Strongly Enhanced Orbital Moments and Anisotropies of Adatoms on the Ag(001) Surface

Abstract

We present ab initio calculations for orbital moments and anisotropy energies of 3d and 5d adatoms on the Ag(001) surface, based on density functional theory, including Brooks' orbital polarization (OP) term, and applying a fully relativistic Korringa-Kohn-Rostoker -Green's function method. In general, we find unusually large orbital moments and anisotropy energies, e.g., in the 3d series, 2.57m B and 174 meV for Co, and, in the 5d series, 1.78m B and 142 meV for Os. These magnetic properties are determined mainly by the OP and even exist without spin-orbit coupling. DOI: 10.1103/PhysRevLett.86.2146 The interest in surface magnetism is primarily caused by the enhancement of the spin moments at surfaces being driven by the reduced coordination. A typical example for this effect is iron, for which the bulk moment (2.15m B ) is enhanced at the (001) [6]. Here the 4d and 5d atoms, being nonmagnetic as impurities in the bulk, show as adatoms very large local moments comparable with the free atom values. Sizable moments also survive when these atoms are incorporated into the first layer.In contrast to the spin magnetism, the orbital magnetism in solids has its origin in the spin-orbit interaction and is closely connected with the magnetocrystalline anisotropy, with magneto-optical effects and magnetic x-ray dichroism. In this paper we address the enhancement of orbital moments at surfaces. It is well known that the orbital moments are "quenched" in the bulk, i.e., strongly suppressed by the crystal field splitting, the strong hybridization with the neighboring atoms, respectively. Thus, calculations yield very small orbital moments: 0.049m B , 0.075m B , and 0.042m B for bcc Fe, hcp Co, and fcc Ni [0.082m B , 0.123m B , and 0.058m B if Brooks' orbital polarization (OP) is included] [7]. At surfaces ab initio calculations show that also the orbital moments are enhanced, to e.g., a value of 0.090m B at the hcp Co(0001) surface (0.158m B if Brooks' OP is included) [7,8]. Even larger orbital moments are obtained for the 3d monolayers, e.g., 0.121m B for a Co monolayer on Cu(100) (0.261m B including OP) [7,9]. Thus, at surfaces the quenching of the orbital moments is less pronounced due to the reduced hybridization. However, it is important to realize that these enhanced orbital moments are still an order of magnitude smaller than the corresponding free atom values, as given by Hund's second rule. Thus, the orbital moments are to a large extent also quenched at the surface, as it seems to be the general rule in metallic environments.However, this rule can have exceptions. Riegel and co-workers [10] have already shown that Fe impurities, being injected into alkali metals, show hyperfine properties which indicate very large orbital moments, probably the full atomic values. The low and more or less constant electron density of the alkali hosts is responsible for this behavior. Here we will predict by density functional calculations that single 3d and 5d transition metal impurities on the Ag(001) surface can have very large or...

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“…In the past, calculations 24 on the magnetocrystalline anisotropy energy ͑MAE͒ of transition-metal adatoms on Ag and Au surfaces ͑where the spin-orbit coupling is strong͒ have shown that Cr and Mn adatoms have a MAE of less than 5 meV. On Ni, the effect should be weaker, as on Cu, where the MAE of a single Co adatom was calculated 25 to be less than 1 meV.…”

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“…In a solid, electron delocalization and crystal field effects compete with these interactions, causing a substantial decrease of m S and partial or total quenching of m L . Theoretical calculations [7] predict such effects to be strongly reduced in TM impurities at non-magnetic surfaces owing to the decreased coordination, with implications also for the appearance of significant magnetic anisotropy (Fig. 1).…”

“…Small clusters deposited on metal surfaces are predicted to have spin (m S ) and orbital (m L ) magnetic moments in between those of bulk compounds and free atoms [5][6][7], and to exhibit strong magnetic anisotropy with characteristic energies (E a ) of the order of 1-10 meV/atom, i.e., a factor 10 3 larger than bulk ferromagnetic metals [7,8]. In the case of single-domain particles, E a determines the orientation and stability of the magnetization and is thus a crucial parameter for most applications of magnetic materials in modern technology.…”

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