M.T. Causa

Depression of the weak-ferromagnetism of CuO2 planes in Gd2CuO4 through Ce and Th doping

Abstract Gd 2−x Ce x CuO oxides belong to a family of recently discovered electron doped cuprate superconductors. The undoped material Gd 2 CuO 4 shows antiferromagnetic ordering of the Cu moments about room temperature. We report here ESR and static magnetization measurements made in ceramic samples which indicate a depression of the ordering temperature at a rate of 18 K/at%Ce, down to the Ce solubility limit of x∽0.16, where the material orders below 180 K. A similar rate of depression has been observed for a sample with Th substitution for Gd ions.

Strong ferro-antiferromagnetic competition and charge ordering in Pr0.67Ca0.33MnO3

Abstract In this article we present d.c. resistivity, a.c. impedance, magnetization and dielectric permittivity measurements in Pr0.67Ca0.33MnO3. A rise in the imaginary part of impedance was detected at the charge ordering transition temperature (T=225xa0K). This indicates an intrinsic capacitive behavior of the sample in the charge ordering state. The a.c. impedance is presented here as a valid and easy tool to characterize the charge ordering transition.

Jahn—Teller effects on the superexchange interactions in LaMnO3

Abstract We present Magnetization and ESR measurements in the paramagnetic phase of ceramic LaMnO 3 with carefully controlled stoichiometry. At T ≈ 600 K, near the Jahn—Teller transition observed in this material, we found an important decrease of the ESR linewidth. We explain this observation considering the crucial role of the lattice distortions which strongly affects the anisotropic and isotropic exchange interactions.

Experimental study of charge ordering transition in Pr0.67Ca0.33MnO3

Abstract Charge ordering and antiferromagnetic transitions in Pr 0.67 Ca 0.33 MnO 3 have been studied by resistivity, thermopower, magnetic susceptibility and capacitance measurements. Strong magnetic competition arising from the distorted perovskite structure leads to a complex behavior. The appearance of the charge ordering coincides with antiferromagnetism at ≈225 K, leading to a large capacitive response of the material.

High temperature susceptibility in electron doped Ca1 xYxMnO3: double exchange versus superexchange

We present a study of the magnetic properties of the electron doped manganites Ca1−x Yx MnO3 (for 0 ≤ x ≤ 0.25) in the paramagnetic regime. For the less doped samples (x ≤ 0.1) the magnetic susceptibility, χ(T), follows a Curie–Weiss (CW) law only for T > 450 K and, below this temperature, χ−1 (T) shows a ferrimagnetic-like curvature. We approached the discussion of these results in terms of a simple mean-field model where double exchange, approximated by a ferromagnetic Heisenberg-like interaction between Mn3+ and Mn4+ ions, competes with classical superexchange. For higher levels of doping (x ≥ 0.15), the CW behaviour is observed down to the magnetic ordering temperature (Tmo) and a better description of χ(T) was obtained by assuming full delocalization of the eg electrons. In order to explore the degree of delocalization as a function of T and x, we analysed the problem through Monte Carlo simulations. Within this picture we found that at high T the electrons doped are completely delocalized but, when Tmo is approached, they form magnetic polarons of large spin that cause the observed curvature in χ−1 (T) for x ≤ 0.1.

Electron spin resonance and magnetization in perovskite and pyrochlore manganites

We have performed electron spin resonance (ESR) and dc susceptibility, χdc(T), measurements in La0.7Ca0.3MnO3 perovskite up to 1000 K. This is adequately described in the paramagnetic regime in the constant coupling approximation assuming an effective Heisenberg-like pair interaction. The ESR linewidth is given by ΔHpp(T)=ΔHpp(∞)[C/Tχ(T)]. The ESR intensity follows the T dependence of χdc(T). We conclude that the relaxation mechanism is essentially the same for the whole studied paramagnetic range. The observed behavior reflects the progressive importance of magnetic clustering on approaching Tc. We have found similar T dependence for La0.7Sr0.3MnO3 and La0.7Pb0.3MnO3 perovskites, and also for the pyrochlore In2Mn2O7.

Copper content and superconductivity of Nd1.85Ce0.15Cu1+δOy

Abstract The superconducting transition of Nd 1.85 Ce 0.15 Cu 1+δ O y samples was studied as a function of the Cu content and processing temperature through magnetic modulated microwave absorption and dc magnetization measurements. Differential thermal analysis and scanning electron microscopy observations show an indication of a small Cu nonstoichiometry with a maximum Cu excess in the T structure, δ max ≲ 0.01. Superconducting transitions were observed only for samples with a Cu content 1+ δ ≥ 1.01. This Cu excess seems to be necessary in order to obtain superconductivity in the Nd-Ce-Cu-O system.

Effect of porosity on FMR linewidth of Ln0.67A0.33MnO3 (Ln La, Pr; A Ca, Sr)

Abstract Magnetization and ferromagnetic resonance (FMR) measurements near the Curie temperature have been taken from sol—gel and ceramic samples of Ln 0.67 A 0.33 MnO 3 (Ln ue5fb Pr, La; A ue5fb Ca, Sr). We demonstrate that the demagnetizing fields arising from the pores in polycrystalline samples cause the FMR line broadening observed below 1.1 T C , with respect to the values of the equivalent single crystals.

The electronic structure of CaMnOx with 2.66 ≤ x ≤ 3.00

Abstract We studied the electronic structure of CaMnOx with 2.66 ≤ x ≤ 3.00 by means of soft X-ray absorption spectroscopy (XAS). The Mn 2p XAS spectra present changes in the shape of the multiplets and chemical shifts. The chemical shifts indicate a decrease of the Mn 3d count as the oxygen content increases. The Mn 2p XAS spectra are calculated projecting the atomic multiplet on the appropriate crystal field. The O 1s XAS spectra show an increase in the absorption intensity just above the bottom of the conduction band. This effect indicates an increase of unoccupied O 2p states for larger oxygen contents. The O 1s XAS spectra are calculated using cluster model calculations which include charge transfer processes. We conclude that the doped holes in the CaMnOx system contain a mixture of metal and ligand character.

Magnetic frustration in Y-doped manganites: Electron spin resonance and magnetization

Magnetic susceptibility (χdc) and electron spin resonance (ESR) measurements on La0.67−xYxCa0.33MnO3 in the paramagnetic (PM) regime Tc⩽T⩽1000u200aK are presented, for 0⩽x⩽0.20. For T>750u200aK,χdc follows a Curie–Weiss (CW) law with C=2.95(15)u200aemuu200aK/mol, and a CW temperature, Θ, showing a monotonic variation as a function of x. The ESR linewidth follows a T dependence given by ΔHpp(T)=[C/Tχdc(T)]ΔH∞. The parameter ΔH∞ increases from 2600 G for x=0 to 3300 G for x=0.20. The correlation with expected variation of the anisotropic spin–spin interactions is discussed. The results appear to be insensitive to the changes of the double-exchange isotropic interaction. A comparison of our experimental results in the PM regime with reported data on the ordered phase indicates a complex magnetic behavior associated with Y substitution. The CW temperature, a measure of the average strength of the magnetic interactions, decreases slowly with x (17% for x=0.20) reflecting the softening of the double exchange due to the bending...