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

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)$_2$ 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 ($T_{\rm C}$) 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, $T_{\rm SP}$, 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 $T_{\rm SP}$ = 150 K and the KTCNQ-Br$_2$ compound showed no evidence of a transition. Both KTCNQ-H$_4$ and KTCNQ-F$_4$ 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$_4$ sample 2 F-$\mu^+$-F states are observed. Another controversial organic based magnet is Ni$_2$TCNQ which was first synthesised in 2007. Here a study of a similar material is reported where the TCNQ has been swapped for TCNQF$_4$ 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)$_2$ 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)$_2$ 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$_4$ based material. This demonstrates the inappropriate nature of Ni(COD)$_2$ as a starting material for metal-organic based magnetic compounds

A 3D antiferromagnetic ground state in a quasi-1D π-stacked charge-transfer system

With the rising interest in organic based materials for spintronic and multiferroic applications it is important to fully understand their electrical and magnetic properties and to identify correlations between their structural and physical attributes. One material that still holds some ambiguity is triethylammonium bis-7,7,8,8-tetracyanoquinodimethane (TEA(TCNQ)2). This charge transfer compound has one electron delocalised across two TCNQ molecules along quasi-1D stacks. Previous work has shown that there is magneto-electrical coupling associated with the magnetic transition at ∼120 K, however the magnetism and magnetic ground-state are not well understood. Within this manuscript we provide evidence for a long range magnetic order that is 3D in nature

No evidence for room temperature ferromagnetism in the high temperature metal-organic material: Ni2TCNQ

The search for ferromagnetic organic-based compounds has been a particular challenge to both chemists and physicists over the past few decades. The synthesis of the Ni2A, where A is an organic acceptor; tetracyanoethene (TCNE), 3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) or 7,7,8,8-tetracyanoquinodimethane (TCNQ) (Jain et al 2007 Nature 445 291), was reported to be a great advancement with claims that the ferromagnetism persisted to well above room temperature. There were, however some substantial flaws in the methodology associated with the synthesis and physical characterisation. Our work solely studies the Ni2TCNQ compound where we find no evidence for the existence of inherent ferromagnetism within the material that was reported in the original paper. Instead, we find that the magnetism is due to superparamagnetic nickel nanoparticles embedded in an amorphous matrix. It is hoped that our work will also show that one must be careful when using Ni(COD)2 as a precursor in the synthesis of magnetic materials and that the usefulness of the reported synthetic method is extremely limited

Understanding the role of electrons in the magnetism of a colossal permittivity dielectric material

Creating new materials that show potential for technological devices is one of the most important and active areas of solid state chemistry and physics. For data storage, multiferroics present some great advantages due to the coupling of their electrical and magnetic properties. The discovery of In and Nb co-doped rutile (Hu et al. Nat. Mater. 12, 821) presented a material that was perfect for capacitive devices; with high permittivity and low loss, which was attributed to localised polarisable defects within the crystal structure, though other work has suggested internal blocking barriers at grain boundaries as being responsible for the dielectric properties. Here we report on the magnetic properties of this material and shown that magnetic ordering occurs at room temperature and below, with the Curie temperature depending upon doping levels. Moreover, muon spin relaxation measurements suggest that the magnetic order is confined to grain boundaries or areas where the defects can cluster. This implies that a strong magneto-electronic coupling could exist within In- and Nb-co-doped rutile close to room temperature, though the inhomogeneous nature of the magnetism suggests that the coupling may be optimised in nanoparticles or thin films where defect clustering could be promoted

Study of the B-site ion behaviour in the multiferroic perovskite bismuth iron chromium oxide

A simple, near-ambient pressure solid-state method was developed to nominally synthesize BiFe0.5Cr0.5O3. The procedure allowed the gram-scale production of multiferroic samples with appreciable purity and large amounts of Cr incorporation that were suitable for systematic structural investigation by neutron, X-ray, and electron diffraction in tandem with physical characterization of magnetic and ferroelectric properties. The rhombohedrally distorted perovskite phase was assigned to the space group R3c by way of X-ray and neutron powder diffraction analysis. Through a combination of magnetometry and muon spin relaxation, it is evident that there is magnetic ordering in the BFCO phase consistent with G-type antiferromagnetism and a TN 400 K. There is no clear evidence for chemical ordering of Fe and Cr in the B-site of the perovskite structure and this result is rationalized by density functional theory and bond valence simulations that show a lowered energy associated with a B-site disordered structure. We believe that our contribution of a new, low-complexity method for the synthesis of BFO type samples, and dialogue about realising certain types of ordering in oxide perovskite systems, will assist in the further development of multiferroics for next-generation devices. Published by AIP Publishing. https://doi.org/10.1063/1.5020305B.R.M., J.L., A.B., D.L.C., T.L., R.L.W., N.N., and Y.L. acknowledge the support of the Australian Research Council (ARC) in the form of Discovery Projects (No. DP160104780). A.B., D.L.C., N.N., D.Y. and authors thank the Australian Nuclear Science and Technology Organisation (ANSTO) for facilities and financial support and also thank the Echidna instrument scientists for their expertise