MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

92 Chapter 4
4. 2. 2 High Temperature resistivity
Above T C , The zero field resistivity (Figure 4-9) of La 0.67 Ca 0.33 MnO 3 fit best
to R h Texp(E a /k b T) predicted by small polaron hopping conductivity [123], as
opposed to that often used for a semiconductor, ln(ρ) ∝ 1/T (Figure 9). Other
(low temperature) forms predicted by variable range hopping [124] or band
semiconductor type conductivity were also fit with two parameters to the data
with less accurate and less consistent results. In contrast to the Ca and Ba
analogs [29, 104] the La 0.67 Sr 0.33 MnO 3 resistivity (Figure 4-10) does not reach a
maximum until about 100K above T C [16].
ln(ρ(10 -3 Ωcm)/T(K))
-3.0
-3.5
-4.0
-4.5
-5.0
-5.5
ln(ρ(10 -3 Ωcm))
2.4
2.2
2.0
1.8
1.6
1.4
1.2
750K
La 0.67 Ca 0.33 MnO 3
Crystal
Film
1.0 1.5 2.0 2.5 3.0 3.5
1000/T (1/K)
750K
a
Crystal
Film
-6.0
0.5 1.0 1.5 2.0
1000/T (1/K)
2.5 3.0 3.5
mW/mg
0.09
0.08
0.07
b
730 740 750 760 770 780
Temperature (K)
Figure 4-9 High temperature resistivity (warming and
cooling) of La 0.67 Ca 0.33 MnO 3 film (LCM17) and crystal in zero
field. Inset a, same data with different abscissa to compare
small polaron and semiconductor models. Inset b, DSC trace
of polycrystalline La 0.67 Ca 0.33 MnO 3 showing the heat of the
high temperature structural transition.