Magnetism, structure and chemical order of small FeRh alloy clusters

Alloying elements with complementary qualities has been a major route in material development since the antiquity. In this project we investigate the structural, electronic and magnetic properties of small FemRhn clusters having N = m + n ? 8 atoms in the framework of a generalized-gradient approximation to density-functional theory. The correlation between structure, chemical order, and magnetic behavior is analyzed as a function of size and composition in view of tailoring their physical behavior for speci c technological purposes. Examples of some of the considered geometries are given below. The 3d-3d and 3d-4d transition metal alloys are particularly interesting since they open the possibility of combining the high stability of 3d magnetism with the stronger spin-orbit in-teraction and resulting magnetic anisotropy energies of the heavier 4d or 5d elements. FeRh in particular shows a very rich phase diagram in the thermodynamic limit whose size dependence deserves special attention. 

The calculations show that the bonding resulting from FeRh nearest neighbor pairs is stronger than RhRh or FeFe bonds. For the rich Fe concentration, when the number of weaker FeFe bonds dominates, one observes that binding energy decreases with increasing m.

The average magnetic moment µN of Fem Rhn increases monotonically, more or less linearly, with the number of Fe atoms. This is a consequence of the larger Fe local moments and the underlying ferromagnetic like magnetic order. Notice, in particular, the enhancement of the magnetic moments of the pure clusters in particular for FeN (m = N), which go well beyond 3µB, the value corresponding to a saturated d-band in the d7s1 con guration. In contrast, the moments of pure RhN are far from saturated except for N = 2 and 7.

Further details may be found in the reference below. An extension of this work concerning the other materials and optimization methods is currently underway.

[1] J. H. Mokkath and G. M. Pastor, First principles study of magnetism, structure and chemical order in small FeRh alloy clusters, submitted to Phys. Rev. B. (2011)