0.21 Chapter 7. magnetic materials.  (Page 4/4)

From Figure 7.8 it is evident Ferromagnets have high positive susceptibility right up to Curie Temperature. Only after Curie Temperature susceptibility drastically falls. Where as Paramagnetic material has a graded response. At very low temperatures susceptibility is high and it gradually falls with rise in temperature.

Section 7.6.1. Classifications of Ferromagnets.

Feromagnets have further sub-classes namely Ferrimagnetic Materials and Anti-ferromagnetic Materials.

In Figure 7.9 Chromium BCC crystal’s Unit Cell is shown. The Body central left directed dipole moment is negated by the right directed dipole moment of 8 corner atoms. This negation results into zero Magnetic Moment because each of the 8 corner atoms make 1/8 contribution to the Unit Cell.

Manganese Oxide is anti-ferromagnetic below Neel Temperature and above Neel Temperature it is Paramagnetic. In Figure 7.10, it is shown below Neel Temperature.

In 1986, high temperature Lanthanite Barium Copper Oxide [La(2-x)Ba(x)CuO4] were found to exhibit superconductivity at 30K shown in Figure 7.11. Yittrium Barium Copper Oxide(YBa2Cu3O6) Superconductors were discovered which exhibited superconductivity at 90K warmer than liquid Nitrogen Temperature. In all these layered compounds it was found that CuO2 planes had anti-ferromagnetic order. This led to a flurry of activities in CuO2 based Ceramics for developing Room Temperature Superconductors. A detailed account of the quest for Room Temperature Superconductor is given in Chapter 8.

In Figure 7.12 the dipole alignments in Ferro, Ferri and Antiferro-magnetic materials are shown. Here T _C refers to Curie Temperature and T _N refers to Neel Temperature.

Curie Temperature refers to Ferromagnetic Materials only. Above Curie Temperature, Ferromagnetic Materials loses its spontaneous magnetization.

In Ferrimagnetic and Anti-Ferromagnetic materials below Neel Temperature, the materials retain their Ferrimagnetism or Antiferromagnetism as the case may be. But above Neel Temperature they become paramagnetic. Thermal fluctuations disturbs the order.

Table 7.2. gives the Neel Temperature and Curie Temperature for some important magnetic materials.

Table 7.2.Neel Temperature for some important Ferri and Anti-Ferromagnetic Materials and Curie Temperatures for Ferromagnetic Materials.

As is evident from the Table 7.14 , Fe, Co and Ni which are the transition elements and which have uncompensated spin electrons are strongly Ferromagnetic Materials

Section 7.6.2. Domains and Hysteresis.

Like Polycrystals or like Ferrielectric materials, in ferromagnetic and ferrimagnetic materials also there are domains of magnetization. Each domain has its own alignment direction and all dipoles in each domain are aligned but the direction of alignment changes from domain to domain as shown in Figure 7.15.

As we move from one domain to another, orientation of the dipoles only gradually change. Thus there is no abrupt changes across the domain walls as shown in Figure 7.16.

When external magnetic field is applied, favourable domains grow at the expense of unfavourable domains as shown in Figure 7.17. Finally at high fields a domain aligned with external field remains. This is an exact analog of the electric poling of Ferro-electric material.

Figure 7.18 gives B-H curve of a ferro-magnet. Red is the Hysteresis Loop. Blue is the initial magnetization. B _r is the remnance Magnetic Flux Density at H=0. H _C is the coercive magnetic field required to completely demagnetize the material.

As is evident from figure 7.19 induced magnetization is perfectly linear in paramagnetic and in diamagnetic materials but is non-linear in ferro/ferromagnetic materials. Hence latter has a hysteresis loop.