Publications by Thorsten Hesjedal

Methane chemical vapor deposition on transition metal/GaAs samples - a possible route to Haeckelite carbon nanotubes?

Surface and Interface Analysis (2011)

MJ Burek, T Hesjedal

An Undergraduate Nanotechnology Engineering Laboratory Course on Atomic Force Microscopy

IEEE Transactions on Education (2010)

D Russo, RD Fagan, T Hesjedal

Magnetic logic element based on an S-shaped Permalloy structure

Applied Physics Letters 96 (2010)

T Hesjedal, T Phung

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. © 2010 American Institute of Physics.

Surface acoustic wave-assisted scanning probe microscopy - A summary

Reports on Progress in Physics 73 (2010)

T Hesjedal

Elastic properties of nanoscopic materials, structures and thin films are important parameters controlling their growth, as well as their optical and electronic properties. Acoustic microscopy is a well-established method for elastic imaging. In order to overcome its micrometer-scale diffraction-limited lateral resolution, scanning probe microscopy-based acoustic near-field techniques have been developed. Among the acoustic modes used for microscopy, surface acoustic waves (SAWs) are especially suited for probing very small and thin objects due to their localization in the vicinity of the surface. Moreover, the study of SAWs is crucial for the design of frequency filter devices as well as for fundamental physical studies, for instance, the probing of composite fermions in two-dimensional electron systems. This review discusses the capabilities and limitations of SAW-based scanning probe microscopy techniques. Particular emphasis is laid on the review of surface acoustic waves and their interaction with elastic inhomogeneities. Scattering, diffraction and wave localization phenomena will be discussed in detail. Finally, the possibilities for quantitative acoustic microscopy of objects on the nanoscale, as well as practical applications, are presented. © 2010 IOP Publishing Ltd.

Investigation of slanted and V-shaped domain walls in MnAs films

Journal of Applied Physics 105 (2009)

R Engel-Herbert, T Hesjedal

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. © 2009 American Institute of Physics.

Magnetic coupling of ferromagnetic stripe arrays: Analytical model for the α-β -phase coexistence regime of MnAs/GaAs(001)

Physical Review B - Condensed Matter and Materials Physics 78 (2008)

R Engel-Herbert, T Hesjedal

We investigate the temperature-dependent hysteresis of the stripe state of MnAs thin films on GaAs(001) in the phase coexistence regime. The underlying magnetic domain structure is described employing an analytic model for stripe arrays with perpendicular anisotropy. In the framework of this model the magnetic properties of the MnAs stripe array can be unraveled as a combined effect of magnetostatic coupling of neighboring ferromagnetic stripes and the tendency to form antiparallel magnetic domains within the individual ferromagnetic stripes. The detailed analysis reveals the balance of demagnetization energy and domain-wall energy for the domain structure. It is capable to quantitatively predict the temperature dependency of the coercive field of MnAs thin films on GaAs(001) in the phase coexistence regime. Further, the analytic model allows for an understanding of the unusual magnetic reversal properties as a consequence of the temperature-driven geometrical variations in the stripe array. Here, it is the energy difference of the single and the multidomain states associated with the geometrical variations, which is the driving factor, rather than the temperature dependence of the magnetic properties themselves. Although the stripe array of MnAs thin films can be in an interstripe as well as in an intrastripe coupling state, the magnetization reversal is entirely determined by interstripe coupling. © 2008 The American Physical Society.

Comparative study of the influence of the solvent on the catalytic growth of carbon nanotubes

Microelectronic Engineering 85 (2008) 156-160

R Engel-Herbert, H Pforte, T Hesjedal

The catalytic growth by chemical vapor deposition is a well-established route to single-wall carbon nanotubes (SWNTs). In this process, the choice and preparation of the metal catalyst determines the nanotube growth. The system Fe/Mo is known to yield a large percentage of SWNTs. In order to make use of SWNTs in electronic or electromechanical devices, the patterned growth relies on lithography techniques like photolithography or electron beam lithography. Many standard lithographic processes, i.e. the combination of photoresist and lift-off procedure, are not compatible with Fe/Mo catalyst solutions, resulting in low SWNT yield. We present a systematic study of the influence of the catalyst solvent on the patterned SWNT growth. Most remarkably, the use of water as a solvent is the basis for integrating SWNT with the established processing techniques due to its compatibility with any lithographic process and the simultaneous high SWNT yield. © 2007 Elsevier B.V. All rights reserved.

