Hopping conductivity in one-dimensional Ca/sub 3/Co/sub 2/O/sub 6/ single crystals

We studied the electronic conductivity and magnetic properties of quasi-one-dimensional Ca₃Co₂O₆ single crystals. The results evidence a variable range hopping conductivity with temperature-induced crossover between one-dimensional (intrachain) and three-dimensional (3D) transport and the opening of a Coulomb gap in the d bands along with the ferromagnetic intrachain ordering. At low temperatures, an applied magnetic field induces a large negative magnetoresistance (MR) in apparent dissociation with the 3D magnetic ordering. Both spin-dependent hopping and field-induced suppression of the Coulomb gap are discussed. The electronic hopping parameters we infer agree remarkably with the accessible Co sites. Surprising narrow peaks corresponding to a transient resistivity decrease are observed on the MR curves. We discuss these in terms of peculiar in-plane magnetization states in an Ising-like Heisenberg antiferromagnetic triangular lattice during the magnetization reversal

Non-saturation of the defect moment of goethite and fine-grained hematite up to 57 Teslas

International audienceDefect moment of antiferromagnets yields the highest remanent coercivity observed among minerals, and previous studies have been unable to reach saturation of isothermal remanent magnetization (IRM) in some goethite and hematite, even up to 20 Teslas, using resistive Bitter magnets. To go further, acquisition of IRM at room temperature has been monitored on various natural and synthetic goethite and hematite samples in pulsed magnetic fields up to 57 Teslas. ``Coarse'' hematite is saturated around 5 T, and low unblocking temperature (TB, i.e. with low crystallinity or Al substitution) goethites saturate around 20 T. Higher TB goethites and a Mn-bearing fine-grained hematite are still not saturated even at 57 T, with only 2 to 10 percent of the maximum IRM acquired in 3 T. Half acquisition fields are mostly above 10 T. This indicates that usual rock magnetic techniques strongly underestimate the contribution of such minerals to remanence. IRM acquisition is strongly irreversible: in some samples a 57 T backfield is unable to erase a previous 38 T IRM. A field induced defect diffusion model is put forward to account for remanence acquisition in these materials