Fe Double Layer Research Articles

Stability and magnetic properties of Fe double layers on Ir (111)

We investigate the interplay between the structural reconstruction and the magnetic properties of Fe double layers on Ir (111) substrate using first-principles calculations based on density functional theory and mapping of the total energies on an atomistic spin model. We show that if a second Fe monolayer is deposited on Fe/Ir (111), the stacking may change from hexagonal-close-packed to bcc (110)-like accompanied by a reduction of symmetry from trigonal to centered rectangular. Although the bcc-like surface has a lower coordination, we find that this is the structural ground state. This reconstruction has a major impact on the magnetic structure. We investigate in detail the changes in the magnetic exchange interaction, the magnetocrystalline anisotropy, and the Dzyaloshinskii-Moriya interaction depending on the stacking sequence of the Fe double layer. Based on our findings, we suggest a technique to engineer Dzyaloshinskii-Moriya interactions in multilayer systems employing symmetry considerations. The resulting anisotropic Dzyaloshinskii-Moriya interactions may stabilize higher-order skyrmions or antiskyrmions.

Pb-induced skyrmions in a double layer of Fe on Ir(111)

Spin-polarized scanning tunneling microscopy reveals that the spin-spiral period of a two-atomic-layer-thick Fe film on Ir(111) roughly doubles upon covering the system with a monolayer of Pb. The spin spirals have a zigzag shape and are guided by structural dislocation lines. In contrast to the spin spiral in the bare Fe double layer, the magnetic state of the Pb-covered Fe film responds to an external magnetic field: the spin spiral breaks up into segments, reminiscent of the spin-spiral state in the Fe triple layer on Ir(111), where the external magnetic field induces magnetic skyrmions. This demonstrates that the tuning of noncollinear spin textures is not only possible by interface engineering based on transition metals, but also with other metallic adlayers such as Pb with a filled $d$ shell.

Inducing skyrmions in ultrathin Fe films by hydrogen exposure

Magnetic skyrmions are localized nanometer-sized spin configurations with particle-like properties, which are envisioned to be used as bits in next-generation information technology. An essential step toward future skyrmion-based applications is to engineer key magnetic parameters for developing and stabilizing individual magnetic skyrmions. Here we demonstrate the tuning of the non-collinear magnetic state of an Fe double layer on an Ir(111) substrate by loading the sample with atomic hydrogen. By using spin-polarized scanning tunneling microscopy, we discover that the hydrogenated system supports the formation of skyrmions in external magnetic fields, while the pristine Fe double layer does not. Based on ab initio calculations, we attribute this effect to the tuning of the Heisenberg exchange and the Dzyaloshinsky–Moriya interactions due to hydrogenation. In addition to interface engineering, hydrogenation of thin magnetic films offers a unique pathway to design and optimize the skyrmionic states in low-dimensional magnetic materials.

Design and performance of an ultra-high vacuum spin-polarized scanning tunneling microscope operating at 30 mK and in a vector magnetic field.

We describe the design and performance of a scanning tunneling microscope (STM) that operates at a base temperature of 30 mK in a vector magnetic field. The cryogenics is based on an ultra-high vacuum (UHV) top-loading wet dilution refrigerator that contains a vector magnet allowing for fields up to 9 T perpendicular and 4 T parallel to the sample. The STM is placed in a multi-chamber UHV system, which allows in situ preparation and exchange of samples and tips. The entire system rests on a 150-ton concrete block suspended by pneumatic isolators, which is housed in an acoustically isolated and electromagnetically shielded laboratory optimized for extremely low noise scanning probe measurements. We demonstrate the overall performance by illustrating atomic resolution and quasiparticle interference imaging and detail the vibrational noise of both the laboratory and microscope. We also determine the electron temperature via measurement of the superconducting gap of Re(0001) and illustrate magnetic field-dependent measurements of the spin excitations of individual Fe atoms on Pt(111). Finally, we demonstrate spin resolution by imaging the magnetic structure of the Fe double layer on W(110).

Distance- and spin-resolved spectroscopy of iridium atoms on an iron bilayer

The induced spin polarization of Ir atoms on a ferromagnetic Fe double layer on W(110) has been investigated with spin-polarized scanning tunneling microscopy. An unoccupied state is observed with a spin polarization exceeding $60\phantom{\rule{0.16em}{0ex}}%$ that is inverted with respect to the Fe layer. This inversion is due to the tunneling gap acting as an orbital and spin filter. Distance dependent measurements show that the spin polarization remains approximately constant over the entire experimentally accessible range, from far in the tunneling regime to 1 \AA{} from the point of contact formation. This is corroborated by density functional theory calculations which show that the inversion of spin polarization occurs within 0.5 \AA{} of the adatom.

