Spin-polarized Tip Research Articles

Optimizing tip-surface interactions in ESR-STM experiments

Electron-spin resonance carried out with scanning tunneling microscopes (ESR-STM) is a recently developed experimental technique that is attracting enormous interest on account of its potential to carry out single-spin on-surface resonance with subatomic resolution. Here we carry out a theoretical study of the role of tip-adatom interactions and provide guidelines for choosing the experimental parameters in order to optimize spin resonance measurements. We consider the case of the Fe adatom on a MgO surface and its interaction with the spin-polarized STM tip. We address three problems: first, how to optimize the tip-sample distance to cancel the effective magnetic field created by the tip on the surface spin, in order to carry out proper magnetic field sensing. Second, how to reduce the voltage dependence of the surface-spin resonant frequency, in order to minimize tip-induced decoherence due to voltage noise. Third, we propose an experimental protocol to infer the detuning angle between the applied field and the tip magnetization, which plays a crucial role in the modeling of the experimental results.

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.

No observation of chiral flux current in the topological kagome metal CsV3Sb5

Compounds with kagome lattice usually host many exotic quantum states, including the quantum spin liquid, non-trivial topological Dirac bands and a strongly renormalized flat band, etc. Recently an interesting vanadium based kagome family $A$V$_{3}$Sb$_{5}$ ($A$ = K, Rb, or Cs) was discovered, and these materials exhibit multiple interesting properties, including unconventional saddle-point driving charge density wave (CDW) state, superconductivity, etc. Furthermore, some experiments show anomalous Hall effect which inspires that there might be some chiral flux current states. Here we report scanning tunneling measurements by using spin-polarized tips. Although we have observed clearly the $2a_0\times2a_0$ CDW and $4a_0$ stripe orders, the well-designed experiments with refined spin-polarized tips do not reveal any trace of the chiral flux current phase in CsV$_3$Sb$_5$ within the limits of experimental accuracy. No observation of the local magnetic moment in our experiments may put an upper bound constraint on the magnitude of magnetic moments induced by the possible chiral loop current which has a time-reversal symmetry breaking along $c$-axis in CsV$_{3}$Sb$_{5}$.

Accurate measurement of atomic magnetic moments by minimizing the tip magnetic field in STM-based electron paramagnetic resonance

Electron paramagnetic resonance (EPR) performed with a scanning tunneling microscope (STM) allows for probing the spin excitation of single atomic species with MHz energy resolution. One of the basic applications of conventional EPR is the precise determination of magnetic moments. However, in an STM, the local magnetic fields of the spin-polarized tip can introduce systematic errors in the measurement of the magnetic moments by EPR. We propose to solve this issue by finding tip-sample distances at which the EPR resonance shift caused by the magnetic field of the tip is minimized. To this end, we measure the dependence of the resonance field on the tip-sample distance at different radiofrequencies and identify specific distances for which the true magnetic moment is found. Additionally, we show that the tip's influence can be averaged out by using magnetically bistable tips, which provides a complementary method to accurately measure the magnetic moment of surface atoms using EPR STM.

Mapping out the magneto-Seebeck coefficient

Spintronics In the Seebeck effect, a temperature difference across a device generates voltage. If the thermal gradient is imposed across a magnetic tunnel junction—with two magnetized layers separated by an insulating tunnel barrier—the magnitude of the generated voltage depends on the relative orientation of the magnetization in the two layers. Transport measurements of this so-called magneto-Seebeck tunneling typically reveal only the signal averaged over the device. Friesen et al. created an atomic-scale version of this experiment by using a scanning tunneling microscope with a spin-polarized tip that they scanned across the surface of a magnetic sample. By heating the tip, they were able to map out the spatial dependence of the spin-resolved Seebeck coefficient. Science , this issue p. [1065][1] [1]: /lookup/doi/10.1126/science.aat7234

Magneto-Seebeck tunneling on the atomic scale.

