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Recently, three-dimensional (3D) magnetic textures have moved into the focus of spintronics as both technologically relevant and physically intriguing on a fundamental level. A rich variety of 3D textures is currently being investigated; however, their unambiguous experimental detection and detailed study remains challenging. In this work, a new type of chiral 3D spin-texture, consisting of two antiferromagnetically coupled Néel bobbers, is explored. The static properties of this structure depend on the chirality of the individual bobbers. Different chirality combinations are studied with regard to their phase stability regions by micromagnetic simulations and compared to antiferromagnetically coupled skyrmion tubes. Furthermore, the coupled internal breathing modes are investigated by application of a periodically alternating external magnetic field. The breathing modes of each studied system possess a unique fingerprint, which might allow for the identification of the resonating spin textures via their dispersion curves. [ABSTRACT FROM AUTHOR]

Epitaxial FePt(001)/MgO/FePt magnetic tunnel junctions with L10-FePt electrodes showing perpendicular magnetic anisotropy were fabricated by molecular beam epitaxial growth. Tunnel magnetoresistance ratios of 21% and 53% were obtained at 300K and 10 K, respectively. Our previous work, based on transmission electron microscopy, confirmed a semi-coherent interfacial structure with atomic steps (Kohn et al., APL 102, 062403 (2013)). Here, we show by x-ray photoemission spectroscopy and first-principles calculation that the bottom FePt/MgO interface is either Pt-terminated for regular growth or when an Fe layer is inserted at the interface, it is chemically bonded to O. Both these structures have a dominant role in spin dependent tunneling across the MgO barrier resulting in a decrease of the tunneling magnetoresistance ratio compared with previous predictions. [ABSTRACT FROM AUTHOR]

Incorporation of magnetic dopants into topological insulators to break time-reversal symmetry is a prerequisite for observing the quantum anomalous Hall (QAHE) effect and other novel magnetoelectric phenomena. GdBiTe3 with a Gd:Bi ratio of 1:1 is a proposed QAHE system, however, the reported solubility limit for Gd doping into Bi2Te3 bulk crystals is between ~0.01 and 0.05. We present a magnetic study of molecular beam epitaxy grown (GdxBi1-x)2Te3 thin films with a high Gd concentration, up to x ≈ 0.3. Magnetometry reveals that the films are paramagnetic down to 1.5 K. X-ray magnetic circular dichroism at the Gd M4,5 edge at 1.5 K reveals a saturation field of ~6T, and a slow decay of the magnetic moment with temperature up to 200 K. The Gd3+ ions, which are substitutional on Bi sites in the Bi2Te3 lattice, exhibit a large atomic moment of ~7 μB, as determined by bulk-sensitive superconducting quantum interference device magnetometry. Surface oxidation and the formation of Gd2O3 lead to a reduced moment of ~4/μB as determined by surface-sensitive x-ray magnetic circular dichroism. Their large atomic moment makes these films suitable for incorporation into heterostructures, where interface polarization effects can lead to the formation of magnetic order within the topological insulators. [ABSTRACT FROM AUTHOR]

Synchrotron radiation based techniques provide unique insight into both the element and time resolved magnetization behavior in magnetic spin systems. Here, we highlight the power of two recent developments, utilizing x-ray scattering techniques to reveal the precessional magnetization dynamics of ordered spin structures in the GHz regime, both in diffraction and reflection configurations. Our recently developed diffraction and reflectometry ferromagnetic resonance (DFMR and RFMR) techniques provide novel ways to explore the dynamics of modern magnetic materials, thereby opening up new pathways for the development of spintronic devices. In this paper we provide an overview of these techniques, and discuss the new understanding they provide into the magnetization dynamics in the chiral magnetic structure in Y-type hexaferrite and the depth dependence to the magnetization dynamics in a [CoFeB/MgO/Ta]4 multilayer. [ABSTRACT FROM AUTHOR]

