Special Issue "Manganese-based Permanent Magnets"

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A special issue of Metals (ISSN 2075-4701).

Deadline for manuscript submissions: closed (31 March 2015)

Special Issue Editor

Guest Editor
Prof. Dr. Ian Baker (Website)

Thayer School of Engineering, Dartmouth College, 14 Engineering Drive, Hanover, NH 03755-8000, USA
Fax: +1 603 646 3856
Interests: microstructural characterization; phase transformations; mechanical properties; magnetic materials

Special Issue Information

Dear Colleagues,

There is a significant gap between the energy product, BHmax, of both the traditional ferrite and AlNiCo permanent magnets of less than 10 MGOe and that of the rare earth magnets of greater than 30 MGOe. This is a gap that Mn-based magnets could potentially fill inexpensively. This special issue presents work on the development of both MnAl and MnBi permanent magnets. Some of the challenges involved in the development of these magnets include improving the compounds’ energy product, increasing the thermal stability of these metastable compounds, and producing them in quantity as a bulk material. These challenges are addressed from both experimental and theoretical points of view in the papers presented here.

Prof. Dr. Ian Baker
Guest Editors

Submission

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Keywords

  • permanent magnets
  • Mn-based magnets
  • maximum energy product
  • MnAl
  • MnBi

Published Papers (6 papers)

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Editorial

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Open AccessEditorial Manganese-based Permanent Magnets
Metals 2015, 5(3), 1435-1436; doi:10.3390/met5031435
Received: 10 August 2015 / Accepted: 10 August 2015 / Published: 11 August 2015
PDF Full-text (279 KB) | HTML Full-text | XML Full-text
Abstract
There is a significant gap between the energy product, BH, where B is the magnetic flux density and H is the magnetic field strength, of both the traditional ferrite and AlNiCo permanent magnets of less than 10 MGOe and that of the [...] Read more.
There is a significant gap between the energy product, BH, where B is the magnetic flux density and H is the magnetic field strength, of both the traditional ferrite and AlNiCo permanent magnets of less than 10 MGOe and that of the rare earth magnets of greater than 30 MGOe. This is a gap that Mn-based magnets could potentially, inexpensively, fill. This Special Issue presents work on the development of both types of manganese permanent magnets. Some of the challenges involved in the development of these magnets include improving the compounds’ energy product, increasing the thermal stability of these metastable compounds, and producing them in quantity as a bulk material.[...] Full article
(This article belongs to the Special Issue Manganese-based Permanent Magnets)

