MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

144 Chapter 7
The related form ρ ∝ -M 2 would also be consistent with our low field data
since in low fields ∆ρ = -H 2 2
σH 2/σ0 and σ0 is nearly constant with respect to
temperature. This form was also observed above T C in La 1-x Sr x CoO 3 [165] and
in small fields for lower T C films [158] of La 0.7 Ca 0.3 MnO 3 . In the temperature
range of this experiment, T > 1.01 T C , 4πM < H, so B = H + 4πM ≈ H and
therefore not very temperature dependent. Thus the resistivity or
conductivity is not proportional to B or B 2 .
7. 2. 2 Magnetoresistance scaling below T C
Below T C , ρ ∞ can be interpreted as due to scattering processes which would
persist if the moments were perfectly ordered. As shown in Figure 7-10, the
resistivity in a infinite field, ρ ∞ , follows the A + BT 2 fit to the low-
temperature, H = 0, intrinsic resistivity described in section 4.2.1. In a true
infinite field (H = ∞) one would expect the resistance to be continuous with a
positive (metallic) slope as T increases through T C . While there is a factor of 2
discrepancy between ρ ∞ (T C ) extrapolated from above (Figure 7-8) and below
(Figure 7-10) this is not unreasonable because two different fitting equations
are used. Particularly above T C , the fitting parameter ρ ∞ may not reflect the
true H = ∞ resistivity because the nonlinearity of M upon H is not taken into
account.
The parameter σ 0 , which can be related to the resistivity in zero field
ρ(H = 0) ≈ ρ ∞ + 1/σ 0 , describes the limiting field-dependent, spin-disorder
scattering processes present as H approaches 0. Unlike ρ ∞ , 1/σ 0 diverges as the
Curie temperature is approached. A critical exponent of about 1.8 (Figure 7-
10) is found: σ 0 (T) ≈ σ 0 (0) (1 - T/T C ) 1.8 , where σ 0 (0) ≈ 2 × 10 -3 mΩcm -1 .