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  • 1
    Electronic Resource
    Electronic Resource
    Nucleation of reversed domains at grain boundaries : 37th Annual conference on magnetism and magnetic materials (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 6510-6512 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Micromagnetic analysis using a finite-element technique confirms that intergrain magnetostatic and exchange interactions drastically affect the magnetization reversal mechanism in nucleation controlled permanent magnets. The investigation of the nucleation fields of magnetically coupled grains emphasizes the important role of nonmagnetic boundary phases and well-aligned grains for the enhancement of coercivity. The long-range magnetostatic interactions between the grains reduce the coercive field of ideally oriented particles. The exchange coupling of misoriented neighboring grains causes a strongly inhomogeneous magnetic state near the interface of the particles which favors the nucleation of reversed domains. Intergrain exchange interactions considerably reduce the coercive field, especially if the angle between the easy axes of adjacent grains is high. An algorithm for adaptive mesh refinement controls the discretization error throughout the calculation of the demagnetization curves.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.352597
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  • 2
    Electronic Resource
    Electronic Resource
    Micromagnetic simulation of domain structures in patterned magnetic tunnel junctions (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 7000-7002 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The magnetization reversal process of patterned magnetic tunnel junctions was investigated using finite element micromagnetics, taking into account the magnetostatic interactions between the pinned and the free layer. Two different reversal modes were observed in the simulations depending on the domain structure for zero applied field. In order to reduce the magnetostatic energy, end domains form in the free layer either in the S state or the C state. If the system is in the S state, the end domains grow under the influence of a reversed field. The end domains touch each other, leading to the reversal of the center. Finally, the residual domains along the edges parallel to the field direction reverse. If the system is in the C state, the growth of the end domains leads to a four domain flux closure structure. The domain with the magnetization in favor of the field direction expands until the free layer becomes reversed at a field. The S state and the C state were found to differ in energy by less than 0.2%. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1360329
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  • 3
    Electronic Resource
    Electronic Resource
    Domain wall pinning in high temperature SM(Co, Fe, Cu, Zr)7-8 magnets (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 4765-4767 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A finite element method was used to simulate domain wall pinning in SmCo5/Sm2Co17 based permanent magnets. The finite element model was built according to the cellular microstructure obtained from transmission electron microscopy investigations. The numerical results show a strong influence of the dimension of the cell boundary phase on the coercive field, which significantly increases with the extension of the 1:5/7-type cell boundary phase. The calculated values of the coercive field are in the range from 1000 to 2000 kA/m assuming a cell size varying from 80 to 160 nm. The difference of the magnetocrystalline anisotropy between cell boundary and cell interior phases is determined by the Cu content of the magnet. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.373152
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  • 4
    Electronic Resource
    Electronic Resource
    Simulation of the particle misalignment of hard magnets after compaction (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 85 (1999), S. 5672-5674 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The simulation of powder compaction for Nd–Fe–B, Sm–Co, and barium ferrite shows the influence of different static field strengths and different mold filling on particle misalignment. The final magnetization differs up to 20%. The processing determines the pressure activated processes in which the particle overcomes a volume barrier, characterized by elastic energy fluctuations. Isostatic compaction shows less pressure activated processes as compared to uniaxial mold pressing. However, an improved magnetization of about 10% owing to isostatic pressing was observed only for spherical particles. The pressure activated processes determine particle density defects such as cracks and pores, misalignment of c axes at the punch side, at corners and at particle density defects, and internal stress areas with cold plastic flow. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.369836
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  • 5
    Electronic Resource
    Electronic Resource
    Overview of Nd–Fe–B magnets and coercivity (invited) : The 40th annual conference on magnetism and magnetic materials (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 5029-5034 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: High performance Nd2Fe14B-based permanent magnets are produced with different composition and various processing techniques. The composition and the processing route influence the complex, multiphase microstructure of the magnets, such as grain size, alignment, and distribution of phases. Grain sizes in the range between 10 and 500 nm are obtained by melt spinning, mechanical alloying, and the HDDR process. Sintered and hot worked magnets exhibit grain sizes above 1 μm. The coercive field is determined by the high uniaxial magnetocrystalline anisotropy as well as the magnetostatic and exchange interactions between neighboring hard magnetic grains. The dipolar interactions between misaligned grains are more pronounced in large-grained magnets, whereas exchange coupling reduces the coercive field in small grained magnets. Transmission electron microscopy has been used to study the influence of substituent and dopant elements on microstructure, coercivity, and corrosion resistance of advanced (Nd,S1)–(Fe,S2)–B:(M1,M2) magnets. The replacement of the Nd-rich intergranular phase by secondary phases formed after doping by M1 and M2 type elements improves the corrosion resistance, especially in large-grained magnets. Secondary, nonmagnetic phases reduce the remanence and the energy product. In addition to the characterization of the microstructure, special attention has been paid to the computer modeling of the interaction between microstructure and coercivity. The simulation of the magnetization reversal process based on the real microstructure reveals a good agreement with experimental values. It is shown that the coercive field depends on grain size, distribution, and misorientation of grains. A strong exchange coupling between hard magnetic grains is desired in nanostructured magnets in order to improve the remanence. This effect is further increased by secondary, soft magnetic phases. Nanocrystalline, composite Nd–Fe–B based magnets show a remanence enhancement, both in experiments and in model calculations. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.361565
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  • 6
    Electronic Resource
    Electronic Resource