From the perspective of surface science, only the topmost atomic layers usually exhibit physical properties that are different to those of the bulk material, whereas the deeper layers are assumed to be bulk-like and remain largely unexplored. Going beyond conventional diffraction and imaging techniques, we have determined the depth dependence of the full 3D spin structure of magnetic skyrmions below the surface of a bulk Cu2OSeO3 sample using the polarization dependence of resonant elastic x-ray scattering (REXS). While the bulk spin configuration showed the anticipated Bloch type structure, it was found that the skyrmion lattice changes to a Néel twisting (i.e., with a different helicity angle) at the surface within a distance of several hundred nm. The exact surface helicity angle and penetration length of this twist have been determined, revealing the detailed internal structure of the skyrmion tube. It was found that the experimental penetration length of the Néel twisting is 7× longer than the theoretical value given by the ratio of J/D. This indicates that apart from the considered spin interactions, i.e., the Heisenberg exchange interaction J and the Dzyaloshinskii-Moriya interaction D, as well as the Zeeman interaction, other effects must play an important role. The findings suggest that the surface reconstruction of the skyrmion lattice is a universal phenomenon, stemming from the breaking of translational symmetry at the interface. [ABSTRACT FROM AUTHOR]

Three-dimensional topological insulators (TIs) are a perfectly tuned quantum-mechanical machinery in which counterpropagating and oppositely spin-polarized conduction channels balance each other on the surface of the material. This topological surface state crosses the bandgap of the TI and lives at the interface between the topological and a trivial material, such as vacuum. Despite its balanced perfection, it is rather useless for any practical applications. Instead, it takes the breaking of time-reversal symmetry (TRS) and the appearance of an exchange gap to unlock hidden quantum states. The quantum anomalous Hall effect, which has first been observed in Cr-doped (Sb,Bi)2Te3, is an example of such a state in which two edge channels are formed at zero field, crossing the magnetic exchange gap. The breaking of TRS can be achieved by magnetic doping of the TI with transition metal or rare earth ions, modulation doping to keep the electronically active channel impurity free, or proximity coupling to a magnetically ordered layer or substrate in heterostructures or superlattices. We review the challenges these approaches are facing in the famous 3D TI (Sb,Bi)2(Se,Te)3 family and try to answer the question whether these materials can live up to the hype surrounding them. [ABSTRACT FROM AUTHOR]

We investigate the domain structure of submicrometer sized ferromagnetic stripes exhibiting in-plane and out-of-plane magnetized areas with magnetic force microscopy (MFM). Two simulation approaches are used to calculate the observed MFM response. The first relies on an analytical solution for the stray field of a bar magnet and the subsequent modeling of the sample as an arrangement of bar magnets. The MFM response is calculated for a realistic tip shape incorporating a distribution of magnetic dipoles. The second, numerical approach is based on a discretization scheme, breaking the tip-sample problem up into cells and then calculating the energy of the magnetic tip-sample interaction. The MFM responses obtained for the ferromagnetic stripe structure are compared. A discussion of the advantages and limitations of the two methods is given in terms of precision, computing time, and flexibility. The numerical method offers shorter computing times and greater flexibility, opening the door for realistic three-dimensional MFM response simulations. The advantage of the analytical method is the investigation of small structures, as its precision is higher for the comparable computational effort. [ABSTRACT FROM AUTHOR]

We have studied the domain behavior of submicrometer wide ferromagnetic stripes by magnetic force microscopy (MFM) in the presence of an in situ magnetic field. MFM images in the demagnetized state show alternatingly magnetized domains fully extended across the stripe. Moreover, domain structures are found to exhibit a substructure across the stripe. Increasing fields drive out the domain walls of the complex domains first, leaving the alternating domains behind. The remaining magnetization process aligns increasing parts of the domains along the field direction by gradually shrinking the width of oppositely magnetized domains rather than by flipping larger areas at once. Micromagnetic simulations confirm the observed behavior. The simulations reveal that flipping of the domains occurs only when a magnetic pinning center is involved. [ABSTRACT FROM AUTHOR]