Guiding Spin Spirals by Local Uniaxial Strain Relief.

We report on the influence of uniaxial strain relief on the spin spiral state in the Fe double layer grown on Ir(111). Scanning tunneling microscopy (STM) measurements reveal areas with reconstruction lines resulting from uniaxial strain relief due to the lattice mismatch of Fe and Ir atoms, as well as pseudomorphic strained areas. Magnetic field-dependent spin-polarized STM measurements of the reconstructed Fe double layer reveal cycloidal spin spirals with a period on the nm scale. Globally, the spin spiral wave fronts are guided along symmetry-equivalent [112̅] crystallographic directions of the fcc(111) substrate. On an atomic scale the spin spiral propagation direction is linked to the [001] direction of the bcc(110)-like Fe, leading to a zigzag shaped wave front. The isotropically strained pseudomorphic areas also exhibit a preferred magnetic periodicity on the nm scale but no long-range order. We find that already for local strain relief with a single set of reconstruction lines a strict guiding of the spin spiral is realized.

First-principles analysis of a homochiral cycloidal magnetic structure in a monolayer Cr on W(110)

The magnetic structure of a Cr monolayer on a W(110) substrate is investigated by means of first-principles calculations based on noncollinear spin density functional theory (DFT). As magnetic ground state we find a long-period homochiral left-rotating spin spiral on top of an atomic-scale antiferromagnetic order of nearest-neighbor atoms. The rotation angle of the magnetic moment changes inhomogeneously from atom to atom across the spiral. We predict a propagation direction along the crystallographic $[001]$ direction with a period length of $|\ensuremath{\lambda}|=14.3\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, which is in excellent agreement with a modulation of the local antiferromagnetic contrast observed in spin-polarized scanning tunneling microscope experiments by Santos et al. [New J. Phys. 10, 013005 (2008)]. We identify the Dzyaloshinskii-Moriya interaction as the origin of the homochiral magnetic structure, competing with the Heisenberg-type exchange interaction and magnetocrystalline anisotropy energy. From DFT calculations we extract parameters for a micromagnetic model and thereby determine a considerable inhomogeneity of the spin spiral, increasing the period length by 6% compared to homogeneous spin spirals. The results are compared to the behavior of a Mn and Fe monolayer and Fe double layer on a W(110) substrate.

Micromagnetic description of the spin spiral in Fe double-layer stripes on W(110)

The surface spin spiral in the Fe double layer on W(110) is investigated within the framework of micromagnetic theory. It is shown that the previously suggested restriction to homogeneous spiral profiles and the consideration of the demagnetizing energy within the concept of an effective magnetic anisotropy do not allow for a consistent description of all experimental observations. After a detailed discussion of the model approaches in the literature, an extended micromagnetic model is proposed. The respective model parameters are derived as fit parameters to experimental data. It is shown that the presented model overcomes the deficiencies of all previous approaches. In particular, it reproduces the hitherto unexplained divergence of the spiral period in narrow Fe double-layer stripes of a width below 15 nm.

Strain relaxation induced spin reorientation in Fe films on W(110)

We studied Fe double layer films on W(110) to investigate the role of strain relaxation in determination of the magnetic ground states. The spin reorientation transition (SRT) between perpendicular and in-plane magnetization takes place with varying density of dislocation lines as imaged with spin polarized scanning tunneling microscopy/spectroscopy. Unexpected domain wall orientation along W[001] and aperiodic magnetic domain patterns suggest that strain relaxation-induced domain has an easy axis along W[001] rather than W[11¯0] in conjunction with the Dzyaloshinskii-Moriya interaction. Furthermore, we confirmed that the SRT can be suppressed by reducing the density of the dislocation lines with lower temperature annealing.

Asymmetric Spin-Wave Dispersion on Fe(110): Direct Evidence of the Dzyaloshinskii-Moriya Interaction

The influence of the Dzyaloshinskii-Moriya interaction on the spin-wave dispersion in an Fe double layer grown on W(110) is measured for the first time. It is demonstrated that the Dzyaloshinskii-Moriya interaction breaks the degeneracy of spin waves and leads to an asymmetric spin-wave dispersion relation. An extended Heisenberg spin Hamiltonian is employed to obtain the longitudinal component of the Dzyaloshinskii-Moriya vectors from the experimentally measured energy asymmetry.

