Half metals are defined by a particular spin of the conduction charge carriers. To have a 100% spin polarized charge carriers of one particular spin requires that a band gap between the two spin states exists and one of the spin states falls on the Fermi energy level. In other words one spin channel becomes conducting while the other becomes insulating. Band gaps can be covalent, or as a result of charge transfer, or a d-d band gaps. Covalent band gaps exist in semiconductors and Heusler alloys where the crystal structure and site occupation are essential. Most of the known half metals in this category are oxides or Heusler alloys which turned out to be weak magnets. In reality there is no direct measurement of the degree of half metalicity in compounds. Hence, most of the experimental work relies on the predictions of numerical band calculations. NiMnSb and Fe1-xCoxSi were predicted to be half metals. Half metalicity (100% spin polarization) is difficult to achieve at finite temperatures and also is affected by crystal defects. In addition bulk properties can be very different from surface and interface properties. So the search is on for half metals with as high spin polarization as possible as they hold the potential of great breakthroughs in the electronic world. These half metals hold the promise for spintronics. The half Heusler NiMnSb and the Fe1-xCoxSi compounds were synthesized using the solid state method and X-ray diffraction was used to identify their crystal structures. Resistivity and the Hall-effect measurements at two temperatures were undertaken. Our results will be discussed.

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