L. Stoleriu

Micromagnetic evaluation of statistical and mean-field interactions in particulate ferromagnetic media

Abstract Samples with Stoner–Wohlfarth ferromagnetic particles interacting only through dipolar magnetostatic fields were computer generated and the effects of the interactions on the magnetisation processes have been analysed. In some geometrical configurations of the particles, magnetostatic mean-field interaction field was also obtained, in agreement with the usual assumption in the moving Preisach model. A micromagnetic analysis of the magnetostatic mean interaction field is made. Classical Δ M and integral generalised Δ M curves have been calculated and the results have been analysed.

Micromagnetic calculation of the transverse susceptibility of patterned media

In this paper, we present a generalized model of the transverse susceptibility (TS) for realistic fine particle systems that may have any type of anisotropy (uniaxial,cubic,or unidirectional), distribution of easy axes orientation, particles positions and volumes and different strength of magnetostatic interactions between particles. The field dependence of TS is obtained using a micromagnetic algorithm based on the Landau-Lifshitz-Gilbert equation. For the uniaxial particles, where the critical curve approach can also be used, we have compared the results of the micromagnetic algorithm and those obtained by integration of Aharonis formula over the orientation distribution. Tests were also made on the calculation of the TS for single particles with various orientation of the easy axis with respect to the AC/DC applied fields.

Magnetization reversal in graded anisotropy Co/Pt multilayers: A first order reversal curve study

The magnetization reversal in graded anisotropy Co/Pt multilayers is studied by first order reversal curve (FORC) diagrams. The graded profile influences the nucleation field in thin films. In thicker films (above the threshold value for stripe domain formation), its effect is masked, as the loop shape depends sensitively on the ratio of film thickness to a characteristic length defined by domain wall and demagnetizing energies. For such thick samples, the FORC distribution shows two positive features that correspond to domain nucleation and annihilation. Between these two peaks, no other distinct features are visible, due to the high reversibility of domain pattern characteristics. In thin continuous films, a marked feature that corresponds to nucleation is accompanied by two features of opposite polarity characteristic of two-phase interactions. In nanostructured samples, the domain effects are suppressed and the FORC is dominated by a single feature elongated along the interaction axis.

New NdFeB-based bulk "exchange spring"-type permanent magnets. Experimental and micromagnetic FORC analysis

Summary form only given. This paper focuses on the possibility of direct casting miniature NdFeB-based bulk permanent magnets in different three-dimensional shapes for various applications, with amorphous structure followed by subsequent optimal annealing. Nd₃Dy₁Fe₆₆Co₁₀B₂₀ bulk nanocomposite permanent magnets in form of rods with diameters ranging from 0.5 to 0.8 mm have been prepared by devitrification annealing of amorphous and partly-amorphous precursors produced by injection die-casting. A fully amorphous structure was obtained for rods with the diameter as large as 0.6 mm. The best-achieved magnetic properties have been obtained for the optimum devitrification (15 min at 630degC) samples with 0.6 mm diameter, and are: H_C,5T = 316 kA/m, mu₀M_r,6T= 1.04 T, M_r/M_max,6T=0.68 and (BH)_max,6T=98 kJ/m³. The samples were characterized using the first-order reversal curve diagram (FORC). The micromagnetic simulation reproduced the experimental results.