Spin-polarized Scanning Tunneling Microscopy Research Articles

Atomic structure and spin polarization at the apex of tips used in spin-polarized scanning tunneling microscopy

To analyze spin-polarized scanning tunneling microscopy (STM) studies quantitatively, we evaluated the atomic structure and spin polarization at the apex of Cr/W and Fe/W tips using field ion microscopy (FIM) and field-emitted electron polarimetry, respectively. The patchwork-patterned H2-FIM images of the Cr/W tip indicated partially developed Cr planes, and the spin polarization at the surface was 10 ± 3% at room temperature. H2-FIM images of the Fe/W tip indicate the crystalline order of Fe layers on the W tip, and its spin polarization was 41 ± 2%. These first results allow us to quantify the spin polarization in spin-dependent STM measurements.

Electric-Field-Induced Magnetic Anisotropy in a Nanomagnet Investigated on the Atomic Scale

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.

スピン偏極走査トンネル顕微鏡&amp;mdash;探針が測定に及ぼす影響を中心にして&amp;mdash;

After a brief introduction of the spin-polarized scanning tunneling microscopy (SP-STM), its measurement techniques are described based on our recent experimental results. We have observed anomalously high spin contrast for epitaxially grown Mn(001)/Fe(001) samples. This phenomenon can be attributed to a single atom or a cluster of Mn attached to the apex of the thin film Fe tip. This interpretation is supported by a recent first-principle calculation of model magnetic tips. These results stress the importance of the identification of the magnetic tip and its precise control at an atomic level for the purpose of SP-STM measurement.

Identifying Magnetic Anisotropy of the Topological Surface State ofCr0.05Sb1.95Te3with Spin-Polarized STM

The surface magnetic property plays a key role in determining magnetic related quantum phenomena of magnetic topological insulators. Using spin-polarized scanning tunneling microscopy, we investigate the surface magnetism and anisotropy of a Cr doped topological insulator: Cr(0.05)Sb(1.95)Te(3). It is found that the topological surface state of Cr(0.05)Sb(1.95)Te(3) is spin polarized in the surface plane while the bulk shows a ferromagnetism with an out-of-plane easy axis. The upper and lower branch of the helical Dirac cone harbors the opposite spin polarization and the polarization at the Dirac point is zero. Our results show the complexity of surface magnetism of magnetic doped topological insulators.

Atomic-scale inversion of spin polarization at an organic-antiferromagnetic interface

Using first-principles calculations, we show that the magnetic properties of a two-dimensional antiferromagnetic transition-metal surface are modified on the atomic scale by the adsorption of small organic molecules. We consider benzene (C6H6), cyclooctatetraene (C8H8) and a small transition metal - benzene complex (BzV) adsorbed on a single atomic layer of Mn deposited on the W(110) surface -- a surface which exhibits a nearly antiferromagnetic alignment of the magnetic moments in adjacent Mn rows. Due to the spin-dependent hybridization of the molecular pz orbitals with the d states of the Mn monolayer there is a significant reduction of the magnetic moments in the Mn film. Furthermore, the spin-polarization at this organic-antiferromagnetic interface is found to be modulated on the atomic scale, both enhanced and inverted, as a result of the molecular adsorption. We show that this effect can be resolved by spin-polarized scanning tunneling microscopy (SP-STM). Our simulated SP-STM images display a spatially-dependent spin-resolved vacuum charge density above an adsorbed molecule -- i.e., different regions above the molecule sustain different signs of spin polarization. While states with s and p symmetry dominate the vacuum charge density in the vicinity of the Fermi energy for the clean magnetic surface, we demonstrate that after a molecule is adsorbed those d-states, which are normally suppressed due to their symmetry, can play a crucial role in the vacuum due to their interaction with the molecular orbitals. We also model the effect of small deviations from perfect antiferromagnetic ordering, induced by the slight canting of magnetic moments due to the spin spiral ground state of Mn/W(110).

