Ferromagnetism and the formation of interlayer As2-dimers in Ca(Fe1-xNix)2As2

TL;DR

This study investigates how As2-dimer formation and weak ferromagnetism evolve in Ca(Fe1-xNix)2As2 compounds, revealing that dimer formation depends on metal-metal bonding strength and ferromagnetism arises from 3d band density, not dimer breaking.

Contribution

It provides new insights into the relationship between As2-dimer formation, magnetic properties, and electronic structure in Ca(Fe1-xNix)2As2, challenging previous theories about dimer breaking and magnetism.

Findings

As-As interlayer distances vary with Ni content, forming dimers in Ni-rich compounds.

Weak ferromagnetism appears near x=0.5, while other compositions are Pauli-paramagnetic.

DFT calculations show dimer formation depends on metal-metal bonding, not antibonding state shifts.

Abstract

The compounds Ca(Fe1-xNix)2As2 with the tetragonal ThCr2Si2-type structure (space group I4/mmm) show a continuous transition of the interlayer As-As distances from a non-bonding state in CaFe2As2 (dAs-As = 313 pm) to single-bonded As2-dimers in CaNi2As2 (dAs-As = 260 pm). Magnetic measurements reveal weak ferromagnetism which develops near the composition Ca(Fe0.5Ni0.5)2As2, while the compounds with lower and higher nickel concentrations both are Pauli-paramagnetic. DFT band structure calculations reveal that the As2-dimer formation is a consequence of weaker metal-metal in MAs4-layers (M = Fe1-xNix) of Ni-richer compounds, and depends not on depopulation or shift of As-As antibonding states as suggested earlier. Our results also indicate that the ferromagnetism of Ca(Fe0.5Ni0.5)2As2 and related compounds like SrCo2(Ge0.5P0.5)2 is probably not induced by dimer breaking as recently suggested, but arises from the high density of states generated by the transition metal 3d bands near the Fermi level without contribution of the dimers.