Spin-polarized Scanning Tunneling Microscopy Research Articles

Imaging antiferromagnetic domain fluctuations and the effect of atomic scale disorder in a doped spin-orbit Mott insulator.

Correlated oxides can exhibit complex magnetic patterns. Understanding how magnetic domains form in the presence of disorder and their robustness to temperature variations has been of particular interest, but atomic scale insight has been limited. We use spin-polarized scanning tunneling microscopy to image the evolution of spin-resolved modulations originating from antiferromagnetic (AF) ordering in a spin-orbit Mott insulator perovskite iridate Sr3Ir2O7 as a function of chemical composition and temperature. We find that replacing only several percent of lanthanum for strontium leaves behind nanometer-scale AF puddles clustering away from lanthanum substitutions preferentially located in the middle strontium oxide layer. Thermal erasure and reentry into the low-temperature ground state leads to a spatial reorganization of the AF puddles, which nevertheless maintain scale-invariant fractal geometry in each configuration. Our experiments reveal multiple stable AF configurations at low temperature and shed light onto spatial fluctuations of the AF order around atomic scale disorder in electron-doped Sr3Ir2O7.

Distorted 3Q state driven by topological-chiral magnetic interactions

We demonstrate that topological-chiral magnetic interactions can play a key role for magnetic ground states in ultrathin films at surfaces. Based on density functional theory we show that significant chiral-chiral interactions occur in hexagonal Mn monolayers due to large topological orbital moments which interact with the emergent magnetic field. Due to the competition with higher-order exchange interactions superposition states of spin spirals such as the 2Q state or a distorted 3Q state arise. Simulations of spin-polarized scanning tunneling microscopy images suggest that the distorted 3Q state could be the magnetic ground state of a Mn monolayer on Re(0001).

Resolving the spin polarization and magnetic domain wall width of (Nd,Dy)2Fe14B with spin-polarized scanning tunneling microscopy

The electronic structure and the domain wall width of industrial (Nd,Dy)2Fe14B hard magnets were investigated using low-temperature, spin-polarized scanning tunneling microscopy (STM) in ultra-high vacuum. In a first step, atomically clean and flat surfaces were prepared. The flat terraces were separated by monatomic steps. Surface termination was identified as the Fe c layer from atomically resolved STM imaging. The electronic density of states and its spin polarization agree well with ab initio predictions of the Fe c layer. High-resolution spin-polarized STM images allowed to finally resolve the domain wall width w of only 3.2 ± 0.4 nm.

Probing Magnetic Exchange Interactions with Helium.

Controlling and sensing spin polarization of electrons forms the basis of spintronics. Here, we report a study of the effect of helium on the spin polarization of the tunneling current and magnetic contrast in spin-polarized scanning tunneling microscopy (SP STM). We show that the magnetic contrast in SP STM images recorded in the presence of helium depends sensitively on the tunneling conditions. From tunneling spectra and their variation across the atomic lattice we establish that the helium can be reversibly ejected from the tunneling junction by the tunneling electrons. The energy of the tunneling electrons required to eject the helium depends on the relative spin polarization of the tip and sample, making the microscope sensitive to the magnetic exchange interactions. We show that the time-averaged spin polarization of the tunneling current is suppressed in the presence of helium and thereby demonstrate voltage control of the spin polarization of the tunneling current across the tip-sample junction.

Robust perpendicular magnetization of Co nanomagnets against alloy composition

${\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}$ bilayer nanomagnets formed on a Cu(111) substrate with the alloy composition of $x=0$, 0.2, 0.5, 0.8, and 1 were investigated using scanning tunneling microscopy (STM). The islands with $x=0$, 0.2, and 0.5 were further studied using spin-polarized STM. With $x=0.2$, the nanoislands exhibit triangular shapes similar to the pure Co islands with an upward shift in the peak energy of the minority $d$ state. On the other hand, at $x=0.5$, the appearance of the nanoislands evolve to a round shape, and the peak energy is distributed irregularly within the islands. A hexagonal shape and inhomogeneous electronic states similar to pure Fe islands are observed on the $x=0.8$ islands. The magnetic field dependence of the spin-sensitive signal revealed that the perpendicular anisotropy of the ${\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}$ alloy islands is persistent up to half of the Fe alloy composition.