Strain in epitaxial MnAs thin films on GaAs(001) substrates plays an important role in the coupled magnetostructural phase transition. As a result of strain, the phase transition from the ferromagnetic α phase to the paramagnetic β phase proceeds over a wide temperature range and the coexisting phases form a periodic stripe array. Employing suitable wet chemical etchants, the two MnAs phases can be etched selectively. Perpendicular to the α-β-stripe structure, the built-up strain relaxes in the course of the etching process by the formation of cracks. The combination of both strain relaxation mechanisms allows for the defined patterning of two-dimensional arrays of nanomagnets. Through micromagnetic investigations, it is possible to identify the location of α- and β-MnAs which helps to clarify two major aspects of the etching process. First, it is possible to determine the etch rates of α- and β-MnAs and follow the complex interplay of strain and phase composition during the etching process. Second, as strain reflects itself in a shifted phase-transition temperature, temperature-dependent micromagnetic studies allow to determine the strain environment of the cracks. [ABSTRACT FROM AUTHOR]

We present an analytic solution for the magnetic field of a bar-shaped permanent magnet. Assuming a constant magnetization, we derive expressions for the stray field in three dimensions. The analytic solutions can be readily applied to field calculation problems for magnetic force microscopy simulations without the need for finite element methods. [ABSTRACT FROM AUTHOR]

We present a detailed study of the influence of ultrasonic surface acoustic waves (SAWs) on point-contact friction. Lateral force microscopy (LFM) and multimode scanning acoustic force microscopy (SAFM) were used to measure and to distinguish between the influence of in-plane and vertical surface oscillation components on the cantilever's torsion and bending. The experiments show that friction can locally be suppressed by Rayleigh-type SAWs. Through the mapping of crossed standing wave fields, the wave amplitude dependence of the friction is visualized within microscopic areas without changing other experimental conditions. Above a certain wave amplitude threshold, friction vanishes completely. We found that the friction reduction effect is caused by the vertical oscillation components of the SAW. Purely in-plane polarized Love waves do not give rise to a significant friction reduction effect. Thus, we conclude that the mechanical diode effect, i.e., the effective shift of the cantilever off of the oscillating surface, is responsible for the SAW-induced lubrication. This explanation is supported by vertical and lateral SAFM measurements: in areas with completely vanishing friction, low frequency vertical cantilever oscillations are still observable, whereas lateral (torsional) cantilever oscillations are no longer excited. Additionally, at very high Rayleigh wave amplitudes an effect of lateral force rectification was observed. It results in a scan direction-independent appearance of the LFM traces. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Si nanowires grow rapidly by chemical vapor deposition on Ti-containing islands on Si surfaces when an abundant supply of Si-containing gaseous precursor is available. The density of wires is approximately the same as the density of the nucleating islands on the Si surface, although at least two different types of islands appear to correlate with very different wire growth rates. For the deposition conditions used, a minority of long, defect-free wires form, along with more numerous wires containing defects. Energy-dispersive x-ray spectroscopy shows that the Ti-containing nanoparticles remain at the tip of the growing wires. The estimated diffusion coefficient of Si in TiSi[sub 2] is consistent with the catalyzing nanoparticle remaining in the solid phase during nanowire growth. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ferromagnetic resonance in thin films and heterostructures is studied by micromagnetic simulation. Using a Fourier transform, we obtain the power spectrum of modes of precession and demonstrate that they agree with experimental results. Additional resonance modes associated with demagnetization effects are observed and studied as a function of the sample dimensions. Sufficiently confined geometries lead to a reorientation of the resonance mode, and a dramatic change in its frequency. Finally, the simulations are extended to exchange-coupled bilayers where energy is transferred between magnetic materials. [ABSTRACT FROM AUTHOR]

The magnetic domain structure of MnAs stripes exhibits a number of domain transitions that are inclined with respect to the system’s easy axis. Among them are laterally confined slanted and V-shaped domain walls, as well as extended zigzag shaped transitions that are running along the stripe axis. The nature of these unusual domain transitions was investigated with the help of micromagnetic simulations and compared with experimental magnetic force microscopy (MFM) images. All types of inclined walls result from the underlying three-dimensional domain structure of the wires. In the domain transition region, the underlying flux closure pattern gets asymmetrically distorted in the cross-sectional wire plane as the system changes its domain types. The results of the simulations are in excellent qualitative agreement with the stray field patterns observed with MFM. [ABSTRACT FROM AUTHOR]

