Influence of grain size on the electrical properties of the double-layered LaSr2Mn2O7 manganite

Magnetization, resistivity, and electron spin resonance (ESR) measurements were carried out to investigate spin dynamics, electrical and magnetic properties of (La 0.5 Pr 0.5 ) 0.7 Pb 0.3 Mn 1− x Cu x O 3 ( x = 0 , 0.02) perovskites. It was found that with Cu addition the Curie temperature decreased from 326 K for x =0 to 300 K for x = 0.0 2 composition. The x = 0 sample underwent a metallic–insulator transition at ∼130 K, whereas the x = 0.0 2 sample with higher resistivity exhibited an insulating behavior as a whole. It was furthermore found that asymmetrical ESR signals at low temperatures became Lorentzian at high temperatures above T min . The temperature dependence of the linewidth, Δ H ( T ) , at T > T min fits well to the one-phonon model, Δ H ( T ) = AT + B . The activation energy E a , determined from the ESR intensity with respect to temperature, are 0.16 and 0.14 eV for x = 0 and 0.02 compositions, respectively. In terms of our experimental results, it is reasonable to conclude that the Cu addition led to a suppression of the ferromagnetism and conductivity of the parent compound ( x = 0 ).

Spin dynamics, electrical and magnetic properties of (La0.5Pr0.5)0.7Pb0.3Mn1−xCuxO3 (x=0, 0.02) perovskites

Bulk polycrystalline layered perovskite sample La2Sm0.4Sr0.6Mn2O7 was prepared by the conventional solid-state reaction technique. Temperature dependence of magnetization measurements in the zero-field-cooled and field-cooled conditions with an applied magnetic field of 100 Oe indicates that layered manganite La2Sm0.4Sr0.6Mn2O7 undergoes a paramagnetic to ferromagnetic transition at Curie temperature Tc∼355 K. The magnetic transition temperature (Tc) is much higher than metal–insulator transition temperature (Tp) obtained from temperature dependence resistivity measurement. The anisotropic exchange interaction may be one of the reasons resulting in a large deviation between Tc and Tp. Field dependence of magnetization measurements showed a high value of magnetization (62 emu/g at 5 K and 42 emu/g at 300 K) in an applied magnetic field of 1 T. At 78 K, a sharp increase of magnetoresistance was observed at low magnetic fields followed by a weak and linear dependence at high fields.

Temperature features of magnetoresistance of layered manganite La2Sm0.4Sr0.6Mn2O7

Electron spin resonance (ESR) spectra of La1–xPbxMnO3 (0.1 ≤ x ≤ 0.5) compounds were recorded at different temperatures. Asymmetrical and distored resonance signals due to ferromagnetic correlations at temperatures T Tmin, where Tmin is the temperature corresponding to the narrowest ESR linewidth. The ESR linewidth with respect to temperature, ΔH (T), for the samples was fitted to the one-phonon process, ΔH (T) = A + BT. We found that B decreased from 5.45 Oe/K for x = 0.1 to 4.61 Oe/K for x = 0.5, indicating the decrease of lattice distortions with the Pb addition. The tem- perature dependence of the ESR intensity, I (T), for the samples was described well to an expression of I (T) = I0 exp (Ea/kBT). In the high-temperature region, 1/I (T) obeyed the Curie–Weiss law. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

ESR study of La1-xPbxMnO3 (0.1 ≤x ≤ 0.5) perovskites

Influence of A-site substitution on ESR spectra of lanthanum manganite perovskites

We have performed EPR and DC magnetization measurements on single crystals of La1.2Sr1.8Mn2O7. Similar to previous authors, we found an increase in the magnetization, M, near 300 K, a temperature well above T C ∼125 K . Previously, this high temperature ( ∼300 K ) feature has been suggested to be intrinsic to the system. We have observed at these high temperatures the appearance of new ferromagnetic resonances, FMR. Within the experimental error the volume of sample required to explain the extra M and the intensity of the FMR is the same. We conclude that the increase in M and the FMR are due to the presence of extrinsic Ruddlesden–Popper (RP) phases and not to intrinsic effects as has been claimed. Furthermore, the resonance field of the EPR line due to the La1.2Sr1.8Mn2O7 phase presents a measurable temperature-dependent anisotropy up to ∼450 K . The anisotropy is proportional to M, independent of the extrinsic RP phase content, and intrinsic to the system. We show that demagnetization effects can explain only part of the anisotropy. The role of short-range magnetic ordering in the observed intrinsic features cannot be disregarded, however.

Extrinsic and intrinsic features above Tc in layered manganite : La1.2Sr1.8Mn2O7

High temperature electron spin resonance (ESR) and magnetic susceptibility (χ) are analyzed for manganites related with colossal magnetoresistance (CMR). The properties of compounds with different crystalline structures: three-dimensional (3D) perovskites, pyrochlore, and La1.2Sr1.8Mn2O7, a two-dimensional layer, are compared. In the paramagnetic regime, and outside the critical regions associated with phase transitions, the temperature dependence of the ESR linewidth presents a universal behavior dominated by the variations of χ(T), ΔHpp(T)=[C/Tχ(T)]ΔHpp(∞). The high temperature limit of the linewidth, ΔHpp(∞), is related to the parameters of the Hamiltonian describing the interactions of the spin system. The role played by magnetic anisotropy, isotropic superexchange, and double exchange is revealed and discussed in the analysis of the experimental data. In CMR and non-CMR pyrochlores, ΔHpp(∞)∝ωp2/J where J is proportional to the Curie–Weiss temperature, including the hybridization mechanism producing C...

Spin dynamics in perovskites, pyrochlores, and layered manganites

We report magnetoresistivity measurements on a La 1.2 Sr 1.8 Mn 2 O 7 single crystal. The charge transport in the ferromagnetic phase is dominated by two-magnon scattering in the high T range and by weak localization effects at lower T , in the non-metallic regime. In the paramagnetic phase, ρ ab ( T ) obeys the adiabatic small polaron hopping mechanism, while ρ c ( T ) follows an Arrhenius behavior with the same activation energy.

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Electronic transport of bilayer manganite La1.2Sr1.8Mn2O7

Electron Spin Resonance Above Tc In Layered Manganites

http://repositorio.unicamp.br/jspui/bitstream/REPOSIP/107320/1/2-s2.0-0001019115.pdf

J S Gardner

Papers

Extrinsic and intrinsic features above Tc in layered manganite : La1.2Sr1.8Mn2O7

Spin dynamics in perovskites, pyrochlores, and layered manganites

Electronic transport of bilayer manganite La1.2Sr1.8Mn2O7

Electron Spin Resonance Above Tc In Layered Manganites

Spin Dynamics In Perovskites, Pyrochlores, And Layered Manganites