Thermal treatment for magneto-resistant materials design

Reader heads of compact discs and computer hard discs or position and magnetic field sensors are some of the applications of magneto-resistant materials, which are normally obtained by costly methods. Precisely in order to solve this problem, university teacher María Luisa Fernández-Gubieda Ruiz, of the University of the Basque Country, is carrying out research into developing a simpler and more effective method for the preparation of these materials, based on their undergoing thermal treatment. The lecturer explained the new method at a seminar recently given at the Department of Physics at the Public University of Navarre.

Granular solids

Fernández-Gubieda explained that magneto-resistance is the change in resistance manifested by certain materials when subjected to a magnetic field. These changes, she added, can be of great importance and in some systems can reach a variation of up to 40 or 50 per cent.

She explained how, at the beginning of the 90’s, the discovery of huge magneto-resistance in granular multilayers and solids sparked enormous interest both for their peculiar magnetic and transfer properties, as for their possible technological applications as magnetic reading reader heads for very high density and velocity reading devices.

Concretely, Fernández-Gubieda’s research has centred on granular solids which are composed of magnetic particles (for example, iron, cobalt, nickel) saturated in a non-magnetic matrix (for example, copper or silver). With these materials, the professor commented, the origin of the magneto-resistance is principally due to the magnetic scattering of the interface carriers, between the magnetic particles and the matrix. Thus, this quality fundamentally depends on the size of the magnetic particles, of their composition and of the interaction between them.

Thermal treatment
To understand the mechanisms controlling magneto-resistance, Fernández-Gubieda carried out an exhaustive study on the development of the microstructure of the granular solids by means of thermal treatment and analysed its influence on the magnetic properties and magneto-transfer.

The study of the structural evolution was carried out using two complementary techniques: high resolution X-ray diffraction and X-ray absorption spectroscopy, using the Sincrotron at Grenoble and the nuclear reactor at Laue-Langevin, both in France.

As a result of this research, Profesor Fernández-Gubieda observed how, at temperatures greater than 500ºC, a re-dissolution of cobalt and copper was produced, giving rise to an increase in the roughness at the interface between a the nanometric particles of cobalt and the copper matrix, which, in turn, produced the drop in magneto-resistance, she concluded.