Charles Johnson

 

Emeritus Professor of Physics, University of Liverpool, England
M.A., Oxford University, England
D. Phil., Oxford University, England

 

Dr Charles Johnson studies glasses and nanoparticles using Mössbauer spectroscopy, most recently using the isotope 151Eu.

The rare earths are an interesting group of elements which form solid compounds with important optical, electrical and magnetic properties which lead to technological applications. They usually form trivalent ions, differing only in the number of electrons n in the 4f shell, ranging from 1 in cerium to 13 in ytterbium. Particularly interesting is europium which can exist in the trivalent (n = 6) as well as the divalent (n = 7) state. Eu2+ is magnetic whereas Eu3+ is non-magnetic, and Eu2+ easily oxidizes to Eu3+. Mössbauer spectroscopy provides a simple method for monitoring the oxidization state, through their different chemical shifts.

A detailed study has been made on Eu-doped ZBLAN fluorochlorozirconate glasses and glass ceramics containing BaCl2. These have uses in optical devices (e.g. fiber lasers and amplifiers), up-converting and downconverting
glass layers for solar cells, and x-ray storage phosphors or scintillators for x-ray detection and imaging (see Dr Jackie Johnson’s report). The effective Debye temperatures for Eu2+ and Eu3+ have been measured. The results were used to optimize the performance of image plates for digital x-ray radiography. This involves maximizing the amount of Eu2+ and determining the conditions required for precipitating BaCl2 nanocrystals containing Eu2+ from a barium chloro-fluorozirconate glass (ZBLAN). On x-irradation in this system Eu2+ stores defects and hence can be used in imaging. Eu3+ scintillates and so is a detector.

Studies have also been made on nanoparticles of europium sulfide, EuS, which is a non-metallic ferromagnet with a Curie temperature of 16.6 K. As well as obtaining data on superparamagnetism, the spectra have been used for monitoring sample preparation in order to improve the purity and obtain monodisperse samples. The magnetic properties of Fe3O4 nanoparticles have been studied. They have applications in medicine (targeting and destroying tumors, enhancing MRI signals) and in information technology. The well-known six-line spectrum of ferromagnetic iron results from the hyperfine interaction between the nuclei and the electrons. In paramagnetic iron compounds this splitting may be observed in large magnetic fields (~ 6 T) produced by superconducting magnets applied at low temperatures (~ 4 K). Magnetic nanoparticles are superparamagnets and the splitting may be observed in much smaller fields (< 1 T) even at room temperature. Such fields are now obtainable with neodymium iron boron permanent magnets and we have observed almost complete magnetic saturation at room temperature. The data enable the number of aligned iron magnetic moments in the nanoparticles to be measured and hence to check their size. In 5 nm particles we find 104 iron atoms.