Spin-polarized Scanning Tunneling Spectroscopy Research Articles

Spin configuration of T14 and A61 states in a manganese-doped wide band gap semiconductor

We probe the spin configuration of manganese's $d$ states, namely $^{4}\mathrm{T}_{1}$ and $^{6}\mathrm{A}_{1}$, in doped semiconductor quantum dots (QDs) through spin-polarized scanning tunneling spectroscopy (SP-STS). We show that the two levels have opposing spin states. Experimentally, we probed the density of states (DOS) of the undoped and manganese-doped ZnS QDs, which have a wide band gap so that the $^{4}\mathrm{T}_{1}$ and $^{6}\mathrm{A}_{1}$ energies lie within the gap. The states were viewed with a nonmagnetic and a spin-polarized tip of a scanning tunneling microscope (STM). While a nonmagnetic tip could inject electrons into the $^{4}\mathrm{T}_{1}$ and withdraw electrons from the $^{6}\mathrm{A}_{1}$ state as well, the DOS derived with a spin-polarized tip evidenced that the tip could ``see'' only one of the two energy levels, whose spin state remained parallel to that of the tip. The intensity of DOS representing the $^{4}\mathrm{T}_{1}$ and $^{6}\mathrm{A}_{1}$ of manganese thereby depended on the mutual alignment of the magnetization vectors representing the tip and the $d$ states. We have inferred that the spin configuration of $^{4}\mathrm{T}_{1}$ and $^{6}\mathrm{A}_{1}$ states is mutually antiparallel in nature; such results have been the rationale of the photoluminescence (PL) emission in such manganese-doped QDs to possess an exceptionally long lifetime.

Origin of Asymmetric Splitting of Kondo Peak in Spin-Polarized Scanning Tunneling Spectroscopy: Insights from First-Principles-Based Simulations.

The spin-polarized scanning tunneling microscope (SP-STM) has served as a versatile tool for probing and manipulating the spintronic properties of atomic and molecular devices with high precision. The interplay between the local spin state and its surrounding magnetic environment significantly affects the transport behavior of the device. Particularly, in the contact regime, the strong hybridization between the SP-STM tip and the magnetic atom or molecule could give rise to unconventional Kondo resonance signatures in the differential conductance (dI/dV) spectra. This poses challenges for the simulation of a realistic tip control process. By combining the density functional theory and the hierarchical equations of motion methods, we achieve first-principles-based simulation of the control of a Ni-tip/Co/Cu(100) junction in both the tunneling and contact regimes. The calculated dI/dV spectra reproduce faithfully the experimental data. A cotunneling mechanism is proposed to elucidate the physical origin of the observed unconventional Kondo signatures.

Spin-polarized imaging of strongly interacting fermions in the ferrimagnetic state of the Weyl candidate CeBi

CeBi has an intricate magnetic phase diagram whose fully polarized state has recently been suggested as a Weyl semimetal, though the role of $f$ states in promoting strong interactions has remained elusive. Here we focus on the less-studied but also time-reversal symmetry-breaking ferrimagnetic phase of CeBi, where our density functional theory (DFT) calculations predict additional Weyl nodes near the Fermi level ${E}_{F}$. We use spin-polarized scanning tunneling microscopy and spectroscopy to image the surface ferrimagnetic order on the itinerant Bi $p$ states, indicating their orbital hybridization with localized Ce $f$ states. We observe suppression of this spin-polarized signature at ${E}_{F}$, coincident with a Fano line shape in the conductance spectra, suggesting the Bi $p$ states partially Kondo screen the $f$ magnetic moments, and this $p\ensuremath{-}f$ hybridization causes strong Fermi-level band renormalization. The $p$-band flattening is supported by our quasiparticle interference measurements, which also show band splitting in agreement with DFT, painting a consistent picture of a strongly interacting magnetic Weyl semimetal.

Indirect Spin-Readout of Rare-Earth-Based Single-Molecule Magnet with Scanning Tunneling Microscopy.

