Atomic structure and domain wall pinning in samarium-cobalt-based 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

Towards engineering the perfect defect in high-performing permanent magnets

Permanent magnets draw their properties from a complex interplay, across multiple length scales, of the composition and distribution of their constituting phases, that act as building blocks, each with their associated intrinsic properties 1. Gaining a fundamental understanding of these interactions is hence key to decipher the origins of their magnetic performance2 and facilitate the engineering of better-performing magnets, through unlocking the design of the “perfect defects” for ultimate pinning of magnetic domains3. Here, we deployed advanced multiscale microscopy and microanalysis on a bulk Sm2(CoFeCuZr)17 pinning-type high-performance magnet with outstanding thermal and chemical stability 4. Making use of regions with different chemical compositions, we showcase how both a change in the composition and distribution of copper, along with the atomic arrangements enforce the pinning of magnetic domains, as imaged by nanoscale magnetic induction mapping. Micromagnetic simulations bridge the scales to provide an understanding of how these peculiarities of micro- and nanostructure change the hard magnetic behaviour of Sm2(CoFeCuZr)17 magnets. Unveiling the origins of the reduced coercivity allows us to propose an atomic-scale defect and chemistry manipulation strategy to define ways toward future hard magnets

The effect of the thermal decomposition reaction on the mechanical and magnetocaloric properties of La(Fe,Si,Co)13

We report on the influence of the Co content in the magnetocaloric system La(Fe,Si,Co)13 on the thermal decomposition (TD) reaction, and subsequently on the magnetocaloric properties. In the course of the TD reaction, the magnetocaloric La(Fe,Si,Co)13 phase reversibly decomposes into α-Fe(Co,Si) and the intermetallic LaFeSi phase, thus enhancing the mechanical properties and therefore the machinability of the compound. The addition of Co significantly speeds up the reaction kinetics. The optimum temperature range for the TD reaction was determined to be 973–1073 K, with the lower and upper limit at 873 K and 1173 K, respectively. With electron microscopy a lamellar microstructure has been found in the decomposed state, indicating a eutectoid-type phase reaction. The width of the lamellae is ∼26 nm in LaFe12Si and decreases with increasing Co content. Three-dimensional atom probe (3DAP) measurements show the enrichment of Co and Si in LaFeSi lamellae. We conclude that the addition of Co somehow decreases the lamellar spacing, which is the main reason for the enhanced TD kinetics. Finally, it is interesting to note that the highly ordered nano-scale mixture of strongly ferromagnetic α-Fe(Co) with the non-ferromagnetic phase induces a significant increase in coercivity, Hc. The shape anisotropy of the thin α-Fe(Co) lamellae yields a semi-hard permanent magnet with a coercivity of ∼100 A cm−1

Electrochemical corrosion study of La(Fe11,6-xSix1,4Mnx)H1,5 in diverse chemical environments

La-Fe-Si-based alloys represent a promising material class for magnetocaloric cooling at ambient temperatures, but contain highly oxophilic elements and are chemically sensitive, which impairs their continued operation in aqueous heat exchange media. The development of protection strategies ensuring long-term stability necessitates a comprehensive understanding of the material's corrosion characteristics. The present work focuses on a such an assessment for hydrogenated La(FeMnSi)13 containing α-Fe to enhance it's mechanical properties. Linear sweep voltammetry served as the main analytical tool and was performed in preferably buffered electrolytes with pH values reaching from moderately acidic to strongly alkaline, in the presence and absence of corrosion-enhancing species (chloride, chelators). α-Fe, La1Fe1Si1, and La(FeSi)13 were employed as reference materials to clarify the passivation pattern of the material in carbonate buffer. Specific chemical compounds with clear mechanistic impact (phosphate acting as precipitate former, hydrazine as oxygen scavenger) were tested alongside commercial corrosion inhibitors to investigate their effects. Our objective is to provide a systematic evaluation of the corrosion properties of the alloy system, building on previous investigations and taking into account its general materials chemistry