ABSTRACT
The application of a novel UV fs Laser Ablation Ionization Mass Spectrometry approach for chemical depth profiling of low-melting point, high surface roughness SnAg solder bump features is presented. The obtained submicrometer resolved three-dimensional compositional data reveal unprecedented information on the distribution of individual elements inside the solder bump matrix. Moreover, the determination of matrix-matched relative sensitivity coefficients allows the first report on quantitative assessment of the SnAg alloy composition. These results significantly contribute to an in-depth understanding of the SnAg plating process. This experimental procedure may find application in future additive performance screening.
ABSTRACT
Using spin-polarized scanning tunneling microscopy and density functional theory, we have studied the magnetic properties of Pd/Fe atomic bilayers on Re(0001). Two kinds of magnetic ground states are discovered due to different types of stacking of the Pd adlayer on Fe/Re(0001). For fcc stacking of Pd on Fe/Re(0001), it is a spin spiral propagating along the close-packed (ΓK[over ¯]) direction with a period of about 0.9 nm, driven by frustrated exchange and Dzyaloshinskii-Moriya interactions. For the hcp stacking, the four-site four-spin interaction stabilizes an up-up-down-down state propagating perpendicular to the close-packed direction (along ΓM[over ¯]) with a period of about 1.0 nm. Our work shows how higher-order exchange interactions can be tuned at interfaces.
ABSTRACT
Step edges represent a local break of lateral symmetry in ultrathin magnetic films. In our experiments, we investigate the spin coupling across atomic step edges on Fe/W(110) by means of spin-polarized scanning tunneling microscopy and spectroscopy. Local modifications of the spin texture toward step edges separating double from single layer areas are observed, and selection rules indicate a chiral spin coupling that significantly changes with the propagation along the [11[over ¯]0] or the [001] crystallographic direction. The findings are explained via anisotropic Dzyaloshinskii-Moriya interactions arising from the broken lateral symmetry at atomic step edges.
ABSTRACT
A magnetic atom in a superconducting host induces so-called Yu-Shiba-Rusinov (YSR) bound states inside the superconducting energy gap. By combining spin-resolved scanning tunneling spectroscopy with simulations we demonstrate that the pair of peaks associated with the YSR states of an individual Fe atom coupled to an oxygen-reconstructed Ta surface gets spin polarized in an external magnetic field. As theoretically predicted, the electron and hole parts of the YSR states have opposite signs of spin polarizations which keep their spin character when crossing the Fermi level through the quantum phase transition. The simulation of a YSR state right at the Fermi level reveals zero spin polarization which can be used to distinguish such states from Majorana zero modes in chains of YSR atoms.
ABSTRACT
The thermal stability of the magnetic nano-skyrmion lattice in the monolayer Fe/Ir(111) is investigated using temperature dependent spin-polarized scanning tunneling microscopy. Our experiments show that the skyrmion lattice disappears at a temperature of T_{c}=27.8 K, indicating a loss of long-range magnetic order. At second-layer iron islands the lattice is pinned and local order persists at temperatures above T_{c}. The findings are explained in terms of the complex magnetic interactions involved in the formation of the skyrmion lattice.
ABSTRACT
Atom manipulation with the magnetic tip of a scanning tunneling microscope is a versatile technique to construct and investigate well-defined atomic spin arrangements. Here we explore the possibility of using a magnetic adatom as a local probe to image surface spin textures. As a model system we choose a Néel state with 120° between neighboring magnetic moments. Close to the threshold of manipulation, the adatom resides in the threefold, magnetically frustrated hollow sites, and consequently no magnetic signal is detected in manipulation images. At smaller tip-adatom distances, however, the adatom is moved towards the magnetically active bridge sites and due to the exchange force of the tip the manipulation process becomes spin dependent. In this way the adatom can be used as an amplifying probe for the surface spin texture.
ABSTRACT
Magnetoelectric coupling is studied using the electric field between the tip of a spin-polarized scanning tunneling microscope and a nanomagnet. Our experiments show that a negative (positive) electric field stabilizes (destabilizes) in-plane magnetization against thermal agitation, whereas it destabilizes (stabilizes) out-of-plane magnetization. We conclude that the electric field E induces a uniaxial anisotropy that favors in-plane magnetization for E<0 and out-of-plane magnetization for E>0. Our experiments demonstrate magnetic manipulation on the atomic scale without exploiting spin or charge currents.
