MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

Electronic and Magnetic Measurements 55
T/T C « ζ 2 (fractional polarization)[81]. A more complete improvement of the
Stoner theory takes into account the effect of spin density fluctuations. Such
improvements are discussed the following section.
3.2.2.2.3 Generalized Model
The self-consistent-renormalization (SCR) theory [79, 84, 85] combines
both single particle and spin wave excitations into generalized interacting
spin density fluctuations. It deals with the coupled modes of spatially
extended spin density fluctuations in a self-consistent fashion. The theory
gives a new mechanism for the Curie-Weiss magnetic susceptibility
associated with the spatially extended modes of spin fluctuations instead of
the local moments as spatially localized spin fluctuations. Furthermore, the
theory is successful quantitatively in explaining various physical properties of
weak itinerant ferro- and antiferromagnets: the theory contains a limited
number of parameters which can be determined experimentally. Thus by the
1970Õs the theory of magnetism was advanced to the stage of having two
established regimes in the opposite extremes, the local moment systems and
weak itinerant ferro- and antiferromagnets. In the former regime the spin
fluctuations are localized in real space while in the latter they are localized in
k-space. In order to unify the two extremes into a single theory an
interpolation theory for the intermediate regime was developed in the 1980Õs
using the functional integral method within the static approximation.