| Figure 1(a) | Figure 1(b) | Figure 1(c) |
Our principal research activity is the preparation and study of new magnetic materials. The incentive for this work is two-fold. First, the study of the fundamental properties of magnets gives valuable insights into the principles that govern the structure and bonding of solids, and tells about co-operative phenomena in solid-state science in general. This has implications that stretch beyond magnetism and into superconductivity and metal-insulator transitions. Second, magnetic materials are of great technological importance, playing an essential role in most information storage equipment and electromotive machines, so we are exploring new routes to thin films and fine particles of a variety of magnetic materials that may have applications in magnetic recording media.
(a) Fundamental magnetism: frustration and spin fluidsThe conventional model of most magnetic materials treats the magnetic moments as vectors with a size and direction, distributed over a regular lattice. The moments may be coupled through forces that favour a parallel or an antiparallel orientation, depending on whether the forces are ferro- or antiferromagnetic respectively. This is illustrated in Figure 1(a) for a square antiferromagnet, where it is possible to arrange the moments so that every nearest-neighbour pair is antiparallel. As chemists we can design and synthesise solids with particular magnetic atoms - and therefore a particular size and symmetry of moment - distributed over lattice sites of a particular topology. We can also control the forces between the moments through the choice of atoms that bridge them, and the geometry of these bridges.
Most of the materials that we try to engineer have layered or chain-like structures, and are designed to test theories of fundamental magnetism. Much theoretical work is formulated for low-dimensional magnets either because the theory is easier in low dimensions, or because some physical properties are unique to chainar or planar magnets. In particular, it is believed that for some lattice geometries, the conventional magnetic ground state that we considered for the square lattice may not be found, and we are also investigating antiferromagnets with frustrated lattices to explore new forms of magnetic order.
| Figure 1(a) | Figure 1(b) | Figure 1(c) |
Consider the triangular antiferromagnet depicted in Figure 1(b). It is impossible to satisfy the requirement that every pair be antiparallel simultaneously and the compromise arrangement with lowest energy has nearest-neighbour moments at 120° to each other (1(c)). This phenomenon is called geometric frustration. In some cases it may prevent the magnet from selecting a unique ground state. Figures 2 and 3 depict part of the so-called Kagome lattice (named after a form of Japanese basket weaving).
| Figure 2 | Figure 3 |
Figure 4
Our measurements of the magnetic structure and excitations indicate that some members of the family show conventional magnetic long-range order with the spin structure depicted in Figure 4, while others (and in particular the hydronium salt in which A = H3O) show a glassy magnetic ground state which is most unusual for a homogenous magnetic material.
(b) Applied magnetism: fine particles and thin films
We are investigating preparative routes to fine iron oxide particles of controlled size and shape. Most of this activity is based on hydrothermal synthesis, but we are extending the technique through microemulsion methods, and using agents that modify the growing surface to direct the morphology. We have also devised a new route to needle-shaped magnetite (Fe3O4) particles in a one-step process starting from a solution of a ferric salt and hydrolysing the reaction mixture in a microwave field. Typical products of such growth processes are shown in Figure 5, depicting 0.2mm needles of hematite which have then been given a protective coating of silica using a sol-gel process. The products of these growth processes are also of interest to work on geomagnetism and magnetic-liquid crystal composites. Clare Peters {Clare.Peters@ed.ac.uk} is working in the group as a Royal Society of Edinburgh Research Fellow, developing methods of growing fine particles of magnetic minerals such as hematite. and studying the properties as a function of size and shape with a view to devising models to extract information from the geomagnetic record left behind by events of geological or archaeological significance.
Figure 5
Figure 6
(c) Facilities for studying magnetic materials
DC magnetic susceptibility is performed with a Quantum Design MPMS2 SQUID magnetometer (1.7 - 350K, 0-1T) while AC susceptibility and heat capacity measurements are conducted through collaborative links. The most incisive probe of magnetic properties is provided by neutron scattering which is performed at the Institut Laue-Langevin in Grenoble and at the ISIS Facility of the Rutherford Appleton Laboratory enjoy good collaborative links within this Department with Dr Winpenny and Dr Robertson, with the Department of Geology and Geophysics, and several other Institutions. We provide a SQUID magnetometry service, running occasional samples for those who do not have access to such an instrument, or providing access to workers who wish to operate the machine themselves.
References
This page was last updated on the 11th May 1998