Polarized neutron reflectivity study of spin vortices formed in Gd-based Langmuir-Blodgett films

Abstract

Results are reported of a polarized neutron reflectivity (PNR) study of Gd-based Langmuir-Blodgett films in the temperature range of 55 mK to 15 K, representing two-dimensional magnets. A model based on in-plane spin vortices can explain the low-field as well as high-field PNR data. The branching of zero-field-cooled and field-cooled magnetization curves are explained by reconfiguration of the vortex structure. The low-field magnetization data is found to follow a power-law behavior as predicted by the Berezinskii-Kosterlitz-Thouless (BKT) transition for a finite-size system. We have also observed that a field of 15 kOe annihilates vortex-antivortex pairs completely to produce a homogeneous phase with saturation moment ~7μ_B per Gd ion below the BKT transition temperature (≃600 mK).