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We compare the physical differences between two atomic configurations,
found in the literature, of the half-Heusler alloys-XMnY, where X is a
transition-metal element and Y is a nonmetallic element. The structural
differences arise from the placement of the X and Y atoms and the
vacancy within the full-Heusler (L2(1)) structure. In one configuration,
Y and Mn are nearest neighbors and the vacancy is at (1/4,3/4,1/4)a (4d)
while in the other configuration, X and Mn are nearest neighbors and the
vacancy is located at (0,0,1/2)a (4b), where a is the lattice constant
of face-centered cube. We suggest that the important difference between
the two configurations is the identity of the transition-metal element
nearest to the non-metal element. Physical properties, in particular the
bonding features, reflect this difference. The general validity of the
modified Slater-Pauling curve, which gives successful zeroth-order
prediction of the magnetic moments of many half-Heusler alloys including
CrMnSb in the second configuration, is tested in the six XMnY alloys
studied here. Their calculated moments obey the predictions by the
modified Slater-Pauling curve for positive moments only. Exceptions
appear for predictions with negative moments. A simple and plausible
physical reason is provided. Furthermore, the possibility of a
non-ferromagnetic phase is examined and the energetics between the
ferromagnetic and non-ferromagnetic phases are compared. (C) 2013
American Institute of Physics. [http://dx.doi.org/10.1063/1.4788825]
