Chapter 9 - Half-Metallic Systems: Complete Asymmetry in Spin Transport

Abstract

There are systems with metallic spin-up and spin-down electrons, that is, both spins contribute to the Fermi level density of state (and transport). For highly spin polarized systems, it can be envisioned having a situation where one spin channel shows metallicity while the other spins are insulating. Such materials, where tunneling currents show 100% spin polarization when used in tunneling devices, have been found and are called half-metals. Half-metals have unusual electronic properties. For one type of spin, the half-metal looks metallic with a well defined Fermi surface. For the opposite spin, there is a gap in the state density. In half-metals, there is a clear gap in the density of states corresponding to one type of spin. These half-metals do conduct electricity but do not show a high field magnetic susceptibility. All half-metals contain more than one element with most of them being oxides or Heusler type alloys. In real half-metals, conduction is carried out almost entirely by one spin channel. There is no clear experimental signal to identify half-metals. If the minority (or majority) gap is a true gap (with zero total density of states) then it is possible to conclude that the majority and minority counts of spin have to be integers per unit cell. The band gap for one spin type is the defining feature of these materials, therefore, the chapter are the three different categories of gaps—covalent band gaps, charge-transfer gaps, and d-d band gaps. Half-metals in different categories listed above respond differently to external parameters such as pressure because of different electronic structures that give rise to half-metallicity in these three categories. The chapter discusses half Heusler alloys: NiMnSb and PtMnSb, full Heusler alloys: Co 2 MnSi, Co 2 MnGe, Co 2 Cr 1-x Fe x Al, chromium dioxide, perovskites and double-perovskites, and multilayers of zincblende half-metals with semiconductors.