A higher saturation magnetization obtained by an increased iron content is essential for yielding larger energy products in rare-earth Sm
Co
-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.
Nd–Fe–B permanent magnets have been coated with 0.6wt.% dysprosium and annealed at various temperatures to study the impact of the temperature-dependent Dy diffusion processes on both the magnetic ...properties and the microstructure. When optimum annealing conditions are applied the Dy processed magnets with initial coercivity of ∼1100kAm−1 yield coercivity increases which can exceed 400kAm−1 without a significant reduction of the remanent magnetic polarization. The improved stability against opposing magnetic fields can be observed up to a depth of ∼3mm along the diffusion direction, restricting the application of the Dy diffusion process to either thin magnets or magnets with tailored coercivity gradients. While in the proximity of the Dy-coated surface, each grain has a Dy-enriched shell with a Dy content of ∼6at.%; the Dy concentration decreases exponentially to ∼1.8at.% after a diffusion depth of 400μm and to ∼1at.% after a diffusion depth of 1500μm, as was found with wavelength dispersive X-ray spectroscopy and scanning transmission electron microscopy–energy dispersive X-ray spectroscopy, respectively. In the vicinity of the Dy-coated surface, the mechanism of the Dy-shell formation is attributed to the melting/solidification of a heavy-rare-earth-rich intermediate phase during high-temperature annealing. This is based on the observation that a constant Dy concentration over the width of the shells was found. Also an epitaxial relation between the Dy-poor core and the Dy-rich shell was observed by electron backscattered diffraction, which is supported by results obtained with Kerr microscopy.
The microstructure of grain boundaries (GBs) in the commercial NdFeB-based alloy for permanent magnets has been studied. It is generally accepted that the unique hard magnetic properties of such ...alloys are controlled by the thin layers of a Nd-rich phase in Nd2Fe14B/Nd2Fe14B GBs. These GB layers ensure the magnetic isolation of Nd2Fe14B grains from each other. It is usually supposed that such GB layers contain metallic Nd or Nd-rich intermetallic compounds. However, the commercial NdFeB-based permanent magnets frequently contain a tangible amount of neodymium oxide Nd2O3 at the triple junctions between Nd2Fe14B grains. The goal of this work was to check whether the Nd2Fe14B/Nd2Fe14B GBs could also contain the thin layers of Nd2O3 oxide phase. Indeed, the screening with EELS-based elemental analysis permitted to observe that some of these Nd-rich layers in Nd2Fe14B/Nd2Fe14B GBs contain not only neodymium, but also oxygen. More detailed analysis of such GBs with high-resolution transmission electron microscopy (HR TEM) showed these GB layers are crystalline and have the lattice of neodymium oxide Nd2O3. In turn, the Lorentz micro-magnetic contrast in TEM permitted to observe that the Nd-oxide GB layers prevent the migration of domain walls from one Nd2Fe14B grain to another during remagnetization. This finding proves that the GB oxide layers, similar to those of metallic Nd or Nd-rich intermetallic compounds, can ensure the magnetic isolation between Nd2Fe14B grains needed for high coercivity. Therefore, the GB oxide layers can be used for further development of NdFeB-based permanent magnets.
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•It has been observed that Nd2Fe14B/Nd2Fe14B GBs in the NdFeB-based alloys for permanent magnets can contain Nd2O3 oxide layers.•It has been also shown that Nd2O3 oxide GB layers, similar to GBs with metallic Nd or intermetallic phases, can effectively ensure the magnetic isolation of Nd2Fe14B grains from each other.•Therefore, the GB oxide layers can be used for further development of NdFeB-based permanent magnets.
The demagnetization behavior of commercial sintered Nd-Fe-B magnets was investigated for field directions inclined by 0/spl deg/, 45/spl deg/, and 90/spl deg/ with respect to the easy axis. For ...demagnetizing field strengths well below the coercivity, the reversible permeability parallel to the easy axis amounts to 1.03-1.05 and perpendicular to the easy axis to 1.12-1.17, depending on the alignment coefficient and the coercivity. For demagnetizing fields applied at an inclination angle of up to 45/spl deg/, irreversible losses start at 90%-95% of the respective coercivity, independent on the alignment coefficient. For an inclination of 90/spl deg/, irreversible losses arise at demagnetizing field strengths of about 130%-170% of the respective coercivity. Here, the demagnetizing stability increases with increasing alignment coefficient.
