Giant magnetoresistance of oxides with perovskite-related structure.

摘要

Magnetoresistance (MR) is investigated in four systems of perovskite-related compounds: Ca(Cu_3−xMn_x)Mn₄O₁₂ , CaMnO_3−δ, Ca_1−xCe_xMnO ₃ and Sr₂Cr_1.2Mo_0.8O_6−δ . Single phase CaCu_3−xMn_4+xO₁₂ (0 ≤ x ≥ 3.0) with a perovskite-related structure was synthesized by a combined sol-gel and solid state reaction methods. CaCu_3−xMn _4+xO₁₂ (0 ≤ x ≥ 2.0) develop a FM moment in the range 340–190 K and exhibit a negative MR at low temperature. CaCu₃Mn ₄O₁₂ with only Mn4+ shows the highest MR effect and increasing Mn3+ content on the B site of Ca2+(Cu2+_3−xMn3+ _x)(Mn4+_4−xMn3+ _x)O₁₂ is deleterious for MR. These results indicate that the Mn3+-O-Mn4+ double exchange mechanism used to explain the properties of substituted LnMnO₃-type phases may not be operating in these compounds. The low O-Mn-O angle (142°) appears to lead to ferromagnetic ordering in CaCu_3−xMn_4+xO ₁₂ via superexchange, rather than the double exchange mechanism operating in substituted LnMnO₃. The dominant MR mechanism in CaCu_3−x Mn_4+xO₁₂ appears to be consistent with intergrain tunneling. CaCu₃Mn₄O₁₂ shows a higher sensitivity to H at low fields, and better temperature stability than other Mn-perovskite CMR materials. These properties are advantageous for device applications.; Titration and Mn-K X-ray absorption spectroscopy (XAS) indicate the reduction of Mn(IV) to Mn(III) with increasing δ in the CaMnO_3−δ system by the creation of two five coordinate Mn(III) sites per O vacancy. The resistivity of the electron doped CaMnO_3−δ (δ ≤ 0.11) decreases as δ increases but remains semiconducting in the range 20–300 K. The superexchange interactions that leads to G-type antiferromagnetic (AF) ordering in the δ = 0, Mn(IV) material still dominates in the O-defect materials, however the AF-domain canting appears hindered. A large ∼40% MR is demonstrated in antiferromagnetic CaMnO_2.89.; In the Ca_1−xCe_xMnO₃ system (x ≤ 0.2), Ce4+ doping leads to n-type majority carrier (i.e. electron doping). The mixed valency of Mn3+/4+ results in a decrease of the resistivity and ferromagnetism via the double exchange mechanism. However, the ferromagnetism could be destroyed by charge ordering, which would increase the resistivity. The charge ordered state may be removed at low temperature; this will lead to lowering of the activation energy at low temperature. Both inter-grain tunneling and charge-ordering-related phenomena are observed in Ca_1−xCe_xMnO₃ as possible mechanism for CMR.; Double perovskite Sr₂Cr_1.2Mo_0.8O _6−δ (δ = 0, 0.2) was prepared by solid state reaction in evacuated quartz tubes. Large magnetoresistance (−43%) is observed in Sr₂Cr_1.2Mo_0.8O_6−δ. The lattice parameter increases for δ ≠ 0. The compounds are n-type narrow gap semiconductors, and are ordered ferrimagnetically below 400 K. The MR behavior could be explained by intra-grain tunneling mechanism.