The magnetic microstructure of single-crystalline MnAs films on GaAs(001) has been investigated. Magnetic force microscopy (MFM) reveals a three-dimensional magnetization pattern that is in disagreement with the simple domain picture observed by surface-sensitive magnetic imaging. Here, we present a consistent micromagnetic picture of MnAs thin films in the ferromagnetic stripe phase, which appears in the course of the phase transition. A number of equilibrium magnetization patterns of the stripes are found that are, in fact, based on flux-closure domain patterns in the basal plane of MnAs. The simulation of a stripe array yields excellent agreement with the measured surface magnetization. The experimentally observed stray field contrast was confirmed by MFM contrast simulations based on these equilibrium magnetization patterns. [ABSTRACT FROM AUTHOR]

Magnetic logic gates based on magnetostatically coupled ferromagnetic structures show a great promise for future information processing. The absolute size and shape of the elementary structures determine their intrinsic and interaction properties and thus have to be chosen appropriately in order to allow for binary logic operations. Here, we present a systematic study of the switching behavior, stray field, and coupling of Permalloy nanostructures as a function of geometrical parameters using micromagnetic simulations. We show that bow-tie shaped elements are promising for magnetic logic gates. [ABSTRACT FROM AUTHOR]

Bi2Te3 nanowires are promising candidates for thermoelectric applications. Vaporliquid-solid growth of these nanowires is straightforward, but the traditional Aucatalyzed method is expected to lead to Au contamination and subsequently crystal defects. Here, we present a comparison of the Au-catalyzed growth method with an alternative method using TiO2. We observe that the latter approach results in perfect quintuple layer nanowires, whilst using Au leads to mixed quintuple and septuple layer structures. Despite these differences, we surprisingly find only a negligible effect on their thermoelectric properties, namely conductivity and Seebeck coefficient. This result is relevant for the further optimization and engineering of thermoelectric nanomaterials for device applications. [ABSTRACT FROM AUTHOR]

We present a nanoscale structural and density functional study of the Mn doped 3D topological insulator Bi2Te3. X-ray absorption near edge structure shows that Mn has valency of nominally 2+. Extended x-ray absorption fine structure spectroscopy in combination with electron energy loss spectroscopy (EELS) shows that Mn is a substitutional dopant of Bi and Te and also resides in the van derWaals gap between the quintuple layers of Bi2Te3. Combination of aberration-corrected scanning transmission electron microscopy and EELS shows that Mn substitution of Te occurs in film regions with increased Mn concentration. First-principles calculations show that the Mn dopants favor octahedral sites and are ferromagnetically coupled. [ABSTRACT FROM AUTHOR]

Nanoscale chiral skyrmions in noncentrosymmetric helimagnets are promising binary state variables in high-density, low-energy nonvolatile memory. Skyrmions are ubiquitous as an ordered, single-domain lattice phase, which makes it difficult to write information unless they are spatially broken up into smaller units, each representing a bit. Thus, the formation and manipulation of skyrmion lattice domains is a prerequisite for memory applications. Here, using an imaging technique based on resonant magnetic x-ray diffraction, we demonstrate the mapping and manipulation of skyrmion lattice domains in Cu2OSeO3. The material is particularly interesting for applications owing to its insulating nature, allowing for electric field-driven domain manipulation. [ABSTRACT FROM AUTHOR]

