H. Rakoto

Charge and orbital order in rare-earth and Bi manganites: a comparison

A comparative investigation of the charge modulation tendencies in R–M–MnO3 and Bi–Sr–MnO3 manganites reveals remarkable differences in these series of materials. By means of magnetic, magnetotransport, synchrotron X-ray and neutron diffraction, as well as pulsed high magnetic fields data, we compare the systematic electronic and magnetic properties of several rareearth-based manganites and bismuth-based Sr manganites. The differences between these types of systems are rationalized in terms of the role that the different electronic structure of bismuth and rare earth plays in these materials.

Spin state transition: the origin of structural, magnetic and metal–insulator transitions in GdBaCo2O5+δ (δ≈0.5)

The metal–insulator transition in GdBaCo2O5:5 at TMIE360 K has been investigated by synchrotron X-ray powder diffraction and magnetic measurements. We have found that,at TMI remarkable structural distortions and magnetic changes take place. These changes evidence a spin state transition of Co ions at this temperature that is the origin of the metal–insulator transition. r 2002 Elsevier Science B.V. All rights reserved.

Crystallographic and magnetic properties of Cu2FeGeSe4 and Cu2FeGeTe4 compounds

X-ray powder diffraction measurements, at room temperature, and magnetic susceptibility measurements, in the temperature range from 2 to 300 K, were made on polycrystalline samples of Cu2FeGeSe4 and Cu2FeGeTe4 magnetic semiconductor compounds. Magnetization measurements at 2, 4.2 and 77 K in magnetic fields up to 35 T were carried out on Cu2FeGeSe4 compounds. From the analysis of the X-ray diffraction lines, it was found that Cu2FeGeSe4 and Cu2FeGeTe4 have, respectively, stannite and monoclinic structures. The resulting 1/X versus T curves showed that Cu2FeGeSe4 is antiferromagnetic with a Neel temperature T N = 20 K while Cu2FeGeTe4 is ferrimagnetic with T N = 160.1 K. The magnetization and susceptibility results obtained on Cu2FeGeSe4 showed the presence of bound magnetic polarons (BMPs) in agreement with earlier studies made on this type of materials [1, 2].

Electronic conductivity in 1D Co spin chain single crystal

We have measured the transport and magnetic properties of the model spin chain single crystal Ca3Co2O6 in high pulsed fields. A crossover between 1D and 3D transport is observed, with the opening of a Coulomb gap below the order temperature of the individual chains. The samples show changes in the variable range hopping transport dimensionality and gap; the magnetic field suppresses the gap, inducing a significant reduction of the resistance, while dimensionality seems governed by temperature. At fields associated with the levels of magnetization of the frustrated triangular lattice formed by the Co lines, and below the critical magnetic temperature, a further increase of the conductivity is observed. This effect is associated with planar magnetic states perpendicular to the lines.

Anomalous magnetocaloric effect in van vleck paramagnet HoVO4 near energy level crossing

The anomalous magnetocaloric effect in singlet rare-earth paramagnets near energy level crossing has been experimentally observed for the first time. The magnetization and differential magnetic susceptibility of Ho1 − xYxVO4 crystals (x = 0, 0.5) measured along the tetragonal axis in a pulsed magnetic field with various change rates near the crossover field, dH/dt, from 3 × 103 to 2.5 × 102 T/s are compared to the data in static magnetic fields down to 0.1 K. These data indicate the large negative magnetocaloric effect in the pulsed magnetic field, so that the crystal with the initial temperature T0 = 4.2 K is cooled much lower than 1 K. The experimental data are qualitatively described in the crystal field model with various interactions including hyperfine interactions and with the known interaction parameters.

Magnetic properties of MnGa2Se4 in the temperature range of 2–300K

Measurements of low field static magnetic susceptibility and of magnetization with pulsed magnetic fields up to 32T have been made as a function of temperature on polycrystalline samples of the compound MnGa2Se4 which has a tetragonal structure. The resulting data have been used to give information on the magnetic spin-flop and magnetic saturation transitions. It has been found that MnGa2Se4 has a zero-field Neel temperature TN of 8.1K and shows a triple point at (7.8K, 2.2T). Details of the magnetic B-T phase diagrams were determined for the phases and the results compared with the predictions of theoretical uniaxial models. The susceptibility χ(T) curves for B=3 and 5T show magnetothermal effects below 4.5K.

