Presentation Information
[P1-49]Coercivity mechanism of rare earth-free Cr substituted Mn1-xCrxAlGe for “thermoelectric permanent magnet” applications
*Andres Martin-Cid1, Babu Madavali1, Fuyuki Ando1, Yuya Sakuraba1, Ken-ichi Uchida1,2, Hossein Sepehri-Amin1 (1. National Institute for Materials Science (NIMS) (Japan), 2. Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo (Japan))
Keywords:
Rare earth free magnet,Transverse thermoelectric,Coercivity mechanism
Permanent magnets are crucial materials for the decarbonization of society, being used in key applications such as electric motors and electric generators. Since their discovery, the permanent magnet market has been dominated by high-performance rare earth (RE) based magnets using RE elements such as Nd, Dy, and Tb. However, the increasing demand and scarcity of some heavy RE elements have raised interest in the development of alternative magnets for specific applications that are low in and/or free of RE elements.
Recently, a new application has been reported for permanent magnet materials related to efficient thermal management making use of the thermoelectric cooling effect [1]. For this, an artificially tilted multilayer device combining a thermoelectric material such as Bi88Sb12 with an anisotropic permanent magnet such as Nd2Fe14B can achieve high transverse thermoelectric conversion performance, reaching record high normalized power density among transverse thermoelectric modules [2]. The use of permanent magnets with finite remanence and coercivity has eliminated the need for an external magnetic field to operate transverse thermoelectric generation based on the anomalous Nernst effect (ANE). Based on this new application, there is room to explore the potential of permanent magnets that are not necessarily among the commercially available permanent magnets but have excellent thermoelectric properties.
In this study, we investigated the potential of bulk MnAlGe-based compound for the “thermoelectric permanent magnet” application. This system is one of the only four uniaxial ferromagnetic materials reported until now with a negative anomalous Nerst coefficient at room temperature, together with Nd2Fe14B-type, MnGa-type [3], and MnBi-type magnets [4]. The MnAlGe system has been shown to have an enhanced uniaxial anisotropy from 2.5×105 J/m3 up to 7.3×105 J/m3 in thin films by Cr substitution of Mn [5]. This material has also been reported to have an increasing negative anomalous Nerst coefficient of up to -0.5 µV/K with the substitution of Mn by Cr in thin films [3]. However, there is no report on their performance in the bulk form.
In this work, we explored the development of permanent magnet properties of the bulk Mn1-xCrxAlGe system for x ranging from 0 to 0.5 and its transverse thermoelectric properties. The bulk isotropic magnets were produced by induction melting and casting the pure constituents followed by melt-spinning, hammer-milling, and hot pressing to obtain nearly fully dense magnets. The parameters of each process were kept the same for all compositions to better compare the effect of Cr substitution in the final properties. X-ray diffraction confirmed that the hot-pressed magnets are composed of a single phase identified as the C38 MnAlGe phase. SEM-EDS analysis verified the target Cr-Mn ratio; however, all samples exhibited a slight Ge depletion. Based on SEM observations, it has been confirmed that the samples exhibit a comparable microstructure characterized by a broad range of grain sizes extending from less than 1 μm to up to 10 μm.
An analysis of the transverse thermoelectric properties by lock-in thermography shows an increase of the ANE coefficient when substituting Mn by Cr, having a maximum value of -0.74×10-6 VK-1 for x = 0.1, decreasing with further substitution of Cr by Mn. On the other hand, the Seebeck coefficient presents a maximum value of 18.5×10-6 VK-1 for x = 0.2. SQUID-VSM measurements show a trend of the coercivity with Cr content similar to that of the Seebeck coefficient, reaching a maximum value of 1.18 T for x = 0.2, followed by a decrease for higher concentrations of Cr. The saturation magnetization presents a similar trend to the coercivity, while the remanent magnetization is almost constant for all compositions around 50% of the saturation magnetization, as expected from isotropic magnets. Focusing on the curve of first magnetization, samples with a Cr concentration of 0.2 or greater show a kink, typically associated with magnetic domain pinning, which was studied based on detailed microstructure characterizations and magnetic domain observations. In this talk, we will discuss the transport properties of bulk Mn1-xCrxAlGe magnets along with their coercivity mechanism.
