Remanence enhancement in nano-structured composite magnets

Abstract

The remanence of an isotropic nanocrystalline exchange-coupled composite system (comprising hard and soft magnetic phases) is investigated analytically within a simple (one-dimensional) micromagnetic model, in which only exchange and anisotropy energies are considered. For a soft grain with size much less than its own Bloch wall width, our analysis shows that the magnetization distribution in the soft phase is linearly dependent upon position, which we show is responsible for the significant enhancement observed in composite systems. Furthermore, we show that a good estimate of the remanence can be provided by a simple formula, which is easily calculated by hand.

An Analytical Model on Remanence Enhancement in Nanosized Permanent/Composite Magnets

“…Our calculation for bulk material (3D) is consistent with the experimental data of 3D isotropic melt-spun Nd2Fe14B by Manaf et al,1 where W = 4.3 nm (theoretical) [3][4][5]. A value of 1226 Gs is adopted as the experimental M0, which is slightly smaller than the theoretical value of 1282 Gs.…”

“…3,5 Here we give a brief description. For simplicity, we consider two neighboring cubic grains in close contact with uniaxial easy axis orientation α 1 and α 2 , defined relative to the applied field H. Here, H, α 1 and α 2 are in the same plane ( Fig.…”

“…[1][2][3][4][5] To further increase the energy product, remanence B r (or M r ) has to be improved because the maximum energy product (B • H) max cannot surpass (B r /2) 2 . [1][2][3][4][5] According to the Stoner-Wohlfarth (SW) model, the ratio of the remanent magnetization M r to the saturation magnetization M 0 is 0.5 if the easy axes are distributed three-dimensionally (3D) isotropic and 2/π if they are 2D random. 6 Recent experiments have shown that isotropic permanent nanomagnets exhibit remanence enhancement (RM), mainly attributed to the inter-grain exchange interaction ignored in the SW model.…”

“…[1][2][3] Later it is found that the composite material, with both permanent and soft magnets, have more significant RM. 4,5 Lots of experiments and simulations have been done on such materials to find an optimum composition as well as the grain size. However, analytical results in this field are still lacking.…”

“…However, analytical results in this field are still lacking. [1][2][3][4][5] In this paper, we investigate the remanence in such materials systematically based on a simple analytical model. A general formula will be derived and discussed and compared with experiments.…”