Research

Jump to: Editorial

Open AccessArticle Epitaxial Growth of Hard Ferrimagnetic Mn3Ge Film on Rhodium Buffer Layer
Metals 2015, 5(2), 910-919; doi:10.3390/met5020910
Received: 23 April 2015 / Revised: 15 May 2015 / Accepted: 21 May 2015 / Published: 2 June 2015
Cited by 2 | PDF Full-text (824 KB) | HTML Full-text | XML Full-text
Abstract
Mn\(_3\)Ge has a tetragonal Heusler-like D0\(_{22}\) crystal structure, exhibiting a large uniaxial magnetic anisotropy and small saturation magnetization due to its ferrimagnetic spin structure; thus, it is a hard ferrimagnet. In this report, epitaxial growth of a Mn\(_3\)Ge film on a Rh [...] Read more.
Mn\(_3\)Ge has a tetragonal Heusler-like D0\(_{22}\) crystal structure, exhibiting a large uniaxial magnetic anisotropy and small saturation magnetization due to its ferrimagnetic spin structure; thus, it is a hard ferrimagnet. In this report, epitaxial growth of a Mn\(_3\)Ge film on a Rh buffer layer was investigated for comparison with that of a film on a Cr buffer layer in terms of the lattice mismatch between Mn\(_3\)Ge and the buffer layer. The film grown on Rh had much better crystalline quality than that grown on Cr, which can be attributed to the small lattice mismatch. Epitaxial films of Mn\(_3\)Ge on Rh show somewhat small coercivity (\(H_{\rm c}\) = 12.6 kOe) and a large perpendicular magnetic anisotropy (\(K_{\rm u}\) = 11.6 Merg/cm\(^3\)), comparable to that of the film grown on Cr. Full article
(This article belongs to the Special Issue Manganese-based Permanent Magnets)
Open AccessArticle Electronic Structure and Maximum Energy Product of MnBi
Metals 2014, 4(3), 455-464; doi:10.3390/met4030455
Received: 30 June 2014 / Revised: 20 August 2014 / Accepted: 21 August 2014 / Published: 29 August 2014
Cited by 5 | PDF Full-text (1209 KB) | HTML Full-text | XML Full-text
Abstract
We have performed first-principles calculations to obtain magnetic moment, magnetocrystalline anisotropy energy (MAE), i.e., the magnetic crystalline anisotropy constant (K), and the Curie temperature (Tc) of low temperature phase (LTP) MnBi and also estimated the maximum [...] Read more.
We have performed first-principles calculations to obtain magnetic moment, magnetocrystalline anisotropy energy (MAE), i.e., the magnetic crystalline anisotropy constant (K), and the Curie temperature (Tc) of low temperature phase (LTP) MnBi and also estimated the maximum energy product (BH)max at elevated temperatures. The full-potential linearized augmented plane wave (FPLAPW) method, based on density functional theory (DFT) within the local spin density approximation (LSDA), was used to calculate the electronic structure of LPM MnBi. The Tc was calculated by the mean field theory. The calculated magnetic moment, MAE, and Tc are 3.63 μB/f.u. (formula unit) (79 emu/g or 714 emu/cm3), −0.163 meV/u.c. (or K = −0.275 × 106 J/m3) and 711 K, respectively. The (BH)max at the elevated temperatures was estimated by combining experimental coercivity (Hci) and the temperature dependence of magnetization (Ms(T)). The (BH)max is 17.7 MGOe at 300 K, which is in good agreement with the experimental result for directionally-solidified LTP MnBi (17 MGOe). In addition, a study of electron density maps and the lattice constant c/a ratio dependence of the magnetic moment suggested that doping of a third element into interstitial sites of LTP MnBi can increase the Ms. Full article
(This article belongs to the Special Issue Manganese-based Permanent Magnets)
Open AccessArticle Phase Transitions in Mechanically Milled Mn-Al-C Permanent Magnets
Metals 2014, 4(2), 130-140; doi:10.3390/met4020130
Received: 30 January 2014 / Revised: 3 April 2014 / Accepted: 4 April 2014 / Published: 17 April 2014
Cited by 5 | PDF Full-text (846 KB) | HTML Full-text | XML Full-text
Abstract
Mn-Al powders were prepared by rapid solidification followed by high-energy mechanical milling. The rapid solidification resulted in single-phase ε. The milling was performed in both the ε phase and the τ phase, with the τ-phase formation accomplished through a heat treatment at [...] Read more.
Mn-Al powders were prepared by rapid solidification followed by high-energy mechanical milling. The rapid solidification resulted in single-phase ε. The milling was performed in both the ε phase and the τ phase, with the τ-phase formation accomplished through a heat treatment at 500 °C for 10 min. For the ε-milled samples, the conversion of the ε to the τ phase was accomplished after milling via the same heat treatment. Mechanical milling induced a significant increase in coercivity in both cases, reaching 4.5 kOe and 4.1 kOe, respectively, followed by a decrease upon further milling. The increase in coercivity was the result of grain refinement induced by the high-energy mechanical milling. Additionally, in both cases a loss in magnetization was observed. Milling in the ε phase showed a smaller decrease in the magnetization due to a higher content of the τ phase. The loss in magnetization was attributed to a stress-induced transition to the equilibrium phases, as no site disorder or oxidation was observed. Surfactant-assisted milling in oleic acid also improved coercivity, but in this case values reached >4 kOe and remained stable at least through 32 h of milling. Full article
(This article belongs to the Special Issue Manganese-based Permanent Magnets)
Open AccessArticle Microstructure and Magnetic Properties of Bulk Nanocrystalline MnAl
Metals 2014, 4(1), 20-27; doi:10.3390/met4010020
Received: 18 December 2013 / Revised: 10 January 2014 / Accepted: 17 January 2014 / Published: 22 January 2014
Cited by 6 | PDF Full-text (1085 KB) | HTML Full-text | XML Full-text
Abstract
MnAl is a promising rare-earth free permanent magnet for technological use. We have examined the effects of consolidation by back-pressure, assisted equal channel angular extrusion processing on mechanically-milled, gas-atomized Mn-46% at. Al powder. X-ray diffraction showed both that the extruded rod consisted [...] Read more.
MnAl is a promising rare-earth free permanent magnet for technological use. We have examined the effects of consolidation by back-pressure, assisted equal channel angular extrusion processing on mechanically-milled, gas-atomized Mn-46% at. Al powder. X-ray diffraction showed both that the extruded rod consisted mostly of metastable τ phase, with some of the equilibrium γ2 and β phases, and that it largely retained the as-milled nanostructure. Magnetic measurements show a coercivity of ≤4.4 kOe and a magnetization at 10 kOe of ≤40 emu/g. In addition, extrusions exhibit greater than 95% of the theoretical density. This study opens a new window in the area of bulk MnAl magnets with improved magnetic properties for technological use. Full article
(This article belongs to the Special Issue Manganese-based Permanent Magnets)
Open AccessArticle Magnetism-Structure Correlations during the ε→τ Transformation in Rapidly-Solidified MnAl Nanostructured Alloys
Metals 2014, 4(1), 8-19; doi:10.3390/met4010008
Received: 13 December 2013 / Revised: 14 January 2014 / Accepted: 16 January 2014 / Published: 21 January 2014
Cited by 6 | PDF Full-text (606 KB) | HTML Full-text | XML Full-text
Abstract
Magnetic and structural aspects of the annealing-induced transformation of rapidly-solidified Mn55Al45 ribbons from the as-quenched metastable antiferromagnetic (AF) ε-phase to the target ferromagnetic (FM) L10 τ-phase are investigated. The as-solidified material exhibits a majority hexagonal ε-MnAl phase [...] Read more.
Magnetic and structural aspects of the annealing-induced transformation of rapidly-solidified Mn55Al45 ribbons from the as-quenched metastable antiferromagnetic (AF) ε-phase to the target ferromagnetic (FM) L10 τ-phase are investigated. The as-solidified material exhibits a majority hexagonal ε-MnAl phase revealing a large exchange bias shift below a magnetic blocking temperature TB~95 K (Hex~13 kOe at 10 K), ascribed to the presence of compositional fluctuations in this antiferromagnetic phase. Heat treatment at a relatively low annealing temperature Tanneal ≈ 568 K (295 °C) promotes the nucleation of the metastable L10 τ-MnAl phase at the expense of the parent ε-phase, donating an increasingly hard ferromagnetic character. The onset of the ε→τ transformation occurs at a temperature that is ~100 K lower than that reported in the literature, highlighting the benefits of applying rapid solidification for synthesis of the rapidly-solidified parent alloy. Full article
(This article belongs to the Special Issue Manganese-based Permanent Magnets)

