MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

38 Chapter 3
Complex materials often have resistivities that are inadequately described
by the simple models given above. For example the cuprate superconductors
and SrRuO 3 show a linear resistivity well above the Ioffe Regel limit [73]. A
more extreme example is Ba 6 Co 25 S 27 which not only has a resistivity
minimum but a less than linear resistivity above the minimum [42]. As
more materials with complex electronic structures are examined, such non-
standard behavior will certainly become more common.
3. 1. 3. 4 Phase transitions
Any precise measurement as a function of temperature can usually detect
or be influenced by a phase transition. Conductivity data in particular can be
quite precise but can also be greatly influenced by subtle electronic, magnetic
or structural changes. Thus one should be cautious when fitting data to very
similar functional forms. Electronic transitions such as metal-insulator
transitions and charge ordering (charge density wave formation) obviously
change the conductivity at the ordering temperature. Magnetic transitions
are also easily observed in the conductivity since local moments act as
scattering centers, or may even help localize/delocalize carriers.
Ferromagnetic metals for instance always show a decrease in resistivity as the
temperature goes below the Curie temperature [73]. Structural phase
transitions usually change the symmetry and volume, which affects the
conduction paths and density of the charge carriers. This will subtlety change
the conductivity at the transition temperature even if the conduction
mechanism remains the same, as shown in section 4.2.5.
Careful measurements can help determine the nature of such phase
transitions. If the phase transition is reversible and not hysteretic, i.e. the data
are the same upon warming and cooling, then the phase transition is
probably a single second order process. If the data is hysteretic then a first
order process is involved. For example, the ferromagnetic and accompanying
metal insulator transition in the manganites appears to be of second order.