Magnetic Edge Anisotropy in Graphenelike Honeycomb Crystals

The independent predictions of edge ferromagnetism and the quantum spin Hall phase in graphene have inspired the quest of other two-dimensional honeycomb systems, such as silicene, germanene, stanene, iridates, and organometallic lattices, as well as artificial superlattices, all of them with electronic properties analogous to those of graphene, but a larger spin-orbit coupling. Here, we study the interplay of ferromagnetic order and spin-orbit interactions at the zigzag edges of these graphenelike systems. We find an in-plane magnetic anisotropy that opens a gap in the otherwise conducting edge channels that should result in large changes of electronic properties upon rotation of the magnetization.

Imaging of spin waves in atomically designed nanomagnets

The spin dynamics of all ferromagnetic materials are governed by two types of collective phenomenon: spin waves and domain walls. The fundamental processes underlying these collective modes, such as exchange interactions and magnetic anisotropy, all originate at the atomic scale. However, conventional probing techniques based on neutron1 and photon scattering2 provide high resolution in reciprocal space, and thereby poor spatial resolution. Here we present direct imaging of standing spin waves in individual chains of ferromagnetically coupled S = 2 Fe atoms, assembled one by one on a Cu2N surface using a scanning tunnelling microscope. We are able to map the spin dynamics of these designer nanomagnets with atomic resolution in two complementary ways. First, atom-to-atom variations of the amplitude of the quantized spin-wave excitations are probed using inelastic electron tunnelling spectroscopy. Second, we observe slow stochastic switching between two opposite magnetization states3, 4, whose rate varies strongly depending on the location of the tip along the chain. Our observations, combined with model calculations, reveal that switches of the chain are initiated by a spin-wave excited state that has its antinodes at the edges of the chain, followed by a domain wall shifting through the chain from one end to the other. This approach opens the way towards atomic-scale imaging of other types of spin excitation, such as spinon pairs and fractional end-states5, 6, in engineered spin chains.

Edge states in graphene-like systems

The edges of graphene and graphene like systems can host localized states with evanescent wave function with properties radically different from those of the Dirac electrons in bulk. This happens in a variety of situations, that are reviewed here. First, zigzag edges host a set of localized non-dispersive state at the Dirac energy. At half filling, it is expected that these states are prone to ferromagnetic instability, causing a very interesting type of edge ferromagnetism. Second, graphene under the influence of external perturbations can host a variety of topological insulating phases, including the conventional quantum Hall effect, the quantum anomalous Hall (QAH) and the quantum spin Hall phase, in all of which phases conduction can only take place through topologically protected edge states. Here we provide an unified vision of the properties of all these edge states, examined under the light of the same one orbital tight-binding model. We consider the combined action of interactions, spin–orbit coupling and magnetic field, which produces a wealth of different physical phenomena. We briefly address what has been actually observed experimentally.

Magnetism and photophysics in supramolecular transition-metal compounds

Based on a synthetic strategy, extended anionic, homo and bimetallic oxalato-bridged transition-metal compounds with two (2D) and three-dimensional (3D) connectivities can be synthesized and crystallized. Thereby, the choice of the templating counterions will determine the crystal chemistry. Since the oxalato bridge is a mediator for both antiferro and ferromagnetic interactions between similar and dissimilar metal ions, long-range magnetic ordering will occur. Examples of the determination of magnetic structures in 2D and 3D compounds by means of elastic neutron scattering methods will be discussed. In addition, due to the possibility of the variation of different metal ions in varying oxidation states, interesting photophysical processes can be observed within the extended three-dimensional host/guest systems.

Strain-induced structural instability in FeRh

We perform density functional calculations to investigate the structure of the intermetallic alloy FeRh under epitaxial strain. Bulk FeRh exhibits a metamagnetic transition from a low-temperature antiferromagnetic (AFM) phase to a ferromagnetic phase at 350 K, and its strain dependence is of interest for tuning the transition temperature to the room-temperature operating conditions of typical memory devices. We find an unusually strong dependence of the structural energetics on the choice of exchange-correlation functional, with the usual local density approximation yielding the wrong ground-state structure, and generalized gradient (GGA) extensions being in better agreement with the bulk experimental structure. Using the GGA we show the existence of a metastable face-centered-cubic-like AFM structure that is reached from the ground-state body-centered-cubic-like AFM structure by following the epitaxial Bain path. We show that the behavior is well described using nonlinear elasticity theory, which captures the softening and eventual sign change of the orthorhombic shear modulus under compressive strain, consistent with this structural instability. Finally, we predict the existence of an additional unit-cell-doubling lattice instability, which should be observable at low temperature.

Magnetic charachterization of surface sediments in the South Atlantic

Surface sediment samples representative for the tropical and subtropical South Atlantic (15°N to 40°S) were investigated by isothermal magnetic methods to delineate magnetic mineral distribution patterns and to identify their predominant Holocene climatic and oceanographic controls. Individual parameters reveal distinct, yet frequently overlapping, regional sedimentation characteristics. A probabilistic ('fuzzy c-means') cluster analysis was applied to five concentration independent magnetic properties assessing magnetite to hematite ratios and diagnostic of bulk and fine-particle magnetite grain size and coercivity spectra. The resultant 10 cluster structures establish an oceanwide magnetic sediment classification scheme tracing the major terrigenous eolian and fluvial fluxes, authigenic biogenic magnetite accumulation in high-productivity areas, transport by ocean current systems, and effects of bottom water velocity on depositional regimes. Distinct dissimilarities in magnetic mineral inventories between the eastern and western basins of the South Atlantic reflect prominent contrasts of both oceanic and continental influences.

