Schedule Nov 19, 2002
Tight-binding v. LDA+U in FeAl and MnAl
Dmitri Papaconstantopoulos, Naval Research Laboratory
http://cst-www.nrl.navy.mil/people/papacon.html
Recently [1], we calculated the Stoner criterion N(E)I for ferromagnetism in FeAl. We found that N(E)I=1.06 at the experimental value of the lattice parameter while it is below 1 at the LDA lattice constant. Other workers[2] 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 [3] 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(E) sufficient to have N(E)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. [4] We also show that a similar calculation for MnAl requires a much larger shift of the t2g parameter to suppress ferromagnetism.
[1] D.A. Papaconstantopoulos and K.B. Hathaway, J. Applied Phys.87,5872 (2000).
[2] D.J. Singh, in Intermetallic Compounds, ed by Westbrook and Fleisher, Wiley (1994)
[3] P. Mohn et al., Phys. Rev. Lett. 87, 196401, (2001)
[4] D.A. Papaconstantopoulos and C.S. Hellberg, Phys. Rev. Lett.89,029701, (2002)

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