Superparamagnetic response of Fe-coated W tips in spin-polarized scanning tunneling microscopy

We performed spin-polarized scanning tunneling spectroscopy on biatomic-layer-high Co nanoislands grown on Cu(111) in magnetic fields oriented normal to the sample surface, with Fe-coated W tips. Increasing the temperature from 10 to 30 K, we observe a reduced slope of the differential conductance around zero field. A quantitative analysis of the field- and temperature-dependent differential conductance data in the framework of superparamagnetism as described by a Langevin function gives an excellent description of the experimental results. The analysis suggests that a Fe nano-apex at the W tip, which is composed of 220–300 Fe atoms, determines the magnetic response of the tip.

Manipulation of spin state of iron porphyrin by chemisorption on magnetic substrates

One of the key factors behind the rapid evolution of molecular spintronics is the efficient realization of spin manipulation of organic molecules with a magnetic center. The spin state of such molecules may depend crucially on the interaction with the substrate on which they are adsorbed. In this paper we demonstrate, using ab initio density functional calculations, that the stabilization of a high spin state of an iron porphyrin (FeP) molecule can be achieved via chemisorption on magnetic substrates of different species and orientations, viz., Co(001), Ni(001), Ni(110), and Ni(111). The signature of chemisorption of FeP on magnetic substrates is evident from broad features in N $K$ x-ray absorption (XA) and Fe ${L}_{2,3}$ x-ray magnetic circular dichroism (XMCD) measurements. Our theoretical calculations show that the strong covalent interaction with the substrate increases Fe-N bond lengths in FeP and hence a switching to a high spin state ($S=2$) from an intermediate spin state ($S=1$) is achieved. Due to chemisorption, ferromagnetic exchange interaction is established through a direct exchange between Fe and substrate magnetic atoms as well as through an indirect exchange via the N atoms in FeP. The mechanism of exchange interaction is further analyzed by considering structural models constructed from ab initio calculations. Also, it is found that the exchange interaction between Fe in FeP and a Ni substrate is almost 4 times smaller than with a Co substrate. Finally, we illustrate the possibility of detecting a change in the molecular spin state by XMCD, Raman spectroscopy, and spin-polarized scanning tunneling microscopy.

Probing the Rashba effect via the induced magnetization around a Kondo impurity

When a single magnetic adatom is deposited on a surface of a metal, it affects the charge and spin texture of the electron gas surrounding it. The screening of the local moment by conduction electrons gives rise to the Kondo effect. Here we investigate the effect of the Rashba spin orbit coupling on the local magnetization density of states (LMDOS) around a Cobalt impurity on a Au(111) surface in a magnetic field. We show that the in-plane component of the LMDOS is exclusively associated with the Rashba spin orbit interaction. This observation can be experimentally exploited to confirm the presence of the Rashba effect on surfaces, such as Au(111), by performing spin-polarized STM measurements around the Kondo impurity.

Spin-Polarized Electron Transport Across Metal-Organic Molecules: A Density Functional Theory Approach.

In the field of molecular spintronics, experimental techniques have achieved a stage where it is feasible to explore the interplay between quantum electron transport and magnetism at the single molecule level. An example is a spin-polarized STM, which can probe local electrical currents through organic molecules deposited on magnetic surfaces. The atomistic complexity of nanoscale systems calls for a first-principles description of spin-dependent transport phenomena, e.g., based on the nonequilibrium Green's function (NEGF) formalism merged with density functional theory (DFT). However, for the case of molecular junctions with transition metal electrodes, a computation of the underlying Kohn-Sham Hamiltonian can be a challenging problem: a simultaneous and accurate description of spin ordered magnetic surfaces together with the electronic structure of a molecule is required. In the present work, we provide a solution for this problem. We present an implementation, within a standard quantum chemistry package, of the NEGF formalism with an efficient approximation for the self-energy, which accounts both for absorbing boundary conditions and for exchange splitting of the energy bands in ferromagnetic electrodes. We demonstrate an ability to simulate a variety of magnetic configurations including nanoscale domain walls, which are realized when a molecule with few spin centers is brought in contact with differently magnetized reservoirs. The magnetoresistance effect arising on the molecular scale is discussed based on examples including Ni atomic-sized contact, electron transport across a prototypical molecular magnet (vanadium-benzene multidecker cluster), and tunneling through Co-phthalocyanine. Furthermore, we verify the stability of magnetically nontrivial solutions against electron correlations within the DFT+U approach.