Rock magnetism of pelagic sediments from DSDP Leg 85 sites

A detailed rock magnetic investigation has been carried out on Deep Sea Drilling Project (DSDP) pelagic sediments from the Central Equatorial Pacific. This comprises hysteresis and thermomagnetic measurements, Lowrie-Fuller test and, for the first time, ferromagnetic resonance (FMR). Nearly stochiometric magnetite in two grain size fractions, single domain (SD) and multi domain (MD), has been deduced to be the carrier of magnetic remanence. Comparatively strong paramagnetic contributions are carried by pyrite, being identified by X-ray analysis. The statistical analysis of paleomagnetic parameters (NRM, MDF, initial susceptibility, Königsberger ratio Q) from a large number (> 1000) of samples, supported by hysteresis measurements, indicates a latitude and sedimentation rate dependent ratio of SD/MD grains. Possible sources for the magnetic constituents are discussed in terms of bacterial, volcanic, meteoritic and authigenic origin.

(Table 1) Helium concentration in sediments

Two decades ago, Merrihue (1964) reported 3He/4He ratios of >10**-4 in ferromagnetic separates from a Pacific deep ocean red clay and concluded that the high ratio is due to extraterrestrial debris amounting to ~1% of the sediment. A decade later Krylov et al. (1973) compiled 3He/4He isotopic data on ocean sediments measured in the Soviet Union and observed that the 3He/4He ratio is generally higher in pelagic sediments where the sedimentation rate is lower. They suggested that the high 3He/4He ratio was attributable to extraterrestrial materials which were concentrated in slowly accumulating ocean floor. However, these important discoveries were almost completely neglected until we re-examined the problem. We have measured 39 sediments from 12 different sites, 10 sites from the western to central Pacific and two sites from the Atlantic Ocean. We find 3He/4He ratios >5 * 10**-5 for six sites, well above the values generally observed in common terrestrial materials. The very high 3He/4He ratio in the sediments is probably due to input of extraterrestrial materials. Input of stratospheric dust of <1 p.p.m., which corresponds to a fallout rate of ~2,000 tons per year, can explain the observation.

(Table S1) FMR and rock magnetic parameters for upper Paleocene to lower Eocene strata, Site Ancora

Previous workers identified a magnetically anomalous clay layer deposited on the northern United States Atlantic Coastal Plain during the Paleocene-Eocene thermal maximum (PETM). The finding inspired the highly controversial hypothesis that a cometary impact triggered the PETM. Here we present ferromagnetic resonance (FMR), isothermal and anhysteretic remanent magnetization, first-order reversal curve, and transmission electron microscopy analyses of late Paleocene and early Eocene sediments in drill core from Ancora, New Jersey. A novel paleogeographic analysis applying a recent paleomagnetic pole from the Faeroe Islands indicates that New Jersey during the initial Eocene had a ~6°-9° lower paleolatitude (~27.3° for Ancora) and a more zonal shoreline trace than in conventional reconstructions. Our investigations of the PETM clay from Ancora reveal abundant magnetite nanoparticles bearing signature traits of crystals produced by magnetotactic bacteria. This result, the first identification of ancient biogenic magnetite using FMR, argues that the anomalous magnetic properties of the PETM sediments are not produced by an impact. They instead reflect environmental changes along the eastern margin of North America during the PETM that led to enhanced production and/or preservation of magnetofossils.

Enhanced spin injection efficiency in ferromagnet/semiconductor tunnel junctions

Within the ballistic transport picture, we have investigated the spin-polarized transport properties of a ferromagnetic metal/two-dimensional semiconductor (FM/SM) hybrid junction and an FM/FM/SM structure using quantum tunnelling theory. Our calculations indicate explicitly that the low spin injection efficiency (SIE) from an FM into an SM, compared with a ferromagnet/normal metal junction, originates from the mismatch of electron densities in the FM and SM. To enhance the SIE from an FM into an SM, we introduce another FM film between them to form FM/FM/SM double tunnel junctions, in which the quantum interference effect will lead to the current polarization exhibiting periodically oscillating behaviour, with a variation according to the thickness of the middle FM film and/or its exchange energy strength. Our results show that, for some suitable values of these parameters, the SIE can reach a very high level, which can also be affected by the electron density in the SM electrode.

The electric field effect on spin injection in tunneling regime

Using the quantum tunneling theory, we investigate the spin-dependent transport properties of the ferromagnetic metal/Schottky barrier/semiconductor heterojunction under the influence of an external electric field. It is shown that increasing the electric field, similar to increasing the electron density in semiconductor, will result in a slight enhancement of spin injection in tunneling regime, and this enhancement is significantly weakened when the tunneling Schottky barrier becomes stronger. Temperature effect on spin injection is also discussed. (C) 2003 Elsevier B.V. All rights reserved.