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 both nonsuperconducting SmFeAsO and superconducting SmFeAsO_(0.9)F_(0.1) single crystals was studied by magnetic torque, magnetization, and magnetoresistance measurements in magnetic fields up to 60 T and temperatures down to 0.6 K. We uncover in both compounds a distinct rearrangement of the antiferromagnetically ordered Sm moments near 35 — 40 T. This is seen in both static and pulsed magnetic fields as a sharp change in the sign of the magnetic torque, which is sensitive to the magnetic anisotropy and hence to the magnetic moment in the ab plane, (i.e., the FeAs layers), and as a jump in the magnetization for magnetic fields perpendicular to the conducting planes. This rearrangement of magnetic ordering in 35 — 40 T is essentially temperature independent and points toward a canted or a partially polarized magnetic state in high magnetic fields. However, the observed value for the saturation moment above this rearrangement, suggests that the complete suppression of the antiferromagnetism related to the Sm-moments would require fields in excess of 60 T. Such a large field value is particularly remarkable when compared to the relatively small Neel temperature T_n (~-) 5 K, suggesting very anisotropic magnetic exchange couplings. At the transition, magnetoresistivity measurements show a crossover from positive to negative field dependence, indicating that the charge carriers in the FeAs planes are sensitive to the magnetic configuration of the rare-earth elements. This points towards a finite magnetic/electronic coupling between the SmO and the FeAs layers which are likely to mediate the exchange interactions leading to the long-range antiferromagnetic order of the Sm ions.