Parity effect in the ground state localization of antiferromagnetic chains coupled to a ferromagnet.

We investigate the ground states of antiferromagnetic Mn nanochains on Ni(110) by spin-polarized scanning tunneling microscopy in combination with theory. While the ferrimagnetic linear trimer experimentally shows the predicted collinear classical ground state, no magnetic contrast was observed for dimers and tetramers where noncollinear structures were expected based on abinitio theory. This striking observation can be explained by zero-point energy motion for even-numbered chains derived within a classical equation of motion leading to nonclassical ground states. Thus, depending on the parity of the chain length, the system shows a classical or a quantum behavior.

Prediction of the bias voltage dependent magnetic contrast in spin-polarized scanning tunneling microscopy

This work is concerned with the theoretical description of the contrast, i.e., the apparent height difference between two lateral surface positions on constant current spin-polarized scanning tunneling microscopy (SP-STM) images. We propose a method to predict the bias voltage dependent magnetic contrast from single point tunneling current or differential conductance measurements, without the need of scanning large areas of the surface. Depending on the number of single point measurements, the bias positions of magnetic contrast reversals and of the maximally achievable magnetic contrast can be determined. We validate this proposal by simulating SP-STM images on a complex magnetic surface employing a recently developed approach based on atomic superposition. Furthermore, we show evidence that the tip electronic structure and magnetic orientation have a major effect on the magnetic contrast. Our theoretical prediction is expected to inspire experimentalists to considerably reduce measurement efforts for determining the bias dependent magnetic contrast on magnetic surfaces.

Spin-polarized scanning tunneling microscopy of the room-temperature antiferromagnetc-FeSi

Antiferromagnetic spin ordering has been revealed by room-temperature spin-polarized scanning tunneling microscopy (SP-STM) in thin epitaxial films of $c$-FeSi on Si(111). Spin polarization of tunneling current for unoccupied states is found to be unusually large ${I}_{\ensuremath{\uparrow}\ensuremath{\uparrow}}/{I}_{\ensuremath{\downarrow}\ensuremath{\uparrow}}=3.8$. Atomically sharp spin-frustration domain walls, developing on the surfaces of nanoscale islands, have been observed on SP-STM images. Our results suggest that antiferromagnetism in $c$-FeSi is driven by Mott-Hubbard transition, and the atomically narrow domain walls are caused by local insulator-to-metal breakdown.

Atomic-scale magnetism of cobalt-intercalated graphene

Using spin-polarized scanning tunneling microscopy and density functional theory, we have studied the structural and magnetic properties of cobalt-intercalated graphene on Ir(111). The cobalt forms monolayer islands being pseudomorphic with the Ir(111) beneath the graphene. The strong bonding between graphene and cobalt leads to a high corrugation within the Moir\'e pattern which arises due to the lattice mismatch between the graphene and the Co on Ir(111). The intercalation regions exhibit an out-of-plane easy axis with an extremely high switching field, which surpasses the significant values reported for uncovered cobalt islands on Ir(111). Within the Moir\'e unit cell of the intercalation regions, we observe a site-dependent variation of the local effective spin polarization. State-of-the-art first-principles calculations show that the origin of this variation is a site-dependent magnetization of the graphene: At top sites the graphene is coupled ferromagnetically to the cobalt underneath, while it is antiferromagnetically coupled at fcc and hcp sites.

Current-Driven Spin Dynamics of Artificially Constructed Quantum Magnets

The future of nanoscale spin-based technologies hinges on a fundamental understanding and dynamic control of atomic-scale magnets. The role of the substrate conduction electrons on the dynamics of supported atomic magnets is still a question of interest lacking experimental insight. We characterized the temperature-dependent dynamical response of artificially constructed magnets, composed of a few exchange-coupled atomic spins adsorbed on a metallic substrate, to spin-polarized currents driven and read out by a magnetic scanning tunneling microscope tip. The dynamics, reflected by two-state spin noise, is quantified by a model that considers the interplay between quantum tunneling and sequential spin transitions driven by electron spin-flip processes and accounts for an observed spin-transfer torque effect.

