A Study of Magnetic Materials Based Upon the Organic Acceptor 7,7,8,8-Tetracyanoquinodimethane

Abstract

The study of organic based materials is a flourishing area of interest as the physical/chemical properties of the compound can be tuned through functionalisation or simple chemical changes to the organic component. This thesis will focus on the magnetic behaviour of metal-organic magnetic materials where a variety of techniques will be used to study the magnetism such as bulk magnetometry and muon spin relaxation. As well as the magnetic properties, some comments will be made on the chemical properties such as molecular structure. The thesis begins with an overview of the theory of magnetism and details regarding experimental techniques.

Ni(TCNQ) is a recently discovered non-solvated metal organic magnet that was reported to show ferromagnetic behaviour below 20 K where there was evidence of a glassy magnetic component. This thesis reports the synthesis of both a protio and deutero form of the material where upon deuteration of the TCNQ molecule, a shift in critical temperature () was observed to a higher temperature by approximately 15%. Diffraction experiments were conducted to attempt to provide information on the atomic structure however this proved unsuccessful. Magnetometry experiments showed a ferromagnetic transition at approximately 20 K in the deuterated and 17 K in the protonated materials where at low temperatures the sample appeared to be a three-dimensional magnetically order material. Muon spin relaxation studies were conducted on the deuterated sample which showed two peaks within the dynamical relaxation in zero-field; one associated with the transition and a low temperature (5 K) spin freezing effect where it is believed there are interactions between magnetic clusters that enter a quasi-static regime. It may be possible that the glassy component and the ferromagnetic behaviour of the material are not due to the same exchange mechanism or magnetic interactions.

KTCNQ is a compound that undergoes a spin-Peierls transition, , at approximately 400 K where below this temperature there is a dimerisation of the TCNQ radical spins that couple antiferromagnetically and the system goes from a conductive to insulating state. In an attempt to tune the TCNQ-TCNQ interactions different materials were synthesised where the protons on the TCNQ ring were substituted for fluorine and bromine atoms. On substitution of the protons with other elements a dramatic shift in Tsp was observed where for the fluorine based compounds = 150 K and the KTCNQ-Br compound showed no evidence of a transition. Both KTCNQ-H and KTCNQ-F were studied further using muon spin relaxation where the transition is clearly modelled using a stretched exponential where an increase in electronic fluctuation rate is shown by a gradual move from an exponential to Gaussian relaxation. At low temperatures the relaxation again changes and within the KTCNQ-F sample 2 F--F states are observed.

Another controversial organic based magnet is NiTCNQ which was first synthesised in 2007. Here a study of a similar material is reported where the TCNQ has been swapped for TCNQF and the magnetic properties are shown to be a result of nickel nanoparticles trapped within a metal-organic or purely organic based matrix. The room temperature ferromagnetism is not strictly due to only the bulk Ni particles as this would result in a blocking temperature below 20 K and so the matrix is shown to play an important role. The size of the Ni nanoparticles was shown to be tuneable when using different solvents within the reaction, generally use of chlori- nated solvents lead to rapid decomposition of the starting material, Ni(COD) and lead to larger nanoparticles, however use of a nitrile based solvent led to Ni clusters that were approximately 1 nm in size and dispersed within the matrix. A novel scaling of the magnetisation curves as a function of field showed that once the ferromagnetic component had been subtracted the matrix or Ni based clusters show an antiferromagnetic ground state at low temperature.

The final chapter describes an investigation of the starting material, Ni(COD) which was studied using a SQUID magnetometer where it was shown that there was a high level of magnetic impurities, which was attributed to small Ni clusters that showed a similar scaling relationship of the magnetisation as for the Ni2TCNQF based material. This demonstrates the inappropriate nature of Ni(COD) as a starting material for metal-organic based magnetic compounds.