Rare-earth based single-molecule magnets are promising candidates for magnetic information storage including qubits as their large magnetic moments are carried by localized 4f electrons. This shielding from the environment in turn hampers a direct electronic access to the magnetic moment. Here, we present the indirect readout of the Dy moment in Bis(phthalocyaninato)dysprosium (DyPc_{2}) molecules on Au(111) using milli-Kelvin scanning tunneling microscopy. Because of an unpaired electron on the exposed Pc ligand, the molecules show a Kondo resonance that is, however, split by the ferromagnetic exchange interaction between the unpaired electron and the Dy angular momentum. Using spin-polarized scanning tunneling spectroscopy, we read out the Dy magnetic moment as a function of the applied magnetic field, exploiting the spin polarization of the exchange-split Kondo state.

Spin-Resolved Quantum Scars in Confined Spin-Coupled Two-Dimensional Electron Gas.

Quantum scars refer to an enhanced localization of the probability density of states in the spectral region with a high energy level density. Scars are discussed for a number of confined pure and impurity-doped electronic systems. Here, we studied the role of spin on quantum scarring for a generic system, namely a semiconductor-heterostructure-based two-dimensional electron gas subjected to a confining potential, an external magnetic field, and a Rashba-type spin-orbit coupling. Calculating the high energy spectrum for each spin channel and corresponding states, as well as employing statistical methods known for the spinless case, we showed that spin-dependent scarring occurs in a spin-coupled electronic system. Scars can be spin mixed or spin polarized and may be detected via transport measurements or spin-polarized scanning tunneling spectroscopy.

Rashba Band Splitting in CH3NH3PbI3: An Insight from Spin-Polarized Scanning Tunneling Spectroscopy.

We report an experimental observation of Rashba splitting in methylammonium lead iodide (MAPbI3). Because of a large spin-orbit coupling parameter in the noncentrosymmetric material, both of the bands were predicted to split having two different spin-textures leading to two different Z-components of total angular momentum (JZ). We use spin-polarized scanning tunneling spectroscopy to probe only one-type of JZ-matched bands throughout the film; dI/dV spectra recorded at many different points of a film however allowed us to spot both the Rashba split-levels and also deliberate on their spin-textures. We observe that the bands split in such a manner that the conduction and valence bands closer to the Fermi energy have the same type of spin-textures (a spin-allowed transition model). Still a low recombination rate of photogenerated carriers in MAPbI3 has been analyzed by considering Wannier-type excitons, a molecular nature of spin-domains from dI/dV images, and therefore a spin-forbidden nature of interdomain transition.

Probing Weakly Hybridized Magnetic Molecules by Single-Atom Magnetometry.

Advances in molecular spintronics rely on the in-depth characterization of the molecular building blocks in terms of their electronic and, more importantly, magnetic properties. For this purpose, inert substrates that interact only weakly with adsorbed molecules are required in order to preserve their electronic states. Here, we investigate the magnetic-field response of a single paramagnetic 5,5'-dibromosalophenatocobalt(II) (CoSal) molecule adsorbed on a weakly interacting magnetic substrate, namely, Fe-intercalated graphene (GR/Fe) grown on Ir(111), by using spin-polarized scanning tunneling microscopy and spectroscopy. We have obtained local magnetization curves, spin-dependent tunneling spectra, and spatial maps of magnetic asymmetry for a single CoSal molecule, revealing its magnetic properties and coupling to the local environment. The distinct magnetic behavior of the Co metal center is found to rely strictly on its position relative to the GR/Fe moiré structure, which determines the level of hybridization between the GR/Fe surface π-system and the molecular orbitals.

Vacuum Resonance States as Atomic-Scale Probes of Noncollinear Surface Magnetism.

The reflection of electrons at noncollinear magnetic surfaces is investigated by spin-polarized scanning tunneling microscopy and spectroscopy on unoccupied resonance states located invacuo. Even for energies up to 20eV above the Fermi level, the resonance states are found to be spin split, exhibiting the same local spin quantization axis as the underlying spin texture. Mapping the spin-dependent electron phase shift upon reflection at the surface on the atomic scale demonstrates the relevance of all magnetic ground state interactions for the scattering of spin-polarized low-energy electrons.

