College Park, Maryland June 6 - 10 , 2004
T1-C4 (9:30 AM): Spin Density Waves in Iron Aluminides
A. S. Arrott, D.R. Noakes (Virginia State University)
The transition from ferromagnetism with increasing Al concentration in the iron-aluminides now appears to be a change from ferromagnetism to a complex spin density wave state. This discovery by neutron diffraction opens a new field of investigation that should have an impact comparable to that of the spin density waves in chromium over the past forty years. If one views the magnetic structure of Cr considering only the atoms on the corners of the body centered cubic lattice, one finds a periodic modulation of the spin polarization over long distances. The same is true for the spin density wave in the iron aluminides. The difference between Cr and the iron aluminides is that the polarization of the center atoms of Cr is almost opposite to that of the corner atoms while for the iron aluminides the polarization of the center atoms is almost parallel to that of the corner atoms. The change from the spin density wave state in the iron aluminides to the ferromagnetic state can be viewed as an increase in the spatial period of the antiferromagnetic structure. This treats the wave vectors about the origin as being the most important ones. But if one takes the wave vectors that are close to the 110 reciprocal lattice vectors as controlling, then the transition is from an incommensurate spin density wave state to a commensurate spin density state where all the atoms are at the positive (or negative) peaks of wave. It is a trivial statement to say that every ferromagnet is such a spin densisty wave, but in the case of the iron aluminides there is a good reason to adopt that view. The transition from ferromagnetism then appears to be the swinging away of the spin density wave vector from matching the 110 reciprocal lattice vectors to reach the observed nearby incommensurate position. This suggests that wave vectors spanning the Fermi surface play an important role in the origin of ferromagnetism in this highly itinerant electron system.
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