55Mn NMR observation of colossal magnetoresistance effect in Sm0.55Sr0.45MnO3.

Temperature dependent 55Mn NMR study of Sm0.55Sr0.45MnO3 is reported. Previous bulk magnetization measurements have shown that below T C ~ 125 K the sample is ferromagnetic metallic (FMM) and above TC it is charge ordered and insulating. In present report, we show that from zero-field NMR a single line double-exchange (DE) signal is observed at temperatures up to 139 K, which is due to a presence of FMM clusters also above T C. The intensity of the DE line follows the temperature dependence of the magnetization measured at 0.01 T. When a magnetic field up to 2 T is applied at 139 K (i.e. 14 K above T C), a strong increase in NMR intensity of the DE line is observed indicating that content of FMM regions increases. This reveals that metallicity is induced in the material by the applied magnetic field and explains the observed colossal magnetoresistance (CMR) effect at the microscopic level. The observation agrees with previous results, which confirm that the percolation of the FMM clusters is responsible for the CMR effect. The shift of the resonant frequency in the applied field is three times smaller compared to decrease expected from gyromagnetic ratio, which indicates an antiferromagnetic coupling between the FMM clusters.

Correlations among magnetic, electrical and magneto-transport properties of NiFe nanohole arrays.

In this work, we use anodic aluminum oxide (AAO) templates to build NiFe magnetic nanohole arrays. We perform a thorough study of their magnetic, electrical and magneto-transport properties (including the resistance R(T), and magnetoresistance MR(T)), enabling us to infer the nanohole film morphology, and the evolution from granular to continuous film with increasing thickness. In fact, different physical behaviors were observed to occur in the thickness range of the study (2 nm < t < 100 nm). For t < 10 nm, an insulator-to-metallic crossover was visible in R(T), pointing to a granular film morphology, and thus being consistent with the presence of electron tunneling mechanisms in the magnetoresistance. Then, for 10 nm < t < 50 nm a metallic R(T) allied with a larger anisotropic magnetoresistance suggests the onset of morphological percolation of the granular film. Finally, for t > 50 nm, a metallic R(T) and only anisotropic magnetoresistance behavior were obtained, characteristic of a continuous thin film. Therefore, by combining simple low-cost bottom-up (templates) and top-down (sputtering deposition) techniques, we are able to obtain customized magnetic nanostructures with well-controlled physical properties, showing nanohole diameters smaller than 35 nm.

XAS and XMCD under high magnetic field and low temperature on the energy-dispersive beamline of the ESRF.

The present paper demonstrates the feasibility of X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) under high magnetic fields up to 26 T and low temperatures down to 5 K on the ID24 energy-dispersive XAS beamline of the ESRF. The pulsed magnetic field set-up, entirely developed at the ESRF, is described as well as the beamline set-up, the synchronization and the measurement procedure. It allows field strengths up to 30 T. Finally, as an example, we report a recent XMCD study at the Re L2 and L3 absorption edges of the double perovskite Sr2CrReO(6).