Half-metallic antiferromagnets

Half-Metallic Antiferromagnetic (HMAFM) Double Perovskites... 

First-principles calculations have been performed to study the electronic band structure and the magnetic properties for thiazole complex, Fe(thiazole)2Cl2. From the total and the partial density of states and the atomic spin magnetic moments, it was found that Fe(thiazole)2Cl2 is a metallic antiferromagnet and has a half-metallic (HM) ferromagnetic metastable state <2006PLE245>. 

BeGeAs2 is a semiconductor with lattice constants a = 0.545 nm, c = 1.097 nm and an indirect gap Eg = 0.63 eV [5]. Impurities change the properties of semiconductor BeGeAs2 to metallic or half-metallic and lead to an appearance of ferromagnetism with the exceptional case of manganese in II site and vanadium in IV site when antiferromagnetic (AFM) state is preferable. The trends obtained are shown in the Fig. 1. The tendency fi-om the Fig. la is very similar to one obtained for other A B c 2 chalcopyrite compounds doped with the same impurities [7]. 

As it was already mentioned, the ordered double perovskite St2FeRe06 is also predicted to have a half-metallic ground state. The observed Ms of 2.7 pB/f-u. at 4.2 K is smaller than expected (3 pB/f-u.) for an antiferromagnetic coupling between Fe (d ) and Subsequent to the... 

Hubbard (13) elucidated a mathematical description of the change from one situation to another for the simplest case of a half-filled s band of a solid. His result is shown in Figure 11. For ratios of W/U greater than the critical value of 2/ /3 then a Fermi surface should be found and the system can be a metal. This critical point is associated with the Mott transition from metal to insulator. At smaller values than this parameter, then, a correlation, or Hubbard, gap exists and the system is an antiferromagnetic insulator. Both the undoped 2-1 -4 compound and the nickel analog of the one dimensional platinum chain are systems of this type. At the far left-hand side of Figure 11 we show pictorially the orbital occupancy of the upper and lower Hubbard bands. 

In what follows we should bear in mind that the generation of a diamagnetic metallic state (irrespective of whether it is a superconductor or not) will not be favored by a half-filled band of electrons. Either a Peierls distortion or the generation of an antiferromagnetic insulating state will result, with a ferromagnet being less likely for the reasons discussed. Superconductivity in these materials is in fact only observed if electrons are removed, or (less commonly to date) added to the half-filled band. Considerable effort is underway to theoreti-... 

This kind of enhancement, which occurs for a half-full band near the point where an antiferromagnetic lattice forms, is quite different from the Stoner enhancement for nearly ferromagnetic metals described in Chapter 3, Section 11. The latter occurs for non-integral occupation of a d-band, and enhances the Pauli susceptibility only, not the specific heat, apart from probably small paramagnon effects. 

NiS2, particularly its behaviour under pressure, has been discussed by Wilson and Pitt (1971) and Wilson (1985). This has two electrons half Ming an eg band, and should therefore behave like V203. It is an antiferromagnetic semiconductor, but shows a metal-insulator transition at a pressure of 46kbar (Mori et al 1973). Thus the transition occurs for a decrease in volume of about 0.4%, with no change... 

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