Room-temperature ferromagnetism enhancement in Fe-doped VSe2 nanosheets synthesized by a chemical method
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The coercivity of anisotropic cast–hot-pressed Pr19Fe74.5B5Cu1.5 magnets is investigated. The microstructure features and virgin magnetization curve reveal a nucleation-controlled coercivity mechanism. Regression analysis shows that the intrinsic coercivity varies inversely as the logarithm of the average grain size: iHc(kOe) = 21.7550 − 6.0517 ln d (μm), which is in good agreement with the nucleation statistical model. Investigation of Cu addition and Nd substitution shows that Cu mainly plays a role of suppressing grain growth during the final annealing. Higher coercivity is thus obtained with Cu addition. Nd19Fe74.5B5Cu1.5 magnets exhibit a much lower coercivity due to their coarse-grained cast structure.
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It has been a puzzle for a century about how ``hard'' (coercive) a ferromagnet can be. Seven decades ago, W. Brown gave his famous theorem to correlate coercivity of a ferromagnet to its magnetocrystalline anisotropy field. However, the experimental coercivity values are far below the calculated level given by the theorem, which is called Brown's Coercivity Paradox. The paradox has been considered to be related to the complex microstructures of the magnets in experiments because coercivity is an extrinsic property that is sensitive to any imperfections in the specimens. To date, coercivity cannot be predicted and calculated by quantitative modeling. In this investigation, we carried out a case study on the high magnetic coercivity of Co nanowires exceeding the magnetocrystalline anisotropy field as predicted by Brown's theorem. It is found that the aspect ratio and diameter of the nanocrystals have a strong effect on the coercivity. When the nanocrystals have an increased aspect ratio, the coercivity is significantly higher than the magnetocrystalline anisotropy field of a hcp Co crystal. Micromagnetic simulations give a coercivity aspect-ratio dependence that is well consistent with the experimental results. It is also revealed that a coercivity limit exists based on the geometrical structures of the nanocrystals that govern the demagnetizing process. The quantitative correlation obtained between the structure and coercivity enables material design of advanced permanent magnets in the future.
Magnetocrystalline anisotropy
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Films of Co-P, varying in composition from 2.5% to 5% P and varying in thickness from 200 to 2500 Å, were prepared by chemical deposition. The coercivity of these films was found to be a function of both P content and thickness. The particle size of these films increased with increasing thickness and was a function of the P content. The films were annealed in a reducing atmosphere at various temperatures up to 500°C. The low-coercivity films undergo a transformation to films exhibiting coercivities of ∼250 Oe at a temperature of 250°C. The intermediate-coercivity films remain essentially unchanged, whereas the high-coercivity films drop sharply in coercivity at ∼400°C to this apparent ``equilibrium coercivity'' of ∼250 Oe. Etching experiments on these samples show no drastic effects on coercivity. The coercivity merely proceeds back along the original coercivity-versus-thickness curves. Structural and magnetic evidence indicates the existence of an equilibrium magnetic structure in these films which is primarily dependent on particle size.
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The nature of domain-wall motion coercivity in magnetic media is studied using a two-dimensional numerical micromagnetic model. It is necessarily assumed that the source of the coercivity is cylindrical anomalies in the material that have anisotropy or exchange parameters which are either larger or smaller than that of the surrounding media. Varying these parameters from those required to obtain the observed coercivity shows that a larger decrease in either parameter is required to obtain the same change in coercivity as a given increase. Thus, the dependence of coercivity on these parameters is nonlinear. It was also found that the computed coercivity decreases with an increase in the separation of the defects, and increases with an increase in defect size. It is shown that a correction for the calculated coercivity is required to account for the statistical distribution in defect sizes and locations. Finally, a suitable variation in the defect parameters can accurately characterize the temperature behavior of the coercivity.
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Single domain
Domain wall (magnetism)
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Magnetocrystalline anisotropy
Aspect ratio (aeronautics)
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Hysteresis
Stoner–Wohlfarth model
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In the present paper, the angular dependence of coercivity for the Sm2Fe17Cx magnets has been investigated in detail. Our results indicate that the angular dependence of the coercivity depends strongly on the coercivity value of the sample. For the sample of low coercivity with short ball milling time, the coercivity increases at first, develops a peak and then decreases with increasing angle between the easy axis and the direction of the external field. In contrast, minimum coercivity behavior has been observed in the sample of larger coercivity with longer ball milling time.
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In the present paper, the angular dependence of coercivity for the Sm2Fe17Cx magnets has been investigated in detail. Our results indicate that the angular dependence of the coercivity depends strongly on the coercivity value of the sample. For the sample of low coercivity with short ball milling time, the coercivity increases at first, develops a peak and then decreases with increasing angle between the easy axis and the direction of the external field. In contrast, minimum coercivity behavior has been observed in the sample of larger coercivity with longer ball milling time.
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Hysteresis
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