The extraordinary Hall balance (EHB) is a general device concept that harnesses the net extraordinary Hall effect (EHE) arising from two independent magnetic layers, which are electrically in parallel. Different EHB behavior can be achieved by tuning the strength and type of interlayer coupling, i.e., ferromagnetic or antiferromagnetic of varying strength, allowing for logic and memory applications. The physics of the EHE in such a multilayered systems, especially the interface-induced effect, will be discussed. A discrepancy between the magnetization and the Hall effect, called the magneto-Hall difference (MHD) is found, which is not expected in conventional EHE systems. By taking advantage of the MHD effect, and by optimizing the materials structure, magnetoresistance ratios in excess of 40,000% can be achieved at room-temperature. We present a new design, the planar EHB, which has the potential to achieve significantly larger magnetoresistance ratios. [ABSTRACT FROM AUTHOR]

Magnetic skyrmion materials have the great advantage of a robust topological magnetic structure, which makes them stable against the superparamagnetic effect and therefore a candidate for the next-generation of spintronic memory devices. Bulk MnSi, with an ordering temperature of 29.5 K, is a typical skyrmion system with a propagation vector periodicity of ~18 nm. One crucial prerequisite for any kind of application, however, is the observation and precise control of skyrmions in thin films at room-temperature. Strain in epitaxial MnSi thin films is known to raise the transition temperature to 43 K. Here we show, using magnetometry and x-ray spectroscopy, that the transition temperature can be raised further through proximity coupling to a ferromagnetic layer. Similarly, the external field required to stabilize the helimagnetic phase is lowered. Transmission electron microscopy with element-sensitive detection is used to explore the structural origin of ferromagnetism in these Mn-doped substrates. Our work suggests that an artificial pinning layer, not limited to the MnSi/Si system, may enable room temperature, zero-field skyrmion thin-film systems, thereby opening the door to device applications. [ABSTRACT FROM AUTHOR]

Breaking time-reversal symmetry through magnetic doping of topological insulators has been identified as a key strategy for unlocking exotic physical states. Here, we report the growth of Bi2Te3 thin films doped with the highest magnetic moment element Ho. Diffraction studies demonstrate high quality films for up to 21% Ho incorporation. Superconducting quantum interference device magnetometry reveals paramagnetism down to 2K with an effective magnetic moment of ~5 μB/Ho. Angle-resolved photoemission spectroscopy shows that the topological surface state remains intact with Ho doping, consistent with the material's paramagnetic state. The large saturation moment achieved makes these films useful for incorporation into heterostructures, whereby magnetic order can be introduced via interfacial coupling. [ABSTRACT FROM AUTHOR]

We report on the controlled modification of relaxation parameters and magnetic moments of epitaxial Fe thin films through Dy doping. Ferromagnetic resonance measurements show that an increase of Dy doping from 0.1% to 5% gives a tripling in Gilbert damping, and more importantly a strongly enhanced anisotropic damping that can be qualitatively understood through the slow-relaxing impurity model. X-ray magnetic circular dichroism measurements show a pronounced suppression of the orbital moment of the Fe with Dy doping, leading to an almost threefold drop in the orbital to spin moment ratio, ml/ms. Doping with Dy can therefore be used to control both dynamic and static properties of thin ferromagnetic films for improved performance in spintronics device applications, mediated through the antiferromagnetic interaction of the 4f and 3d states. [ABSTRACT FROM AUTHOR]

We present a miniaturized molecular beam epitaxy (miniMBE) system with an outer diameter of 206 mm, optimized for flexible and high-throughput operation. The three-chamber system, used here for oxide growth, consists of a sample loading chamber, a storage chamber, and a growth chamber. The growth chamber is equipped with eight identical effusion cell ports with linear shutters, one larger port for either a multi-pocket electron beam evaporator or an oxygen plasma source, an integrated cryoshroud, retractable beam-flux monitor or quartz-crystal microbalance, reflection high energy electron diffraction, substrate manipulator, main shutter, and quadrupole mass spectrometer. The system can be combined with ultrahigh vacuum (UHV) end stations on synchrotron and neutron beamlines, or equivalently with other complex surface analysis systems, including low-temperature scanning probe microscopy systems. Substrate handling is compatible with most UHV surface characterization systems, as the miniMBE can accommodate standard surface science sample holders. We introduce the design of the system, and its specific capabilities and operational parameters, and we demonstrate the epitaxial thin film growth of magnetoelectric Cr2O3 on c-plane sapphire and ferrimagnetic Fe3O4 on MgO [ABSTRACT FROM AUTHOR]

