Two-dimensional magnetic ordering in a multilayer structure

M K MUKHOPADHYAY and M K SANYAL

Surface Physics Division, Saha Institute of Nuclear

Physics, 1/AF Bidhan Nagar, Kolkata 700 064, India

E-mail: milank.sanyal@gmail.com

Abstract. The effect of confinement from one, two or from all three

directions on magnetic ordering has remained an active field

of research for almost 100 years. The role of dipolar

interactions and anisotropy are important to obtain, the

otherwise forbidden, ferromagnetic ordering at finite

temperature for ions arranged in two-dimensional (2D) arrays

(monolayers). We have demonstrated that conventional

low-temperature magnetometry and polarized neutron scattering

measurements can be performed to study short-range

ferromagnetic ordering of in-plane spins in 2D systems using

a multilayer stack of non-interacting monolayers of

gadolinium ions formed by Langmuir--Blodgett (LB) 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 lowering

temperature and then reaches saturation following a

$T\ln(\bt T)$ dependence. The $T\ln(\bt T)$ dependence

of magnetization has been predicted from spin-wave theory of

2D in-plane spin system with ferromagnetic interaction. The

experimental findings reported here could be explained by

extending this theory to a temperature domain of $\bt T < 1$.

Keywords. Two-dimensional magnetism; neutron scattering;

Langmuir--Blodgett films; sub-Kelvin magnetometry.

PACS Nos 75.70.Ak; 75.60.Ej; 75.50.Xx