The tunneling of spin-polarized electrons across a magnetic tunnel junction driven by a temperature gradient is a fundamental process for the thermal control of electron spin transport. We experimentally investigated the atomic-scale details of this magneto-Seebeck tunneling by placing a magnetic probe tip in close proximity to a magnetic sample at cryogenic temperature, with a vacuum as the tunneling barrier. Heating the tip and measuring the thermopower of the junction while scanning across the spin texture of the sample lead to spin-resolved Seebeck coefficients that can be mapped at atomic-scale lateral resolution. We propose a spin detector for spintronics applications that is driven solely by waste heat, using magneto-Seebeck tunneling to convert spin information into a voltage that can be used for further data processing.

The design and the performance of an ultrahigh vacuum 3He fridge-based scanning tunneling microscope with a double deck sample stage for in-situ tip treatment

Scanning tunneling microscope (STM) is a powerful tool for studying the structural and electronic properties of materials at the atomic scale. The combination of low temperature and high magnetic field for STM and related spectroscopy techniques allows us to investigate the novel physical properties of materials at these extreme conditions with high energy resolution. Here, we present the construction and the performance of an ultrahigh vacuum 3He fridge-based STM system with a 7 Tesla superconducting magnet. It features a double deck sample stage on the STM head so we can clean the tip by field emission or prepare a spin-polarized tip in situ without removing the sample from the STM. It is also capable of in situ sample and tip exchange and preparation. The energy resolution of scanning tunneling spectroscopy at T = 310 mK is determined to be 400 mK by measuring the superconducting gap with a niobium tip on a gold surface. We demonstrate the performance of this STM system by imaging the bicollinear magnetic order of Fe1+xTe at T = 5 K

Dynamical Negative Differential Resistance in Antiferromagnetically Coupled Few-Atom Spin Chains.

We present the appearance of negative differential resistance (NDR) in spin-dependent electron transport through a few-atom spin chain. A chain of three antiferromagnetically coupled Fe atoms (Fe trimer) was positioned on a Cu_{2}N/Cu(100) surface and contacted with the spin-polarized tip of a scanning tunneling microscope, thus coupling the Fe trimer to one nonmagnetic and one magnetic lead. Pronounced NDR appears at the low bias of 7mV, where inelastic electron tunneling dynamically locks the atomic spin in a long-lived excited state. This causes a rapid increase of the magnetoresistance between the spin-polarized tip and Fe trimer and quenches elastic tunneling. By varying the coupling strength between the tip and Fe trimer, we find that in this transport regime the dynamic locking of the Fe trimer competes with magnetic exchange interaction, which statically forces the Fe trimer into its high-magnetoresistance state and removes the NDR.

Sensing Noncollinear Magnetism at the Atomic Scale Combining Magnetic Exchange and Spin-Polarized Imaging.

Storing and accessing information in atomic-scale magnets requires magnetic imaging techniques with single-atom resolution. Here, we show simultaneous detection of the spin-polarization and exchange force with or without the flow of current with a new method, which combines scanning tunneling microscopy and noncontact atomic force microscopy. To demonstrate the application of this new method, we characterize the prototypical nanoskyrmion lattice formed on a monolayer of Fe/Ir(111). We resolve the square magnetic lattice by employing magnetic exchange force microscopy, demonstrating its applicability to noncollinear magnetic structures for the first time. Utilizing distance-dependent force and current spectroscopy, we quantify the exchange forces in comparison to the spin-polarization. For strongly spin-polarized tips, we distinguish different signs of the exchange force that we suggest arises from a change in exchange mechanisms between the probe and a skyrmion. This new approach may enable both nonperturbative readout combined with writing by current-driven reversal of atomic-scale magnets.

