Nov 19, 2002
Tight-binding v. LDA+U in FeAl and MnAl
Dmitri Papaconstantopoulos, Naval Research Laboratory
Recently , we calculated the Stoner criterion N(Eƒ)I for ferromagnetism
in FeAl. We found that N(Eƒ)I=1.06 at the experimental value of the lattice
parameter while it is below 1 at the LDA lattice constant. Other workers
have determined a magnetic moment of 0.7mB again at the experimental value
of “a”. Since experimentally B2-FeAl is believed to be non-magnetic there
is great interest in resolving this issue. A new theoretical study 
applying the LDA+U approach argues that there is a range of U values that
causes a lowering of the Fe-t2g states and a reduction of N(Eƒ) sufficient
to have N(Eƒ)I<1. We want to advocate that as long as the value of U is
treated in the LDA+U theory as an adjustable parameter there is nothing to
be gained by using this complex formalism. An accurate 3-center-orthogonal
tight-binding Hamiltonian that reproduces very well the LDA band structure
leads to exactly the same result as the LDA+U theory by varying the value of
the t2g Fe on-site parameter which plays the role of U.  We also show
that a similar calculation for MnAl requires a much larger shift of the t2g
parameter to suppress ferromagnetism.
-  D.A. Papaconstantopoulos and K.B. Hathaway, J. Applied Phys.87,5872
-  D.J. Singh, in Intermetallic Compounds, ed by Westbrook and Fleisher, Wiley (1994)
-  P. Mohn et al., Phys. Rev. Lett. 87, 196401, (2001)
-  D.A. Papaconstantopoulos and C.S. Hellberg, Phys. Rev. Lett.89,029701, (2002)
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