    Two- and three-dimensional calculation of remanence enhancement of rare-earth based composite magnets (invited) : The 6th joint magnetism and magnetic materials (MMM)−intermag conference (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 7053-7058 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Micromagnetic calculations using a finite element technique rigorously describe the magnetic properties of novel, isotropic rare-earth-based composite magnets. Numerical results obtained for a composite material of Nd2Fe14B, SmCo5 or Sm2(Fe0.8Co0.2)17N2.8 and α-Fe particles show that remanence, coercivity, and coercive squareness sensitively depend on microstructural features. Interparticle exchange interactions enhance the remanence by about 60% with respect to noninteracting particles for a mean-grain size approaching the exchange length of the soft magnetic phase and a significant percentage of α-Fe. On the other hand, exchange interactions between the phases suppress the nucleation of reversed domains and thus preserve a high coercive field. Therefore, optimally structured, isotropic composite magnets show remarkably high energy products exceeding 400 kJ/m3.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.358026
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  • 7
    Electronic Resource
    Electronic Resource
    Numerical micromagnetics in hard magnetic and multilayer systems (invited) : The 40th annual conference on magnetism and magnetic materials (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 6458-6463 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Micromagnetic finite element calculations using a magnetic vector potential to treat long-range dipolar interactions describe the interactive magnetization processes which determine the magnetic properties of fine-grained magnetic materials. Micromagnetic models give a quantitative treatment of the correlation between the microstructure and the magnetic properties of melt-spun Nd2Fe14B magnets and of Co/Pt multilayers. In nanocrystalline permanent magnets, intergrain exchange interactions cause nonuniform magnetic states which increase the remanence and act as nucleation sites. Remanence increases with decreasing grain size. Remanence enhancement of about 15% with respect to noninteracting particles can be achieved for an average grain size D≤20 nm. On the other hand, the nucleation field reduces to about 20% of the anisotropy field. Once a reversed domain has been nucleated, strong internal stray fields cause the expansion of the domain over several grains. The nucleation field of Co/Pt multilayers increases with improving texture. Spatial fluctuations of the magnetocrystalline anisotropy energy create barriers for domain wall motion. The pinning field of reversed domains increases with increasing misalignment and with increasing grain size. The quality of texture and the grain size significantly influence the jaggedness of domains in Co/Pt multilayers. Large grains and strong deviations of the easy directions from the film normal deteriorate the smoothness of domains and increase the transition width. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.361973
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  • 8
    Electronic Resource
    Electronic Resource
    Microstructural analysis of strip cast Nd–Fe–B alloys for high (BH)max magnets : The 7th joint MMM-intermag conference on magnetism and magnetic materials (1998)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 83 (1998), S. 6396-6398 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: High energy density magnets 〉400 kJ/m3 are increasingly used in many applications. Conventional casting techniques for sintered magnets reveal the formation of a high quantity of α-Fe and large Nd-rich regions. New techniques, like strip casting, produce homogeneous and fine scaled microstructures and are already used for producing high (BH)max magnets. The fast cooling rate during strip casting suppresses the formation of α-Fe dendrites and of large Nd-rich pockets. Directional solidification causes a formation of columnar grains containing a typical arrangement of hard magnetic Nd2Fe14B regions and Nd-rich regions. The Nd regions occur as intragranular platelets as well as intergranular phases. Intragranular lamellae show a periodicity which corresponds to a eutectoidal solidification according to the composition of the liquid and are directed parallel to the temperature gradient during solidification. The lamellae show an average width of 150 nm, a spacing of 3 μm, and a length up to the size of the hard magnetic grains. The fine separation of the hard magnetic and Nd phases is advantageous for the milling of the alloy after hydrogen decripitation and improves sinterability of magnets. Although the microstructure of strip cast alloys is much finer than that of ordinary cast alloys, the alignment of the powder is not deteriorated and Br is not reduced due to a sufficient large interlamellar spacing between the Nd-rich platelets that enables the formation of single crystal powder particles after milling. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.367557
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  • 9
    Electronic Resource
    Electronic Resource
    Micromagnetic simulation of 360° domain walls in thin Co films (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 5517-5519 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A moving mesh finite element technique is applied to simulate the formation and annihilation of 360° wall structures in thin Co films. Adaptive refinement and coarsening of the finite element mesh controls the discretization error during the simulation of domain wall movement. Elements are refined in regions with high variation of the magnetization, whereas elements are taken out where the magnetization is uniform. The calculated Neel walls have very wide tails and have an extension of about 15–20 nm. The motion of domain walls through the sample gives rise to the formation of 360° domain walls. They are formed when a Bloch line in a domain wall is caught at a nonmagnetic defect in the sample. Pinholes with an extension greater than 6 nm are sufficient to trap a Bloch line. The width of the 360° walls is found to be in the range of 40 to 50 nm. The stability of the 360° walls depends on the strength of the pinning of the Bloch lines. The field required to annihilate the 360° walls increases linearly with the size of the nonmagnetic defect. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.373390
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  • 10
    Electronic Resource
    Electronic Resource
    Dynamic FE simulation of μMAG standard problem No. 2 (invited) (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 85 (1999), S. 5819-5821 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: μMAG standard problem No. 2 was studied using a three-dimensional finite element simulation based on the solution of the Gilbert equation. Asymptotic boundary conditions were imposed in order to compute the demagnetizing fields and a Gilbert-damping parameter (α=1.0) was used to drive the system towards equilibrium. The coercivities observed for the thin elongated platelet on which standard problem No. 2 is based show a slight dependence on its size. The width of the particle was varied from 1 to 30 times the exchange length while keeping the aspect ratio of 5:1:0.1 unchanged. An external field is applied parallel to the [111] direction, giving values of the coercive field ranging from −0.056 to −0.04 in units of the saturation magnetization Ms. With the external field applied parallel to the long axis of the particle a strong dependence of the coercivity on its size is found which can be attributed to different reversal mechanisms. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.369930
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