MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

Critical Transport and Magnetization of La 0.67 Ca 0.33 MnO 3
competition between ferromagnetic and antiferromagnetic interactions. As
pointed out by DeGennes, there is competition between double exchange
(ferromagnetic) and superexchange (antiferromagnetic) as will be discussed
further in section 7.1.4. One of the early theories of double exchange
presented by Anderson and Hasagawa [7] also predicts a downward curvature
in 1/χ vs. T.
It is not too surprising that two different values of γ (0.7 and 0.9) are
obtained from the same data. The two methods weight the data differently
and as discussed above, the true critical region where γ is a constant has
probably not been reached.
7. 1. 3 Positive nonlinear susceptibility
A peculiarity of the T > T C isotherms of Figure 7-1, which likely affects the
determination of γ, is their negative slope as they approach M 2 = 0. This slope
is proportional to the nonlinear susceptibility. For M = χH + χ 3 H 3 , χ is the
linear susceptibility and χ 3 is the third-order nonlinear magnetic
susceptibility. The slope of a T > T C isotherm in an M 2 vs. H/M Arrott plot as
M 2 approaches zero is given by -χ 4 /χ 3 . For a normal ferromagnet, χ > 0 while
χ 3 < 0 giving a positive slope for all T > T C isotherms. However in
La 0.67 Ca 0.33 MnO 3 a negative slope is found which implies that χ 3 > 0. This can
also be seen in a plot of M vs. H, which is shown in Figure 7-1. A positive χ 3
produces the faster than linear increase in M with H which leads to an
inflection point in the M vs. H curve due to the saturation of M for large H.
This effect is not likely due to the sample preparation since it is seen in both
the polycrystal and float-zone crystal samples. Arrott plots of SrRuO 3
(Appendix A) at similar temperatures do not show χ 3 > 0, which indicates that
the effect is not due to the measurement system. A more pronounced
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