The Role of Unconvential Magnetism in the Insulator-Metal Transition of Perovskite Cobaltites

In the perovskite cobaltite LaCoO 3 , the magnetic interactions resulting from the three possible spin states of the Co 3+ ion (S= 0, 1 and 2) are sensitively affected by the lattice. Doping of charge carrier into this Mott insulator leads to two different effects: a) the lattice adjusts to accommodate the charges and b) different magnetic exchange may occur, which ultimately have consequences on the magnetism and electrical conductance. Thus, understanding the unconventional magnetism exhibited in this system and the insulator-metal transition that appears in the doped compounds is a challenge due to the complexity caused by the spin state, spin correlations and the magnetic exchange associated with Co 3+ ions. In this dissertation, the magnetic and lattice effects in doped ACoO 3 (A = La, Pr) are investigated on an atomic scale by neutron scattering techniques. In the hole-doped compounds, La 1-x A x CoO 3 (A= Ca, Sr, Ba), the magnetic phase separation (ferromagnetic and incommensurate or antiferromagnetic clusters of nanometer scale) and its trend are characterized. It is found that this trend is strongly correlated with the A-site dopant. A hypothesis is proposed to explain how the magnetic phase separation is driven by the lattice inhomogeneity and how the insulator-metal transition comes about. In LaCo 1-y Ni y O 3 , an enhanced compression of the oxygen octahedron along the trigonal axis is invoked to break the degeneracy of the magnetic Co 3+ without the local lattice breaking the trigonal symmetry. The magnetic correlation is found to be exclusively ferromagnetic in the spin glass state, and it is related to a magnetic excitation at ~ 1.1 meV down to T 0 K with a trace amount of doping. The nature of this excitation is argued to be associated with an indirect magnetic exchange. Two iii possibilities are discussed: a) a spin-flip excitation associated with a super-exchange ferromagnetic coupling via Co 3+ -O 2- -Ni 3+ . The coupling can be confined in one or two dimensions resulting in a high probability of charge localization; b) a weak ferromagnetic coupling mediated by Ni 3+ ions spins via Co 3+ -e - -Co 3+ within the localization range. Lastly, in PrCoO 3 , the atomic structure and neutron low energy spectra suggest that the thermal spin state transition may occur at a temperature above 300 K, which is much higher than what was previously observed in LaCoO 3 . To conclude, this thesis provides a microscopic approach to understanding the spinlattice interactions in this strongly correlated electron system that may be of broad interest to the field. To the years of A.D. 2005 to 2011.

Note: Abstract extracted from PDF text