Dynamical Shiba states from precessing magnetic moments in ans-wave superconductor

We study theoretically the dynamics of a Shiba state forming around precessing classical spin in an s-wave superconductor. Utilizing a rotating wave description for the precessing magnetic impurity, we find the resulting Shiba bound state quasi-energy and the spatial extension of the Shiba wavefunction. We show that such a precession pertains to dc charge and spin currents flowing through a normal STM tip tunnel coupled to the superconductor in the vicinity of the impurity. We calculate these currents and find that they strongly depend on the magnetic impurity precession frequency, precession angle, and on the position of the Shiba energy level in the superconducting gap. The resulting charge current is found to be proportional to the difference between the electron and hole wavefunctions of the Shiba state, being a direct measure for such an asymmetry. By dynamically driving the impurity one can infer the spin dependence of the Shiba states in the absence of a spin-polarized STM tip

Spin-polarized tunneling into helical edge states: Asymmetry and conductances

We consider tunneling from the spin-polarized tip into the Luttinger liquid edge state of quantum spin Hall system. This problem arose in the context of the spin and charge fractionalization of an injected electron. Renormalization of the dc conductances of the system is calculated in the fermionic approach and scattering states formalism. In the lowest order of the tunneling amplitude we confirm previous results for the scaling dependence of conductances. Going beyond the lowest order we show that the interaction affects not only the total tunneling rate, but also the asymmetry of the injected current. The helical edge state forbids the backscattering, which leads to the possibility of two stable fixed points in the renormalization group sense, in contrast to the Y-junction between the usual quantum wires.

Preparation of magnetic tips for spin-polarized scanning tunneling microscopy onFe1+yTe

The interplay of electronic nematic modulations, magnetic order, superconductivity and structural distortions in strongly correlated electron materials calls for methods which allow characterizing them simultaneously - to allow establishing directly the relationship between these different phenomena. Spin-polarized STM enables studying both, electronic excitations as well as magnetic structure in the same measurement at the atomic scale. Here we demonstrate preparation of magnetic tips, both ferromagnetic and antiferromagnetic, on single crystals of FeTe. This opens up preparation of spin-polarized tips without the need for sophisticated ultra-high vacuum preparation.

Electronic structure for a topological insulator with a smoothly varying step on surface

We study the penetration depth (PD) of surface state into the bulk, the local density of states (LDOS) and the spin-polarized STM tip tunneling conductance (TG) for a smoothly varying step described by an asymptotic model, on the surface of a three-dimensional (3D) topological insulator (TI). Using curvilinear coordinates for calculation, we find that the PD shows two peaks near step edges with maximum surface curvature and a dip value of flat surface at step middle with zero curvature. In contrast, the LDOS exhibits a double valley (dip) pattern, in which it rapidly declines from the highest value for flat surface to the dip value near step edges, and then shows a small peak at step middle. This interesting small peak is in good agreement with experimentally observed LDOS peak within a step of definite span on a 3D TI surface. Moreover, the calculated surface spin-polarized STM tip TG also confirms the LDOS qualitatively. The findings here may provide a further understanding of 3D TI surface with step line defects.

Bias asymmetry in the conductance profile of magnetic ions on surfaces probed by scanning tunneling microscopy

The conductance profiles of magnetic transition-metal atoms, such as Fe, Co, and Mn, deposited on surfaces and probed by a scanning tunneling microscope (STM), provide detailed information on the magnetic excitations of such nanomagnets. In general, the profiles are symmetric with respect to the applied bias. However, a set of recent experiments has shown evidence for inherent asymmetries when either a normal or a spin-polarized STM tip is used. In order to explain such asymmetries, here we expand our previously developed perturbative approach to electron-spin scattering to the spin-polarized case and to the inclusion of out of equilibrium spin populations. In the case of a magnetic STM tip, we demonstrate that the asymmetries are driven by the nonequilibrium occupation of the various atomic spin levels, an effect that is reminiscent of electron spin transfer. In contrast, when the tip is not spin polarized, such a nonequilibrium population cannot be built up. In this circumstance, we propose that the asymmetry simply originates from the transition metal ion density of states and from the uneven electronic coupling between the scattering magnetic atoms and both the tip and the substrate. These effects are included in the formalism as a nonvanishing real component to the spin-scattering self-energy.

