Grain-size dependent demagnetizing factors in permanent magnets

Bance, Simon; Seebacher, Bernhard; Schrefl, T.; Exl, Lukas; Winklhofer, Michael; Hrkac, G.; Zimányi, Gergely T.; Shoji, Tetsuya; Yano, Masao; Sakuma, Noritsugu; Ito, Masaaki; Kato, Akira; Manabe, Akira

doi:10.1063/1.4904854

Cited by 85 publications

(30 citation statements)

References 28 publications

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“…The increased coercive field of the misaligned model structure (Figure 12), compared to the aligned one, is in good agreement with the measured data. The small discrepancy between the measured and simulated coercive fields can be attributed to the uniform size of distribution grains [40,41] and GBs and other microstructural features, which were not taken into account in the simulated model structure, for example, soft magnetic inclusions in the grains, which can act as a nucleation site for reversed magnetic domains [42], and a reduction of the magnetocrystalline anisotropy of the hard magnetic grains near intergranular phases as a molecular dynamic study revealed [27,43].…”

Section: Micromagnetic Simulations Of the Influence Of The Gbmentioning

confidence: 96%

A Combined TEM/STEM and Micromagnetic Study of the Anisotropic Nature of Grain Boundaries and Coercivity in Nd-Fe-B Magnets

Zickler

Fidler

Bernardi³

et al. 2017

Advances in Materials Science and Engineering

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The nanoanalytical high resolution TEM/STEM investigation of the intergranular grain boundary phase of anisotropic sintered and rapidly quenched heavy rare earth-free Nd-Fe-B magnet materials revealed a difference in composition for grain boundaries parallel (large Fe-content) and perpendicular (low Fe content) to the alignment direction. This behaviour vanishes in magnets with a high degree of misorientation. The numerical finite element micromagnetic simulations are based on the anisotropic compositional behaviour of GBs and show a decrease of the coercive field with an increasing thickness of the grain boundary layer. The magnetization reversal and expansion of reversed magnetic domains primarily start as Bloch domain wall at grain boundaries parallel to thec-axis and secondly as Néel domain wall perpendicular to thec-axis into the adjacent hard magnetic grains. The increasing misalignment of grains leads to the loss of the anisotropic compositional behaviour and therefore to an averaged value of the grain boundary composition. In this case the simulations show an increase of the coercive field compared to the anisotropic magnet. The calculated coercive field values of the investigated magnet samples are in the order ofμ0HcJ=1.8 T–2.1 Tfor a mean grain boundary thickness of 4 nm, which agrees perfectly with the experimental data.

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Section: Micromagnetic Simulations Of the Influence Of The Gbmentioning

confidence: 96%

A Combined TEM/STEM and Micromagnetic Study of the Anisotropic Nature of Grain Boundaries and Coercivity in Nd-Fe-B Magnets

Zickler

Fidler

Bernardi³

et al. 2017

Advances in Materials Science and Engineering

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“…The reduction in coercivity due to grain shape was recently investigated by Bance at al. [36], where it was shown that, for a 50 nm grain diameter, an aligned cubic grain has a 0.85 reduction in coercivity with respect to a dodecahedral grain shape. Additionally, grain easy axis misalignment reduces coercivity in real magnets.…”

Section: Grain Boundary Diffused Magnetic Grainsmentioning

confidence: 99%

Thermal Activation in Permanent Magnets

Bance

Fischbacher

Kovacs

et al. 2015

JOM

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The coercive field of permanent magnets decays with temperature. At non-zero temperature the system can overcome a finite energy barrier through thermal fluctuations. Using finite element micromagnetic simulations, we quantify this effect, which reduces coercivity in addition to the decrease of the coercive field associated with the temperature dependence of the anisotropy field, and validate the method through comparison with existing experimental data.

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“…The motivation is to explore the possible role of the place where strain/stress appears and the associated micromagnetic mechanism. The grain shape effects have been recently investigated for achieving high coercivity [53][54][55][56]. Here we considered four types of prism grains with triangular, rectangular, hexagonal, and circular sections.…”

Section: Single-grain Nd-fe-b Magnetsmentioning

confidence: 99%

Multiscale Examination of Strain Effects in Nd-Fe-B Permanent Magnets

Zhang

Gutfleisch

2017

Phys. Rev. Applied

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We have performed a combined first-principles and micromagnetic study on the strain effects in Nd-Fe-B permanent magnets. First-principles calculations on Nd2Fe14B reveal that the magnetocrystalline anisotropy (K) is insensitive to the deformation along c axis and the ab in-plane shrinkage is responsible for the K reduction. The predicted K is more sensitive to the lattice deformation than what the previous phenomenological model suggests. The biaxial and triaxial stress states have a greater impact on K. Negative K occurs in a much wider strain range in the ab biaxial stress state. Micromagnetic simulations of Nd-Fe-B magnets using first-principles results show that a 3 − 4% local strain in a 2-nm-wide region near the interface around the grain boundaries and triple junctions leads to a negative local K and thus remarkably decreases the coercivity by ∼ 60% or 3 − 4 T. The local ab biaxial stress state is more likely to induce a large loss of coercivity. In addition to the local stress states and strain levels themselves, the shape of the interfaces and the intergranular phases also makes a difference. Smoothing the edge and reducing the sharp angle of the triple regions in Nd-Fe-B magnets would be favorable for a coercivity enhancement.

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Grain-size dependent demagnetizing factors in permanent magnets

Cited by 85 publications

References 28 publications

A Combined TEM/STEM and Micromagnetic Study of the Anisotropic Nature of Grain Boundaries and Coercivity in Nd-Fe-B Magnets

A Combined TEM/STEM and Micromagnetic Study of the Anisotropic Nature of Grain Boundaries and Coercivity in Nd-Fe-B Magnets

Thermal Activation in Permanent Magnets

Multiscale Examination of Strain Effects in Nd-Fe-B Permanent Magnets

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