Composition controlled spin polarization in Co_1−xFe_xS₂alloys

Abstract

The transition metal (TM) chalcogenides of the form TMX(2) (X = S or Se) have been studied for decades due to their interesting electronic and magnetic properties such as metamagnetism and metal-insulator transitions. In particular, the Co(1-x)Fe(x)S(2) alloys were the subject of investigation in the 1970s due to general interest in itinerant ferromagnetism. In recent years (2000-present) it has been shown, both by electronic structure calculations and detailed experimental investigations, that Co(1-x)Fe(x)S(2) is a model system for the investigation of highly spin polarized ferromagnetism. The radically different electronic properties of the two endpoint compounds (CoS(2) is a narrow bandwidth ferromagnetic metal, while FeS(2) is a diamagnetic semiconductor), in a system forming a substitutional solid solution allows for composition control of the Fermi level relative to the spin split bands, and therefore composition-controlled conduction electron spin polarization. In essence, the recent work has shown that the concept of 'band engineering' can be applied to half-metallic ferromagnets and that high spin polarization can be deliberately engineered. Experiments reveal tunability in both sign and magnitude of the spin polarization at the Fermi level, with maximum values obtained to date of 85% at low temperatures. In this paper we review the properties of Co(1-x)Fe(x)S(2) alloys, with an emphasis on properties of relevance to half-metallicity. Crystal structure, electronic structure, synthesis, magnetic properties, transport properties, direct probes of the spin polarization, and measurements of the total density of states at the Fermi level are all discussed. We conclude with a discussion of the factors that influence, or even limit, the spin polarization, along with a discussion of opportunities and problems for future investigation, particularly with regard to fundamental studies of spintronic devices.

High-pressure study of the phase transition in the itinerant ferromagnetCoS2

“…However CoS 2 occupies a special place among the others weak itinerant magnets like ZrZn 2 , Ni 3 Al, MnSi, (Fe,Co)Si, etc. The point is that CoS 2 becomes a nearly half metal at the ferromagnetic phase transition, when exchange splitting causes a significant decrease of the density of states at the Fermi level for minority spins, which influences its transport and thermodynamic properties 7,8 . That is why contrary to normal behavior, the electrical resistivity of CoS 2 increases upon transitioning to the spin-ordered ferromagnetic state, forming a hump below the phase-transition point, and the volume anomaly at the phase transition turned out to be so evident 9 .…”

Reactive sputter deposition of pyrite structure transition metal disulfide thin films: Microstructure, transport, and magnetism

“…Initial depositions and optimization of sputtering conditions were done for CoS 2 , primarily due to our prior experi-ence with this material in bulk form [8][9][10] and in films synthesized by ex situ sulfidation of Co. 30,31 A. CoS 2…”

“…41 The M s is noteworthy however as it is >95% of the bulk value. [8][9][10] Recall that the density of such films is %96% of the bulk single crystal value, meaning that the M s achieved via optimized reactive sputter deposition is essen-tially bulk-like. Confirmation of bulk-like M s is important as it is well-established that it reflects, indirectly, the bulk spin polarization at the Fermi surface, high values of which are the main motivation for the study of Co 1Àx Fe x S 2 .…”

“…5 Moving further along the first row transition metals, CoS 2 , a t 2g 6 e g 1 system, is a ferromagnetic metal with a Curie temperature (T C ) near 120 K. 6 Recent work has shown that this material forms a substitutional solid solution with FeS 2 yielding Co 1Àx Fe x S 2 , a material in which the position of the Fermi level with respect to the exchange-split d bands can be fine-tuned by x. [7][8][9][10] This leads to a composition-tunable highly spinpolarized ferromagnet of considerable interest for fundamental studies in spintronics. NiS 2 is similarly interesting as its t 2g 6 e g 2 configuration is thought to lead to a Mott insulating antiferromagnetic ground state, intensively studied in bulk crystals [e.g., Refs.…”

“…The desire to exploit the high conduction electron spin polarization of Co 1Àx Fe x S 2 in heterostructured devices for fundamental studies in spintronics provides a good example. [8][9][10] In this case, the availability of closely latticematched, thermodynamically compatible transition metal disulfides with such a wide variety of electronic and magnetic ground states opens up the exciting possibility of a new class of "all sulfide" epitaxial heterostructures. Obvious opportunities include the potential for large magnetoresistive phenomena in Co 1Àx Fe x S 2 /FeS 2 /Co 1Àx Fe x S 2 or Co 1Àx Fe x S 2 / ZnS 2 /Co 1Àx Fe x S 2 magnetic tunnel junctions, or efficient spin injection in Co 1Àx Fe x S 2 /FeS 2 or Co 1Àx Fe x S 2 /CuS 2 -based spin transport devices.…”