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There is a large gap in energy product (BHmax) between permanent magnets based on rare-earth elements (REE) and those which are REE-free. Although the need for REE-magnets in the highest performance devices is indisputable, there are many other applications with lower magnetic demand for which REE-magnets are still employed, in the absence of REE-free alternatives which can meet the requirements. Thus, providing new REE-free options with BHmax values within this gap would allow reducing the use of these critical resources. A common approach to address the mentioned issue is trying to enhance the properties of REE-free hard phases (e.g., ferrites) by combining them with soft magnetic phases in a composite material.
In the present work, the nature of the magnetic coupling of two magnetic phases (hard-soft) in atomic contact has been studied. First, the magnetic domain configuration of an isolated SrFe12O19 platelet shaped nanoparticle (hard phase) was studied by means of photoemission electron microscopy (PEEM) combined with x-ray absorption (XAS) and magnetic circular dichroism (XMCD) at the CIRCE beamline at the ALBA Synchrotron Light Facility,[1] concluding the particle was in a single-domain state with the magnetization direction perpendicular to the platelet plane. Afterwards, a 1.2 nm layer of metallic Co (soft phase) was deposited on top of the platelet by molecular beam epitaxy (MBE) and thorough characterization of the sample at the same synchrotron beamline revealed the magnetization of the soft metallic overlayer remained in-plane despite the out of plane magnetization displayed by the hard platelet. Thus, the two phases presented completely uncorrelated magnetic domain patterns.[2]
Our experiments therefore suggest that the two magnetic phases must not be exchange coupled to one another, which is somewhat unexpected for a system in which: (i) intimate contact between the phases is presumed, as the Co overlayer is directly grown onto the SrFe12O19 surface, and (ii) the soft phase dimension is clearly below the threshold size for rigid coupling. However, the dominant energy component here appears to be the shape anisotropy term, which for this specific case is calculated to overcome the Zeeman energy of the perpendicular magnetic field generated by the hard ferrite.
[1] L. Aballe, M. Foerster, E. Pellegrin, J. Nicolas, S. Ferrer, J. Synchrotron Radiat. 2015, 22, 745.
[2] G. D. Soria, C. Granados-Miralles, A. Mandziak, P. Jenuš, M. Saura-Múzquiz, M. Christensen, M. Foerster, L. Aballe, J. F. Fernández, J. de la Figuera, A. Quesada, J. Phys. D. Appl. Phys. 2021, 54, 054003.

REPM 2021: The 26th International Workshop on Rare-Earth and Future Permanent Magnets and their Applications

Investigating the magnetic coupling of hard (SrFe12O19) and soft (Co) phases in atomic contact

It is our great pleasure to invite you to attend the 26th International Workshop on Rare Earth and Future Permanent Magnets and Their Applications (REPM2021) which will be held virtually on June 7-10, 2021. Please note, this is an updated schedule due to COVID-19 travel restrictions which forced us to postpone the conference from summer of 2020. The REPM workshop takes place every two years and is the premier conference on permanent magnets covering a wide range of topics including:

* Rare earth magnets
* Rare earth-free magnets
* Raw materials, mining and recycling
* Advanced synthesis, processing and characterization techniques
* Theoretical modeling of coercivity
* Fundamental properties of advanced magnetic materials
* Thin film magnets
* Applications of permanent magnets

***Workshop***
The workshop will be chaired by Professor George C. Hadjipanayis, University of Delaware, Newark, DE and Dr. Jinfang Liu, President & COO of Electron Energy Corporation, Landisville, PA.
The objectives of the workshop are to bring together engineers and scientists who work on permanent magnets and their applications to present their latest results and exchange ideas on a wide range of relevant topics including resources, raw materials, recycling, processing, fundamental and hard magnetic properties and applications of rare earth and future magnets.

