Magnetic softness

Hysteresis curves for a magnetically hard and a magnetically soft ferromagnetic material. S = saturation magnetization, R = remanent magnetization, K = coercive force... 

The substrate was also found to influence the properties of the electrolessly deposited vertical media CoNiMnP, CoNiReMnP, and CoNiReP. The c-axis orientation had a larger degree of perpendicular orientation for films deposited on electroless NiP than for those deposited on Cu foil, presumably because of the smaller roughness of the former substrate [43]. The double-layer (magnetically soft interface, magnetically hard bulk) properties of CoNiReP deposited on a NiMoP underlayer [57] have already been discussed. 

Films of CoB have been prepared by electroless deposition. Chang et al. [25] deposited magnetically soft amorphous films, which could be annealed to give materials with an Hc of 250 Oe. Depending on the annealing temperature, the films crystallized as the hep or fee modifications of Co. Matsui and co-workers [22] obtained crystalline materials in the as-deposited state, the crystalline characteristics being determined by processing conditions. A maximum HQ of 300 Oe was observed for films with 10.0 preferred orientation. 

Energy losses in soft magnetic materials arise due to both hysteresis and eddy currents, as described in the previous section. Eddy current losses can be reduced by increasing the electrical resistivity of the magnetic material. This is one reason why solid-solution iron-silicon alloys ( 4% Si) are used at power frequencies of around 60 Hz and why iron-nickel alloys are used at audio frequencies. Some magnetically soft ferrites (see Section 6.2.2.1) are very nearly electrical insulators and are thus immune to eddy current losses. Some common soft magnetic materials and their properties are listed in Table 6.19. Soft magnetic alloys are described further in Section 6.2.1.6. 

Magnetically soft Fe-Ni alloys can have their properties altered by heat treatment. The compound NisFe undergoes an order-disorder transformation at about 500°C. Since the susceptibility of the ordered phase is only about half that of the disordered phase, a higher susceptibility is realized when the alloy is quenched from 600°C, a process that retains the high-temperature, disordered structure. Heat treatment of Fe-Ni alloys in a magnetic field further enhances their magnetic characteristics (see Figure 6.61), and the square hysteresis loop of 65 Permalloy so processed is desirable in many applications. A related alloy called Supermalloy (see Table 6.19) can have an initial susceptibility of approximately one million. 

Fig. 9.11 Hysteresis loops illustrating the distinction between magnetically soft and hard ...

Generally speaking, the spinel ferrites have low magnetic anisotropies and are magnetically soft exceptions are those containing Co2+ which is itself strongly... 

Abstract The focus of this chapter is primarily directed towards nanocrystalline soft magnetic materials prepared by crystallization of amorphous precursors. The key elements involved in the development of this class of materials are three-fold (i) theoretical models for magnetic softness in nanostructures (ii) nanostructure-property relationships and (iii) nanostructural formation mechanisms. This chapter surveys recent research on these three areas with emphasis placed on the principles underlying alloy design in soft magnetic nanostructures. 

Following this introductory section, we will overview the development of random anisotropy models and discuss the origin of the magnetic softness in nanostructures. The nanostructural formation process and alloy development in the Fe-M-B-(Cu) alloys to which less attention has been addressed in the previous reviews, will be another focal point in this chapter. 

Magnetically soft materials are used as magnetic cores for the purpose of magnetic flux multiplication. Major requirements for the materials include high saturation magnetization (Js = pi0Ms,), small coercivity (Hc) and high permeability pi). 

A/m is the coercivity originating from the induced anisotropies. The grain size dependence of Hc in Fig. 4(a) suggests that the grain refinement below 10 - 15 nm in this alloy system may not be beneficial to better magnetic softness unless care is taken over the reduction of the effect of the induced anisotropies. 

The magnetically soft nanostructure in F1NEMET or NANOPERM is a direct result of primary crystallization of amorphous ribbons. Here, we discuss this type of crystallization process with emphasis on the nano-structural formation upon crystallization. 

It is worth mentioning here that a compound-free bcc-Fe nanostructure with grain sizes smaller than L0 can be obtained even in a binary Fe Zrg alloy [3, 60] although the initial permeability of nanocrystalline Fe92Zr8 (2 x 103 at 1 kHz) is similar to that of magnetically soft Fe-Si steels. Weak... 

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