Studies of Magnetism in Rhenium and Manganese Based Perovskite Oxides

Abstract

<p>The bulk of this thesis consists of studies of geometric frustration in S = 1/2 FCC perovskites based upon the chemical formula A₂BReO₆. The magnetism of these materials is expected to exhibit geometric frustration, a situation in which the ideal spin arrangements cannot be achieved for antiferromagnetic interactions between adjacent spins. It is proposed that subtle quantum effects are driving these systems to unique ground states in the absence of chemical disorder. Both compounds Sr₂CaReO6 and Sr₂MgReO₆ exhibit spin glass behaviour at low temperatures (TG ~14 K and TG -50 K respectively), in which the magnetic moments freeze out in random orientations instead of an ordered array. This work shows that these materials possess several unconventional properties, which suggest that interesting spin dynamics may be present. Other perovskite and perovskite-related materials studied in this thesis include the magnetoresistive CaMn03_o and the "pillared" material La₅Re₃MnO₁₆. Neutron diffraction studies have shown that both CaMnO2.94 and CaMn02.89 order at TN - 125 K, but possess unique yet related magnetic structures. CaMnO2.94 orders into a simple Gtype magnetic structure, as observed in the compound CaMnO₃. The slightly more doped sample CaMn02.89, on the other hand, orders into a magnetic structure related to the Gtype, and involves a Mn³/Mn⁴⁺ charge ordering over every four lattice spacings. The new material La₅Re₃MnO₁₆ consists of layers of comer shared ReO₆ and MnO₆ octahedra that are separated by layers of Re₂O₁₀ dimer units. Metal-metal bonding involving Re atoms have been postulated for these dimers which separate the Re/Mn layers by approximately 10 Å. The magnetic behaviour exhibited by this new class of materials is rich and complex. Despite the large distances separating the perovskite layers, the Re and Mn magnetic moments order into a ferrimagnetic Q = (0, 0, 1/2) structure below a relatively high TN of 161 K. There may be an additional spin rearrangement at lower temperatures as evidenced by weak magnetic Bragg peaks below ~ 50 K.</p>