Atomic-scale coexistence of short-range magnetic order and superconductivity in Fe1+ySe0.1Te0.9

The ground state of the parent compounds of many high temperature superconductors is an antiferromagnetically (AFM) ordered phase, where superconductivity emerges when the AFM phase transition is suppressed by doping or application of pressure. This behaviour implies a close relation between the two orders. Understanding the interplay between them promises a better understanding of how the superconducting condensate forms from the AFM ordered background. Here we explore this relation in real space at the atomic scale using low temperature spin-polarized scanning tunneling microscopy (SP-STM) and spectroscopy. We investigate the transition from antiferromagnetically ordered $\mathrm{Fe}_{1+y}\mathrm{Te}$ via the spin glass phase in $\mathrm{Fe}_{1+y}\mathrm{Se}_{0.1}\mathrm{Te}_{0.9}$ to superconducting $\mathrm{Fe}_{1+y}\mathrm{Se}_{0.15}\mathrm{Te}_{0.85}$. In $\mathrm{Fe}_{1+y}\mathrm{Se}_{0.1}\mathrm{Te}_{0.9}$ we observe an atomic-scale coexistence of superconductivity and short-ranged bicollinear antiferromagnetic order.

Step-Edge-Induced Anisotropic Chiral Spin Coupling in Ultrathin Magnetic Films.

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.

Tuning interfacial spin filters from metallic to resistive within a single organic semiconductor family

A metallic spin filter is observed at the interface between $\mathrm{Al}{\mathrm{q}}_{3}$ adsorbates and a Cr(001) surface. It can be changed to a resistive (i.e., gapped) filter by substituting Cr ions to make $\mathrm{Cr}{\mathrm{q}}_{3}$ adsorbates. Spin-polarized scanning tunneling microscopy and spectroscopy show these spin-dependent electronic structure changes with single molecule resolution. Density functional theory calculations highlight the structural and electronic differences at the interfaces. For $\mathrm{Al}{\mathrm{q}}_{3}$, a charge-transfer interaction with the substrate leads to a metallic spin filter. For $\mathrm{Cr}{\mathrm{q}}_{3}$, direct covalent interactions mix molecular orbitals with the substrate surface state to make two well-separated interfacial hybrid orbitals.

Exploring a Proximity-Coupled Co Chain on Pb(110) as a Possible Majorana Platform.

Linear chains of magnetic atoms proximity coupled to an s-wave superconductor are predicted to host Majorana zero modes at the chain ends in the presence of strong spin–orbit coupling. Specifically, iron (Fe) chains on Pb(110) have been explored as a possible system to exhibit topological superconductivity and host Majorana zero modes [Nadj-PergeS. et al., Science2014, 346, 602−60725278507]. Here, we study chains of the transition metal cobalt (Co) on Pb(110) and check for topological signatures. Using spin-polarized scanning tunneling spectroscopy, we resolve ferromagnetic order in the d bands of the chains. Interestingly, also the subgap Yu–Shiba–Rusinov (YSR) bands carry a spin polarization as was predicted decades ago. Superconducting tips allow us to resolve further details of the YSR bands and in particular resonances at zero energy. We map the spatial distribution of the zero-energy signal and find it delocalized along the chain. Hence, despite the ferromagnetic coupling within the chains and the strong spin-orbit coupling in the superconductor, we do not find clear evidence of Majorana modes. Simple tight-binding calculations suggest that the spin–orbit–split bands may cross the Fermi level four times which suppresses the zero-energy modes.