To regsiter for this event, please follow the link below.

poster

Exchange bias in bulk FeNi-NiO nanocompounds processed by severe plastic deformation

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

Synthesis of La-Zn co substituted M-type ferrite under high oxygen pressure

Hard magnetic Sm1+xFe9.5Nb0.1Nδ (x=0, 0.05, 0.10, 0.15) powders were synthesized by melt-spinning, annealing and nitriding. The structure, magnetic properties and thermal stability have been investigated by means of powder X-ray diffraction, vibrating sample magnetometer, HRTEM and flux meter. By changing the Sm content and correspondingly adjusting the ratio of TbCu7–type structure and Th2Zn17–type structure, the magnetic properties can be tuned accordingly and a series of high performance Sm-Fe-N magnetic powders can be obtained. The maximum energy product obtained was 15.50 MGOe associated with a coercivity of 9.1 kOe for the SmFe9.5Nb0.1Nδ, and 13.50 MGOe associated with the highest intrinsic coercivity (HcJ) of 13.3 kOe for the Sm1.1Fe9.5Nb0.1Nδ. After exposed at 120℃ for 200 hours, the irreversible flux loss of Sm1.1Fe9.5Nb0.1Nδ bonded magnets is -4.5%, which have increased 40% compared with the Sm1+xFe9.5Nb0.1Nδ(x=0) sample.These properties for Sm-Fe-N magnetic powders are close to those of NdFeB commercial powders, which have never been reported in Sm based isotropic magnetic powders.

Controlled magnetic properties and thermal stability by tuning TbCu7 /Th2Zn17 phase in isotropic Sm-Fe-Nb-N compounds

Magnetic properties, crystal structure, and microstructure of melt spun LaCo4.9M0.1 ribbons (M = Ti, Zr, and Hf) are studied. Very low iHc = 0.8 kOe is obtained for binary LaCo5 ribbons due to the coexistence of LaCo13 and La2Co7 phases with LaCo5 phase. iHc of the ribbons is significantly enhanced to 11.6 kOe for M = Ti, 8.1 kOe for M = Zr, and 9.0 kOe for M = Hf, respectively. A slight Ti, Zr, or Hf substitution for Co is effective to suppress the formation of La2Co7 and/or LaCo13 phases, stabilize LaCo5 (1:5) phase, and accordingly enhance the coercivity of the ribbons significantly. The preferential occupancy of M at 3g sites in 1:5 crystal, indicated by Rietveld refinement of XRD, may improve magnetocrystalline anisotropy field, thus enhance coercivity, and slightly decrease Curie temperature. The refined grain size with M substitution from 50-120 nm to 10-30 nm also contributes to remarkable improvements of iHc and also the squareness of demagnetization curve. Besides, the coercivity of LaCo5-xMx ribbon is mainly governed by nucleation mechanism. The result of this study provides an effective way to significantly enhance coercivity for the isotropic LaCo5-based nanocomposite ribbons.

Phase stability and coercivity enhancement of melt spun LaCo5 ribbons with Ti, Zr, or Hf substitution

Fabrication of high heat resistant Sm-Fe-N magnetic powder by reduction-diffusion process

Phosphogypsum is a waste (or by-product) of phosphoric acid production [1]. This waste represents an economic potential thanks to a multitude of probable valuations such as rare earth recovery [2] that our SCIMATOP laboratory is interested in.
The rare earth content, the most effective extraction voice and the rate of their recovery are the main objectives of our study.
In the first step of our work, we focused simultaneously on the physic-chemical characterization of Moroccan phosphogypsum and the development of an optimal process for the solubilization of rare earths. the realization of this process required an experimental plan of 35 reactions where the treated phosphogypsum (drying + grinding + sieving) was exposed to acid attack by different percentages of sulfuric acid (from 10% up to 15%) by completing our experimental design, we were able to determine the total rare earth content contained in the Moroccan phosphogypsum and determine the rare earth recovery rate for each of the reactions carried out.
By analyzing the previous results, we have been able to deduce the optimal conditions for theleaching of rare earths by sulfuric acid (%H2SO4= 15%; T= 53 °C; S/L = 1/4, t = 40 min; A= 200 rpm.)
This work represents an essential stage of extraction of the rare earths by their solubilization and recovery in a liquid phase from which the final extraction step will take place.