ABSTRACT
We investigate the electronic and magnetic properties of single Fe, Co, and Ni atoms and clusters on monolayer graphene (MLG) on SiC(0001) by means of scanning tunneling microscopy (STM), x-ray absorption spectroscopy, x-ray magnetic circular dichroism (XMCD), and ab initio calculations. STM reveals different adsorption sites for Ni and Co adatoms. XMCD proves Fe and Co adatoms to be paramagnetic and to exhibit an out-of-plane easy axis in agreement with theory. In contrast, we experimentally find a nonmagnetic ground state for Ni monomers while an increasing cluster size leads to sizeable magnetic moments. These observations are well reproduced by our calculations and reveal the importance of hybridization effects and intra-atomic charge transfer for the properties of adatoms and clusters on MLG.
ABSTRACT
We demonstrate using inelastic scanning tunneling spectroscopy and simulations based on density functional theory that the amplitude and sign of the magnetic anisotropy energy for a single Fe atom adsorbed onto the Pt(111) surface can be manipulated by modifying the adatom binding site. Since the magnitude of the measured anisotropy is remarkably small, up to an order of magnitude smaller than previously reported, electron-hole excitations are weak and thus the spin excitation exhibits long lived precessional lifetimes compared to the values found for the same adatom on noble metal surfaces.
ABSTRACT
A combined experimental and theoretical study of doping individual Fe atoms into Bi(2)Se(3) is presented. It is shown through a scanning tunneling microscopy study that single Fe atoms initially located at hollow sites on top of the surface (adatoms) can be incorporated into subsurface layers by thermally activated diffusion. Angle-resolved photoemission spectroscopy in combination with ab initio calculations suggest that the doping behavior changes from electron donation for the Fe adatom to neutral or electron acceptance for Fe incorporated into substitutional Bi sites. According to first principles calculations within density functional theory, these Fe substitutional impurities retain a large magnetic moment, thus presenting an alternative scheme for magnetically doping the topological surface state. For both types of Fe doping, we see no indication of a gap at the Dirac point.
ABSTRACT
Chirality is a fascinating phenomenon that can manifest itself in subtle ways, for example in biochemistry (in the observed single-handedness of biomolecules) and in particle physics (in the charge-parity violation of electroweak interactions). In condensed matter, magnetic materials can also display single-handed, or homochiral, spin structures. This may be caused by the Dzyaloshinskii-Moriya interaction, which arises from spin-orbit scattering of electrons in an inversion-asymmetric crystal field. This effect is typically irrelevant in bulk metals as their crystals are inversion symmetric. However, low-dimensional systems lack structural inversion symmetry, so that homochiral spin structures may occur. Here we report the observation of magnetic order of a specific chirality in a single atomic layer of manganese on a tungsten (110) substrate. Spin-polarized scanning tunnelling microscopy reveals that adjacent spins are not perfectly antiferromagnetic but slightly canted, resulting in a spin spiral structure with a period of about 12 nm. We show by quantitative theory that this chiral order is caused by the Dzyaloshinskii-Moriya interaction and leads to a left-rotating spin cycloid. Our findings confirm the significance of this interaction for magnets in reduced dimensions. Chirality in nanoscale magnets may play a crucial role in spintronic devices, where the spin rather than the charge of an electron is used for data transmission and manipulation. For instance, a spin-polarized current flowing through chiral magnetic structures will exert a spin-torque on the magnetic structure, causing a variety of excitations or manipulations of the magnetization and giving rise to microwave emission, magnetization switching, or magnetic motors.
ABSTRACT
Scanning tunneling spectroscopy is used to study the real-space local density of states of a two-dimensional electron system in a magnetic field, in particular within higher Landau levels. By Fourier transforming the local density of states, we find a set of n radial minima at fixed momenta for the nth Landau levels. The momenta of the minima depend only on the inverse magnetic length. By comparison with analytical theory and numerical simulations, we attribute the minima to the nodes of the quantum cyclotron orbits, which decouple in a Fourier representation from the random guiding center motion due to disorder. Adequate Fourier filtering reveals the nodal structure in real space in some areas of the sample with relatively smooth potential disorder.
ABSTRACT
We report a transverse conical spin spiral as the magnetic ground state of a double-layer Mn on a W(110) surface. Using spin-polarized scanning tunneling microscopy, we find a long-range modulation along the [001] direction with a periodicity of 2.4 nm coexisting with a local row-wise antiferromagnetic contrast. First-principles calculations reveal a transverse conical spin-spiral ground state of this system which explains the observed magnetic contrast. The canting of the spins is induced by higher-order exchange interactions, while the spiraling along the [001] direction is due to frustrated Heisenberg exchange and Dzyaloshinskii-Moriya interaction.