Discovery and characterization of a new type of domain wall in a row-wise antiferromagnet

Antiferromagnets have recently moved into the focus of application-related research, with the perspective to use them in future spintronics devices. At the same time the experimental determination of the detailed spin texture remains challenging. Here we use spin-polarized scanning tunneling microscopy to investigate the spin structure of antiferromagnetic domain walls. Comparison with spin dynamics simulations allows the identification of a new type of domain wall, which is a superposition state of the adjacent domains. We determine the relevant magnetic interactions and derive analytical formulas. Our experiments show a pathway to control the number of domain walls by boundary effects, and demonstrate the possibility to change the position of domain walls by interaction with movable adsorbed atoms. The knowledge about the exact spin structure of the domain walls is crucial for an understanding and theoretical modelling of their properties regarding, for instance, dynamics, response in transport experiments, and manipulation.

Spin-spiral state of a Mn monolayer on W(110) studied by soft x-ray absorption spectroscopy at variable temperature

The noncollinear magnetic state of epitaxial Mn monolayers on tungsten (110) crystal surfaces is investigated by means of soft x-ray absorption spectroscopy, to complement earlier spin-polarized STM experiments. X-ray absorption spectra (XAS), x-ray linear dichroism (XLD) and x-ray magnetic circular dichroism (XMCD) Mn L23-edge spectra were measured in the temperature range from 8 to 300 K and compared to results of fully-relativistic ab initio calculations. We show that antiferromagnetic (AFM) helical and cycloidal spirals give rise to significantly different Mn L23-edge XLD signals, enabling thus to distinguish between them. It follows from our results that the magnetic ground state of a Mn monolayer on W(110) is an AFM cycloidal spin spiral. Based on temperature-dependent XAS, XLD and field-induced XMCD spectra we deduce that magnetic properties of the Mn monolayer on W(110) vary with temperature, but this variation lacks a clear indication of a phase transition in the investigated temperature range up to 300 K - even though a crossover exists around 170 K in the temperature dependence of XAS branching ratios and in XLD profiles.

Odd spin frustration in Cr(001) films thinner than three nanometers revealed by spin-polarized scanning tunneling microscopy

We have studied the surface structure and morphology of epitaxial Cr films on Au(001) substrate with Cr thickness (${d}_{\mathrm{Cr}}$) below 3 nm. We have characterized the films prepared under various growth conditions by using scanning tunneling microscopy and spectroscopy (STM/STS), low-energy electron diffraction, and Auger electron spectroscopy. The good growth conditions of the Cr(001) films, which have atomically flat terraces without surface segregation of Au and with distinct surface states, are obtained by the following two methods. (I) ${d}_{\mathrm{Cr}}=3$ nm film realized by two-step growth, i.e., first 1.5 nm of Cr deposited at room temperature ($\ensuremath{\sim}290$ K) and an additional 1.5 nm Cr deposition at 570 K and (II) ${d}_{\mathrm{Cr}}=1.5$ nm film by room temperature growth and subsequently post-annealing at 470 K. The magnetic imaging of (I) ${d}_{\mathrm{Cr}}=3$ nm Cr(001) film was performed by means of spin-polarized STM/STS. The observed magnetic image indicates that the topological antiferromagnetic (AF) order appears in a series of adjacent terraces and two types of spin frustration caused by screw dislocations lead to modifications of the topological AF order. One type of spin frustration is accompanied by narrow domain walls between two screw dislocations. The other type is a complex spin frustration caused by a cluster of an odd number of screw dislocations. The difference of the two types of spin frustration is explained through a micromagnetic simulation. We have also identified a large spin-frustrated area, consisting of a cluster of multiple number of screw dislocations.

Spin-Polarized Yu-Shiba-Rusinov States in an Iron-Based Superconductor.

Yu-Shiba-Rusinov (YSR) bound states appear when a magnetic atom interacts with a superconductor. Here, we report on spin-resolved spectroscopic studies of YSR states related with Fe atoms deposited on the surface of the topological superconductor FeTe_{0.55}Se_{0.45} using a spin-polarized scanning tunneling microscope. We clearly identify the spin signature of pairs of YSR bound states at finite energies within the superconducting gap having opposite spin polarization as theoretically predicted. In addition, we also observe zero-energy bound states for some of the adsorbed Fe atoms. In this case, a spin signature is found to be absent indicating the absence of Majorana bound states associated with Fe adatoms on FeTe_{0.55}Se_{0.45}.