Spin crossover in Fe(phen)2(NCS)2 complexes on metallic surfaces

In this review, we give an overview on the spin crossover of Fe(phen)2(NCS)2 complexes adsorbed on Cu(100), Cu2N/Cu(100), Cu(111), Co/Cu(111), Co(100), Au(100), and Au(111) surfaces. Depending on the strength of the interaction of the molecules with the substrates, the spin crossover behavior can be drastically changed. Molecules in direct contact with non-magnetic metallic surfaces coexist in both the high- and low-spin states but cannot be switched between the two. Our analysis shows that this is due to a strong interaction with the substrate in the form of a chemisorption that dictates the spin state of the molecules through its adsorption geometry. Upon reducing the interaction to the surface either by adding a second molecular layer or inserting an insulating thin film of Cu2N, the spin crossover behavior is restored and molecules can be switched between the two states with the help of scanning tunneling microscopy. Especially on Cu2N, the two states of single molecules are stable at low temperature and thus allow the realization of a molecular memory. Similarly, the molecules decoupled from metallic substrates in the second or higher layers display thermally driven spin crossover as has been revealed by X-ray absorption spectroscopy. Finally, we discuss the situation when the complex is brought into contact with a ferromagnetic substrate. This leads to a strong exchange coupling between the Fe spin in the high-spin state and the magnetization of the substrate as deduced from spin-polarized scanning tunneling spectroscopy and ab initio calculation.

Spin-Polarized Electron Transfers in Organic/Inorganic Hybrid (Rectifying) Junctions

We formed a heterojunction based on a monolayer of nickel phthalocyanine (NiPc) and another layer of cobalt-doped ZnO (Co@ZnO) nanoparticles on an electrode. By tunneling current through a monolayer of the components that in turn yielded density of states (DOS) of the semiconductors, we located energy levels of NiPc, which were electron donating in nature, and also the band edges of the n-type nanoparticles. Energies of the components in a heterojunction showed that there would be a preferential transfer of electrons from Co@ZnO to NiPc in junctions formed in either sequence (NiPc|Co@ZnO and Co@ZnO|NiPc). We then proceeded to form heterojunctions with the magnetization vector of Co@ZnO and the easy magnetization axis of the metalorganic parallel to each other. When such aligned junctions were characterized with spin-polarized scanning tunneling spectroscopy (SP-STS), we observed that the configuration acted as a unique diode applicable to spin-polarized electrons.

Symmetry-selected spin-split hybrid states inC60/ferromagneticinterfaces

The understanding of orbital hybridization and spin-polarization at the organic-ferromagnetic interface is essential in the search for efficient hybrid spintronic devices. Here, using first-principles calculations, we report a systematic study of spin-split hybrid states of C$_{60}$ deposited on various ferromagnetic surfaces: bcc-Cr(001), bcc-Fe(001), bcc-Co(001), fcc-Co(001) and hcp-Co(0001). We show that the adsorption geometry of the molecule with respect to the surface crystallographic orientation of the magnetic substrate as well as the strength of the interaction play an intricate role in the spin-polarization of the hybrid orbitals. We find that a large spin-polarization in vacuum above the buckyball can only be achieved if the molecule is adsorbed upon a bcc-(001) surface by its pentagonal ring. Therefore bcc-Cr(001), bcc-Fe(001) and bcc-Co(001) are the optimal candidates. Spin-polarized scanning tunneling spectroscopy measurements on single C$_{60}$ adsorbed on Cr(001) and Co/Pt(111) also confirm that both the symmetry of the substrate and of the molecular conformation have a strong influence on the induced spin polarization. Our finding may give valuable insights for further engineering of spin filtering devices through single molecular orbitals.

Electronic and magnetic effects of a stacking fault in cobalt nanoscale islands on the Ag(111) surface

By utilizing spin-polarized scanning tunneling microscopy (STM) and spectroscopy, we observe the coexistence of perpendicularly and in-plane magnetized cobalt nanoscale islands on an Ag(111) surface. The magnetization direction has the relationship with the observed moir\'e-corrugation amplitude on the islands; the islands with stronger moir\'e corrugation show perpendicular magnetization, and the ones with weaker moir\'e corrugation exhibit in-plane magnetization. We calculate the magnetic anisotropy energy for various stackings of a Co nanostructure based on density functional theory, and we reveal that perpendicular magnetic anisotropy is reduced drastically with increasing fcc stacking faults in the Co layer. Simulated STM images reproduce the moir\'e-corrugation difference observed experimentally when the stacking difference between perpendicularly and in-plane magnetized islands is considered. These theoretical analyses strongly suggest that the electronic and magnetic differences between the two types of islands are caused by the presence of fcc stacking faults in the intrinsic hcp stacking of Co.

