TOPICAL REVIEW: Nanomagnetics

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.

Two- and three-dimensional calculation of remanence enhancement of rare-earth based composite magnets (invited)

Exchange hardening in nano-structured two-phase permanent magnets

There are two main thrusts towards new permanent-magnet materials: improving extrinsic properties by nanostructuring and intrinsic properties by atomic structuring. Theory-both numerical and analytical-plays an important role in this ambitious research. Our analysis of aligned hard-soft nanostructures shows that soft-in-hard geometries are better than hard-in-soft geometries and that embedded soft spheres are better than sandwiched soft layers. Concerning the choice of the hard phase, both a high magnetization and a high anisotropy are necessary. As an example of first-principle research, we consider interatomic Mn exchange in MnAl and find strongly ferromagnetic intralayer exchange, in spite of the small Mn-Mn distances.

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Predicting the Future of Permanent-Magnet Materials

Micromagnetic calculations are used to investigate coercivity and energy products of magnets consisting of an aligned hard‐magnetic skeleton phase and a soft‐magnetic phase with high saturation magnetization. Compared to the present‐day theoretical limit of 516 kJ/m3 for single‐phase Nd2Fe14B, the energy product in suitable nanostructured Sm2Fe17N3/Fe65Co35 composites is predicted to be as high as 1090 kJ/m3. The influence of the skeleton’s texture and shape is discussed, and aligned nanocrystalline two‐phase magnets are compared with remanence‐enhanced isotropic magnets. In particular, it is shown how the nucleation‐based analytical approach breaks down in the isotropic limit. Finally, we outline conceivable processing methods and discuss potential applications of ‘‘megajoule’’ magnets.

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Aligned two‐phase magnets: Permanent magnetism of the future? (invited)

The linearized micromagnetic equation for inhomogeneous nucleation in planar soft magnetic regions is derived and interpreted as a one-dimensional Schrodinger equation. Based on the Kronig-Penney model of quantum mechanics, the nucleation field for a periodic multilayer composed of two alternating layers with different crystalline anisotropies is determined.



Die linearisierte mikromagnetische Gleichung fur die inhomogene Keimbildung in ebenen weichmagnetischen Bereichen wird abgeleitet, und es wird gezeigt, das diese der eindimensionalen Schrodingergleichung entspricht. Ausgehend vom Kronig-Penney-Modell der Quantenmechanik wird das Keimbildungsfeld einer periodischen Vielfachschicht, die aus zwei alternierenden Schichten mit verschiedenen Kristallanisotropien zusammengesetzt ist, berechnet.

Nucleation Fields in Periodic Multilayers

Taking into account the anisotropy constants up to the sixth order the critical fields for the demagnetization mode of uniform rotation in uniaxial materials are derived. The boundary curves separating regions of material parameters with distinct critical behaviour are determined. The relation between uniform rotation and FOMP is discussed in the context of elementary catastrophe theory.



Unter Berucksichtigung der Anisotropiekonstanten sechster Ordnung werden die kritischen Felder fur die Entmagnetisierungsmode der gleichformigen Drehung in einachsigen Materialien abgeleitet. Die Grenzkurven zwischen Bereichen von Materialparametern mit unterschiedlichem kritischem Verhalten werden bestimmt. Der Zusammenhang zwischen der gleichformigen Drehung und FOMP wird im Rahmen der elementaren Katastrophentheorie diskutiert.

S. Nieber

Papers

Nucleation Fields in Periodic Multilayers

Micromagnetics of Critical Fields and Elementary Catastrophe Theory