Polarized neutron scattering and sub-Kelvin magnetization measurements in two-dimensional gadolinium stearate Langmuir-Blodgett films

Abstract

The role of dipolar interactions and anisotropy are important to obtain, otherwise forbidden, ferromagnetic ordering at a finite temperature for ions arranged in two-dimensional (2D) arrays (monolayers). We performed magnetization measurements with conventional low temperature magnetometry and with polarized neutron scattering on a multilayer stack of noninteracting monolayers of gadolinium ions formed by the Langmuir-Blodgett technique. The spontaneous magnetization could not be detected in the heterogeneous magnetic phase observed here and the saturation value of the net magnetization was found to depend on the sample temperature and applied magnetic field. The net magnetization rises exponentially with a lowering temperature and then reaches saturation following a T ln(βT) dependence. The T ln(βT) dependence of the magnetization has been predicted from the spin-wave theory of a 2D in-plane spin system with the ferromagnetic interaction above βT=1. The experimental findings reported here could be parametrized by extending the fit of this expression to a temperature domain of βT<1. Complete understanding of our observations will require further theoretical development.