Cadmium arsenide (Cd3As2) is a material well-known for its very high room-temperature carrier mobility. Recently, it has also been shown to be a three-dimensional Dirac semimetal--the three-dimensional analogue of graphene. Here, we present a detailed structural study of the self-catalyzed growth of Cd3As2 nanowires. The crystal structure is confirmed using X-ray diffraction and transmission electron microscopy. Scanning electron microscopy and energy-dispersive X-ray spectroscopy are used to gain insight into the vapor-solid growth mechanism. The role of group II and V elements is reversed in contrast to III-V and other II-V systems, and the tips are found to be As-rich. [ABSTRACT FROM AUTHOR]

We report the growth of Mn-doped Bi2Se3 thin films by molecular beam epitaxy (MBE), investigated by x-ray diffraction (XRD), atomic force microscopy (AFM), SQUID magnetometry and x-ray magnetic circular dichroism (XMCD). Epitaxial films were deposited on c-plane sapphire substrates by co-evaporation. The films exhibit a spiral growth mechanism typical of this material class, as revealed by AFM. The XRD measurements demonstrate a good crystalline structure which is retained upon doping up to ~7.5 atomic-% Mn, determined by Rutherford backscattering spectrometry (RBS), and show no evidence of the formation of parasitic phases. However an increasing interstitial incorporation of Mn is observed with increasing doping concentration. A magnetic moment of 5.1 μβ/Mn is obtained from bulk-sensitive SQUID measurements, and a much lower moment of 1.6 μβ/Mn from surface-sensitive XMCD. At ~2.5 K, XMCD at the Mn L2,3 edge, reveals short-range magnetic order in the films and indicates ferromagnetic order below 1.5 K. [ABSTRACT FROM AUTHOR]

We present the catalyst-free growth of binary Bi2Te3 topological insulator nanostructures on c-plane sapphire substrates by molecular beam epitaxy. Dense arrays of single-crystalline nanostructures, growing along the [110] direction, are obtained for substrate temperatures ranging from -- 180 °C to 260 °C. The growth rate and shape of the nanostructures are highly temperaturedependent. The microscopic study of the nanostructures and their relationship to the underlying Bi2Te3 thin film gives an insight into the growth mechanism. [ABSTRACT FROM AUTHOR]

Materials with layered van der Waals crystal structures are exciting research topics in condensed matter physics and materials science due to outstanding physical properties associated with their strong two dimensional nature. Prominent examples include bismuth tritelluride and triselenide topological insulators (TIs), which are characterized by a bulk bandgap and pairwise counterpropagating spin-polarized electronic surface states. Angle-resolved photoemission spectroscopy (ARPES) of ex-situ grown thin film samples has been limited by the lack of suitable surface preparation techniques. We demonstrate the shortcomings of previously successful conventional surface preparation techniques when applied to ternary TI systems which are susceptible to severe oxidation. We show that in-situ cleaving is a simple and effective technique for preparation of clean surfaces on ex-situ grown thin films for high quality ARPES measurements. The method presented here is universally applicable to other layered van der Waals systems as well. [ABSTRACT FROM AUTHOR]

We present a comparative study of TiO2-based and Au catalysts for the physical vapor deposition of (Bi1-xSbx)2Se3 topological insulator nanowires. The standard Au nanoparticle catalyst was compared to five TiO2 nanoparticle based catalysts (anatase, rutile, P-25, high surface area anatase, and TiO2 supported Au particles). The use of Au nanoparticles seriously harms the properties of nanowires, thereby limiting their application. In contrast, TiO2 based catalysts lead to the residue-free growth of nanowires with a higher degree of crystallinity. Homogeneous nanowire ensembles are achieved with the mixed phase P-25 catalyst, and a possible growth mechanism is proposed. [ABSTRACT FROM AUTHOR]

