Observation of spin-wave characteristics in the two-dimensional ferromagnetic ordering of in-plane spins

TL;DR

This study uses magnetometry and neutron scattering to observe ferromagnetic ordering of in-plane spins in 2D gadolinium monolayers, confirming spin-wave theory predictions and revealing temperature-dependent magnetization behavior.

Contribution

It demonstrates a novel experimental approach to study 2D ferromagnetism and verifies theoretical predictions about spin-wave behavior in in-plane 2D spin systems.

Findings

Net magnetization depends on applied magnetic field.

Magnetization rises exponentially with decreasing temperature.

Saturation follows a T ln(βT) dependence.

Abstract

The role of dipolar interactions and anisotropy are important to obtain, otherwise forbidden, ferromagnetic ordering at finite temperature for ions arranged in two-dimensional (2D) arrays (monolayers). Here we demonstrate that conventional low temperature magnetometry and polarized neutron scattering measurements can be performed to study ferromagnetic ordering of in-plane spins in 2D systems using a multilayer stack of non-interacting monolayers of gadolinium ions. The spontaneous magnetization is absent in the heterogenous magnetic phase observed here and the saturation value of the net magnetization was found to depend on the applied magnetic field. The net magnetization rises exponentially with lowering temperature and then reaches saturation following a $T\ln(\beta T)$ dependence. These findings verify predictions of the spin-wave theory of 2D in-plane spin system with ferromagnetic interaction and will initiate further theoretical development.