References:

[1]: H. SFAR FELFOUL, P. CLASTRES, M. BEN OUEZDOU and A. CARLES-GIBERGUES (2002): Properties and prospects of phosphogypsum valorization (the example of Tunisia) Proceedings of International Symposium on Environmental Pollution Control and Waste Management 7-10 January 2002, Tunis (EPCOWM’2002), p.510-520

[2]: Fernando Pereira (2013): Phosphoric acid production by hydrochloric acid etching of phosphate ores with reduction of environmental pollution and recovery of rare earths as by-products. PhD thesis, Geochemistry. Mines National School of Saint-Etienne, French.


Optimization of Rare Earth Leaching from Phosphogypsum exposed to sulfuric acid attack

Coercivity of Sm2Fe17N3 as a sintered magnet has so far been only a few percent of the huge anisotropic magnetic field. Controlling grain boundary properties is a promising strategy to improve the coercivity. In powder metallurgical processes, coating raw powders with different materials can be a powerful methodology for grain boundary control. However, if the powder surface covered with the oxide film is coated, the coating phase and the magnetic phase do not contact with each other directly, so that a sufficient effect may not be obtained. Furthermore, iron oxides contained in the surface oxide film undergo a redox reaction with Sm2Fe17N3 at elevated temperatures, precipitating soft magnetic iron phase, which causes coercivity deterioration. For these reasons, the coating should be applied to surfaces without oxide film.

In this study, first we have established a versatile technique for coating powders with an oxide-free surface. By using the technique, we coated Sm2Fe17N3 powders with 20 non-magnetic metals, and investigated the effects on the coercivity. A simple heat treatment was applied to the coated powders instead of sintering, to clarify pure effects of coating excluding inter-grain coupling issues. 

Sm2Fe17N3 fine powders were prepared by jet-milling coarse powders. Coating was performed by arc-plasma deposition (APD) and DC magnetron sputtering. The powder was continuously stirred during deposition to ensure uniform coating. The whole process of the sample preparation was undertaken in a low-oxygen atmosphere. After the heat treatments, the sample powders were subjected to evaluations. Magnetic properties of the powders were measured as bonded magnets by using a vibrating sample magnetometer.

Scanning electron microscopy with energy dispersive X-ray spectrometry and X-ray photoelectron spectroscopy (XPS) suggested that an arc-plasma deposited Zn layer on Sm2Fe17N3 powder was highly uniform and had nearly 100 % coverage. Moreover, even the particles inside an agglomerate were fully coated with Zn, which suggested the agglomerates dynamically changed its constituent particles during the coating. 

An XPS depth-profile analysis confirmed that the powder had an oxide-free direct metal-metal interface between the coating layer and the main phase, in case of coating on a Sm2Fe17N3 powder prepared in the low-oxygen atmosphere. 

The effects of coating on the coercivity were evaluated first with Zn, Ti, and Al as the coating materials and both with APD and DC sputtering as the coating methods. With all the three coating metals, the sputter-coated powders showed higher coercivity than the raw powders. The APD powders had lower coercivity than the sputter-coated powders, and in the case of Ti, coercivity was even lower than that of the raw powder. Transmission electron microscopy and nano-beam electron diffraction suggested that the high energy particles deposited by APD damaged the magnetic phase. We therefore exclusively used the sputtering method for the rest of the experiments. 

It was found that sputter-coating as thick as several nm makes the coercivity increase, almost regardless of the coating element. However, further coercivity changes resulted from subsequent heat treatment at 500 °C depended strongly on the element. The mechanism of the former phenomenon is supposed to be a common and universal one. The latter one seems, on the other hand, to be related directly to the chemical details of each element. We are currently conducting research to clarify the factors controlling the coercivity change by coating and heat treatment.

Coercivity Changes in Sm2Fe17N3 Powders by Direct Coating with Nonmagnetic Metals on Oxide-free Powder Surfaces

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

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. 


Investigating the magnetic coupling of hard (SrFe12O19) and soft (Co) phases in atomic contact

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Exchange bias in bulk FeNi-NiO nanocompounds processed by severe plastic deformation

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