SATURATION MAGNETIZATION AND CURIE TEMPERATURE
The maximum magnetization in a ferromagnet when all the atomic magnetic moments have been aligned as much as possible is called the saturation magnetization Msat. In the iron crystal, for example, this corresponds to each Fe atom with an effective spin magnetic moment of 2.2 Bohr magnetons aligning in the same direction to give a magnetic field /XtfMsat or 2.2 T. As we increase the temperature, lattice vibrations become more energetic, which leads to a frequent disruption of the alignments of the spins. The spins cannot align perfectly with each other as the temperature increases due to lattice vibrations
0 0.2 0.4 0.6 0.8
Figure 8.21 Normalized saturated magnetization versus reduced temperature T/Tc where Tc is the Curie temperature (1043 K).
Randomly agitating the individual spins. When an energetic lattice vibration passes through a spin site, the energy in the vibration may be sufficient to disorientate the spin of the atom. The ferromagnetic behavior disappears at a critical temperature called the Curie temperature, denoted by Tc, when the thermal energy of lattice vibrations in the crystal can overcome the potential energy of the exchange interaction and hence destroy the spin alignments. Above the Curie temperature, the crystal behaves as if it were paramagnetic. The saturation magnetization Msat, therefore, decreases from its maximum value Msat(0) at absolute zero of temperature to zero at the Curie temperature. Figure 8.21 shows the dependence of Msat on the temperature when Msat has been normalized to M>at(0) and the temperature is the reduced temperature, that is, T/Tc — At T/Tc = 1, Msat = o. When plotted in this way, the ferromagnets cobalt and nickel follow closely the observed behavior for iron. We should note that since for iron Tc = 1043 K, at room temperature, T/Tc = 0.29 and Msat is very close to its value at Afsat(0).
Since at the Curie temperature, the thermal energy, of the order of kTc, is sufficient to overcome the energy of the exchange interaction Eex that aligns the spins, we can take kTc as an order of magnitude estimate of Eex. For iron, Etx is ~0.09 eV and for cobalt this is ~0.1 eV.
Table 8.3 summarizes some of the important properties of the ferromagnets Fe, Co, Ni, and Gd (rare earth metal).
Table 8.3 Properties of the ferromagnets Fe, Co, Ni, and Gd
Fe
|
Co
|
Ni
|
Gd
| |
Crystal structure
|
BCC
|
HCP
|
FCC
|
HCP
|
Bohr magnetons per atom
|
2.22
|
1.72
|
0.60
|
7.1
|
A#„t(0) (MA m“1)
|
1.75
|
1.45
|
0.50
|
2.0
|
^sat ^ sat(T)
|
2.2
|
1.82
|
0.64
|
2.5
|
Tc
|
770 °C
|
1127 °C
|
358 °C
|
16 °C
|
1043 K
|
1400 K
|
631 K
|
289 K
|
Magnetic Domains: Ferromagnetic Materials
Reference: http://elektroarsenal.net/saturation-magnetization-and-curie-temperature.html
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