Thermally activated coercivity in core-shell permanent magnets

Bance, Simon; Fischbacher, Johann; Schrefl, T.

doi:10.1063/1.4916542

Cited by 61 publications

(25 citation statements)

References 25 publications

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“…This discrepancy was recently overcome by introducing a nucleus at the surface of the polycrystalline model 25) . In addition, thermal uctuations can further reduce the coercivity, ~15% for room temperature calculations as reported by Bance et al 26,27) , which was not taken into account in this simulation by Sepehri-Amin et al 23) . In the Nd-Fe-B sintered magnets, the surface grains have almost no coercivity due to the defects created during surface machining/polishing or surface oxidation and the magnetization of the surface grains reverse at a lower magnetic eld than the nucleation eld 28) .…”

Section: Effect Of Grain Size On Coercivity and Thermal Stability Of mentioning

confidence: 68%

Micromagnetic Simulations of Magnetization Reversals in Nd-Fe-B Based Permanent Magnets

2016

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Finite element micromagnetic simulation was employed to explain how the microstructure of Nd-Fe-B permanent magnets such as grain size, grain shape, and grain boundary composition in uence the magnetization reversals and coercivity. Micromagnetic simulations showed that local demagnetization factor decreases as grain size decreases, which is attributed to a higher coercivity in ne-grained anisotropic permanent magnets. Lower demagnetization factor is also responsible for a lower temperature dependence of coercivity in the magnets with a smaller grain size. It was also found that the reduction of the magnetization of the grain boundary phase in hot-deformed Nd-Fe-B magnets leads to the coercivity enhancement due to a stronger pinning force against magnetic domain wall motion. The coercivity of Nd-Fe-B magnets cannot be enhanced by the reduction of the grain size alone unless the grain boundary phase become non-ferromagnetic, indicating that the role of the grain boundary phase is more pronounced in the Nd-Fe-B magnets with a smaller grain size.

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Section: Effect Of Grain Size On Coercivity and Thermal Stability Of mentioning

confidence: 68%

Micromagnetic Simulations of Magnetization Reversals in Nd-Fe-B Based Permanent Magnets

2016

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“…The temperature dependence of coercivity is generally due to two factors: the temperature-dependent magnetocrystalline anisotropy and the thermal fluctuations at finite temperatures. Bance et al considered both of these two factors in their micromagnetic simulations and found that thermal fluctuations can reduce the coercivity by 15% at room temperature and by 25% at 450 K. 25,26 Here, the grain shape is the focus and the thermal fluctuations are ignored. We take the temperature dependent magnetocrystalline anisotropy and saturation magnetization 37 to simulate the temperature dependence of coercivity, as shown in Fig.…”

Section: Resultsmentioning

confidence: 99%

“…However, to the best of our knowledge, a systematic study on the grain shape effect in Nd-Fe-B is still meaningful according to the literatures. Bance et al [25][26][27] used the cubic or dodecahedral single grain covered by a defect layer to study the defect thickness dependent angular dependence of coercivity and the thermally activated coercivity in Nd-Fe-B by micromagnetic simulations. Based on the sub-micron Nd 2 Fe 14 B single and perfect grain with a rectangular prism shape, Thielsch et al 23 applied micromagnetic simulations to reveal the dependence of coercivity on length ratios.…”

Section: Introductionmentioning

confidence: 99%

Micromagnetic simulations on the grain shape effect in Nd-Fe-B magnets

Gutfleisch

2016

Journal of Applied Physics

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Micromagnetic simulations were performed to study the effect of grain shape and defect layer in Nd-Fe-B magnets. It was found that the coercivity can be increased by a factor of ∼2 by changing the grain shape from the triangular prism to the spheroid. Both the anisotropy field contribution and the shape contribution to the coercivity, and thus also the final coercivity, were found to decrease in the order: spheroid > circular prism > hexagonal prism > square prism > triangular prism. Sputtered columnar grains and hot-deformed platelet grains with a constant volume were also considered. Results show that the coercivity initially increases with the aspect ratio and then nearly saturates above the ratio of ∼4. Simulations of multigrain ensembles showed that depending on the grain shape, compared to the case of single grain, a further decrease of ∼10%–45% in the coercivity is induced by magnetostatic coupling

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“…For Nd-Fe-B thin film models an activation volume of 200e800 nm 3 was determined, which correspond to a spherical activation radius of 3.63e5.76 nm [43]. Micromagnetic simulations further demonstrate, that a 4 nm thick (Dy 47 ,Nd 53 ) 2 Fe 14 B shell is sufficient to compensate the deterioration in coercivity due to a 2 nm thick soft magnetic defect zone [48].…”

Section: Discussionmentioning

confidence: 99%