Fe Stripes Research Articles

Insulator-to-Metallic Spin-Filtering in 2D-Magnetic Tunnel Junctions Based on Hexagonal Boron Nitride.

We report on the integration of atomically thin 2D insulating hexagonal boron nitride (h-BN) tunnel barriers into magnetic tunnel junctions (2D-MTJs) by fabricating two illustrative systems (Co/h-BN/Co and Co/h-BN/Fe) and by discussing h-BN potential for metallic spin filtering. The h-BN is directly grown by chemical vapor deposition on prepatterned Co and Fe stripes. Spin-transport measurements reveal tunnel magneto-resistances in these h-BN-based MTJs as high as 12% for Co/h-BN/h-BN/Co and 50% for Co/h-BN/Fe. We analyze the spin polarizations of h-BN/Co and h-BN/Fe interfaces extracted from experimental spin signals in light of spin filtering at hybrid chemisorbed/physisorbed h-BN, with support of ab initio calculations. These experiments illustrate the strong potential of h-BN for MTJs and are expected to ignite further investigations of 2D materials for large signal spin devices.

Excitations of interface pinned domain walls in constrained geometries

We report a theoretical investigation of the equilibrium pattern and the spectra of head-to-head and Neel domain walls of flat Fe and Py stripes, exchange coupled with a vicinal antiferromagnetic substrate. We show that the domain wall excitation spectrum is tunable by the strength of the interface field. Furthermore, strong interface coupling favors localized wall excitations.

Frustration-driven C4 symmetric order in a naturally-heterostructured superconductor Sr2VO3FeAs

A subtle balance between competing interactions in iron-based superconductors (FeSCs) can be tipped by additional interfacial interactions in a heterostructure, often inducing exotic phases with unprecedented properties. Particularly when the proximity-coupled layer is magnetically active, rich phase diagrams are expected in FeSCs, but this has not been explored yet. Here, using high-accuracy 75As and 51V nuclear magnetic resonance measurements, we investigate an electronic phase that emerges in the FeAs layer below T0 ~ 155 K of Sr2VO3FeAs, a naturally assembled heterostructure of an FeSC and a Mott-insulating vanadium oxide. We find that frustration of the otherwise dominant Fe stripe and V Neel fluctuations via interfacial coupling induces a charge/orbital order in the FeAs layers, without either static magnetism or broken C4 symmetry, while suppressing the Neel antiferromagnetism in the SrVO3 layers. These findings demonstrate that the magnetic proximity coupling stabilizes a hidden order in FeSCs, which may also apply to other strongly correlated heterostructures.

Spin wave surface states in 1D planar magnonic crystals

We have investigated surface spin wave states in 1D planar bi-component magnonic crystals, localized on the surfaces resulting from the breaking of the periodic structure. The two systems have been considered: the magnonic crystal with periodic changes of the anisotropy field in an exchange regime and the magnonic crystal composed of Fe and Ni stripes in a dipolar regime with exchange interactions included. We chose the symmetric unit cell for both systems to implement the symmetry related criteria for existence of the surface states. We investigated also the surface states induced by the presence of perturbation of the surface areas of the magnonic crystals. We showed that the system with modulated anisotropy is a direct analog of the electronic crystal. Therefore, the surface states in both systems have the same properties. For the surface states existing in magnonic crystals in dipolar-exchange regime we demonstrated that the spin waves show distinct differences in comparison to the electronic crystals, which are due to long-range dynamic dipolar interactions. We found that tuning of the strength of magnetization pinning, resulting from the surface anisotropy or dipolar effect, is vitally important for the existence of surface states in magnonic crystals.

Electronic and magnetic properties at the edges of nanostructures in an electric field: ab initio study

State of the art ab initio calculations of the electronic and magnetic properties at the edges of magnetic nanostructures in an external electric field are presented in this paper. Our results for the Fe stripes on Fe(0 0 1) reveal the existence of spin-polarized edge states. A spatially inhomogeneous electronic structure is found at the edge. We demonstrate that the spin-dependent screening density varies greatly at the atomic scale. Tuning of the spin-polarization by the external electric field is demonstrated.

Anisotropic Magnetoresistance of an Epitaxial Fe Stripe Grown on MgO/GaAs

The anisotropic magnetoresistance of a [100] oriented Fe stripe grown on MgO/GaAs was investigated in order to elucidate the magnetization switching of patterned Fe. A 15 nm thick Fe film was epitaxially grown on MgO/GaAs layers using molecular beam epitaxy and a 50 microm x 4 microm shaped Fe stripe was patterned along its magnetic easy axis of [100]. Negative (positive) resistance peaks appear at room (low) temperature in magnetoresistance measurement. A reversal in sign of the peak was evidently observed with decreasing temperature. Such a reversal of resistance peak is caused by the competition between Lorentz force (ordinary) and spin-orbit (extraordinary) dominated scattering processes in a magnetic domain, which is significantly affected by temperature.