Thin film GdBiTe3 has been proposed as a candidate material in which to observe the quantum anomalous Hall effect. As a thermal non-equilibrium deposition method, molecular beam epitaxy (MBE) has the ability to incorporate large amounts of Gd into Bi2Te3 crystal structures. High-quality rhombohedral (GdxBi1-x)2Te3 films with substitutional Gd concentrations of x ≤ 0.4 were grown by MBE. Angle-resolved photoemission spectroscopy shows that the topological surface state remains intact up to the highest Gd concentration. Magnetoresistance measurements show weak antilocalization, indicating strong spin orbit interaction. Magnetometry reveals that the films are paramagnetic with a magnetic moment of 6.93 μB per Gd3+ ion. [ABSTRACT FROM AUTHOR]

Magnetic devices have shown the potential to be used not only as storage elements but also as nonvolatile and programmable logic devices. We present a magnetic logic device element—the S state element—that consists of a single magnetic layer. Its output can be controlled by orthogonal magnetic inputs. The reconfigurable logic element can be easily integrated with common magnetoresistive device concepts, such as spin valves or magnetic tunnel junctions. Using Permalloy as an example, we demonstrate the feasibility of magnetologic operation through micromagnetic simulations. [ABSTRACT FROM AUTHOR]

The micromagnetic properties of MnAs thin films grown on the (111)B-oriented GaAs surface have been investigated. Compared to films grown on (001) surfaces, these films exhibit completely different domain patterns, as the c axis of the hexagonal unit cell is oriented normal to the surface. In the course of the first order phase transition, ferromagnetic α-MnAs forms a network of quasihexagonal areas separated by β-MnAs. We present an analysis of the micromagnetic properties based on imaging and simulations. We observe closure domains that either appear as a vortex-like state or a stripe structure. [ABSTRACT FROM AUTHOR]

Over a wide temperature range of 30 °C around room temperature, MnAs films on GaAs(001) semiconductor substrates break up into ordered arrays of submicron-sized ferromagnetic α and paramagnetic β wires. Both the hard and the easy axis of magnetization (perpendicular to the wires) lie in the film plane and a large variety of complex domain patterns are found in micromagnetic investigations with magnetic force microscopy (MFM). A systematic analysis of the domain configurations is given and the most likely configurations are identified through MFM contrast simulations. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present measurements of the influence of local strain on the phase transition behavior of epitaxial MnAs films on GaAs(001). As shown previously, stripes of ferromagnetic α-MnAs and paramagnetic β-MnAs coexist around room temperature. Temperature-dependent atomic force and magnetic force microscopy reveals that the characteristic temperature T[sup *], at which the as-grown films transform to the paramagnetic β-phase, is locally shifted up towards the value of unstrained bulk MnAs. The film areas exhibiting a higher T[sup *] were identified as regions in which the strain in the MnAs film was allowed to relax. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

From highly sensitive superconducting quantum interference device magnetometry and magnetic force microscopy, we deduce a small out-of-plane magnetization component of MnAs/GaAs(001) films. Its temperature dependence is substantially different from the dominating in-plane magnetization, particularly as it is still detectable above the phase transition temperature of MnAs films. Our measurements indicate that the out-of-plane component is due to small isolated magnetic “grains” within the film. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present variable-temperature magnetic force microscopy (VT-MFM) studies of epitaxially grown MnAs films on GaAs(001). Around a critical temperature of T[sub c]=40 °C, the MnAs film undergoes a first order structural phase transition. Due to the strain involved, ferromagnetic a-MnAs and paramagnetic β-MnAs phases coexist as stripes along MnAs[0001]. The dimensions of the α-phase change from isolated dots at higher temperatures to well ordered stripes at lower temperatures. VT-MFM allows a close look at the evolution of domain patterns of MnAs micromagnets. [ABSTRACT FROM AUTHOR]

We examine the scattering of surface acoustic waves (SAWs) by single dots, periodic and locally damped two-dimensional dot lattices. Employing the scanning acoustic force microscope, SAW fields are imaged with nanometer resolution. We study the influence of a roughly wavelength-sized single dot on SAW diffraction. In order to distinguish between forward- and backscattered components, we insonify the dot with the pump and probe beam under 0° and 90°. We furthermore analyze the SAW diffraction by a regular dot array. The wave field appears to be localized around the dots. Adding surface distortions, the regular SAW localization pattern brakes down in the vicinity of the distortion. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Examines the submicron visualization of surface acoustic wave fields within interdigital acoustic wave devices. Use of scanning acoustic force microscope to detect surface acoustic waves; Dependence of deflection on the amplitude of the surface oscillation; Influence of mass loading on oscillation amplitude.

