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Final Activity Report Summary - TANSAS (Tailoring Nanomagnetism via Symmetry-Breaking at Surfaces)

Nanofabrication is becoming a technological reality for the electronics industry as the road map of the magnetic data storage industry predicts a demand that will require nanometre length magnetic structures within the next 5 years. Breakthroughs in local tailoring of nanomagnetism had generally been reported via reduced dimensional structures. This project aimed to extend this approach by tailoring properties via reduced dimensionality and symmetry breaking at surfaces in order to yield novel magnetic behaviour at the nanoscale. The main approach used in the project was to utilise molecular systems which possess magnetic information, and to immobilise these at surfaces so that the molecule remains intact and creates a well-organised monolayer. Specifically the project addressed the following tasks:
Task 1: Creation of novel nanomagnetic interfaces.
Task 2: Probing the nanomagnetic interfaces.
Task 3: Using first-principles theoretical models to understand the behaviour of these interfaces.

The following systems proved to be the most successful for creating such interfaces.
1. Porphyrin-metal system which contain a metal ion with a magnetic moment. Porphyrin molecules essentially consist of a tetrapyrrole macrocycle and play a central function in oxygen transport (haem), electron transport (cytochrome c) and photosynthesis (chlorophyll) in living systems. They are highly robust and can be functionalised in a wide range of ways by substituent groups attached to the ring and, importantly, by the metal ions that are embedded within the centre of the ring and coordinated to the N atoms. If these ions are magnetic (Fe, Co, Ni) then these molecules can act as carriers of magnetic information. Our work from scanning tunnelling microscopy (STM) and reflection absorption infrared spectroscopy on Co-tetraphenyl porphyrin TPP on Cu(110) showed that 2D organised molecular domains could be created. First principle calculations showed that the Co interacts strongly with the Cu metal substrate. This suggest that this although the spin moment is quenched, this system may display interesting Kondo effects. On the basis of this work, a range of other porphyrin systems were designed to control the interaction of the magnetic ion with the surface.

2. An organic molecular system with radical group. Carboxypyrrolidine nitroxide (3-carboxyproxyl, 3CP) is a paramagnetic molecule possessing an unpaired spin associated with an unpaired electron in a pi* orbital of the NO group. Second, the carboxyl (COOH) functionality of the 3CP molecule enjoys a strong bonding interaction with copper, thus offering a means of anchoring the molecule robustly to the surface. Importantly, the spin on the nitroxide is stabilised by the presence of methyl groups attached to the ring which prevent intermolecular reactions that would pair the spins or molecule-surface interaction that would quench the spin.

Our work from scanning tunnelling microscopy (STM), reflection absorption infrared spectroscopy, and periodic density functional theory (DFT) calculations reveals that the molecule is robustly anchored to the surface via the formation of two Cu-O bonds between the carboxylate functionality and specific short-bridge adsorption sites on the Cu(110) surface. The adsorbed organic radicals appear in STM as discrete entities on the surface and can be imaged with submolecular resolution showing the system possesses low dimensionality and low symmetry. The steric repulsion of the methyl groups with the surface orient the molecular ring almost perpendicular with respect to the surface forcing the NO radical away from the surface. DFT calculations show the survival of the unpaired spin localized on the NO radical with a calculated magnetic moment for the adsorbed molecule of 0.86 ?B, with the spin moment exclusively localised on the radical, being almost equally shared between the oxygen and the nitrogen atom.

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United Kingdom
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