Simulation of spin-polarized scanning tunneling microscopy images of nanoscale non-collinear magnetic structures

We apply an efficient method to calculate spin-polarized scanning tunneling microscopy (SP-STM) images of nanostructures with complex non-collinear magnetic order. The model is based on the spin-polarized version of the Tersoff–Hamann model of STM and the independent orbital approximation for the surface electronic structure. For its application, only the knowledge of the arrangement of the magnetic moments of the surface atoms is required. In spite of its simplifications, calculated SP-STM images of periodic collinear and non-collinear magnetic spin structures are in many cases in excellent agreement with those obtained from computationally much more demanding ab initio calculations. Especially for surfaces of chemically equivalent atoms, the atomic scale SP-STM images are dominated by the magnetic structure and depend much less on the accurate electronic structure. This suggests the application of the method to more complex non-collinear magnetic structures such as domain walls in antiferromagnets, spin-spiral states, spin glasses, or disordered states. Based on the model, we predict SP-STM images of helical spin-spiral states in ultra-thin films. PACS 68.37; 75.70; 75.30     

Spin-polarized scanning tunneling microscopy with antiferromagnetic probe tips

We have performed low temperature spin-polarized scanning tunneling microscopy (SP-STM) of two monolayers Fe on W(110) using tungsten tips coated with different magnetic materials. We observe stripe domains with a magnetic period of 50 +/- 5 nm. Employing Cr as a coating material we recorded SP-STM images with an antiferromagnetic probe tip. The advantage of its vanishing dipole field is most apparent in external magnetic fields. This new approach resolves the problem of the disturbing influence of a ferromagnetic tip in the investigation of soft magnetic materials and superparamagnetic particles.     

Atomic-scale magnetic domain walls in quasi-one-dimensional Fe nanostripes

Fe nanostripes on W(110) are investigated by Kerr magnetometry and spin-polarized scanning tunneling microscopy (SP-STM). An Arrhenius law is observed for the temperature dependent magnetic susceptibility indicating a one-dimensional magnetic behavior. The activation energy for creating antiparallel spin blocks indicates extremely narrow domain walls with a width on a length scale of the lattice constant. This is confirmed by imaging the domain wall by SP-STM. This information allows the quantification of the exchange stiffness and the anisotropy constant.     

自旋极化扫描隧道显微镜的研究进展

自旋极化扫描隧道显微镜(spin-polarized scanning tunneling microscope,SP-STM)将扫描隧道显微镜(scanning tunneling microscope,STM)的实空间分辨率和对自旋敏感的磁成像技术结合起来,已经成为人们研究纳米磁性物理的最有效工具之一.文章介绍了SP-STM的工作原理及其在低维磁性物理领域的应用和最新进展,如对磁性薄膜、磁性纳米岛、磁性原子及不共线结构的研究等,并对我国在这一领域的研究现状和发展前景进行简要评述.     

Spin-polarized scanning tunneling microscopy through an adsorbate layer: Sulfur-covered Fe/W(1 1 0)

Spin-polarized scanning tunneling microscopy and spectroscopy (SP-STM/STS) has been performed on clean and sulfur-covered three-dimensional Fe islands on W(1 1 0). Upon dosing with H₂S the island surface is covered with 1/3 ML S leading to a c(3 × 1) reconstruction. The characteristic magnetic vortex structure is observable before and after dosing, even though the electronic structure of the surface is modified as is shown by SP-STS.     

Recent advances in spin-polarized scanning tunneling microscopy

Considerable progress in the field of spin-polarized scanning tunneling microscopy (SP-STM) has been achieved recently by gaining a high degree of control with regard to properties of the tunneling tip. It is found that by choosing the appropriate material for the magnetic thin film coating of the tip sensitivity to either the samples in-plane or perpendicular magnetization component can be achieved. Using SP-STM in external magnetic fields, domains and domain walls of two atomic layers thick Fe on a W(110) substrate are studied. A residual domain of enhanced stability against remagnetization is observed. Furthermore, a new imaging mechanism is identified which allows the use of even non-magnetic tungsten tips to observe contrast between magnetic domains and domain walls. The effect exploited is a modification of the electronic band structure which is induced by spin–orbit coupling. PACS 75.50.Ak; 75.75.+a; 68.37.Ef     

