Magnetic ordering in two dimensional materials

Asaolu, Isaac Ayoola (1983) Magnetic ordering in two dimensional materials. University of Southampton, Doctoral Thesis.

Abstract

Theory predicts that magnetic ordering exists in three, but not in one, dimensional magnetic materials. In two dimensional (2d) materials on the other hand, the magnetic state is model dependent. If the interactions between the magnetic atoms of a two dimensional material are of the isotropic Heisenberg or the X-Y planar modal, Mermin and Wagner (1966) have proved that magnetic ordering is not possible. However Onsager (1944) has shown that ordering can exist in a 2d material if the interactions between its magnetic atoms are of the Icing type. In addition magnstocrystalline anisotropy, dipolar interactions, and three dimensionality crossover in two dimensional materials have been shown to violate Mermin and Wagner's conditions to give rise to a magnetic ordering. In this project, the aim was to construct a nearly ideal two dimensional magnetic material, characterise its magnetic properties and suggest possible device applications. The Langmuir Blodgett (LB) technique provides a suitable means of architecturally fabricating a stable two dimensional array of magnetic atoms. The 2d material thus constructed is shown to have the high integrity required for this project. Pomerantz (1980) has used electron spin resonance to show the existence of antiferromagnetism mixed with canted ferromagnetism at 2K in a two layer 2d manganese stearate film, fabricated on a cadmium stearate substrate by the LB technique. Haseda et al (1977) on the other hand reported that they used a SQUID (Superconducting Quantum Interference Device) to observe a transition to an ordered magnetic state in a 2d manganese stearate film at 0.3K. This project aims to resolve this discrepancy by measuring directly the magnetic moment of a two dimensional manganese stearate film, using a very sensitive Has SQUID magnetometer. This thesis describes the assembly and testing of the SQUID magnetometer, and establishes the necessary regimes for measuring samples bearing very small magnetic moments. The Has SQUID magnetometer developed for this work is suitable for the future study of rare earth and transitional materials in 2d configuration, which may lead to fruitful investigations of useful magnetic and electronic device applications.

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