Electronic structure of CMR manganites (invited)
- Center for Materials Science Engineering, ETC 9.102, University of Texas at Austin, Austin, Texas 78712-1063 (United States)
A colossal negative magnetoresistance (CMR) occurs in manganites at a first-order ferromagnetic transition. The Mn{sup 4+} and high-spin Mn{sup 3+} ions each contain localized t{sup 3} configurations; the t{sup 3}{endash}p{sub {pi}}{endash}t{sup 3} superexchange interactions are antiferromagnetic. The orbital degeneracy of localized Mn{sup 3+}:t{sup 3}e{sup 1}, {sup 5}E{sub g} configurations is lifted by cooperative static or dynamic Jahn{endash}Teller deformations. Strong e-electron coupling to oxygen displacements, static or dynamic, introduces ferromagnetic e{sup 1}{endash}p{sub {sigma}}{endash}e{sup 0} interactions either via superexchange or via a stronger double exchange. At the first-order ferromagnetic transition, a change from {tau}{sub h}{approx}{h_bar}/W{sub {sigma}}{gt}{omega}{sub R}{sup {minus}1} to {tau}{sub h}{lt}{omega}{sub R}{sup {minus}1} occurs within mobile molecular units, where W{sub {sigma}} is the bandwidth for states of e-orbital parentage and {omega}{sub R}{sup {minus}1} is the period of the optical-mode lattice vibration that traps a mobile hole as a small-polaron Mn{sup 4+} ion. T{sub C} increases with the fraction of double-exchange couplings, and this fraction increases with W{sub {sigma}} and {omega}{sub R} at the transition from polaronic to itinerant-electron behavior below T{sub C}. The bandwidth W{sub {sigma}}{approximately}{epsilon}{sub {sigma}}{lambda}{sub {sigma}}{sup 2}cos{phi}{l_angle}cos({theta}{sub ij}/2){r_angle} depends on the covalent mixing parameter {lambda}{sub {sigma}}, which increases with pressure, as well as on the Mn{endash}O{endash}Mn bond angle (180{degree}{minus}{phi}), which increases with the tolerance factor t that measures the equilibrium bond-length mismatch, and on the angle {theta}{sub ij} between neighboring spins so that W{sub {sigma}} increases with the spontaneous magnetization on cooling below T{sub C}. (Abstract Truncated)
- OSTI ID:
- 544297
- Report Number(s):
- CONF-961141-; ISSN 0021-8979; TRN: 9709M0293
- Journal Information:
- Journal of Applied Physics, Vol. 81, Issue 8; Conference: 41. annual conference on magnetism and magnetic materials, Atlanta, GA (United States), 12-15 Nov 1996; Other Information: PBD: Apr 1997
- Country of Publication:
- United States
- Language:
- English
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