Magnesium diboride (MgB₂) is a novel superconducting material with a critical temperature of about 39 K. The material was discovered in January 2001, and it soon attracted the interest of many research groups (The most cited reference is probably J. Nagamatsu et al., "Superconductivity at 39 K in magnesium diboride", Nature 410, p. 63, 2001). Extremely interesting is that MgB₂ is a simple metallic compound with a relatively high T_c. Although the operation temperature is below the boiling point of liquid nitrogen, recent advances in cooling technology have made such low temperatures acceptable and commercial cryocoolers for these temperatures already exist.

Figure 1. Structure of magnesium diboride

In addition, magnesium diboride is considered to solve most of the problems associated with the HTS materials. HTS compounds are brittle ceramics that are difficult to deposit in a thin-film form and to process into electronics components. Superconducting MgB₂ films have been deposited on silicon using laser ablation but their critical temperatures have so far been around 25 K. Also sapphire substrates have been used, and the films on these substrates have showed much higher critical temperatures (35-39 K). We have started the deposition experiments by studying the effect of various deposition parameters on the growth rate and the surface quality of the films. The major difficulties in the deposition work are due to the high volatility of magnesium and boron and their tendency to form oxides even with tiny amounts of oxygen. These problems can be avoided by depositing the films either in an ultrahigh vacuum or in the presence of some reducing background gas (such as Ar/H₂) and keeping the deposition and annealing temperatures sufficiently low but not too low; the optimal parameter window is quite limited.

A much larger research area than thin films are superconducting cables. Magnesium diboride wires are typically produced by the well-known Powder-In-Tube (PIT) method (the principle of the technique is described in page http://www.wtec.org/loyola/scpa/05_02.htm together with other preparation techniques for high-temperature-supeconductor tapes) , and the superconducting properties, especially the critical current density, of the drawn and rolled cables are comparable to those made of conventional superconductors NbTi and Nb₃Sn. However, the applicability of MgB₂ is, according to the present knowledge, limited to low fields (B < 3.5 T) but in the low-field regime low price and easy fabrication makes the material a good alternative to low-temperature superconductors.