Distinguishing antiferromagnetic spin sublattices via the spin Seebeck effect

Determining the magnetic structure of antiferromagnets is a fundamental challenge for the basic understanding and development of antiferromagnetic spintronic devices. However, apart from techniques that probe individual spins in antiferromagnets directly (such as spin-polarized scanning tunneling microscopy), there are few techniques that can provide independent information for each antiferromagnetic sublattice. Here, the authors demonstrate that spin Seebeck signals from the antiferromagnetic insulator Cr${}_{2}$O${}_{3}$ are controlled by surface spins, and independently detect the orientation of the two different sublattices purely electrically.

Experimental identification of two distinct skyrmion collapse mechanisms

Magnetic skyrmions are key candidates for novel memory, logic, and neuromorphic computing. An essential property is their topological protection caused by the whirling spin texture as described by a robust integer winding number. However, the realization on an atomic lattice leaves a loophole for switching the winding number via concerted rotation of individual spins. Hence, understanding the unwinding microscopically is key to enhance skyrmion stability. Here, we use spin polarized scanning tunneling microscopy to probe skyrmion annihilation by individual hot electrons and obtain maps of the transition rate on the nanometer scale. By applying an in-plane magnetic field, we tune the collapse rate by up to four orders of magnitude. In comparison with first-principles based atomistic spin simulations, the experiments demonstrate a radial symmetric collapse at zero in-plane magnetic field and a transition to the recently predicted chimera collapse at finite in-plane field. Our work opens the route to design criteria for skyrmion switches and improved skyrmion stability.

Image of Dynamic Local Exchange Interactions in the dc Magnetoresistance of Spin-Polarized Current through a Dopant.

We predict strong, dynamical effects in the dc magnetoresistance of current flowing from a spin-polarized electrical contact through a magnetic dopant in a nonmagnetic host. Using the stochastic Liouville formalism we calculate clearly defined resonances in the dc magnetoresistance when the applied magnetic field matches the exchange interaction with a nearby spin. At these resonances spin precession in the applied magnetic field is canceled by spin evolution in the exchange field, preserving a dynamic bottleneck for spin transport through the dopant. Similar features emerge when the dopant spin is coupled to nearby nuclei through the hyperfine interaction. These features provide a precise means of measuring exchange or hyperfine couplings between localized spins near a surface using spin-polarized scanning tunneling microscopy, without any ac electric or magnetic fields, even when the exchange or hyperfine energy is orders of magnitude smaller than the thermal energy.

Stacking-Dependent Spin Interactions in Pd/Fe Bilayers on Re(0001).

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 0.9nm, 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 1.0nm. Our work shows how higher-order exchange interactions can be tuned at interfaces.

Local probes for charge-neutral edge states in two-dimensional quantum magnets

The bulk-boundary correspondence is a defining feature of topological states of matter. However, for quantum magnets such as spin liquids or topological magnon insulators a direct observation of topological surface states has proven challenging because of the charge-neutral character of the excitations. Here we propose spin-polarized scanning tunneling microscopy as a spin-sensitive local probe to provide direct information about charge neutral topological edge states. We show how their signatures, imprinted in the local structure factor, can be extracted by specifically employing the strengths of existing technologies. As our main example, we determine the dynamical spin correlations of the Kitaev honeycomb model with open boundaries. We show that by contrasting conductance measurements of bulk and edge locations, one can extract direct signatures of the existence of fractionalized excitations and non-trivial topology. The broad applicability of this approach is corroborated by a second example of a kagome topological magnon insulator.

High-resolution tunneling spin transport characteristics of topologically distinct magnetic skyrmionic textures from theoretical calculations

