Complex magnetism of iron monolayers on hexagonal transition metal surfaces from first principles

Abstract

Using first-principles calculations, we demonstrate that an Fe monolayer can assume very different magnetic phases on hcp ͑0001͒ and fcc ͑111͒ surfaces of 4d-and 5d-transition metals. Due to the substrates' d-band filling, the nearest-neighbor exchange coupling of Fe changes gradually from antiferromagnetic ͑AFM͒ for Fe films on Tc, Re, Ru, and Os to ferromagnetic on Rh, Ir, Pd, and Pt. In combination with the topological frustration on the triangular lattice of these surfaces the AFM coupling results in a 120°Néel structure for Fe on Re and Ru and an unexpected double-row-wise AFM structure on Rh, which is a superposition of left-and right-rotating 90°spin spirals. DOI: 10.1103/PhysRevB.79.094411 PACS number͑s͒: 75.70.Ak, 71.15.Mb Triggered by the discovery of the giant-magnetoresistance effect and the demand to realize spintronic device concepts, 1 magnetic nanostructures on surfaces have been a focus of experimental and theoretical research for more than 20 years now. In particular, there has been a tremendous effort to grow ultrathin transition-metal films on metal surfaces and to characterize and explain their magnetic properties. It is now generally believed that these structurally simple systems are well understood and more complex nanostructures such as atomic chains, clusters, or molecules on surfaces have moved into the spotlight of today's research. [2][3][4][5][6][7] Therefore, it came as a big surprise when it was experimentally shown that the prototypical ferromagnet Fe becomes a two-dimensional ͑2D͒ antiferromagnet on the W͑001͒ surface. 8 Combining spin-polarized scanning tunneling microscopy and first-principles calculations, it has been further demonstrated that complex magnetic order can be obtained even in single monolayer ͑ML͒ magnetic films on nonmagnetic substrates. For example, recently a spin-spiral state was discovered for a Mn ML on W͑110͒ ͑Ref. 9͒ and for a Mn ML on W͑001͒ ͑Ref. 10͒ and a nanoscale magnetic structure was found for an Fe ML on Ir͑111͒. 11 Surfaces of 4d-and 5d-transition metals ͑TMs͒ such as W, Re, Ru, or Ir have been particularly attractive from an experimental point of view as ultrathin 3d-TM films can often be grown pseudomorphically and without intermixing. 12-16 However, there has been controversy in the past about reports concerning dead magnetic layers and absence of magnetic order in ultrathin films on these surfaces. 14,15 The fundamental key to many unresolved puzzles may be the itinerant character of TMs resulting in competing exchange interactions beyond nearest neighbors and higher-order spin interactions beyond the Heisenberg model. The latter interactions have been proposed to play a role in transition metals; however, to our knowledge, no unambiguous proof of their importance has been given.Here, we use first-principles calculations to demonstrate that a hexagonal Fe ML can assume very different magnetic phases on a triangular lattice provided by hcp ͑0001͒ and fcc ͑111͒ surfaces of 4d-and 5d-transition metals, which are also experimentally accessib...

Spin-correlations and magnetic structure in an Fe monolayer on 5dtransition metal surfaces

“…For the FCC(001) substrates we find that the d interlayer distance is fairly constant, and the absolute value of δ increases with increasing a. Concomitantly, the spin magnetic moment of Fe monotonically increases from Ir towards Au due to the increasing dband filling of the substrate [8].…”

“…Another mechanism that can affect the magnetic ordering of a monolayer is the hybridization between the monolayer and the substrate. Hardrat et al demonstrated the change of the exchange coupling and the magnetic ordering of an Fe ML on HCP(0001) and FCC(111) surfaces of 4d and 5d transition metals due to the change in d-band filling [8]. When increasing the atomic number of the substrate element, the d-band of the substrate moves downwards with respect to the Fermi level while the energetic position of the Fe 3d band remains roughly unchanged.…”

Tailoring magnetic skyrmions in ultra-thin transition metal films

“…4 can be understood on the basis of the hybridization of Fe 3d states with Ir 5d states and Pd 4d states. First-principles calculations show that the NN exchange interaction in a hexagonal Fe ML depends on the 4d or 5d band filling of the substrate and can be tuned from ferro-to antiferromagnetic 33 . On the (111) surface of Ir, which has one electron less in the d band than Pd, the NN exchange is still ferromagnetic but reduced significantly with respect to Fe/Pd(111) (J 1 ¼ 5.7 versus 23.3 meV).…”

Magnetism of 3dtransition-metal monolayers on Rh(100)

“…49 Also, frustration effects, like on Rh(111) surfaces, could lead to interesting magnetic structures. 50 From the LDOS it can be seen how hybridization affects the magnetic order in these systems: Compared to Ag or Pd(100) substrates, the broadening of the d band is enhanced, leading to a tendency in favor of AFM ordering for Fe but FM for Cr. Calculations of the magnetic anisotropy showed that only V and Cr monolayers on Rh(100) have an out-of-plane easy axis in their ground state, while the magnetization of Mn, Fe, Co, and Ni is oriented in plane.…”