MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

Introduction 5
has been widely adopted it will be employed it here where CMR is defined as
ΔR/R(H) > 10. CMR materials often refers to all manganite perovskites.
Although the films are quite stable and the measurements reproducible
even after several months, it is clear that growth and annealing conditions
greatly influence the properties of the manganite films [27]. Furthermore, the
electrical and magnetic properties of the CMR films are often very different
than those of the materials produced by bulk ceramic techniques or single
crystals with the same nominal composition. Thus, in order to understand
these materials, one should distinguish between the properties intrinsic to
perfect crystalline R 1-x A x MnO 3 and those caused by microstructure, strain,
disorder and/or compositional variations.
From the work described in chapter 4, it is concluded that the low
temperature, CMR phenomenon is not intrinsic to the thermodynamically
stable phases with composition La 0.67 Sr 0.33 MnO 3 or La 0.67 Ca 0.33 MnO 3 .
In chapter 5 the effect of the rare earth magnetism is shown for the case
R = Gd in Gd 0.67 Ca 0.33 MnO 3 . The possibility of structural distortions at T C are
considered for this compound.
1.3 Double Exchange
The theory of double exchange is concerned with the exchange process
involving d-band carriers in a mixed valent oxides. First postulated by Zener
3+ 2+
3+ 4+
[6] to explain the properties of ( La1− A )( Mn1− Mn )O
x x x x 3 [1, 2, 4], the theory of
double exchange was formulated by Anderson and Hasegawa [7] and
DeGennes [8]. The compounds at the two ends of the series are
unique to the manganates. Doped EuO and EuS show magnetoresistances
of 10 4 %, using the above definition, and therefore can be considered a
CMR material. Furthermore, it has been shown that in some Chevrel
phase compounds [26], a magnetic field makes the material
superconducting - which would make them Òsuper-magnetoresistanceÓ
(SMR) materials.