Magnetic frustration in the spinel compounds GeNi2O4 and GeCo2O4

In the spinel compounds AB2O4, the B sites form a pyrochlore lattice. GeCo2O4 and GeNi2O4 have been investigated using high magnetic field magnetization as well as neutron diffraction measurements. Both compounds become antiferromagnetic at low temperature (around 23 and 12K, respectively) with the same propagation vector. The magnetization then presents two spin flop fields and one huge saturation field (above 50T). The Curie–Weiss temperature is positive for the Co spinel and negative for the Ni one. A model taking into account the first neighbor interaction J1 together with two different third neighbor interactions J2 and J3 is able to account for the common observed magnetic behavior. J1 is ferromagnetic while J2 and J3 are antiferromagnetic, in agreement with Goodenough–Kanamori–Anderson rules. These competing interactions are responsible for the frustration in these compounds.

Structural, spin state, and magnetic transitions in GdBaCo2O5+δ (δ≈0.5)

Abstract Structural, spin state, magnetic and field-induced transitions have been studied in GdBaCo 2 O 5.5 over a wide range of temperatures. We have found that, concomitant with a metal–insulator transition at 370 K , remarkable structural changes take place. These changes evidence the onset of orbital order at this temperature. Magnetization measurements, and the observation of a field-induced transition, below 250 K , by applying high magnetic fields, confirm that, below this temperature, an antiferromagnetic arrangement of the magnetic moments is present.

H–T diagrams of Ln1−xCaxMnO3 (x=1/2, 1/3) in pulsed fields up to 50 T

Well characterized Ln1−xCaxMnO3 manganites (Ln=La, Pr, Nd, Sm) with x=1/2 and x=1/3 have been investigated by magnetization measurements in pulsed magnetic fields up to 50 T. Using such high fields all the CO phase boundaries were identified in the temperature range 4–300 K. The transition fields at 4.2 K have been determined in a series of polycrystalline samples with x=1/2 and different effective bandwidths. They vary from Hc=8 T for La to ≈30 T for Sm. In this article we have focused on selected H–T phase diagrams. In particular, the results on Pr1/2Ca1/2MnO3, with the ideal 1:1 electron/hole ratio, are compared with Pr2/3Ca1/3MnO3 where the carrier concentration has been significantly reduced. The observed anomalies in the Hc(T) curves are discussed in the light on neutron diffraction data.

Crystallographic characterization and magnetic properties of the MnIn2(1−z)Ga2zSe4 alloy system

X-ray powder diffraction, differential thermal analysis DTA and magnetic susceptibility χ measurements were carried out on polycrystalline samples of the alloy system MnIn2(1−z)Ga2zSe4. Magnetization measurements at 2 K in magnetic fields up to 35 T were made on the MnIn2Se4 and MnGa2Se4 compounds. From the lattice parameter values, the limits of single-phase solid solution were estimated. At room temperature, the solid solutions extend over the range 0<z<0.30 for the δ-phase and from 0.75 to 1.0 for the η-phase. Values of TN, the Neel temperature, were obtained from the cusp in the χ versus T curves. Values of Curie–Weiss temperature θ were determined from the 1/χ versus T curves for the single-phase samples in each range. The magnetic results indicate that for 0<z<0.30, the Mn is randomly distributed over the cation sublattice, the phase having space group R3m and the compound MnIn2Se4 (z=0) was found to be spin-glass, with Tg=2.75 K and θ=−94 K. The high z single phase η alloys show an ordered distribution of the Mn2+ ions on the cation sublattice and were antiferromagnetic showing ideal Curie–Weiss behavior, with TN=8 K and the Curie–Weiss temperature θ=−24 K.