Saturation magnetisations

Figure 2 shows the two hysteresis loops for a medium and a head material. The coercivity, the saturation magnetisation, Af or iaduction, B, remanent magnetisation, M or iaduction, B, and the permeabiHty, p, differ for the two materials. 

The slope of the hysteresis loop in is also an important parameter. From this slope, the parameter S can be derived (17). In Figure 3 a part of the hysteresis loop (M as a function of the appHed field H) is given. The point at which M is constant as the function of the appHed field is defined as saturation magnetisation (M.). From the slope at can be written tan0 = = 1/ 1 — S ) ot dM/dH = — S ). Thus the S is defined in... 

Values of the Saturation Magnetisation, 0 Multiple Magnetic States... 

Figure 8.1. Correlation between (a) magnetic Gibbs energy, (b) magnetic specific heat (from Nishizawa 1992) and (c) change of saturation magnetisation for pure Fe (from Nishizawa 1978).

The saturation magnetisation 0, in pb per atom, of many materials often corresponds to non-integral values of s. This is due to contributions other than s being involved, for example polarised conduction electrons. It is, therefore, general practice to substitute the experimental value of the saturation magnetisation at 0 K, /3b, for 2s in Eq. (8.3) which leads to (Miodownik 1977)... 

Here, Ms is the saturation magnetisation. Notice, that apparently, the stress transfer from the kapton substrate to the deposited film was ignored (compare eq. (IS) with eq. (1 Id)). A consistent result was obtained by determining the anisotropy from the hysteresis loops measured in the hard and easy magnetisation directions, using a vibrating sample magnetometer. 

Clearly, this equation allows to determine As through the values of the stress a, the resonant-field shift A Hr and the saturation magnetisation Ms. Details of the experimental set-up may be found in the review papers by Bushnell et al. (1992), Le Gall et al. (1989) and in Vukadinovic s thesis (Vukadinovic 1988). Unfortunately, up to now the measurements are limited to room temperature. 

Fig. 37. Saturation magnetisation of the TbFe/Fe and TbFe/FeCo multilayers as a function of the transition-metal sublayer-thickness, in comparison to a simple model for exchange coupled layers considering either parallel or antiparallel coupling of the TbFe and the transition metal layers. After Quandt and Ludwig (1997).

Coercivity and saturation magnetisation parameters are influenced by domain state (and hence by grain size of the magnetic fraction). Domain state in materials where the remanence is dominated by magnetite can be determined by position on a Day plot (Day et al., 1977) on such a plot, stability of remanence increases towards the upper left comer. Samples BC4, RC2, RC3, and RC5, all of which show evidence in their demagnetisation behaviour for the presence of a reversed-polarity component, plot closest to the stable part of the Day plot (Fig. 10). Samples BCl, BC3, BC5, and RCl 1, which do not show a clearly isolated reversed component, plot in less stable positions. Samples RC4 and RCl2, which have complex demagnetisation, plot as outliers, well to the less stable side of the plot. 

Ignoring for the moment the slightly open loop near zero applied field (Figure 3.24a), the sample appears to be antiferromagnetically ordered in zero applied field and to switch to a magnetic state at a critical field (He,) of around 3000 G. Because there are two unpaired electrons per formula unit (one on the iron and one on the organic acceptor), the saturation magnetisation is expected to be around 2 x (5585 emu G mol ). 

Fig. 11.1 The reduced internal magnetic field HrSXlJHJf)) plotted as a function of the reduced temperature T/Tq. HJS>) is the value of the field at absolute zero and Tc is the Curie temperature. The dots represent Mbssbauer data, the solid line saturation magnetisation data, and the upper dashed line the n.m.r. measurements. The lower dashed line is drawn with the temperature scale expanded tenfold. [Ref. 1. Fig. 8]...

Tn- The temperature dependence of H, below the Curie point of Tc — 773°C agrees well with saturation-magnetisation data and n.m.r. data (Fig. 11.1). Independent measurement of the spectrum at room temperature in terms of frequency calibration gave = —1-716 and a value for go (= 2/value refers of course to the bulk material, and it is very encouraging to note the excellent agreement with the value derived from n.m.r. measurements [3] of 45-46 MHz which by contrast... 

Table 4.9. Curie temperature and saturation magnetisation of transition metals.

Fig. 4.19. Temperature dependence of the saturation magnetisation for various spinels. Note that M, is given in emu/g (cgs units). To transform into A/m, magnetisation values have to be multiplied by the density of the corresponding ferrite and then by 10. (Adapted from Smit Wijn, 1961.)...

Fig. 4.21. Saturation magnetisation (in Bohr magnetons per formula unit) of various spinel ferrites at low temperature, as a function of Zn substitution (Gorier, 1954 Guillaud Sage, 1951).

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