MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

Introduction 7
For x = 0, the situation is slightly more complex. Most reports claim that
stoichiometric LaMnO 3 is an antiferromagnet insulator [29]. It is apparently
difficult to prepare stoichiometric LaMnO 3 which likes to lose oxygen or be
rich in lanthanum. Off-stoichiometric LaMnO 3 will contain mixed-valent
manganese and could then be metallic and ferromagnetic. Stoichiometric
LaMnO 3 contains entirely Mn 3+ which has 4 d electrons. The first 3 fill the
spin-up t 2g band, as in CaMnO 3 , while the remaining electron half-fills the
spin-up e g band. The e g band is apparently further split, resulting in an
insulator. There are several ways the band could be split, any or all of which
may be the cause of the insulating behavior. First of all, a half-filled band is
susceptible to splitting due to the Mott correlation effect Ð producing a Mott
insulator. Secondly, the structure is not entirely cubic particularly for the end
members. Such a distortion raises the degeneracy of the t 2g and e g orbitals.
This is known as a Jahn-Teller splitting. Finally, the unit cell relevant to the
electronic structure may be doubled, which will split the e g band in half. The
magnetic structure, by virtue of the antiferromagnetism, has a doubled cell,
which may affect the electronic structure.
At finite values of x there will be x holes (or 1 - x electrons) in the spin-up
e g band. These holes should be free to move and provide a large conductivity.
If, however, the intra-atomic exchange, which holds the spins of all the d
electrons on a given ion parallel, is stronger than the Òhopping integral,Ó
then the hopping can only take place between pairs of ions on which the t 2g
spins (Mn 4+ core) are parallel. Otherwise the two sites have different energies.
The difference in energy is proportional to -cos(θ/2), where θ is the angle
between the neighboring core spins. Since free carriers gain kinetic energy by
being itinerant, this provides a type of exchange mechanism which holds the
two core spins parallel. Conversely, the more parallel the core spins are
aligned, the easier it is for the carriers to become itinerant. Since a magnetic
field has a large effect of aligning ferromagnetically coupled magnetic spins