{Rearrangement of the antiferromagnetic ordering at high magnetic fields in SmFeAsO and SmFeAsO$_{0.9}$F$_{0.1}$ single crystals

摘要

The low-temperature antiferromagnetic state of the Sm-ions in bothnonsuperconducting SmFeAsO and superconducting SmFeAsO$_{0.9}$F$_{0.1}$ singlecrystals was studied by magnetic torque, magnetization, and magnetoresistancemeasurements in magnetic fields up to 60~T and temperatures down to 0.6~K. Weuncover in both compounds a distinct rearrangement of the antiferromagneticallyordered Sm-moments near $35-40$~T. This is seen in both, static and pulsedmagnetic fields, as a sharp change in the sign of the magnetic torque, which issensitive to the magnetic anisotropy and hence to the magnetic moment in the$ab$-plane, ({\it i.e.} the FeAs-layers), and as a jump in the magnetizationfor magnetic fields perpendicular to the conducting planes. This rearrangementof magnetic ordering in $35-40$~T is essentially temperature independent andpoints towards a canted or a partially polarized magnetic state in highmagnetic fields. However, the observed value for the saturation moment abovethis rearrangement, suggests that the complete suppression of theantiferromagnetism related to the Sm-moments would require fields in excess of60~T. Such a large field value is particularly remarkable when compared to therelatively small N\'{e}el temperature $T_{\rm N}\simeq5$~K, suggesting veryanisotropic magnetic exchange couplings. At the transition, magnetoresistivitymeasurements show a crossover from positive to negative field-dependence,indicating that the charge carriers in the FeAs planes are sensitive to themagnetic configuration of the rare-earth elements. This is indicates a finitemagnetic/electronic coupling between the SmO and the FeAs layers which arelikely to mediate the exchange interactions leading to the long rangeantiferromagnetic order of the Sm ions.