MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

94 Chapter 4
measurements (Figure 4-6 and Figure 4-7), where there is no error in relative
temperature. Both La 0.67 Ca 0.33 MnO 3 and La 0.67 Sr 0.33 MnO 3 have T MR ≈ T C
within a few Kelvin. For La 0.67 Ca 0.33 MnO 3 T MI is approximately equal to T C ≈
T MR . La 0.67 Sr 0.33 MnO 3 however shows a much more gradual transition to a
hopping conductivity-like transport with T MI (approximately 455K) well above
T C = 360K.
At T C a maximum in dρ(H=0)/dT is expected for a metallic ferromagnet
[125]. This is observed for both compounds studied within experimental
uncertainty. The added resistance at a ferromagnetic Curie temperature is
due to electron scattering off thermally disordered spins and, particularly in
the case of the manganites, polaron formation. Since a magnetic field can
easily suppress spin fluctuations in the critical region, the resistance
associated with magnetic disorder will be reduced in a magnetic field. In a
good metal such as Fe or SrRuO 3 [126] this normally is a small effect of about a
few percent. It has been shown theoretically that this effect is much larger for
a semimetal (or semiconductor) at T C , particularly within the double
exchange model [8, 125, 127, 128]; however, it has been recently been pointed
out that double exchange alone can not account for the large
magnetoresistance [10, 11]. Nevertheless, such an explanation has been used
to explain the magneto-transport properties of semiconducting magnetic
chalcogenides [129] such as EuO [130], Gd 2 S 3 [131], and various spinels [129, 132-
135] where the resistance may drop by several orders of magnitude at T C .
4. 2. 4 Hall effect
The Hall effect data at 5 K (Figure 4-11) were analyzed according to the
equation R = R 0 + H·R H + |H|·R MR , where R H is the Hall resistance and R MR is
the magnetoresistance. The contribution due to the anomalous Hall effect
was not detected at this temperature. From the simple single band interpreta-