A high frequency oscillating Au (111) surface was measured with atomic resolution using a modified scanning tunneling microscope. On the atomic scale propagating surface acoustic waves lead to oscillations of atoms on elliptical trajectories, with the axes being determined by the material parameters of the surface. Since those oscillation frequencies are much higher than the scan frequencies the topography contrast is reduced. This basic problem is solved by measuring a stroboscopic snapshot seeing a defined state of oscillation. The atomic resolution of the phase and the amplitude contrast is explained by the superposition of the surface topography and the oscillation trajectory. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Magnetic reversal in a (110)-oriented [DyFe2(60 Å)/YFe2(240 Å)]×15 multilayer film is investigated using magnetometry, micro-magnetic modeling, and element-specific soft x-ray magnetic circular dichroism. At temperatures between ∼60 and 120 K, the magnetic reversal involves a two-step process. It is shown that the reversal mechanism can be described as switching from an in-plane [001] reversed anti-ferromagnetic state, to an out-of-plane [100] transverse spring exchange state, and finally to an in-plane [001] magnetic exchange spring state. [ABSTRACT FROM AUTHOR]

We present an experimental method for the high-resolution imaging of the excitation and propagation of surface acoustic waves (SAWs) on anisotropic piezoelectric substrates. By employing a scanning acoustic force microscope (SAFM), we are able to image acoustic waves that are excitable by a single circular electrode pair source through the mixing with well-defined reference plane waves. We show amplitude and phase images of the point-source wave field, containing the angular dependence of the phase velocity of these modes, as well as their electromechanical coupling strength. The SAFM allows easy access to acoustic material properties, which are important for the design of commercial SAW devices. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present scanning acoustic force microscopy (SAFM) mixing experiments of differently polarized surface acoustic waves (SAW) with noncollinear propagation directions. The phase velocities of the SAWs are measured at a submicron lateral scale, employing a multimode SAFM that is capable of detecting the wave's normal and in-plane oscillation components. Hereby, the down conversion of the surface oscillations into cantilever vibrations due to the nonlinearity of the tip-sample interaction is utilized. The simultaneous determination of the phase velocities within a microscopic sample area is demonstrated for the mixing of Rayleigh and Love waves on the layered system SiO[sub 2]/ST-cut quartz. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a scanning acoustic force microscope (SAFM) for the study of surface acoustic wave (SAW) phenomena on the submicron lateral scale. Until now, SAWs with in-plane oscillation components could only be studied effectively via nonvanishing out-of-plane oscillation contributions. By operating the microscope in lateral force mode, where both bending and torsion of the cantilever are detected, additional amplitude-dependent signals are found, which are due to the interaction with purely in-plane polarized surface oscillations. To demonstrate the capabilities of this type of SAFM, Love waves were studied on the surface of layers deposited on ST-cut quartz with SAW propagation perpendicular to the crystal X-axis. The phase velocity of the wave as well as the amplitude of a standing wave field was measured and compared to calculated values. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The intrinsic stress and morphology of the Stranski–Krastanow system Ge/Si(111) have been investigated at deposition temperatures of 700–950 K. In a broad range of intermediate temperatures, only one distinct decline of stress is observed at the onset of three-dimensional islanding. Supported by a recent transmission electron microscopy study, the results demonstrate that the strain of Ge/Si(111), where the substrate surface in contrast to Ge/Si(001) is the glide plane for dislocations, is relieved by incorporation and continuous rearrangement of dislocations during the island stage. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]