Spin-Polarized Scanning Tunneling Microscopy: STM Magnetic Imaging of Nano Materials

Spin-Polarized Scanning Tunneling Microscopy: STM Magnetic Imaging of Nano Materials

Non-equilibrium finite temperature dynamics of magnetic quantum systems: applications to spin-polarized scanning tunneling microscopy

We calculated the real-time non-equilibrium dynamics of quantum spin systems at finite temperatures. The mathematical framework originates from the C*-approach to quantum statistical mechanics and was applied to the samples investigated by means of spin-polarized scanning tunneling microscopy. Quantum fluctuations around thermal equilibrium were analyzed and calculated. The time-averaged expectation values agree with the time-averaged experimental data for magnetization curves. The method was used to investigate the dynamics of a sample for shorter times than the resolution time of the experimental setup. Furthermore, predictions of the relaxation times of single spins on metallic and semiconductor surfaces are made. To check the validity of our model, we compared our results with experimental data obtained from Fe adatoms on InSb and from Co adatoms on Pt(111) and found good agreement. Approximated thermalization was found numerically for the expectation values of the spin operators.

単一ナノ分子・磁気抵抗素子

Giant magnetoresistance (GMR) through a single phthalocyanine molecule (H2Pc) was studied by means of a spin-polarized scanning tunneling microscopy (STM) technique. Single molecular magnetic junction was fabricated by contacting a ferromagnetic STM tip to a single molecule on nano magnets. We obtained GMR of +60% and −50% through [Co/H2Pc/Co(111)] and [Fe/H2Pc/Mn(001)] junctions, respectively, in ultra-high vacuum at 4.6 K. Single organic molecules can be a new material for spintronics.

スピン偏極STMを用いたAg(111)上における酸素分子の磁性

スピン偏極STMを用いたAg(111)上における酸素分子の磁性

A complex magnetic structure of ultrathin Fe films on Rh (001) surfaces

We conducted a structural and magnetic analysis of ultrathin Fe films on Rh (001) surfaces by using low electron energy diffraction (LEED), magneto-optical Kerr effects (MOKE) and spin-polarized scanning tunneling microscopy (SP-STM). The films in the investigated thickness range up to 6 monolayers (ML) are pseudomorphic to the Rh (001) substrate. While Fe films thinner than 3 ML grow layer-by-layer at room temperature (RT), Fe films thicker than 4 ML form islands. 1 ML Fe films do not show any hysteresis loops even at low temperature. Polar hysteresis loops for the 2 ML and 3 ML thick films appear at low temperatures. When 1 ML thick Fe films were studied by Cr- and Fe-coated W tips, a (2×3) and stripe structures were observed, respectively. The structures originate from a complex magnetic structure of 1 ML Fe. Based on the SP-STM results we propose a spin configuration model of a 1 ML Fe film.

New insights into nano-magnetism by spin-polarized scanning tunneling microscopy

We study the magnetization reversal and the position dependence of the spin-dependent electronic properties of nm small bilayer Co islands on Cu(111) by spin-polarized scanning tunneling microscopy in magnetic fields at low temperatures of 8K. The analysis of the energy barrier of magnetization reversal from measurements of the switching field suggests a crossover of the magnetization reversal mode with increasing island size around 7500 atoms from exchange-spring behavior to domain wall formation. The quantitative analysis of the island size dependence of the energy barrier indicates an inhomogeneous magnetic anisotropy of the island. The island rim is magnetically soft, whereas the center shows a pronounced effective anisotropy of 0.148meV/atom. We speculate that this inhomogeneity of the magnetic anisotropy might be a consequence of the spatial dependence of the spin-dependent electronic properties. We measure a spin-polarization and a tunnel magneto resistance ratio of opposite sign at the rim as compared to the island center.