A spin-selective approach for surface states at Co nanoislands

During recent years the surface electronic states of cobalt nanoislands grown on Cu(111) and Au(111) have been extensively studied and still yield fascinating results. Among magnetic surfaces, cobalt islands are particularly appealing because of their spin-polarized electronic states near the Fermi energy, involving localized d states of minority character, as well as free-like s–p states of majority character. We show here that these states are a sensitive probe to minute changes of structural details such as strain and stacking, and therefore constitute an ideal playground to study the interplay between structural and spin-related properties. Due to their size, cobalt islands on Cu(111) offer the additional opportunity to host single-magnetic adsorbates suitable for spin-polarized scanning tunneling microscopy and spectroscopy (SP-STM and SP-STS). We establish here that, in an energy interval just below the Fermi level, the spin-polarization of a transition-metal atom is governed by surface-induced states opposite in sign compared to the island, while the spin-polarization of Co-Phthalocyanine molecules is governed by molecular states. This opens up interesting perspectives for controlling and engineering spin-polarized phenomena at the nanoscale.     

Structural and magnetic properties of CrSb compounds: NiAs structure

The structural and magnetic properties of CrSb compounds with NiAs structure have been studied by means of the Korringa-Kohn-Rostoker (KKR) band structure method. An analysis of the structural and magnetic stability has been performed on the basis of total energy calculations for various magnetic states. The magnetic properties at finite temperature have been investigated by means of Monte Carlo simulations on the basis of a classical Heisenberg Hamiltonian and the exchange coupling parameters calculated from first principles. This approach allowed us to determine the critical temperature in good agreement with experiment.     

Characteristics of phase transitions in crystals having a triangular structure in a magnetic field

Analysis of the field dependence of the order parameters and comparison with experimental data on the behavior of the magnetic characteristics of iron phosphide compounds in the stability region of the metamagnetic phase provides a basis for identifying the type of magnetic structure that can exist in this case. The invariants responsible for first-order magnetic phase transitions in crystals having a triangular magnetic structure are identified from the entire rational basis of invariants. Fiz. Tverd. Tela (St. Petersburg) 39, 940–945 (May 1997)     

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     

Spatially ordered arrays of magnetic nanowires: Polarized-neutron scattering investigation

The structure and magnetic properties of magnetically ordered arrays of magnetic nanowires formed on the basis of porous aluminum oxide films have been investigated by the small-angle polarized-neutron scattering method. Small-angle scattering intensity patterns exhibit several diffraction maxima (up to the third reflection order), which correspond to scattering from the highly ordered porous structure of the matrix and on the hexagonal packing of magnetic nanowires. The observed reflections are imposed on noticeable diffuse scattering associated with the defects of the structure. A theoretical solution is obtained for describing neutron diffraction in the dynamical limit on the superstructure of magnetic nanowires incorporated into the diamagnetic matrix. Several contributions to scattering that have been analyzed are the nonmagnetic (nuclear) contribution, the magnetic contribution depending on a magnetic field, and the nuclear-magnetic interference indicating the correlation between the magnetic and nuclear structures. A detailed pattern of the remagnetization of an ordered array of the magnetic nanowires has been obtained and it has been shown that polarized neutron scattering provides information inaccessible by the standard magnetometric methods.     

Propagation of solar cosmic rays in loop-like interplanetary magnetic field structures

Features of propagation of relativistic solar cosmic rays in magnetic clouds have been considered on the basis of model calculations. Magnetic clouds have a structure of magnetic flux ropes and are extended from the Sun to the Earth via coronal mass ejections. Features of propagation of particles of different energies in a magnetic cloud are discussed. The propagation of high-energy solar protons in the loop-like structure of the interplanetary magnetic field in the event of October 28, 2003 is analyzed.     

Magnetic domain structure and thermal stabilization of laser treatment zones in soft magnetic materials

A combined effect of laser treatment and introduced fine-grained weakly magnetic impurity Mg–P–B defects on the magnetic structure and physical properties of anisotropic electrotechnical materials has been investigated. Specific features of changes in the type and behavior of the magnetic domain structure under different types of deformation (laser irradiation, scratching, and introduction of interstitial defects) have been revealed. The physical basis and optimum conditions of increase in thermal stability of local laser treatment zones in soft magnetic alloys have been determined. The obtained results open the prospects of decreasing magnetic losses in soft magnetic alloys and producing magnetic materials with a high level of physical and mechanical properties that are more resistant to operating conditions.     

The structure factor of polydisperse magnetic fluids

This work is devoted to the development of the theoretical model, permitting to find the pair correlation function and the structure factor of magnetic fluids taking into account the polydispersity of magnetic particles. The magnetic fluid was modeled as a system of bidisperse dipolar hard spheres. We compared the results obtained on the basis of monodisperse and bidisperse models. The consideration of the particle polydispersity affects the behavior of the structure factor: the value of the first peak decreases and becomes broader in comparison to the monodisperse case.     

