A. Maignan

MAGNETORESISTANCE IN THE OXYGEN DEFICIENT LNBACO2O5.4 (LN=EU, GD) PHASES

New “112” phases, LnBaCo2O5.4, with an ordered oxygen deficient perovskite structure, derived from the YBaFeCuO5-type were studied for Ln=Eu, Gd. The appearance of giant negative magnetoresistance in this structural type is demonstrated. Resistance ratio R0/R7 T reaches at least 10 at 10 K, i.e., is significantly larger than those observed in the other cobalt perovskites, such as La1−xSrxCoO3. These properties are linked to an original magnetic behavior of these materials that exhibit two types of transition—antiferromagnetic to ferromagnetic, and ferromagnetic to paramagnetic—as T increases. This magnetic behavior may be related to a possible I.S. and L.S. spin ordering of trivalent cobalt in pyramidal and octahedral coordinations, respectively.

Single crystal study of the one dimensional Ca \(\)Co \(\)O \(\) compound: five stable configurations for the Ising triangular lattice

Abstract:Single crystals of the one-dimensional phase Ca3Co2O6 of several mm length have been grown. The magnetic study of such a crystal confirms the previous observations on polycrystalline samples: it consists of a triangular lattice of ferromagnetic [Co2O6] chains ( K) antiferromagnetically coupled ( K). The dynamic of these chains array, probed by AC susceptibility, is very slow as shown from the large shift of the freezing temperature from 12 K to 16.5 K as the excitation frequency increases by three orders of magnitude (100 to 103 Hz). The origin of this effect is believed to be the result of different arrangements with close energies for the chain ferromagnetic moments on the triangular lattice. Five stable magnetic configurations have been evidenced by the magnetization as a function of applied field curves registered at 2 K. Their relative magnetizations correspond to m=1/4, 1/2, 1, 2, 3 where m=3 represents the ferromagnetic ordering of three chains on the same triangle, each chain having a m=1 magnetization. A magnetic phase diagram is finally proposed.

Cation disorder and size effects upon magnetic transitions in Ln0.5A0.5MnO3 manganites

A systematic study of the magnetic properties of the manganites Ln0.5(A,A′)0.5MnO3 (A,A′=Ba, Ca, Sr) has been carried out. The variations of TC and TN vs two parameters, the average size of the interpolated cations 〈rA〉 and the mismatch effect represented by the variance σ2, have been studied for the series of manganites La0.5(A,A′)0.5MnO3, Pr0.5(A,A′)0.5MnO3, Nd0.5(A,A′)0.5MnO3 and Sm0.5(A,A′)0.5MnO3 (A,A′=Ba, Ca, Sr), involving at most three different cations on the perovskite A site. The results obtained for other (Ln, Ln′)0.5Sr0.5MnO3(Ln, Ln′=Gd, Sm Y, Pr) have been taken into consideration. A σ2−〈rA〉 diagram has been established, which displays the different types of magnetic behaviors that can be exhibited by Ln0.5A0.5MnO3 manganites, i.e., when the temperature is decreasing, either a paramagnetic (PM) to ferromagnetic (FM) transition, or PM to FM followed by FM to antiferromagnetic transitions, or a PM to weak ferromagnetic transition.

Valence, spin, and orbital state of Co ions in one-dimensional Ca3Co2O6 : An x-ray absorption and magnetic circular dichroism study

We have investigated the valence, spin, and orbital state of the Co ions in the one-dimensional cobaltate Ca3Co2O6 using x-ray absorption and x-ray magnetic circular dichroism at the Co-L2,3 edges. The Co ions at both the octahedral Cooct and trigonal Cotrig sites are found to be in a 3+ state. From the analysis of the dichroism we established a low-spin state for the Cooct and a high-spin state with an anomalously large orbital moment of 1.7B at the Co trig ions. This large orbital moment along the c-axis chain and the unusually large magnetocrystalline anisotropy can be traced back to the double occupancy of the d2 orbital in trigonal crystal field.

Strongly correlated properties of the thermoelectric cobalt oxide Ca3Co4O9

We have performed both in-plane resistivity, Hall effect, and specific heat measurements on the thermoelectric cobalt oxide

Magnetoresistance and magnetothermopower properties of Bi/Ca/Co/O and Bi(Pb)/Ca/Co/O misfit layer cobaltites

{\mathrm{Ca}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{9}

Extension of colossal magnetoresistance properties to small A site cations by chromium doping in Ln0.5Ca0.5MnO3 manganites

. Four distinct transport regimes are found as a function of temperature, corresponding to a low temperature insulating one up to

Cation size-temperature phase diagram of the manganites Ln0.5Sr0.5MnO3

{T}_{\mathit{min}}\ensuremath{\approx}63\phantom{\rule{0.3em}{0ex}}\mathrm{K}

Ising Magnetism and Ferroelectricity in Ca3CoMnO6

, a strongly correlated Fermi liquid up to

Up to 50 000 per cent resistance variation in magnetoresistive polycrystalline perovskites Ln23Sr13MnO3 (Ln=Nd; Sm)

{T}^{*}\ensuremath{\approx}140\phantom{\rule{0.3em}{0ex}}\mathrm{K}