Half-Metallic Ferromagnetic Oxides
- PDF / 474,139 Bytes
- 5 Pages / 612 x 792 pts (letter) Page_size
- 25 Downloads / 182 Views
Half-Metallic
Ferromagnetic Oxides
J.M.D. Coey and C.L. Chien Abstract Half-metals are unusual ferromagnets that have electrons at the Fermi level in a single spin state, either spin up or spin down. Of potential interest as sources and analyzers of polarized electrons in spintronic devices, they are usually identified from spin-dependent band-structure calculations. We present a classification scheme for half-metals and then discuss methods for measuring spin polarization based on point contacts or tunnel junctions with ferromagnetic or superconducting counter electrodes. Oxide examples include CrO2, the best-studied half-metal. The half-metallicity tends to be destroyed by increasing temperature and by structural defects. The half-metals that currently offer the best prospects for spintronics applications are those with the highest Curie temperatures, such as magnetite, Fe3O4, and perhaps oxide semiconductors such as Co-doped ZnO. Keywords: Andreev reflection, CrO2, Fe3O4, ferromagnetic oxides, half-metals, spinpolarized materials, spintronics, tunneling.
Introduction Half-metals are ferromagnets with an unusual band structure in which only half of the electrons are conducting. For electrons of one spin (either spin up a or spin down b), they are normal metals with a Fermi surface, but electrons of the opposite spin have a gap in their density of states at the Fermi level N(EF), so they cannot contribute to conduction. With only one spin band at EF, half-metals are 100% spin-polarized. Traditional electronic devices manipulate the electrons’ charge and ignore their spin. The first generation of spintronic devices are two-terminal magnetoresistors based on ferromagnetic multilayers.1 They include spin-valve read heads and nonvolatile magnetic random-access memory (MRAM). A spin valve is a device in which the flow of electrons is controlled by changing the direction of magnetization of part of the device. Spin valves are based on the giant magnetoresistance effect and have been incorporated into most computer hard-disk read heads. MRAM is based on magnetic tunnel junctions and is poised to challenge the dominance of dynamic random-access memory (DRAM).2 The
720
spin-dependent conduction of these devices depends firmly on the spin polarization of the ferromagnetic layers, which are usually alloys of Fe, Co, and Ni with a polarization of about 40%.3 Materials with higher polarization can dramatically enhance device performance and may be necessary for a new generation of threeterminal devices, such as spin transistors. No element is a half-metal. Cobalt and nickel have a d band split by on-site exchange interactions in such a way that all five 3da subbands are filled, and only the 3db subbands cross the Fermi level; however, an unsplit 4s conduction band means that both a and b electrons are present at EF. To create a half-metal, the bottom of the 4s band has to be raised above EF. A good way to achieve this is to form an oxide. Strong overlap of the metal 4s and oxygen 2p states creates a hybridization gap of a
Data Loading...