Environment-dependent noncollinear magnetic orders and spin-wave spectra of Fe chains and stripes

The ground-state magnetic properties of free-standing Fe wires having linear, zigzag, and rectangular geometries are investigated in the framework of density-functional theory. The stability of various wires geometries and types of magnetic orders is analyzed as a function of the spin-density-wave (SDW) number $q$. In particular, we consider ferromagnetic (FM) and antiferromagnetic (AF) collinear states as well as periodic and alternating noncollinear (NC) states. The relaxed interatomic bond lengths $d$, frozen-magnon dispersion relations $\ensuremath{\Delta}E(q)$, local magnetic moments ${\ensuremath{\mu}}_{i}$, spin-polarized electronic densities of states ${\ensuremath{\rho}}_{d\ensuremath{\sigma}}(\ensuremath{\varepsilon})$, and effective exchange couplings ${J}_{ij}$ are determined. Iron zigzag chains show FM order ($q=0$) at the equilibrium lattice parameter $a$ with $\ensuremath{\Delta}E(q)$ increasing monotonously with $q$. In contrast, biatomic rectangular ladders (RLs) develop spontaneously a spiral SDWs along the wire, keeping FM dimer couplings in the perpendicular direction. This NC order is further stabilized by compressing the wire. However, stretched RLs show FM order. In all cases, strong ${\ensuremath{\mu}}_{i}$ are found within the Wigner-Seitz spheres, which decrease moderately with increasing $q$. The electronic structures of collinear and NC states are analyzed by comparing the density of states ${\ensuremath{\rho}}_{d\ensuremath{\sigma}}(\ensuremath{\varepsilon})$ for different representative values of $q$. Effective exchange-interaction parameters ${J}_{ij}$ between the local moments ${\ensuremath{\mu}}_{i}$ and ${\ensuremath{\mu}}_{j}$ are derived by fitting the ab initio magnon dispersion relation $\ensuremath{\Delta}E(q)$ to a classical Heisenberg model. The interplay between the various competing ${J}_{ij}$ is shown to depend decisively on the lattice parameter and wire geometry. Comparing the ab initio results within a model phase diagram provides useful insights on the magnetic order of Fe wires from a local perspective.

Giant magnetic anisotropy energy and coercivity in Fe island and atomic wire on W(110)

We have directly investigated the giant magnetic anisotropy energy and coercivity of monolayer (ML) Fe islands and stripes on flat and stepped W(110) surfaces using x-ray magnetic circular dichroism. Both for islands and stripes, the magnetic anisotropy energy is $\ensuremath{\sim}$1.0 meV/atom, independent of the coverage below 0.5 ML. On the contrary, the coercive field of the islands rapidly drops from 4.3 T at 0.25 ML to 1.9 T at 0.50 ML, while that of the stripes moderately degrades from 3.5 T at 0.15 ML ($\ensuremath{\sim}$3 atom rows) to 3.0 T at 0.50 ML. We explain the contrastive behavior for the islands and stripes by different nucleation and remagnetization processes. Considering the enhanced Fe moment, we find an unprecedented large value of the energy product $B{H}_{\mathrm{max}}=5$ MJ$/$m${}^{3}$.

Ferromagnetic resonance and magnetooptic study of submicron epitaxial Fe(001) stripes

We present a combined magnetooptic and ferromagnetic resonance study of a series of arrays of single-crystalline Fe stripes fabricated by electron beam lithography on epitaxial Au(001)/Fe(001)/MgO(001) films grown by pulsed laser deposition. The analysis of the films revealed a clear four-fold magnetocrystalline anisotropy, with no significant presence of other anisotropy sources. The use of a large series of arrays, with stripe widths between 140 and 1000 nm and separation between them of either 200 nm or 500 nm, allowed studying their magnetization processes and resonance modes as well as the effects of the dipolar interactions on both. The magnetization processes of the stripes were interpreted in terms of a macrospin approximation, with a good agreement between experiments and calculations and negligible influence of the dipolar interactions. The ferromagnetic resonance spectra evidenced two types of resonances linked to bulk oscillation modes, essentially insensitive to the dipolar interactions, and a third one associated with edge-localized oscillations, whose resonance field is strongly dependent on the dipolar interactions. The ability to produce a high quality, controlled series of stripes provided a good opportunity to achieve an agreement between the experiments and calculations, carried out by taking into account just the Fe intrinsic properties and the morphology of the arrays, thus evidencing the relatively small role of other extrinsic factors.

