MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

62 Chapter 3
Curie temperature, the temperature where the linear extrapolation of the 1/χ
curve crosses 1/χ = 0. Near the critical temperature however, 1/χ is not
necessarily linear in T since 1/χ = (T - T C ) γ where γ is usually large than 1.
Thus the paramagnetic Curie temperature is usually slightly larger than the
true critical temperature.
In principle the thermodynamic critical temperature can be found by
fitting the divergence of all the measured quantities and using the scaling
relations. This assumes a true critical region has been reached where the
critical exponents do not change. The simpler method using the Arrot plots,
described in the previous section (3.2.2.2.6) can give as accurate results. Thus,
the Arrott T C is probably the best simple way of finding the critical
temperature.
The most basic definition of T C for a ferromagnet is the temperature where
the spontaneous magnetization drops to zero (as the temperature is raised).
Unfortunately due to demagnetization effects, the zero applied field
magnetization will drop to zero at T C faster and less uniformly than expected.
Nevertheless, the temperature where the magnetization, measured in zero or
small applied field, is often used to measure the critical temperature and is
called here the remnant T C .
In an applied field, the magnetization does not drop to zero because of the
large susceptibility near T C . However, below T C the magnetization has a
negative curvature while above T C it has a positive curvature. Thus the
inflection point of the M vs. T curve (denoted here as the inflection T C ) is
often used to measure the Curie temperature. In the mean field model, the
inflection T C is quite close to the real critical temperature. Figure 3-7 shows
the calculated M vs. T of SrRuO 3 in various fields. In a 5 T applied field, the
inflection point is 1.6 K higher than the true T C used for the calculation. This
agrees well with the experimental data shown in Figure 3-9, where the