Although our in-house materials research traditionally majored upon metal purification and crystal growth, over the past few years we have added an extra dimension by growing and studying metal oxide crystals using a four mirror image furnace that was awarded to the University in 2002.
A schematic of the four mirror image furnace is shown below.
In this system, the basic concept is that four ellipsoidal mirrors are used to focus the light from halogen lamps onto a vertically held rod shaped sample to produce a molten zone, which is then moved along the sample in order to grow a single crystal. (The maximum operating temperature is approximately 22000C; and gas pressures of up to 10 bar (1.0 MPa) can be used within the growth chamber).
The use of light heating makes the technique suitable for both conducting and non-conducting materials, a feature that distinguishes image furnace growth from float zoning using induction or electron beam heating, both of which are limited to use with conductors. Although light heating is not suited to all materials (it struggles, for example, with those metallic samples that exhibit a high reflectivity in the infra red region), it is particularly convenient and efficient for those oxides and semiconductors that absorb infrared easily. Thus, probably the group of materials for which image furnace growth has been most used are what may be termed functional oxides, crystals of which find applications in lasers, electronic and optical devices, catalysts, solid oxide fuel cells, memory and magnetic devices, oxygen and magneto-optical sensors and as superconducting materials.
As an example, rare earth orthoferrites RFeO3 (where R is a rare earth ion), which we have grown using the image furnace, crystallize in an orthorhombically distorted perovskite structure and have attracted interest due to their novel magnetic and magneto-optic properties. They are currently the subject of much research aimed at a better understanding of properties of the magnetic subsystems and how interactions between them depend on external parameters, such as temperature, field, pressure, etc.. Of particular interest is their domain wall dynamics, for the velocity of the domain wall motion in orthoferrites (at up to 20 km/s) is reported to be the highest known in any magnetically ordered media, meaning that orthoferrites show much promise for use in various innovative micro-technological devices such as magnetic sensors, magneto-optical current sensors, light spot position measurers, magneto-optical rotational speed sensors and fast latching optical switches.
A typical ErFeO3 single crystal grown in our laboratory using the image furnace, along with its magnetocrystalline anisotropy data, are shown here.
Magnetocrystalline anisotropy of high quality ErFeO3 crystals