ABSTRACT
The robustness of the gapless topological surface state hosted by a 3D topological insulator against perturbations of magnetic origin has been the focus of recent investigations. We present a comprehensive study of the magnetic properties of Fe impurities on the prototypical 3D topological insulator Bi(2)Se(3) using local low-temperature scanning tunneling spectroscopy and integral x-ray magnetic circular dichroism techniques. Single Fe adatoms on the Bi(2)Se(3) surface, in the coverage range ≈ 1% of a monolayer, are heavily relaxed into the surface and exhibit a magnetic easy axis within the surface plane, contrary to what was assumed in recent investigations on the supposed opening of a gap. Using ab initio approaches, we demonstrate that an in-plane easy axis arises from the combination of the crystal field and dynamic hybridization effects.
ABSTRACT
The indirect controlled displacement of an antiferromagnetic domain wall by a spin current is studied by Landau-Lifshitz-Gilbert spin dynamics. The antiferromagnetic domain wall can be shifted both by a spin-polarized tunnel current of a scanning tunneling microscope or by a current driven ferromagnetic domain wall in an exchange coupled antiferromagnetic-ferromagnetic layer system. The indirect control of antiferromagnetic domain walls opens up a new and promising direction for future spin device applications based on antiferromagnetic materials.
ABSTRACT
A theoretical concept of local manipulation of magnetic domain walls is introduced. In the proposed procedure, a domain wall is driven by a spin-polarized current induced by a magnetic tip, as used in a scanning tunneling microscope, placed above a magnetic nanostripe and then moved along its long axis with a current flowing through the vacuum barrier. The angular momentum from the spin-polarized current exerts a torque on the magnetic moments underneath the tip and leads to a displacement of the domain wall. Particularly, the manipulation of a ferromagnetic 180° transverse domain wall has been studied by means of Landau-Lifshitz-Gilbert dynamics and Monte Carlo simulations. Different relative orientations of the tip and the sample magnetization have been considered.
ABSTRACT
The influence of a high spin-polarized tunnel current onto the switching behavior of a superparamagnetic nanoisland on a nonmagnetic substrate is investigated by means of spin-polarized scanning tunneling microscopy. A detailed lifetime analysis allows for a quantification of the effective temperature rise of the nanoisland and the modification of the activation energy barrier for magnetization reversal, thereby using the nanoisland as a local thermometer and spin-transfer torque analyzer. Both the Joule heating and spin-transfer torque are found to scale linearly with the tunnel current. The results are compared to experiments performed on lithographically fabricated magneto-tunnel junctions, revealing a very high spin-transfer torque switching efficiency in our experiments.
ABSTRACT
We demonstrate that well prepared and characterized Cr tips can provide atomic resolution on the bulk NaCl(001) surface with dynamic atomic force microscopy in the noncontact regime at relatively large tip-sample separations. At these conditions, the surface chemical structure can be resolved yet tip-surface instabilities are absent. Our calculations demonstrate that chemical identification is unambiguous, because the interaction is always largest above the anions. This conclusion is generally valid for other polar surfaces, and can thus provide a new practical route for straightforward interpretation of atomically resolved images.
ABSTRACT
We demonstrate that magnetic exchange force spectroscopy allows for a quantitative determination of the distance-dependent magnetic exchange interaction across a vacuum gap. Experiments were performed on the antiferromagnetic Fe monolayer on W(001) with magnetically sensitive tips and compared to first-principles calculations performed for different cluster tip models. For stable tips, which can be distinguished from unstable tips by analyzing the dissipation signal, very good agreement with theory is observed.
ABSTRACT
We have performed single-atom magnetization curve (SAMC) measurements and inelastic scanning tunneling spectroscopy (ISTS) on individual Fe atoms on a Cu(111) surface. The SAMCs show a broad distribution of magnetic moments with 3.5 µB being the mean value. ISTS reveals a magnetization excitation with a lifetime of 200 fsec which decreases by a factor of 2 upon application of a magnetic field of 12 T. The experimental observations are quantitatively explained by the decay of the magnetization excitation into Stoner modes of the itinerant electron system as shown by newly developed theoretical modeling.