MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

Critical Transport and Magnetization of La 0.67 Ca 0.33 MnO 3
magnetic field H dependence of the resistivity is shown to be more fully
described in terms of a magnetoconductive circuit, ρ = ρ ∞ + 1/(σ 0 + σ H 2H 2 ) for
T > T C and ρ = ρ ∞ + 1/(σ 0 + σ H |H|) for T < T C , where ρ ∞ , σ 0 , σ H 2 and σ H are
parameters. In order to determine if the magnetoconductivity, σ H 2H 2 and
σ H |H| can replaced by a common term proportional to M 2 as suggested above,
the temperature dependencies and magnitudes of σ H 2, σ H and M need to be
examined.
Alternatively, it has been found in several cases that the correlation
ρ ∝ exp[-M(H,T)/M E ] is a good fit to the data [24, 114, 115]. This formulation
does not yet have a good theoretical understanding.
In this chapter, which will be published separately, the temperature and
field dependence of the magnetization and magnetoresistance, above and
below T C are reported. A well annealed La 0.67 Ca 0.33 MnO 3 thin film on a
LaAlO 3 substrate grown by MOCVD was used for transport measurements as
described in chapter 4. DC conductivity measurements were performed using
the method of Van der Pauw [52, 53]. Only the longitudinal
magnetoresistance data are shown. The transverse magnetoresistance is
nearly identical to the longitudinal as was discussed in chapter 6. After
stabilizing the temperature, repeated resistance R vs. H curves were measured
and fit with three parameters (ρ ∞ , σ 0 , and σ H 2 or σ H ) to ρ = ρ ∞ + 1/(σ 0 + σ H 2H 2 )
for T > T C and ρ = ρ ∞ + 1/(σ 0 + σ H |H|) for T < T C . The data fit well to these
forms except the few degrees near T C (where a combination of the two forms
is better) and at low temperatures. For T < 50 K the magnetoresistance shows
2
no sign of saturating (ρ = [ρ∞ + 1/σ0 ] - σH |H|/σ0 ), so only two parameters
2
([ρ∞ + 1/σ0 ] and σH /σ0 ) can be extracted.
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