Spin-Dependent Conductance in Co/C60/Co/Ni Single-Molecule Junctions in the Contact Regime

The spin-dependent conductance in Co/C60/Co/Ni single-molecule magnetic tunnel junctions has been measured by combining spin-polarized scanning tunneling spectroscopy and current-displacement measurements using an ultrahigh vacuum low temperature scanning tunneling microscope (STM) at 5 K. With an electrical contact between molecule and electrodes, the measured junction conductance is 0.02–0.13 G0 in the low-conductance state and 0.12–0.76 G0 in the high-conductance state, respectively. The investigated single-molecule junctions exhibit large tunnel magnetoresistance (TMR) ratios higher than −60%. A variation of TMR from −63% to −94% has been observed due to different Co/Ni electrodes.

Spin-Polarized Scanning Tunneling Spectroscopy of Diluted Magnetic Semiconductor Quantum Dots

We have formed ultrathin films of cobalt-doped ZnO quantum dots (QDs), aligned their magnetic domains, and recorded spin-polarized scanning tunneling spectroscopy (SP-STS) with a nickel tip whose magnetization vector was also aligned. From the SP-STS, we have calculated normalized density of states (NDOS) of the diluted magnetic semiconductor (DMS) QDs. We have observed that the intensity of peaks in NDOS spectra has depended on the mutual orientation of the magnetization vectors of the tip of the scanning tunneling microscope (STM) and the domains of the QDs. A higher intensity in the NDOS spectrum is observed in tip–QD systems having their domains aligned parallel to each other as compared to their antiparallel configuration. We have performed a range of control experiments to validate the observation. We hence infer that the orientation of the magnetization vector or the magnetic state of ferromagnetic QDs relative to that of the tip of a STM can be read or probed by recording SP-STS.

The impact of structural relaxation on spin polarization and magnetization reversal of individual nano structures studied by spin-polarized scanning tunneling microscopy

The application of low temperature spin-polarized scanning tunneling microscopy and spectroscopy in magnetic fields for the quantitative characterization of spin polarization, magnetization reversal and magnetic anisotropy of individual nano structures is reviewed. We find that structural relaxation, spin polarization and magnetic anisotropy vary on the nm scale near the border of a bilayer Co island on Cu(1 1 1). This relaxation is lifted by perimetric decoration with Fe. We discuss the role of spatial variations of the spin-dependent electronic properties within and at the edge of a single nano structure for its magnetic properties.

Interface-assisted molecular spintronics

Molecular spintronics, a field that utilizes the spin state of organic molecules to develop magneto-electronic devices, has shown an enormous scientific activity for more than a decade. But, in the last couple of years, new insights in understanding the fundamental phenomena of molecular interaction on magnetic surfaces, forming a hybrid interface, are presenting a new pathway for developing the subfield of interface-assisted molecular spintronics. The recent exploration of such hybrid interfaces involving carbon based aromatic molecules shows a significant excitement and promise over the previously studied single molecular magnets. In the above new scenario, hybridization of the molecular orbitals with the spin-polarized bands of the surface creates new interface states with unique electronic and magnetic character. This study opens up a molecular-genome initiative in designing new handles to functionalize the spin dependent electronic properties of the hybrid interface to construct spin-functional tailor-made devices. Through this article, we review this subject by presenting a fundamental understanding of the interface spin-chemistry and spin-physics by taking support of advanced computational and spectroscopy tools to investigate molecular spin responses with demonstration of new interface phenomena. Spin-polarized scanning tunneling spectroscopy is favorably considered to be an important tool to investigate these hybrid interfaces with intra-molecular spatial resolution. Finally, by addressing some of the recent findings, we propose novel device schemes towards building interface tailored molecular spintronic devices for applications in sensor, memory, and quantum computing.