Erratum: Magnetic Fe stripes created by self-organized MnAs template: Stripe edge pinning and high-frequency properties [Phys. Rev. B80, 155427 (2009)

Received 27 September 2010DOI:https://doi.org/10.1103/PhysRevB.82.209901©2010 American Physical Society

Magnetic Fe stripes created by self-organized MnAs template: Stripe edge pinning and high-frequency properties

Self-organization is an interesting route to the fabrication of nanostructured magnetic materials. Here we show that, near room temperature, an ultrathin Fe film deposited on a suitable MnAs template spontaneously breaks into a ``lateral'' superlattice of magnetic stripes. The magnetic superstructure originates from the temperature-dependent morphological change in the substrate: an epitaxially grown MnAs/GaAs(001) film, whose groove-ridge structure was investigated by scanning tunneling microscopy. Owing to the stray magnetic fields produced by the underlying MnAs template, the Fe stripe domains have opposite magnetizations, and behave essentially as independent magnetic entities because of strong stripe edge pinning. This is shown dramatically in terms of a split microwave resonance that can be controlled with an external magnetic field, as proved by Brillouin light-scattering data and analysis of the Fe spin-wave frequencies. Additionally, the potential for device applications of such lateral magnetic superlattices, displaying an ``inverse'' exchange-spring behavior, is discussed.

Combined Electron Resonance Driven by an All-Oscillating Potential of Patterned Magnets

A novel mechanism is proposed for the phenomenon of combined electron resonance. It is shown that the spatially localized microwave fields of an Fe stripe array mediate the intense electronic transitions involving the changes in both spin and orbital quantum numbers when the electron moves along a cyclotron orbit in a semiconductor (e.g., InGaAs-based) quantum well. This discovery bridges the fields of spintronics and quantum computing, paving the way for conceptually new hybrid devices based on ferromagnetic and semiconductor structured materials.

Thermal coercivity mechanism in Fe nanoribbons and stripes

We investigate the influence of thermally activated process on coercivity values of long Fe nanostripes. By means of the Lagrangian multiplier technique and the micromagnetic approach, we evaluate energy barriers separating the two magnetization states of long Fe nanostripes, varying their width from 30to250nm. As the width of nanostripes decreases, the reversal time, evaluated through the Arrhenius–Neel law, becomes comparable to the measurement time scale (characteristic for typical magnetometer) for fields below the values obtained through zero-temperature micromagnetic approach. We found appreciable variation of the coercivity due to thermal activation for stripe widths below 100nm.

Atomic-scale self-organization of Fe nanostripes on stepped Cu(111) surfaces: Molecular dynamics and kinetic Monte Carlo simulations

Growth of Fe nanostripes on a vicinal Cu(111) surface is investigated on the atomic scale by performing molecular dynamics and kinetic Monte Carlo simulations. We involve in our study the kinetic mechanisms of atomic incorporation recently reported by Mo et al. [Phys. Rev. Lett. 94, 155503 (2005)]. The atomistic processes responsible for the interlayer mass transport and the formation of Fe stripes of 1 ML height are identified. We demonstrate that strain relaxations at steps have a strong impact on the self-assembly of one-dimensional Fe atomic structures on vicinal Cu(111).

Nonreciprocal spin wave spectroscopy of thin Ni–Fe stripes

The authors report on the observation of nonreciprocal spin wave propagation in a thin (∼200nm) patterned Ni–Fe stripe. The spin wave transmission spectrum is measured using a pair of microstrip lines as antennas. The nonreciprocity of surface wave dispersion brought about by an adjacent aluminum ground leads to a nonreciprocal coupling of the antennas. The effects of Ni–Fe film conductivity, thickness, and reflections caused by the lateral confinement of the magnetic stripe are discussed. The nonreciprocity observed in this structure can potentially be used to realize nonreciprocal microwave devices on silicon.

Stray-field-induced modification of coherent spin dynamics

Electron spins in an InGaAs semiconductor quantum well are used as a magnetometer of magnetic stray fields from patterned Fe stripes. Using time-resolved Faraday rotation, the coherent precession of quantum-well spins in the inhomogeneous field below the Fe stripes is measured for varying magnetic fields. Comparing with reference stripes made of Au, we find an enhancement of the spin precession frequency proportional to the Fe magnetization, in line with a decrease of the spin decay time, which is attributed to the inhomogeneous magnetic stray field in the quantum-well layer.