Fine-grained, heavy rare earth free Nd-Fe-B sintered magnets were prepared from He jet milled powders with an average particle size of 1.5µm by low temperature sintering at 920°C or 980°C. A ...coercivity of >1600kA/m was achieved for an average grain size of 1.68µm. Transmission electron microscopy showed that the distribution and composition of intergranular and grain boundary junction phases was similar to that in conventionally processed magnets. Microstructural analysis on different length scales revealed the occurrence of abnormal grain growth, which is unexpected for sintering temperatures below 1000°C. A larger area fraction of abnormal grains was observed in the sample sintered at 920°C compared to that sintered at 980°C. Microtexture investigation showed a better crystallographic alignment of the abnormal grains compared to the fine-grained matrix, which is explained by a size dependent alignment of the powder particles during magnetic field alignment prior to sintering. Slightly larger particles in the initial powder show a better alignment and will act as nucleation sites for abnormal grain growth. Magneto-optical Kerr investigations confirmed the lower switching field of the abnormal grains compared to the fine-grained matrix. The demagnetisation curve of the sample sintered at 920°C showed reduced rectangularity and this was attributed to a cooperative effect of the larger fraction of abnormal grains with low switching field and, as a minor effect, a reduced degree of crystallographic texture in this sample compared to the material sintered at 980°C, which did not show the reduced rectangularity of the demagnetisation curve.
•He Jet milling to reduce Nd-Fe-B grain size and to enhance coercivity.•Normal and abnormal grain growth observed for low temperature sintering.•Well oriented abnormal grown grains explained by size dependent field alignment.•Poor rectangularity is caused by low nucleation field of abnormal grown grains.
The grain size and magnetic properties of sintered Nd 14.3 Fe 78.9 TM 1.2 B 5.6 (TM: Co, Cu, Al, Ga) magnets have been examined in dependence on the powder particle size and distribution. The mean ...grain size decreases almost linearly with the mean particle size and seems to become more homogeneous with a decreasing width of the particle size distribution. The coercivity increases with decreasing grain size, which can be represented by a potential regression curve
We present a comprehensive study of the magnetocaloric materials series La(Fe
1−
x
Co
x
)
11.9Si
1.1 with
0.055
<
x
<
0.122
. The ferromagnetic samples were manufactured using a novel powder ...metallurgy process by which industrial scale production is feasible. This new production method makes the materials more attractive as magnetic refrigerants for room temperature magnetic refrigeration. The Curie temperature of the compounds can be easily tuned by altering the Co content and all samples have little magnetic anisotropy and present a second-order magnetic transition so that thermal and magnetic hysteresis is absent. For all seven samples, we have calculated the magnetic entropy change,
Δ
S
M
, from initial curve measurements and measured the adiabatic temperature change,
Δ
T
ad
, directly. In addition, for two of the samples, we determined the heat capacity as a function of applied magnetic field and the thermal conductivity. Where relevant, the results are compared with those of Gd, the benchmark material for room temperature magnetic refrigeration.
► Impact of magnetization state of NdFeB magnets on their corrosion. ► Free corrosion in low concentrated H
2SO
4: mechanical degradation. ► Anodic polarization: magnetic field reduces current ...density. ► Magnetic field localizes corrosion attack. ► Pseudo-passive behaviour at high anodic potentials.
The corrosion behaviour of uncoated NdFeB permanent magnets in the non-magnetized and magnetized state was comparatively investigated in H
2SO
4 solutions by potentiodynamic and potentiostatic polarization experiments. Depth profiles of the corroded surfaces were recorded and an effect of magnetization on the localization of the corrosion attack was identified. The anodic behaviour is discussed on the basis of previously reported results on the corrosion of neodymium and iron and on basis of the magnetic forces acting on the electrochemical system.
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