Münster 1999 – wissenschaftliches Programm

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AM: Magnetismus

AM 10: Postersitzung

AM 10.8: Poster

Dienstag, 23. März 1999, 16:15–20:00, F\"urstenberghaus

Effective conductivity and magnetic order in (Ho,Ca)MnO₃ and (Y,Ca)MnO₃ — •K. Knorr, W. Prandl, and H. Schmid — Institut f"ur Kristallographie Universit"at T"ubingen, Charlottenstra"se 33, 72070 T"ubingen

(Ho_xCa_1−x)MnO₃ belongs to the family of compounds with high conductivity, ferromagnetic behaviour and potentially high magnetoresistance. We observe a gradual transition of metallic behaviour at low Ho concentrations (with an optimum around x = 0.1) to pure semiconductivity at higher concentrations. The observed resistivities extend over more than seven decades, from 2· 10⁻³ Ohm·cm to 5· 10⁴ Ohm·cm. The metallic phases are canted antiferromagnets with the ferromagnetic component increasing for concentrations 0 ≤ x ≤ 0.12. Beyond x = 0.15 the spin-canting breaks down, and pure antiferromagnetism occurs. We correlate this phenomenon with a transition from orthorhombic (Pnma) to monoclinic (P2₁/m) crystal structures in which the e_g electrons carrying the current are evidently progressively (with increasing x) immobilized. The system (Y,Ca)MnO₃ is very similar to (Ho,Ca)MnO₃: this excludes any influence due to the paramagnetism of Ho on the observed phenomena and points out the importance of ionic radii and cell volume effects in this series of compounds.
We acknowledge the support by the Deutsche Forschungsgemeinschaft (Proj. PR44/8-1).