New Half-Metals

Metals (like copper and aluminum) can carry an electrical current; insulators on the other hand, (like glass) do not. Can a material be simultaneously a metal and an insulator? Surprisingly – the answer is yes! Materials have two different types of electrons – “up” spin and “down” spin. The electrical properties of these two different types of electrons are often quite different in ferromagnets. The most striking difference is seen in “half-metals”. These are ferromagnets in which electrons of one spin type act as if they are in a metal and those of the other spin type act as if they are in insulator. Scientists working at the MRSEC in the MINT Center at the University of Alabama have predicted two new families of half-metals. One is a variation on a well known family of materials called “Heusler” alloys. MINT scientists have developed simple recipes for mixing many different elements to make an infinite number of new half-metallic systems based on the simple body centered cubic crystal structure. The other family of half-metals is based on the spinel crystal structure (AB2X4) where A=Cu or Cd, B=Cr, and X=S or Se. The half-metals can be obtained by using combinations of Cu and Cd on the A sites or by replacing some of the S or Se atoms with Cr or Br or vacancies. These new half-metals may be used to make new types of “spintronic” devices. Spintronics is a new science and technology that utilizes the electron’s spin as well as its charge.