Westener Double Exchange Case Study

When an eg electron moves between adjacent Mn ions, the JT induced lattice distortion moves with it. This coupling between lattice distortion and eg electron is known as Polaron. The Jahn Teller distortions are only favorable when a lot of manganese atoms have a half filled eg shell. This makes this effect doping dependent. The degeneracy within the t2g and eg states can also be lifted by deformations due to externally imposed strain.

1.2.4 Zener Double Exchange

The correlation between metallic conduction and ferromagnetism in manganite was introduced by Jonker and Van Santen [10]. In mixed-valent manganites, the conduction process between two Mn ions was explained by C. Zener in 1951 [Rana thesis-19,20] and therefore called “Zener-Double

This is due to the crystal field created by the cubic symmetry surrounding the Mn site. As shown in fig. 3, conduction takes place between adjacent Mn3+ and Mn4+ ions. Mn3+ has an eg electron on dz2 orbital whereas Mn4+ has vacant eg electron. The eg electron of Mn3+ experiences a net potential difference with respect to vacant site of Mn4+. Hence, it has a tendency to hop to the vacant site. This hopping process is possible through the ligand oxygen, which has two 2p, spin up and spin down electrons in outermost orbital [Rana thesis]. This hopping process is possible through the ligand oxygen, which has two 2P (spin Q and spin R) electrons in outermost orbital. The eg electron of Mn3+ hops to spin up orbital of oxygen ion. According to Hund’s rule, an orbital can occupy only one electron, the oxygen ion accepts the eg electron of Mn3+ ion by giving its own spin up electron to Mn4+ ion. The basic mechanism in this process is the doping of a d-hole from Mn4+ or electron doping from d-shell of Mn3+ to Mn4+ like Mn3+ via the oxygen, so that the Mn3+and Mn4+ions change places [thker thesis-9,11]. The itinerant eg electron retains its spin up state due to Hund’s rule coupling and therefore, this is conduction with spin memory

On any deviation from this magnetic order between the spins of adjacent Mn ions, the ZDE decreases and after certain critical spin disorder, the ZDE vanishes. For instance, if the spins of adjacent Mn ions are in antiferromagnetic order, there will be no ZDE. Goodenough pointed out that ferromagnetism is governed not only by double exchange, but also by the nature of the superexchange interaction (SE). Mn+3-O-Mn+4 superexchange interaction is ferromagnetic while the Mn+3-O-Mn+3 and Mn+4-O-Mn+4 interactions are antiferromagnetic [Vacchani theis-18]. The relation of the ZDE with the spin magnetic order of the Mn ions was given by Anderson and Hasegawa [Rana thesis-31] in 1961, who have defined the transfer integral depending on the angle between the spin moments of adjacent Mn