HYBRIDIZATION IMPURITY MODEL FOR MN IONS

Manganese ions are believed to play an important role in the magnetore sistance of Lah LaMnO3-based systems. We consider a Mn impurity in the mixed-valence state of the two magnetic configurations Mn4+ and Mn3The Mn4+ is represented by a spin S=3/2 consisting of three localized d electrons in the t(2g) orbitals with their spins ferromagnetically c oupled to maximize the total spin, The Mn3+ configuration has an addit ional localized d electron in one of the e(g) orbitals to form a total spin S+1/2. The e(g) electron hybridizes with the conduction electron s and the multiple occupancy of the e(g) level is excluded by a large Coulomb energy. This gives rise to a quadrupolar Kondo effect, which c ompensates for the orbital degrees of freedom into a quadrupolar singl et. We study the lifting of the degeneracy of the e(g) levels due to a small quadrupolar field, which is related to the local Mn-O bond leng ths. In the spin-polarized limit (ferromagnetic lattice) the model is solved exactly. In a small magnetic field the underscreening of the sp in via the usual Kondo effect interferes with the quadrupolar Kondo ef fect, yielding an enhanced Kondo temperature. The quadrupolar suscepti bility gradually increases with the magnetic field, as a consequence o f the breaking up of the spin compensation. [S0163-1829(98)04617-7].