Connection between magnetism and structure in Fe double chains on the Ir(100) surface

Riccardo Mazzarello and Erio Tosatti

Phys. Rev. B 79, 134402 – Published 3 April 2009

Abstract

The magnetic ground state of nanosized systems such as Fe double chains, chains recently shown to form in the early stages of Fe deposition on Ir(100), is generally nontrivial. Using ab initio density functional theory we find that the straight ferromagnetic (FM) state typical of bulk Fe as well as of isolated Fe chains and double chains is disfavored after deposition on Ir(100) for all the experimentally relevant double chain structures considered. So long as spin-orbit coupling (SOC) is neglected, the double chain lowest energy state is generally antiferromagnetic (AFM), a state which appears to prevail over the FM state due to Fe-Ir hybridization. Successive inclusion of SOC adds two further elements, namely, a magnetocrystalline anisotropy and a Dzyaloshinskii-Moriya (DM) spin-spin interaction; the former stabilizing the collinear AFM state and the latter favoring a long-period spin modulation. We find that anisotropy is most important when the double chain is adsorbed on the partially deconstructed Ir(100)—a state which we find to be substantially lower in energy than any reconstructed structure—so that in this case the Fe double chain should remain collinear AFM. Alternatively, when the same Fe double chain is adsorbed in a metastable state onto the $(5 \times 1)$ fully reconstructed Ir(100) surface, the FM-AFM energy difference is very much reduced and the DM interaction is expected to prevail, probably yielding a helical spin structure.

Received 14 January 2009

DOI:https://doi.org/10.1103/PhysRevB.79.134402

Authors & Affiliations

Riccardo Mazzarello^1,* and Erio Tosatti^1,2

¹SISSA, Via Beirut 2/4, 34014 Trieste, Italy and DEMOCRITOS-INFM, Via Beirut 2/4, 34014 Trieste, Italy
²ICTP, Strada Costiera 11, 34014 Trieste, Italy

^*Corresponding author. Present address: Department of Chemistry and Applied Biosciences, Computational Science, ETH Zurich, USI Campus, via Giuseppe Buffi 13, CH-6900 Lugano, Switzerland; riccardo.mazzarello@phys.chem.ethz.ch

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Vol. 79, Iss. 13 — 1 April 2009

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Images

Figure 1
(Color online) Total energy per atom of FM and AFM free-standing Fe single chains as a function of Fe-Fe distance. Dashed vertical line corresponds to the theoretical interatomic distance of the Fe chain deposited on Ir(100).Reuse & Permissions
Figure 2
(Color online) Top and side views of the reconstructed Ir(100) surface with the studied configurations for the dimer chain. Configurations $C_{1}$ and $C_{4}$ correspond to Fe chains adsorbed on the troughs of $(1 \times 5) Ir (100)$ , whereas $C_{2}$ corresponds to Fe chains sitting on the hills of $(1 \times 5) Ir (100)$ . DEC is the partially deconstructed structure, wherein the Fe atoms sit on the hollow sites of a quasisquare, locally deconstructed Ir(100). A and B indicate the Ir atoms nearest neighbors of Fe. Vertical displacements have been exaggerated for clarity purposes.Reuse & Permissions
Figure 3
(Color online) Ferromagnetic, antiferromagnetic, and nonmagnetic density of states of the $C_{1}$ REC structure projected onto Fe atoms. Dashed lines indicate the DOS of free-standing, double Fe wires. In the AFM case, the PDOS were calculated by projecting onto all of the Fe atoms, i.e., both those with positive magnetic moments and those with negative ones: as a consequence, the PDOS of spin-up and spin-down electrons are the same.Reuse & Permissions
Figure 4
(Color online) Ferromagnetic, antiferromagnetic, and nonmagnetic density of states of the partially deconstructed structure projected onto Fe atoms. Dashed lines indicate the DOS of free-standing, double Fe wires. In the AFM case, the PDOS were calculated by projecting onto all of the Fe atoms, i.e., both those with positive magnetic moments and those with negative ones: as a consequence, the PDOS of spin-up and spin-down electrons are the same.Reuse & Permissions
Figure 5
(Color online) Total energy per Fe atom of “toy” DEC FM and AFM structures consisting of Fe atoms and nearest-neighbor Ir atoms as a function of the magnetic moment $m$ on a Fe atom. These calculations were performed by adding a penalty functional to the total energy in order to constrain the local magnetic moment around a Fe atom. For small $m$ , which corresponds to small magnetic fields and small staggered magnetic fields for the FM and AFM case, respectively, the dependence of the energy on $m$ is quadratic and the coefficient of the quadratic term is inversely proportional to the FM or AFM susceptibility.Reuse & Permissions
Figure 6
(Color online) View of the spin-structure of the reconstructed $C_{1}$ and the partially deconstructed DEC Ir(100) configuration: Fe magnetic moments in $C_{1}$ are expected to form a right-handed cycloidal spin spiral, whereas in DEC a collinear AFM state with moments parallel to the chains should prevail.Reuse & Permissions

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