Planned Papers

The below list represents only planned manuscripts. Some of these manuscripts have not been received by the Editorial Office yet. Papers submitted to MDPI journals are subject to peer-review.

Type of Paper: Article
Title: Options for Manganese-Based Alloys as Permanent Magnets
Author: Steve Constantinides
Affiliation: Arnold Magnetic Technologies Corp. 770 Linden Avenue, Rochester, NY 14625, USA
Abstract:
Binary and ternary alloys of the naturally ferromagnetic elements (iron, nickel and cobalt) have been exhaustively investigated for use as permanent magnets. A limited number of binary manganese alloys has also been investigated most notably MnAl and MnBi. An attempt to commercialize MnAl was made in 1979 by Matsushita. Shortly after the announcement of their product, neodymium iron boron was discovered and work on the manganese alloys ceased. With shortages and high prices of the rare earth elements, there is a renewed interest in Mn-based alloys. This work will review past investigation into Mn alloys for magnets and identify the potential for commercialization of the more promising alloys.

Type of Paper: Article
Title:
Effect of Compositions and Heat Treatment on Magnetic Properties of MnBi
Author:
Jun Cui
Affiliation:
Pacific Northwest National Laboratory, PO Box 999, Richland, WA 99352, USA
Abstract:
MnBi has the potential to replace some of the rare-earth based permanent magnetic material. The coercivity of MnBi increases with increasing temperature, making it a good candidate for high temperature application. MnBi itself has relatively low saturation magnetization, about 8 kG at room temperature and 7 kG at 200°C. It must be exchange coupled with soft phases such as FeCo or Co to attain higher magnetization and higher energy product. The first step toward MnBi based composite magnet is to obtain high quality hard phase in large quantity and in submicron sizes. However, MnBi is difficult to obtain in high purity, partly because the reaction between Mn and Bi is peritectic, and partly because Mn reacts readily with oxygen. In addition there is a eutectic reaction at 262°C, which causes MnBi to decompose during the bulk magnet fabrication process. Compositions and heat treatments have drastic effect to the phase contents and magnetic properties of the obtained MnBi material. In this paper, we report our effort on obtaining high performance MnBi hard phase through composition and heat treatment optimization. To date, high purity MnBi (>90%) can be routinely produced in large quantity. The optimum composition is Mn55.9Bi44.1. The heat treated powder exhibits 74 emu/g saturation magnetization at room temperature with 9 T applied field. After proper alignment, the energy product of the powder reached 11.9 MGOe, and that of the sintered bulk magnet reached 7.8 MGOe at room temperature.

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