Inelastic transport detection of spin quantum tunneling and spin relaxation in single-molecule magnets in the absence of a magnetic field

In a scanning tunneling microscope configuration (STM), we study theoretically the effects of quantum tunneling of magnetization (QTM) as well as transport-induced spin relaxation on the electron transport through a single-molecule magnet deposited on a metallic surface. It is shown that for the STM with a high spin-polarized tip, the inelastic current is completely determined by the QTM channel at low temperatures and by the spin relaxation channel at higher temperatures, from which the tunnel splitting and spin-relaxation rate can be directly determined in spite of the absence of a magnetic field. The two types of different mechanisms can be distinguished from each other by current and/or noise spectra.

A theoretical analysis of the spin dynamics of magnetic adatoms traced by time-resolved scanning tunneling spectroscopy

Inelastic scanning tunneling microscopy (STM) has recently been shown (Loth et al 2010 Science329 1628) to be extendable to access the nanosecond, spin-resolved dynamics of magnetic adatoms and molecules. Here we analyze this novel tool theoretically by considering the time-resolved spin dynamics of a single adsorbed Fe atom excited by a tunneling current pulse from a spin-polarized STM tip. The adatom spin configuration can be controlled and probed by applying voltage pulses between the substrate and the spin-polarized STM tip. We demonstrate how, in a pump–probe manner, the relaxation dynamics of the sample spin is manifested in the spin-dependent tunneling current. Our model calculations are based on the scattering theory in a wave-packet formulation. The scheme is non-perturbative and, hence, is valid for all voltages. The numerical results for the tunneling probability and the conductance are contrasted with the predictions of simple analytical models and compared with experiments.

Atomic magnetism revealed by spin-resolved scanning tunnelling spectroscopy

This review focuses on recent advances in the magnetic imaging of atoms adsorbed on a nonmagnetic solid surface (adatoms) by means of spin-resolved scanning tunnelling spectroscopy (SP-STS). Magnetic field dependent spectroscopy using magnetically stable spin-polarized tips has been pushed to enable magnetometry on the single atomic-spin limit. We give a detailed review of the technique for the example of Co adatoms on Pt(1 1 1). We discuss the issues concerning the basic magnetic properties of individual adatoms as well as concerning their substrate mediated interactions that have been addressed.

Spin-Polarized Tunneling between Optically Pumped GaAs(110) Surface and Spin-Polarized Tip

We performed spin-polarized scanning tunneling spectroscopy (SP-STS) on the optically pumped GaAs(110) surface with the Fe-coated W tip to investigate the spin-polarized tunneling effect between them. The optical effects that could be superimposed on the spin-dependent signal were carefully investigated with the nonmagnetic W tip. In the SP-STS experiment, the intensity and circular-polarization of the light were simultaneously modulated, and the modulation signals of the tunneling current were compared in order to distinguish the spin dependent component. We demonstrated that the spin-dependent tunneling effect could be detected when we carefully choose the energy range to avoid the parasitic optical effect.

Spin-Polarized Tunneling between Optically Pumped GaAs(110) Surface and Spin-Polarized Tip

Spin-Polarized Tunneling between Optically Pumped GaAs(110) Surface and Spin-Polarized Tip

Measurement of Fast Electron Spin Relaxation Times with Atomic Resolution

Single spins in solid-state systems are often considered prime candidates for the storage of quantum information, and their interaction with the environment the main limiting factor for the realization of such schemes. The lifetime of an excited spin state is a sensitive measure of this interaction, but extending the spatial resolution of spin relaxation measurements to the atomic scale has been a challenge. We show how a scanning tunneling microscope can measure electron spin relaxation times of individual atoms adsorbed on a surface using an all-electronic pump-probe measurement scheme. The spin relaxation times of individual Fe-Cu dimers were found to vary between 50 and 250 nanoseconds. Our method can in principle be generalized to monitor the temporal evolution of other dynamical systems.