substituted with C (compositions: Mn58Al34-xC8+x x = 0, 5, 10) and in the other series the C-content was constant, while Al was substituted with Mn (compositions: Mn60+xAl21-xC19 x = 0,5 ,10). The alloys were arc-melted in 10 g ingots, followed by a homogenisation at 1100°C in Ar-atmosphere for 5 days and were finally quenched into water. The chemical composition of the homogenised materials was determined using Inductively Coupled Plasma Optical Emission Spectroscopy for Mn- and Al-contents and by carrier-gas hot-extraction for C-content. In Fig. 1 and Fig. 2 the measured values of Mn-content were used for alloys produced in this study. Structural information about the phases in the alloys was gathered from XRD measurements, using a Bruker diffractometer with a Co-Kα radiation. Analysis of the XRD patterns was done using FullProf Suite software. Local chemical analysis of the microstructure was done by EDX-measurements using a Gemini Leo 1530 SEM. The magnetic properties of the alloys were measured using the VSM-mode of a Quantum Design PPMS. M(T)-curves were measured at an applied field of 0.1 T in a temperature range from 10 K to 900 K with a heating and cooling rate of 5Kmin-1. Furthermore, hysteresis loops up to an applied field of 14 T were measured at room temperature and at 100 K. In Fig.1 the TC of Mn3AlC is plotted against the Mn content of the phase from various sources. The Mn-content is related to the Mn3AlC formula unit containing 5 atoms. The curves represent the work of different groups that investigated the influence of composition on the TC. The results of the current work are shown in separate branches, since the two different series of alloys focus on different changes of the composition. The substitution of Al with C in the current work (solid black squares) shows a nearly constant TC at around 290 K, despite the change of the overall Mn-content. The same kind of substitution was done by Tong et al. [7], who reported a somewhat larger influence on TC: a change of 60 K was found in going from a composition of 2.75 to 3.0 Mn atoms per formula unit. Contrary to that, the substitution of Al with Mn in the current work (unfilled black squares) shows a drastic increase of the TC from around 355 K to 765 K. This behaviour of the TC is similar to the increase of the TC shown by the work of Wang et al. [5] and Butters et al. [6], who substituted Al with Mn as well. The work of Si et al. [8] focused on the properties of Mn4C, which shows a similar crystal structure to Mn3AlC. The TC of the Mn4C phase seems to fit well to the dependence of TC of Mn3AlC, which could confirm that a full substitution of Al with Mn is possible. Fig. 2 shows the magnetisation (M) as a function of Mn-content measured in the current work and data taken from Wang et al. [5]. The data for M was taken at 1.5 T, for which the samples are close to saturation. The Mn-content is related to the Mn3AlC formula unit. The M values are taken at 100 K. The curves represent the work of our group and the work of Wang et al. Compared to Fig. 1 we do not consider alloys of the first series, since they do not contain Mn3AlC only which means a change of M is connected to the phase fraction of Mn3AlC and not only connected to a change of composition. The alloy series in which Al was replaced by Mn shows a maximum of 92 Am2kg-1 at 3.4 Mn atoms per unit formula. Compared to the work of Wang et al. [5], who found a maximum for M of 90 Am2kg-1 for a stoichiometric composition the maximum of magnetisation is shifted to higher values of Mn atoms per unit cell. In the current work two different alloy series were used to study the effect of composition on the TC and M of Mn3AlC. The results confirm the dramatic increase in TC on substituting Al with Mn reported in the literature [5] and the decrease in M of Mn3AlC on increasing Mn content. The ferromagnetic nature of the Mn3AlC phase may influence the magnetic properties of τ-MnAl-C magnets, particularly for low temperature applications. To investigate this in more detail, further studies are needed.

**References:**

[1] Feng, L. et al. Acta Materialia 199, 155-168 (2020) [2] Ohtani, T. et al. IEEE Transactions on Magnetics 13, 1328–1330 (1977) [3] Dierkes, H. et al. Inorganic Chemistry 56, 1045-1048 (2017) [4] Wang, B. et al. Journal of Applied Physics 108, 093925 (2010) [5] Wang, X. et al. Journal of Magnetics 24 123-127(2019) [6] Butters, R. P.; Myers H.P. CI. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science 46, 895-902 (1955) [7] Tong, P. et al. Solid State Communications 138, 64-67 (2006) [8] Si, P. et al. Applied Physcis Letter 112 192407 (2018)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/87556979e163a2e8decd39c3937fb997.jpg)
Fig. 1: Curie temperature of Mn3AlC depending on the Mn content per formula unit.

![](https://assets.underline.io/uploads/markdown_image/1/image/91b633b019279b6a999f1d50417249b0.jpg)
Fig. 2: Measured magnetisation at an applied field of 1.5 T at 100 K for different MnAlC-alloys.

 The Dependence of the Intrinsic Magnetic Properties of Mn3AlC on its Chemical Composition

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Presentations

Recent Progress in the Development of MnAl-C Magnets

Microstructural Defects in Hot-Deformed and As-Transformed τ-MnAl-C using TEM

Formation of Carbide Phases in MnAl-C Magnets

The Dependence of the Intrinsic Magnetic Properties of Mn3AlC on its Chemical Composition

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