Relations between the Ruderman-Kittel-Kasuya-Yosida interaction electron concentration and crystal structure

The Ruderman-Kittel-Kasuya-Yosida interaction between localized magnetic spins on the basis of perfectly free conduction electron is calculated as a function of the Fermi wave vector of the conduction electrons. The expression of the interaction according to us converges rapidly, furthermore is strongly related to the detailed crystal structure. The q-dependence of exchange interaction j(q) between conduction- and localized magnetic electron is also taken into account. The result of the calculation is compared successfully with the change of magnetic structures, with θ_p and T_c or T_N of compounds having the CsCl structure, but less successfully with gadolinium compounds having the NaCl structure. These facts are discussed in conjunction with the difference of the RKKY interaction in the cscl and the NaCl structure.     

Half-metallic diluted antiferromagnetic semiconductors

The possibility of half-metallic antiferromagnetism, a special case of ferrimagnetism with a compensated magnetization, in the diluted magnetic semiconductors is highlighted on the basis of the first-principles electronic structure calculation. As typical examples, the electrical and magnetic properties of II-VI compound semiconductors doped with 3d transition metal ion pairs--(V, Co) and (Fe, Cr)--are discussed.     

Electron scattering by a ring-shaped barrier in magnetic fields

We investigate the transport phenomena through a region containing a ring-shaped barrier in a quasi-one-dimensional quantum wire in magnetic fields. The calculated magnetoconductance curve shows a periodic dip structure, which is superimposed upon by another quasi-periodic dip structure. The current distributions for resonant states and the magnetoconductance features are well explained on the basis of the magnetic field dependence of the eigenvalue in the two-dimensional system.     

Symmetry analysis in neutron diffraction studies of magnetic structures: 1. A phase transition concept to describe magnetic structures in crystals

On the basis of phase transition symmetry theory there has been developed an efficient method of calculating the possible magnetic structures liable to arise from the paramagnetic phase of the crystal. Every magnetic structure is described by the superposition of the basis functions of the irreducible representation incorporated in the magnetic representation of the crystal symmetry group. Convenient formulas are given to calculate the basis functions and composition of the magnetic representation. These formulas permit calculation by referring only to the tables of space group irreducible representations. The technique is illustrated on the example of magnetic structures in crystals with the symmetry group D⁶_3d.     

On the magnetic structure of PrMn2O5: a neutron diffraction study

The long-range magnetic ordering of PrMn(2)O(5) has been studied on polycrystalline samples from neutron diffraction and specific heat measurements. The onset of antiferromagnetic ordering is observed at T(N) ≈ 25 K. In the temperature interval 18 K < T < 25 K the magnetic structure is defined by the propagation vector k(1) = (1/2,0,0). Below 18 K, some additional magnetic satellites appear in the NPD patterns, which are indexed with k(2) = (0,0,1/2). Therefore, below 18 K the magnetic structure consists of two independent magnetic domains, defined by the propagation vectors k(1) and k(2). The magnetic structure of the k(1)-domain is given by the basis vectors (C(x),0,0) and (C(x)',0,0) for Mn(4h) and Mn(4f), respectively. In the k(2)-domain, the magnetic structure is defined by the basis vectors (0,0,G(z)) and (F(x)',G(y)',0) for Mn(4h) and Mn(4f), respectively. At T = 1.5 K, for the magnetic phase associated with k(1), the magnetic moments of the Mn atoms at the 4h and 4f sites are 1.82(7) and 1.81(6) μ(B), respectively; for the magnetic phase associated with k(2), the magnetic moments for the Mn(4h) and Mn(4f) atoms are 0.59(5) and 2.62(5) μ(B), respectively.     

Magnetic circular dichroism from the impurity band in III-V diluted magnetic semiconductors

The magnetic circular dichroism of III-V diluted magnetic semiconductors, calculated within a theoretical framework suitable for highly disordered materials, is shown to be dominated by optical transitions between the bulk bands and an impurity band formed from magnetic dopant states. The real-space Green's functions incorporate spatial correlations in the disordered conduction band and valence-band electronic structure, and include extended and localized states on an equal basis. Our findings reconcile unusual trends in the experimental magnetic circular dichroism in III-V diluted magnetic semiconductors with the antiferromagnetic p-d exchange interaction between a magnetic dopant spin and its host.