Epitaxial Heusler Alloys: New Materials for Semiconductor Spintronics
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Epitaxial Heusler Alloys: New Materials for Semiconductor Spintronics
V, or Cr), and Z (a Group III, IV, or V element such as Al, Ga, In, Si, Ge, Sn, As, or Sb). For the half Heusler alloy (XYZ with the C1b crystal structure), the X1 sublattice is empty. If both the X2 and Y sublattices are empty, the zinc-blende structure of a III–V compound semiconductor is formed (Figure 1a, bottom structure). The similarities in the chemistries of the Heusler alloys suggest high mutual solubility, allowing for lattice parameter and property tuning through alloy formation1 such as X1xXx Y1yYyZ 1zZz and X1xXx2Y1yYy Z 1zZz, where the unprimed and primed symbols represent two different elements within the same sublattice.
Chris Palmstrøm Abstract Ferromagnetic materials that have Curie temperatures above room temperature, crystal structures and lattice matching compatible with compound semiconductors, and high spin polarizations show great promise for integration with semiconductor spintronics. Heusler alloys have crystal structures (fcc) and lattice parameters similar to many compound semiconductors, high spin polarization at the Fermi level, and high Curie temperatures. These properties make them particularly attractive for injectors and detectors of spin-polarized currents. This review discusses the progress and issues related to integrating full and half Heusler alloys into ferromagnetic compound semiconductor heterostructures. Keywords: crystallographic structure, epitaxial Heusler alloys, intermetallic compounds, magnetic properties, molecular-beam epitaxy (MBE), semiconductors, single crystals, spin-polarized materials, spintronics, thin films.
Introduction An ideal spintronic material would be one that enables the transport of only one spin carrier across the interface between it and an unpolarized material, without any spin-flip scattering. This concept has been the driving force for investigating epitaxial single-crystal magnetic material/ semiconductor heterostructures with minimal interfacial defects from, for example, magnetic dead layers formed by interfacial reactions, dislocations, or other spinflip scattering mechanisms. A great advantage of compound semiconductors is the ability to control both the lattice parameter and electronic properties such as the bandgap through alloy formation, which has led to the development of bandgap engineering and a number of electronic and optoelectronic devices including lasers, light-emitting diodes, high-electron-mobility transistors, and heterojunction bipolar transistors.
MRS BULLETIN/OCTOBER 2003
The Heusler alloys are a large class of materials that may offer similar tailoring possibilities for magnetic materials. The crystal structures and corresponding lattice parameters of a number of Heusler alloys are shown in Figure 1. Their lattice parameters span the same length scale as most of the compound semiconductors, suggesting the possibility for close lattice matching to most semiconductors. The Heusler structures consist of four interpenetrating
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