Cation ordering in ABO3 perovskites can lead to interesting and useful phenomena such as ferrimagnetism and high magnetoresistance by spin polarized conduction in Sr2FeMoO6. We have used high pressures and temperatures to synthesize the cation ordered AA′BB′O6 perovskites CaMnFeReO6, CaMnMnReO6, and Ca(Mn0.5Cu0.5)FeReO6. These have columnar A/A′ and rocksalt B/B′ cation orders, as found in the recently discovered double double perovskite MnNdMnSbO6, and partial Mn/Cu order over tetrahedral and square planar A′ sites in Ca(Mn0.5Cu0.5)FeReO6 demonstrates that 'triple double' cation order is possible. Neutron diffraction reveals complex ferrimagnetic orders in all three materials; CaMnFeReO6 and Ca(Mn0.5Cu0.5)FeReO6 have large room temperature magnetizations with low temperature switching of magnetoresistance in the latter material, and CaMnMnReO6 displays a high coercivity of 1.3 T at low temperatures.
CaMnFeTaO6 has been synthesised at 1200 °C under 10 GPa pressure. Powder neutron diffraction shows that CaMnFeTaO6 adopts a double double perovskite structure (tetragonal space group P42/n, lattice parameters a = 7.683(3) and c = 7.685(7) Å) with cation disorder at all transition metal sites. Magnetization measurements reveal an apparent ferro-or ferri-magnetic transition at Tm1 = 51 K and a susceptibility peak at Tm2 = 20 K, but no long-range magnetic order is observed by neutron diffraction down to 1.5 K. This is attributed to formation of superparamagnetic clusters of ferrimagnetically ordered spins below Tm1 that freeze into a cluster spin glass at Tf =Tm2. AC magnetisation measurements confirm the cluster spin glass ground state. Disorder from substitution of Fe 2+ for Mn 2+ and Fe 3+ /Ta 5+ inversion disrupts the network of superexchange interactions leading to the cluster spin glass ground state, in contrast to other P42/n double double perovskites where long range magnetic order is stabilised.
Two new CaMnBSbO6 (B = Cr and Fe) high pressure double double perovskites are reported. Spins in CaMnCrSbO6 order below TC = 49 K into a collinear ferrimagnetic arrangement with spins in the xy plane. CaMnFeSbO6 has the same ordered magnetic structure below TC1 = 55 K, but also shows a second magnetic transition at TC2 = 21 K where the spins reorient towards the z axis. This may reflect a greater degree of magnetic disorder for B = Fe and A-B intersite charge transfer may also be significant.
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