Unconventional magnetization textures and domain-wall pinning in Sm–Co magnets

Abstract

Some of the best-performing high-temperature magnets are Sm–Co-based alloys with a microstructure that comprises an $$\hbox {Sm}_2\hbox {Co}_{17}$$ Sm 2 Co 17 matrix and magnetically hard $$\hbox {SmCo}_5$$ SmCo 5 cell walls. This generates a dense domain-wall-pinning network that endows the material with remarkable magnetic hardness. A precise understanding of the coupling between magnetism and microstructure is essential for enhancing the performance of Sm–Co magnets, but experiments and theory have not yet converged to a unified model. Here, transmission electron microscopy, atom probe tomography, and nanometer-resolution off-axis electron holography have been combined with micromagnetic simulations to reveal that the magnetization state in Sm–Co magnets results from curling instabilities and domain-wall pinning effects at the intersections of phases with different magnetic hardness. Additionally, this study has found that topologically non-trivial magnetic domains separated by a complex network of domain walls play a key role in the magnetic state by acting as nucleation sites for magnetization reversal. These findings reveal previously hidden aspects of magnetism in Sm–Co magnets and, by identifying weak points in the microstructure, provide guidelines for improving these high-performance magnetic materials.

Temperature dependence of magnetization processes in Sm(Co, Fe, Cu, Zr)z magnets with different nanoscale microstructures

“…All hysteresis loops have a shape typical for Sm(Co,Fe,Cu,Zr) z magnets, which does not change with temperature, as demonstrated for RT in Figure 6a and for 400 • C in Figure 6b. When the external magnetic field is applied in the opposite direction to demagnetize the remanent state, there is an initial dip in the magnetization (between 0 and 0.5 T), hypothesized to be due to the Z phase reversing its magnetization before the rest of the magnet 26 . Figure 6c shows the temperature dependence of the saturation magnetization (solid symbols) and remanence (hollow symbols) for all three samples.…”

“…However, the magnetic properties of the lamellar Z phase are not well studied, because they differ from their bulk values. A recent study showed that the Z phase has an insignificant magnetic anisotropy compared to the other two phases, and proposed that increasing the thickness of the Z phase would decrease the coercivity of the magnet 26 . Furthermore, the size of the Sm 2 Co 17 cells affects the coercivity and the arXiv:2101.12076v1 [cond-mat.mtrl-sci] 28 Jan 2021 remanence, because it determines the chemical composition of the cell walls 27 .…”

“…Simultaneously, this decreases the Curie temperature of the cell walls, as well as the overall remanence, so the effect of Cu and cell size on the high-temperature magnetic properties remains unclear. Finally, magnetization reversal can start with DW nucleation at the cell walls or Z phase 26,31,32 , meaning that the magnetization processes in Sm(Co,Fe,Cu,Zr) z can be either DW pinning or nucleation, or both. In order to fully understand the behavior of Sm(Co,Fe,Cu,Zr) z magnets, it is necessary to investigate how the aforementioned microstructural features give rise to DW pinning and nucleation, and what determines which one of them is the dominant magnetization process.…”

XPEEM and MFM Imaging of Ferroic Materials

“…[1][2][3][4][5] However, scientific interest alone provides sufficient reason for imaging these types of spatial variation. The spatial variations of order parameter could arise in any samples with structural/chemical inhomogeneity, [6,7] in large samples due to the formation of ferroic domains, [1,8] in small samples due to defects and boundaries, [9] and in thin-film multilayers that support topological defects of…”

“…[ 1–5 ] However, scientific interest alone provides sufficient reason for imaging these types of spatial variation. The spatial variations of order parameter could arise in any samples with structural/chemical inhomogeneity, [ 6,7 ] in large samples due to the formation of ferroic domains, [ 1,8 ] in small samples due to defects and boundaries, [ 9 ] and in thin‐film multilayers that support topological defects of magnetic [ 10–12 ] and polar [ 13,14 ] order. Topological defects include skyrmions and vortices (5–100 nm in diameter), which can be electrically/magnetically manipulated, and used to carry topologically protected information in memory and logic devices.…”