Real-Space Observation of a Right-Rotating Inhomogeneous Cycloidal Spin Spiral by Spin-Polarized Scanning Tunneling Microscopy in a Triple Axes Vector Magnet

Using spin-polarized scanning tunneling microscopy performed in a triple axes vector magnet, we show that the magnetic structure of the Fe double layer on W(110) is an inhomogeneous right-rotating cycloidal spin spiral. The magnitude of the Dzyaloshinskii-Moriya vector is extracted from the experimental data using micromagnetic calculations. The result is confirmed by comparison of the measured saturation field along the easy axis to the respective value as obtained from Monte Carlo simulations. We find that the Dzyaloshinskii-Moriya interaction is too weak to destabilize the single domain state. However, it can define the sense of rotation and the cycloidal spiral type once the single domain state is destabilized by dipolar interaction.

Temperature-driven spin reorientation transition inFe∕Mo(110)nanostructures

Using low-temperature spin polarized scanning tunneling microscopy and spectroscopy, we observed a temperature-driven spin-reorientation transition (SRT) in Fe double layer (DL) nanostructures grown by step-flow growth on Mo(110). Magnetization components along the vertical and horizontal directions were detected with $4∕16$ ML Co∕10 ML $\mathrm{Au}∕\mathrm{W}(110)$ tips with out-of-plane (4 ML Co) and in-plane (16 ML Co) magnetic sensitivities. The magnetic easy axis of the Fe DL nanostructures continuously rotates from the vertical direction at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ to an in-plane direction at $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The rotation angle is independent of the size of the Fe DL nanostructures. Both out-of-plane and in-plane magnetic asymmetries show a strong decrease at the SRT.

Comparative study of magnetization reversal in isolated and strayfield coupled microcontacts

Ferromagnetic microcontacts are key components for future spintronic devices in full metal as well as in hybrid ferromagnet/semiconductor systems. Control of the micromagnetic behavior and especially the reversal process is crucial for the functionality of such devices. We have prepared isolated and strayfield coupled micron sized rectangular Ni∕Fe double layer contacts on silicon nitride membranes. High-resolution magnetic microscopy studies in external fields are performed on identical samples comparing full field magnetic transmission x-ray microscopy and magnetic-force microscopy. The results of both techniques are in good agreement. We find evidence for a strayfield-induced coupling of the domain structure in adjacent contacts in accordance with micromagnetic simulations.

First-principles study of noncommutative band offsets atα−Cr2O3/α−Fe2O3(0001)interfaces

Using first-principles density functional theory, we have modeled the atomic, electronic and magnetic structure of epitaxial interfaces between alpha-hematite and alpha-chromia (corundum structure) in the hexagonal (0001) basal plane. Our model was a superlattice with a period of about 27.5 \AA{}, corresponding to the shortest-period superlattice considered in a recent series of experiments [Chambers et al., Phys. Rev. B 61, 13223 (2000)]. Two different epitaxial interface structures were studied: $(i)$ an oxygen plane separating an Fe double layer from a Cr double layer or $(\mathrm{ii})$ a metal double layer split between Fe and Cr. We found that these two structures are close in total energy but have distinct spin structure and different valence band offsets [chromia above hematite by 0.4 and 0.6 eV for $(i)$ and $(\mathrm{ii}),$ respectively], possibly explaining the experimental noncommutative band offset seen in this system $(0.3\ifmmode\pm\else\textpm\fi{}0.1\mathrm{eV}$ for hematite grown atop chromia, and $0.7\ifmmode\pm\else\textpm\fi{}0.1\mathrm{eV}$ for the reverse).

Temperature dependence of tunnel magnetoresistance

Electric transport through magnetic tunnel junctions (MTJs) has been studied at various temperatures to gain understanding of the transport mechanisms in such devices. Between 15 and 400 K, MTJs with Al2O3 barriers have been tested at low voltage (barrier height: 2.0–2.1 eV, barrier width: 1.5 nm). For the soft-magnetic electrode a sputtered 1 nm Co/6 nm Fe double layer was used. The hard-magnetic electrode is realized with a 1.5 nm Co/1.0 nm Cu/1.0 nm Co system. Antiferromagnetic coupling between the two Co layers leads to a high saturation field. The 1.5 nm Co layer is used as the second electrode of the MTJ. The conductance increases with growing temperature while the tunnel magnetoresistance (TMR) shows a slight decrease. For interpretation of the results, the temperature dependence of direct tunneling, of the hopping conductance via trapped states, and of the interface magnetization have to be taken into consideration. The dominant factor for the TMR is proportional to 1−BT3/2 and follows the temperature dependence of the interface magnetization. The experimental data allow us to separate transport mechanisms and characterize the junction quality. At room temperature the spin-independent hopping conductance of our junctions is calculated to be less than 10% of the total conductance. Concerning the magnetic properties, a ferromagnetic orange-peel coupling corresponding to a field of about 4 Oe (0.3 kA/m) was found at 15 K, which decays exponentially with increasing temperature to less than 0.6 Oe (0.05 kA/m) at 300 K. The coercive field of the soft layer also shows an exponential decay.