Complex magnetism in ultra-thin films: atomic-scale spin structures and resolution by the spin-polarized scanning tunneling microscope

In this paper we present a density functional theory investigation of complex magnetic structures in ultra-thin films. The focus is on magnetically frustrated antiferromagnetic Cr and Mn monolayers deposited on a triangular lattice provided by a Ag (111) substrate. This involves non-collinear magnetic structures, which we treat by first-principles calculations on the basis of the vector spin-density formulation of the density functional theory. We find for Cr/Ag (111) a coplanar non-collinear periodic 120° Néel structure, for Mn/Ag (111) a row-wise antiferromagnetic structure, and for Fe/Ag (111) a ferromagnetic structure as magnetic ground states. The spin-polarized scanning tunneling microscope (SP–STM) operated in the constant-current mode is proposed as a powerful tool to investigate complex atomic-scale magnetic structures of otherwise chemically equivalent atoms. We discuss a recent application of this operation mode of the SP–STM on Mn/W (110), which led to the first observation of a two-dimensional antiferromagnet on a non-magnetic metal. The future potential of this approach is demonstrated by calculating SP–STM images for different magnetic structures of Cr/Ag (111). The results show that the predicted non-collinear magnetic ground state structure can clearly be discriminated from competing magnetic structures. A general discussion of the application of different operation modes of the SP–STM is presented on the basis of the model of Tersoff and Hamann. Received: 07 May 2001 / Accepted: 23 July 2001 / Published online: 3 April 2002     

Oxy- und Aminomethylenmeldrumsäuren

Die Darstellung von Oxy- und Aminomethylenmeldrumsäuren wird beschrieben, ihre IR- und NMR-Spektren angegeben und aufgrund der HMO--Elektronenstrukturen interpretiert. Ein qualitativer Zusammenhang der Streckschwingungsfrequenzen der Mehrfachbindungen des Acylalringes und der entsprechenden -Bindungsordnungen und der Ladung des Acylalringes wird beobachtet. Das Umsetzungsprodukt aus Hydroxymethylenmeldrumsäure und Brady-Reagens wird als 2,4-Dinitrophenylhydrazyl-methylenmeldrumsäure erkannt.

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Mit 1 Abbildung.

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Herrn Prof. Dr.F. Wessely zum 70. Geburtstag in dankbarer Verehrung gewidmet. Die Autoren.

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a)14. Mitt.:G. Billek, O. Saiko, A. Stephen undF. Wessely, Mh. Chem.97, 63 (1966); b) 16. Mitt.:G. A. Bihlmayer, F. J. Kunz undO. E. Polansky, Mh. Chem.97, 1293 (1966).     

Reaction of 1,4-pentadiyne with diethylaminotrimethylstannane

The reaction between 1,4-pentadiyne and diethylaminotrimethylstannane leads predominantly to (E)-2-diethylamino-4-trimethylstannyl-2,4-pentaenyne (with either a 1/1 or 1/2 mol ratio of 1,4-pentadiyne and diethylaminotrimethylstannane). With 1/2 stoicheiometry 1,1,5-tris(trimethylstannyl)penta-1,2-dien-4-yne is formed as a minor product. ¹³C and ¹¹⁹Sn NMR data are reported.     

One-dimensional spin-polarized quantum-wire states in Au on Ni(110)

Au chain structures have been prepared on Ni(110). Au6 s,p-derived features in photoemission spectra are identified as quantum-wire states due to their strong dispersion along the chains and absence of dispersion perpendicular to the chains in agreement with our ab initio calculation of the electronic structure. Spin analysis reveals that the states have minority-spin character showing that the confinement of electrons in the chain structure depends on the electron spin.     

Thermal collapse of spin polarization in half-metallic ferromagnets

We propose two novel approaches to study the temperature dependence of the magnetization and the spin polarization at the Fermi level in magnetic compounds, and apply them to half-metallic ferromagnets. We reveal a new mechanism, where the hybridization of states forming the half-metallic gap depends on thermal spin fluctuations and the polarization can drop abruptly at temperatures much lower than the Curie point. We verify this for NiMnSb by ab initio calculations. The thermal properties are studied by mapping ab initio results to an extended Heisenberg model which includes longitudinal fluctuations and is solved by a Monte Carlo method.     

Observation of a complex nanoscale magnetic structure in a hexagonal Fe monolayer

We have observed a novel magnetic structure in the pseudomorphic Fe monolayer on Ir(111). Using spin-polarized scanning tunneling microscopy we find a nanometer-sized two-dimensional magnetic unit cell. A collinear magnetic structure is proposed consisting of 15 Fe atoms per unit cell with 7 magnetic moments pointing in one and 8 moments in the opposite direction. First-principles calculations verify that such an unusual magnetic state is indeed lower in energy than all solutions of the classical Heisenberg model. We demonstrate that the complex magnetic structure is induced by the strong Fe-Ir hybridization.     

Electronic states of moiré modulated Cu films

We examined by low-energy electron diffraction and scanning tunneling microscopy the surface of thin Cu films on Pt(111). The Cu/Pt lattice mismatch induces a moiré modulation for films from 3 to about 10?ML thickness. We used angle-resolved photoemission spectroscopy to examine the effects of this structural modulation on the electronic states of the system. A series of hexagonal- and trigonal-like constant energy contours is found in the proximity of the Cu(111) zone boundaries. These electronic patterns are generated by Cu sp-quantum well state replicas, originating from multiple points of the reciprocal lattice associated with the moiré superstructure. Layer-dependent strain relaxation and hybridization with the substrate bands concur to determine the dispersion and energy position of the Cu Shockley surface state.     

Exchange Interactions at Surfaces of Fe, Co, and Gd

Magnetic exchange interactions at low-index surfaces of bcc iron, hcp cobalt, and hcp gadolinium are studied using ab initio electronic structure calculations. Interlayer exchange couplings derived from total-energy differences are enhanced at the surfaces over their bulk counterparts. This trend is in contrast to a surface reduction of on-site exchange parameters formulated within a classical Heisenberg model. A particular attention is paid to the sensitivity of exchange interactions at a Gd(0001) surface to relaxation of interlayer distances. The calculated results do not provide support for recently observed surface enhancement of the Curie temperature of the Gd metal.     

Revealing antiferromagnetic order of the Fe monolayer on W(001): spin-polarized scanning tunneling microscopy and first-principles calculations

We prove that the magnetic ground state of a single monolayer Fe on W(001) is c(2x2) antiferromagnetic, i.e., a checkerboard arrangement of antiparallel magnetic moments. Real space images of this magnetic structure have been obtained with spin-polarized scanning tunneling microscopy. An out-of-plane easy magnetization axis is concluded from measurements in an external magnetic field. The magnetic ground state and anisotropy axis are explained based on first-principles calculations.     

Three-dimensional spin structure on a two-dimensional lattice: Mn/Cu(111)

Based on first-principles vector spin-density total-energy calculations of the magnetic and electronic structure of Cr and Mn transition-metal monolayers on the triangular lattice of a (111) oriented Cu surface, we propose for Mn a three-dimensional noncollinear spin structure on a two-dimensional triangular lattice as magnetic ground state. This new spin structure is a multiple spin-density wave of three row-wise antiferromagnetic spin states and comes about due to magnetic interactions beyond the nearest neighbors and due to higher order spin interactions (i.e., four spin). The magnetic ground state of Cr is a coplanar noncollinear periodic 120 degrees Néel structure.