References
[1] K. Uchida, et al., Adv. Energy Mater. 2024, 14, 2302375
[2] F. Ando, et al., Energy Environ. Sci., 2025, in press.
[3] K. Ito, T. Kubota and K. Takanashi, Phys. Rev. Applied 2024, 21, 054012
[4] A. Sola, et al., AIP Advances 2023 13, 035231
[5] Takahide Kubota, et al., Appl. Phys. Express 2019, 12, 103002
Recently, a new application has been reported for permanent magnet materials related to efficient thermal management making use of the thermoelectric cooling effect [1]. For this, an artificially tilted multilayer device combining a thermoelectric material such as Bi88Sb12 with an anisotropic permanent magnet such as Nd2Fe14B can achieve high transverse thermoelectric conversion performance, reaching record high normalized power density among transverse thermoelectric modules [2]. The use of permanent magnets with finite remanence and coercivity has eliminated the need for an external magnetic field to operate transverse thermoelectric generation based on the anomalous Nernst effect (ANE). Based on this new application, there is room to explore the potential of permanent magnets that are not necessarily among the commercially available permanent magnets but have excellent thermoelectric properties.
In this study, we investigated the potential of bulk MnAlGe-based compound for the “thermoelectric permanent magnet” application. This system is one of the only four uniaxial ferromagnetic materials reported until now with a negative anomalous Nerst coefficient at room temperature, together with Nd2Fe14B-type, MnGa-type [3], and MnBi-type magnets [4]. The MnAlGe system has been shown to have an enhanced uniaxial anisotropy from 2.5×105 J/m3 up to 7.3×105 J/m3 in thin films by Cr substitution of Mn [5]. This material has also been reported to have an increasing negative anomalous Nerst coefficient of up to -0.5 µV/K with the substitution of Mn by Cr in thin films [3]. However, there is no report on their performance in the bulk form.
In this work, we explored the development of permanent magnet properties of the bulk Mn1-xCrxAlGe system for x ranging from 0 to 0.5 and its transverse thermoelectric properties. The bulk isotropic magnets were produced by induction melting and casting the pure constituents followed by melt-spinning, hammer-milling, and hot pressing to obtain nearly fully dense magnets. The parameters of each process were kept the same for all compositions to better compare the effect of Cr substitution in the final properties. X-ray diffraction confirmed that the hot-pressed magnets are composed of a single phase identified as the C38 MnAlGe phase. SEM-EDS analysis verified the target Cr-Mn ratio; however, all samples exhibited a slight Ge depletion. Based on SEM observations, it has been confirmed that the samples exhibit a comparable microstructure characterized by a broad range of grain sizes extending from less than 1 μm to up to 10 μm.
An analysis of the transverse thermoelectric properties by lock-in thermography shows an increase of the ANE coefficient when substituting Mn by Cr, having a maximum value of -0.74×10-6 VK-1 for x = 0.1, decreasing with further substitution of Cr by Mn. On the other hand, the Seebeck coefficient presents a maximum value of 18.5×10-6 VK-1 for x = 0.2. SQUID-VSM measurements show a trend of the coercivity with Cr content similar to that of the Seebeck coefficient, reaching a maximum value of 1.18 T for x = 0.2, followed by a decrease for higher concentrations of Cr. The saturation magnetization presents a similar trend to the coercivity, while the remanent magnetization is almost constant for all compositions around 50% of the saturation magnetization, as expected from isotropic magnets. Focusing on the curve of first magnetization, samples with a Cr concentration of 0.2 or greater show a kink, typically associated with magnetic domain pinning, which was studied based on detailed microstructure characterizations and magnetic domain observations. In this talk, we will discuss the transport properties of bulk Mn1-xCrxAlGe magnets along with their coercivity mechanism.
References
[1] K. Uchida, et al., Adv. Energy Mater. 2024, 14, 2302375
[2] F. Ando, et al., Energy Environ. Sci., 2025, in press.
[3] K. Ito, T. Kubota and K. Takanashi, Phys. Rev. Applied 2024, 21, 054012
[4] A. Sola, et al., AIP Advances 2023 13, 035231
[5] Takahide Kubota, et al., Appl. Phys. Express 2019, 12, 103002
