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AM: Magnetismus

AM 13: Postersitzung: Dünne Schichten (1–22), Magnetowiderstand (23–40), Phasenübergänge (41–55), Mikromagnetismus (56–68), Spektroskopie (69–77), Nanokristalline Materialien (78–82), Anisotropie (83–86), Schmelzen (87–90), Weitere Bereiche (91–100)

AM 13.65: Poster

Dienstag, 28. März 2000, 16:00–20:00, A

Evidence for magnetic polarons in A_1−xCa_xMnO₃, A=Ho,Y — •Stefan Mogck, Carsten Schinzer, Helmut Schmid, and Wolfram Prandl — Inst. für Kristallographie, Charlottenstr. 33, 72070 Tübingen

The title compounds crystallize in the GdFeO₃ structure with a small monoclinic distortion. For concentrations 0.655 ≤ x ≤ 0.75 we find from neutron diffraction the coexistence of commensurate as well as incommensurate antiferromagnetic order, but no indication of ferromagnetism. Indications of ferromagnetism are also absent in the longitudinal magnetization M’. The transverse magnetization M”, measured in a rotating magnetic field, shows, nevertheless, the onset of local magnetic order at T_c≈ 133 K in accordance with the neutron observations. In addition M” exhibits three maxima at T₁=101 K,T₂=108 K and T₃=128 K, respectively which do not depend on the composition x. Annealing of the samples repeatedly at 1400 ^∘C for 24 hours lowers the magnitude of these maxima without, however, any shift of the T_j. Neither nuclear or magnetic neutron nor X-ray Bragg intensities reveal any anomaly at T_j: this observation points towards a local origin of the effect. We conjecture that freshly prepared powder samples

may contain some Ca²⁺_m (m=1,2,...) clusters which act as traps for magnetic polarons. These clusters do not have translational symmetry, and so do not show up in the Bragg intensities. Only after repeated annealing steps the pellets enter into a homogeneous (Ho, Ca) distribution.