Remanence enhancement

Example Remanence Enhancement in Exchange-Spring Systems... 

Generally, isotropic permanent magnets with a remanence ration JJJS > 0.5 are referred as remanence enhanced materials. However, the theoretical limit for the remanence of noninteracting grains depends on the crystal symmetry, the crystallographic orientation of the easy directions, and the volume fraction and the saturation polarization of the phases. The remanence ratio a two-phase nanocomposite magnet is... 

Figure 6. Remanence enhancement in a two-phase Nd2Fe 4B/Fe3B magnet containing 343 grains. Left Finite element model of the grain structure. Right Magnetization distribution in a slice plane for zero applied field. The arrows denote the magnetization direction projected on a slice plane.

Finally, remanence enhancement has been achieved by producing a coherent nanocrystalline structure in mechanically alloyed isotropic Sm7Fe93-nitride powders consisting of a-Fe and Sm2Fei7N3 [100],... 

Figure 3. Remanence enhancement in exchange-coupled single -phase nanostructures - ( ) Numerical modeling [117, 118] and (o) calculation according to Eq. 13.

The individual measurements of the hard grain hysteresis cycles by XMCD have revealed that the hard phase magnetisation show a significant remanence enhancement (Mr/Ms 0.6) [123], This observation implies that the soft phase magnetisation shows remanence enhancement as well. This is in agreement with experimental data on other nanocomposite systems. 

The micro structure of the magnets in which remanence enhancement can be observed is composed of fine grains about 10 nm in diameter, which is three times smaller than that of normal melt-quenched powders. The grain size can be controlled by additives such as Si, Y, Cu, Ga and Zr. The BH)nax gradually increases with decreasing grain size. 

Hadjipanayis et al. (1995), Neu et al. (1996) and Miao et al. (1996) have studied the formation of Nd2Fei4B-i-a-Fe nanoeomposites, which are equilibrium mixtures on the Fe-Nd2Fei4B tie line. They observed remanence enhancement in samples with average grain sizes aroimd 20-30 nm. Coercivity, for example, was about 360 kA/m in the sample of Nd3.5Fe9iNb2B3.5 composition (Hadjipanayis et al. 1995). 

The magnetization behaviour of a typical remanenee-enhanced magnet is shown in fig. 15 (Smith et al. 1996). Remanence enhancement is evidenced by the high value of Mr relative to the saturation magnetization at high values of applied field. Values of Mj/Ms exceeding 0.75 have been reported in several two-phase systems (table 7). Such values of... 

Fig. 15. Magnetization curve for remanence enhanced a-Fe+Sm2(CoFe)i7 (Smith et al. 1996).

MJM generally require crystallite sizes less than 20 nm. Microstructures of remanence-enhanced alloys are similar to that shown in fig. 9. 

The crystallite size and fraction of soft phase are key parameters influencing the degree of remanence enhancement and coercivity. In fig. 16 the effect of annealing temperature on the coercivity, and remanence of two-phase a-Fe/Sm2Fei7N2.6 is shown. Both and Af, decrease with increasing temperature above 873 K due to the increase in crystallite size, and two-phase magnetic behaviour is observed in samples... 

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