MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

Electronic and Magnetic Measurements 57
The major advancement of the SCR theory is the development of a new
mechanism for the Curie-Weiss magnetic susceptibility applicable to itinerant
electron ferromagnets. The paramagnetic Curie-Weiss susceptibility arises
essentially from the opposition of the magnetic energy to order the spins and
the thermal energy to disorder them. The magnetization is then proportional
to µ B H/k B T, giving the inverse susceptibility H/M linear in T. In the case of
the weak itinerant electron ferromagnets there are no localized spins above
T C so the susceptibility cannot arise from this mechanism. Instead, the
changing mean-square local amplitude of the spin fluctuation provides the
Curie-Weiss susceptibility.
The SCR expression for the inverse magnetic susceptibility contains a
2
contribution from the mean-square local amplitude of the spin fluctuation SL [84] which can be derived from the fluctuation-dissipation theorem.
2
Calculations have shown that SL increases almost linearly with temperature
above T C giving rise to the Curie-Weiss law. This is in contrast with the local
2
moment mechanism where SL is a constant. The Curie constant of the new
mechanism is related to the band structure around the Fermi surface and is
independent of the saturation moment at T = 0. This Curie-Weiss law should
hold even for paramagnetic metals when they are very close to the
ferromagnetic instability, in contrast to that predicted by paramagnon theories
[84].
2
The temperature variation of SL (T), which determines the new Curie-
Weiss mechanism is strongest when the stiffness of the longitudinal spin
fluctuations is small. When the longitudinal stiffness is small, a relatively
2
rapid increase in SL (T) with inverse temperature is expected above TC and it
should saturate at a certain value determined by the band structure. After
saturation the spin fluctuations behave like local moments with a certain
degree of short range order, since the local amplitude of the spin density is
almost fixed. This phenomenon is called temperature-induced local