Optical reflectance anisotropy studies of Fe nanostructures grown on vicinal W(110)

AbstractA vicinal W(110) substrate, offcut by 1.4° in the [1$ \bar 1 $0] direction, has been used as a template to grow Fe nanostructures under UHV conditions. For sub‐monolayer coverages, deposition of Fe on the substrate at room temperature, followed by annealing, forms continuous Fe stripes aligned with the step edges, which run along [001]. In situ reflection anisotropy spectroscopy (RAS) between 1.5 eV and 5.0 eV shows the damping of the W(110) surface state with increased Fe coverage. Broad and featureless step and Fe nanostructure contributions are identified in the 4 eV to 5 eV spectral region, but no strong signature of the aligned nanostructures or steps is found, in contrast to recent RAS studies of vicinal Cu(111). Capping by Ag for ex situ characterization was investigated by using an eclipsing mask to produce a checkerboard pattern of 0 to 3 ML Fe coverage in 0.25 ML steps, followed by deposition of 18 ML of Ag at room temperature. RAS of the capped structure is dominated by the response of the Ag interfaces. The W(110)/Ag interface shows a distinctive feature at 3.4 eV, which diminishes rapidly with increasing Fe coverage and may be related to the sharp peak observed at 3.9 eV on clean Ag(110) surfaces. The absence of other significant RAS features in this spectral region that can be clearly associated with the aligned Fe nanostructures indicates that characterization of capped structures by other epioptic techniques, such as optical second‐harmonic generation, needs to be explored. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Step-decorated Ferromagnetic Fe Nanostripes on Pt(997)

We investigate the step decoration growth and magnetic properties of Fe grown on a Pt(997) vicinal single crystal by means of reflection high energy electron diffraction (RHEED), scanning tunneling microscopy (STM), and the surface magneto-optical Kerr effect (SMOKE). Pt(997) has 2 nm wide, (111)-oriented terraces separated by ordered, monoatomic steps along the $[1\overline{1}0]$ direction. The Fe is grown at room temperature as wedges with thickness ranging from 0 to 4 monolayers (ML). RHEED and STM show that Fe stripes form at low coverage due to step decoration. SMOKE data taken along a wedge indicate that the Fe is ferromagnetic above 0.2 ML with its magnetic easy axis canted from surface normal direction. As temperature is increased near the Curie temperature, there is an irreversible spin reorientation to the perpendicular out-of-plane direction. The canting can be understood as arising from a competition between an in-plane, step-induced magnetic anisotropy and a perpendicular surface anisotropy associated with extended terraces due to the polarization of the proximal Pt. Above 1.7 ML the perpendicular easy axis reorients to in-plane, along the step direction, due to the dominance of the shape anisotropy. The magnetic anisotropy energy is extracted by fitting the temperature dependence of the coercivity data.

Magnetic anisotropy versus morphology in Fe films deposited on ultrathin iron silicides

The correlation between growth and magnetic anisotropy of thin Fe films deposited on ultra thin iron silicide template grown on Si(111) is investigated by means of scanning tunnelling microscopy and magneto optical Kerr effect experiments. The morphology of the film strongly depends on the Fe beam orientation relative to the sample normal. Whereas the Fe clusters are randomly distributed at normal incidence, oblique evaporation leads to the formation of elongated Fe islands perpendicular to the Fe beam. These distinct growth modes lead to drastically different magnetic anisotropies. An in-plane six-fold anisotropy magnetic easy axis has been evidenced for films deposited at normal incidence. The films evaporated at oblique incidence show an uniaxial in-plane magnetic easy axis parallel to the Fe stripes. This uniaxial anisotropy originates from long-ranged dipolar interactions. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin waves quantization in patterned exchange-coupled double layers

Behaviour of thermal spin waves (SW) localized on an array of Fe(20 nm)/Au(1 nm)/Fe(20 nm) stripes has been studied. Measurements of spectra of SW resonances on 1.0 and 1.5 μm wide stripes carried out by means of Brillouin light scattering (BLS) technique have revealed strong positive exchange coupling ( A 12 = 0.8 erg / cm 2 ) between the two ferromagnetic layers. An improved analytical theoretical technique, simple but efficient, has been developed. It allows taking into account interlayer exchange interaction, affecting the relative position of the two dispersion branches, surface optic (SO) and surface acoustic (SA), as well as dipolar interactions governing the shape of each dispersion branch and “effective dipolar- pinning conditions” at the stripe edges for relatively thick films. To this end, the terms responsible for dipolar “cross-talk” between non-identical components in two layers, have been added. Theoretical predictions are in good agreement with experimental results.