Spontaneous magnetisation

Go Binary and Ternary Alloyed Thin Films. Most of the thin-film media for longitudinal and perpendicular recording consist of Co—X—Y binary or ternary alloys. In most cases Co—Cr is used for perpendicular recording while for the high density longitudinal media Co—Cr—X is used X = Pt, Ta, Ni). For the latter it is essential to deposit this alloy on a Cr underlayer in order to obtain the necessary in-plane orientation. A second element combined with Co has important consequences for the Curie temperature (T ) of the alloy, at which the spontaneous magnetisation disappears. The for... 

Clearly, the critical parameter that must be evaluated is simply the Modified Stoner Factor N(Ef)Ixc - if this exceeds unity then the magnetic energy will become negative and spontaneous magnetisation will become favourable. This is the so-called Stoner Criterion . Janak [6] has calculated l c for several elements, providing an insightful commentary on trends in this quantity across the periodic table. 

For the spontaneous magnetisation the two conditions must be fulfilled simultaneously hence... 

In addition to a trivial solution (MJMS 0 = 0 for rj = 0) the non-trivial solution is obtained either graphically or by an iterative procedure. It exists, however, only below the critical temperature Tc. Above Tc the spontaneous magnetisation vanishes (the equation has only a trivial solution). 

This equation can be solved by an iterative procedure for each (Tc/T) value. The spontaneous magnetisation decreases with temperature according to Fig. 7.11 it escapes at T > Tc. For spins s = 1/2 a simpler transcendent equation is obeyed... 

Fig. 7.11. Temperature behaviour of spontaneous magnetisation for a ferromagnet individual curves correspond to spins 1/2 (solid), 3/2 (long dashed), 5/2 (medium dashed) and 7/2 (short dashed).

The magnetic behaviour of a ferrimagnetic compound is not very different from that of a ferromagnetic one, especially when the magnetisation of one site dominates. The main differences may be seen in the thermal dependence of spontaneous magnetisation. 

Fig. 4.10, instead of in the form of latent heat at the transition. This results in a magnetisation curve with small variations at low temperatures and a large decrease as T approaches Tq, Fig. 4.11. The concepts of reduced magnetisation and reduced temperature, m = M(T)fM(0), and t = T/Tq, respectively, are used to compare materials with different spontaneous magnetisations and Curie points. M T) represents the magnetisation value at temperature T, and M(0) the value at 0 K. Since in ferromagnets magnetisation is always maximum for 0 K, a decreasing curve is obtained.

Fig. 4.11. Temperature dependence of the inverse susceptibility fon (a) a ferromagnetic solid, showing a spontaneous magnetisation (ot T Tc (b) an antiferromagnetic material 7J is the Neel temperature. (Adapted from Cullity, 1972.)...

The molecular field can be introduced in Eq. (4.18) to investigate thermal effects on spontaneous magnetisation ... 

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