High-resolution tunneling electron spin transport properties (longitudinal spin current (LSC) and spin transfer torque (STT) maps) of topologically distinct real-space magnetic skyrmionic textures are reported by employing a 3D-WKB combined scalar charge and vector spin transport theory in the framework of spin-polarized scanning tunneling microscopy (SP-STM). For our theoretical investigation metastable skyrmionic spin structures with various topological charges (Q=-3,-2,-1,0,1,2) in the (Pt0.95Ir0.05)/Fe/Pd(111) ultrathin magnetic film are considered. Using an out-of-plane magnetized SP-STM tip it is found that the maps of the LSC vectors acting on the spins of the magnetic textures and all STT vector components exhibit the same topology as the skyrmionic objects. In contrast, an in-plane magnetized tip generally does not result in spin transport vector maps that are topologically equivalent to the underlying spin structure, except for the LSC vectors acting on the spins of the skyrmionic textures at a specific relation between the spin polarizations of the sample and the tip. The magnitudes of the spin transport vector quantities exhibit close relations to charge current SP-STM images irrespectively of the skyrmionic topologies. Moreover, we find that the STT efficiency (torque/current ratio) acting on the spins of the skyrmions can reach large values up to ~25 meV/μA (~0.97 h/e) above the rim of the magnetic objects, but it considerably varies between large and small values depending on the lateral position of the SP-STM tip above the topological spin textures. A simple expression for the STT efficiency is introduced to explain its variation. Our calculated spin transport vectors can be used for the investigation of spin-polarized tunneling-current-induced spin dynamics of topologically distinct surface magnetic skyrmionic textures.

Spin-Polarized Scanning Tunneling Microscopy Study of Non-Collinear Magnetic Coupling in Layerwise Antiferromagnetic Mn(001) Ultra-Thin Films on Fe(001) due to Interface Roughening

Spin-Polarized Scanning Tunneling Microscopy Study of Non-Collinear Magnetic Coupling in Layerwise Antiferromagnetic Mn(001) Ultra-Thin Films on Fe(001) due to Interface Roughening

Interlayer magnetism in Fe3−xGeTe2

Fe$_{3-x}$GeTe$_2$ is a layered van der Waals magnetic material with a relatively high ordering temperature and large anisotropy. While most studies have concluded the interlayer ordering to be ferromagnetic, there have also been reports of interlayer antiferromagnetism in Fe$_{3-x}$GeTe$_2$. Here, we investigate the interlayer magnetic ordering by neutron diffraction experiments, scanning tunneling microscopy (STM) and spin-polarized STM measurements, density functional theory plus U calculations and STM simulations. We conclude that the layers of Fe$_{3-x}$GeTe$_2$ are coupled ferromagnetically and that in order to capture the magnetic and electronic properties of Fe$_{3-x}$GeTe$_2$ within density functional theory, Hubbard U corrections need to be taken into account.

Towards skyrmion-superconductor hybrid systems

Spin-polarized scanning tunneling microscopy is used to identify the magnetic state of different thin films on a Re(0001) substrate, which becomes superconducting below 1.7 K. All magnetic films contain an Fe/Ir interface, which is known to facilitate the emergence of non-collinear magnetic order. For Fe monolayers on ultra-thin Ir films of different thickness we find several different atomic-scale magnetic states. For Pd/Fe bilayers we find nano-scale spin spirals as magnetic ground states for Ir thicknesses of three and four atomic layers. In applied magnetic fields skyrmions emerge, and in remanence non-trivial magnetic textures survive. This demonstrates the possibility to prepare skryrmion-hosting magnetic films on superconducting substrates.

Probing the pinning strength of magnetic vortex cores with sub-nanometer resolution

Understanding interactions of magnetic textures with defects is crucial for applications such as racetrack memories or microwave generators. Such interactions appear on the few nanometer scale, where imaging has not yet been achieved with controlled external forces. Here, we establish a method determining such interactions via spin-polarized scanning tunneling microscopy in three-dimensional magnetic fields. We track a magnetic vortex core, pushed by the forces of the in-plane fields, and discover that the core (~ 104 Fe-atoms) gets successively pinned close to single atomic-scale defects. Reproducing the core path along several defects via parameter fit, we deduce the pinning potential as a mexican hat with short-range repulsive and long-range attractive part. The approach to deduce defect induced pinning potentials on the sub-nanometer scale is transferable to other non-collinear spin textures, eventually enabling an atomic scale design of defect configurations for guiding and reliable read-out in race-track type devices.

Discovery of Magnetic Single- and Triple-q States in Mn/Re(0001).

We experimentally verify the existence of two model-type magnetic ground states that were previously predicted but so far unobserved. We find them in Mn monolayers on the Re(0001) surface using spin-polarized scanning tunneling microscopy. For fcc stacking of Mn the collinear row-wise antiferromagnetic state occurs, whereas for hcp Mn a three-dimensional spin structure appears, which is a superposition of three row-wise antiferromagnetic states known as the triple-q state. Density-functional theory calculations elucidate the subtle interplay of different magnetic interactions to form these spin structures and provide insight into the role played by relativistic effects.