Evolution of magnetic domain structures and coercivity in high-performance SmCo 2:17-type permanent magnets

Sm2(Co,Cu,Fe,Zr)17 permanent magnets with their three-phase precipitation structure (cells, cell walls, and lamellae) show two characteristic features which so far are difficult to interpret but which are the prerequisites for high-temperature applications: (1) The hard magnetic properties only develop during the final step of the three-step annealing procedure consisting of homogenization, isothermal aging, and cooling. (2) Depending on the composition and on the annealing parameters, the temperature dependence of the coercivity can be easily changed from the conventional monotonic to the recent nonmonotonic behavior showing coercivities up to 1T even at 500K. The magnetic hardening during cooling is due to the fact that the cell walls order chemically and structurally during the cooling process. From an analysis of electron diffraction patterns of the superimposed structures existing before and after cooling it could be proven that a phase transition from a phase mixture of defective phases 2:17, 2:7, a...

Micromagnetism and the microstructure of high-temperature permanent magnets

A higher saturation magnetization obtained by an increased iron content is essential for yielding larger energy products in rare-earth Sm2Co17-type pinning-controlled permanent magnets. These are of importance for high-temperature industrial applications due to their intrinsic corrosion resistance and temperature stability. Here we present model magnets with an increased iron content based on a unique nanostructure and -chemical modification route using Fe, Cu, and Zr as dopants. The iron content controls the formation of a diamond-shaped cellular structure that dominates the density and strength of the domain wall pinning sites and thus the coercivity. Using ultra-high-resolution experimental and theoretical methods, we revealed the atomic structure of the single phases present and established a direct correlation to the macroscopic magnetic properties. With further development, this knowledge can be applied to produce samarium cobalt permanent magnets with improved magnetic performance. Understanding the factors that determine the properties of permanent magnets, which play a central role in many industrial applications, can help in improving their performance. Here, the authors study how changes in the iron content affect the microstructure of samarium cobalt magnets.

Atomic structure and domain wall pinning in samarium-cobalt-based permanent magnets.

Rapidly quenched Sm(CobalCu0.08Fe0.22Zr0.02)8.5 (Cu-/Fe-rich) and Sm(CobalCu0.05Fe0.10Zr0.03)8.5 (Cu-/Fe-poor) ribbons have been prepared by means of the melt-spinning technique. By applying an appropriate annealing procedure a microstructure similar to that of sintered magnets can be obtained. The energy dispersive x-ray microanalysis of the compositional dependence near the cell boundaries suggests a model for the profile of the crystal anisotropy constants responsible for the magnetic hardening. The Cu-/Fe-rich alloy shows a normal temperature dependence of coercivity with a negative temperature coefficient, but the Cu-/Fe-poor ribbons show a positive temperature coefficient in the temperature range from 400–700 K. The different temperature coefficients are discussed in terms of a pinning model.

Melt-spun precipitation-hardened Sm2(Co,Cu,Fe,Zr)17 magnets with abnormal temperature dependence of coercivity

Orientation by solidification in a magnetic field A new process to texture SmCo compounds used as permanent magnets

The alloys with composition around Sm11.5Co58.4Fe22.5Cu5.3Zr2.4 (at%) present hard magnet properties when their microstructure consists of a network of cells based on Sm2(Co, Fe)17, limited by boundaries based on Sm(Co, Cu)5 and cut by platelets based on (Sm, Zr)Co3. The genesis of this microstructure is studied by transmission electron microscopy of heat-treated specimens. The fractions and compositions of the phases formed are evaluated from atomic balance equations. The microstructure forms by nucleation growth of ordered R2:17 cells in the 1:7 matrix, then by nucleation of 1:3 platelets on the twin boundaries in R2:17. The processes are likely limited by diffusion.



Les alliages de composition voisine de Sm11,5Co58,4Fe22,5Cu5,3Zr2,4 ont des caracteristiques exceptionnelles d'aimants permanents quand leur microstructure est constituee d'un reseau de cellules a base de Sm2(Co, Fe)17, limitees par des parois a base de Sm(Co, Cu)5 et decoupees par des plaquettes a base de (Sm, Zr)Co3. La genese de cette microstructure est etudiee par caracterisation, par microscopie electronique en transmission, d'echantillons traites thermiquement. La composition des phases formees est evaluee a partir d'equations de bilans atomiques. La microstructure se construit par nucleation-croissance de cellules de phase ordonnee R2:17, puis par nucleation de plaquettes de phase 1:3 sur les joints de mǎcles de la phase R2:17. Les reactions apparaissent limitees par diffusion.

C. Maury

Papers

Genesis of the cell microstructure in the Sm(Co, Fe, Cu, Zr) permanent magnets with 2:17 type