Handbook of Magnetism and Advanced Magnetic Materials: Fundamentals and theory ; vol. 2, Micromagnetism ; vol. 3, Novel techniques for characterizing and preparing samples ; vol. 4, Novel materials ; vol. 5, Spintronics and magnetoelectronics

in Handbook of Magnetism and Advanced Magnetic Materials: Fundamentals and theory ; vol. 2, Micromagnetism ; vol. 3, Novel techniques for characterizing and preparing samples ; vol. 4, Novel materials ; vol. 5, Spintronics and magnetoelectronics, (2007)

T Hesjedal, K Ploog

At the basis of future applications of spin electronics are ferromagnetic films that have a Curie temperature above room temperature, a crystal structure that allows for epitaxial growth on common semiconductor surfaces, and a high degree of spin polarization at the Fermi level. A class of ternary compounds, the so-called Heusler alloys, combine these requirements as they are lattice-matched to many compound semiconductors, have a compatible crystal structure (face-centered cubic), and show high Curie temperatures. Moreover, calculations suggested that some Heusler alloys may belong to the magnetic class of half-metals that is characterized by a 100% spin polarization at the Fermi level. We review the work on epitaxial-Heusler alloy films on semiconductor surfaces. Special emphasis is laid on molecular-beam epitaxy (MBE), as this growth method allows for an in situ control of the growth and structure of the material. Taking Co2FeSi on GaAs as an example, the structural and magnetic properties of MBE-grown samples will be discussed.

Micromagnetic properties of epitaxial MnAs films on GaAs surfaces

Physica Status Solidi (C) Current Topics in Solid State Physics 4 (2007) 1763-1766

T Hesjedal, R Engel-Herbert, DM Schaadt, KH Ploog

We present a systematic study of the micromagnetic properties of MnAs deposited by molecular-beam epitaxy on GaAs(001) and GaAs(111)B surfaces. In epitaxial MnAs films, the strain state in MnAs-on-GaAs(001) (anisotropic) and MnAs-on-GaAs(111)B (isotropic) has a strong influence on the magneto-structural phase transition and thus the micromagnetic properties. The ferromagnetic α and the β phase coexist over a wide temperature range exhibiting self-organized, magnetically coupled nanostructures. Independent of the substrate orientation, magnetic flux-closure domain patterns are formed in the basal plane of MnAs. The spatial distribution of the phases in equilibrium (stripes and quasi-hexagonal islands, respectively) stabilizes various magnetic states, which were found experimentally and confirmed by micromagnetic simulations. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.

Growth of carbon nanotubes on GaAs

Materials Letters 61 (2007) 4631-4634

R Engel-Herbert, Y Takagaki, T Hesjedal

Hybrid carbon nanotube-semiconductor systems offer unique properties by combining the advantages of one-dimensional conductors with the broad opportunities of semiconductor technology. Thus, it is desirable to incorporate the nanotube growth in III-V semiconductor systems. We present the directed growth of carbon nanotubes from prepatterned CrNi catalyst structures on GaAs. © 2007 Elsevier B.V. All rights reserved.

Systematic investigation of Permalloy nanostructures for magnetologic applications

Journal of Applied Physics 101 (2007)

R Engel-Herbert, SA Haque, T Hesjedal

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. © 2007 American Institute of Physics.

Three-dimensional magnetic flux-closure domain patterns in MnAs thin films on GaAs(001)

Journal of Applied Physics 101 (2007)

R Engel-Herbert, T Hesjedal, DM Schaadt

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. © 2007 American Institute of Physics.

CVD synthesis and purification of single-walled carbon nanotubes using silica-supported metal catalyst

Materials Letters 61 (2007) 2589-2593

R Engel-Herbert, H Pforte, T Hesjedal

We report the growth of single-walled carbon nanotubes (SWNTs) from silica aerogel by methane chemical vapor deposition (CVD). Bimetallic Fe/Mo supported by amorphous silica forms nanostructures that are catalytically active at the growth temperature. Despite the flexibility gained by using silica as a support matrix, the as-grown nanotube samples need to be further purified for applications in nanodevices. Our experiments show that the treatment in hydrofluoric acid selectively removes the silica matrix,while the amorphous carbon deposits can be selectively removed by oxidation. The influence of the oxidation and etching parameters on the SWNT yield is discussed. The optimized purification procedure renders SWNTs only. © 2006 Elsevier B.V. All rights reserved.

Effects of anisotropic exchange on the micromagnetic domain structures

Physica Status Solidi (B) Basic Research 244 (2007) 1271-1279

DM Schaadt, R Engel-Herbert, T Hesjedal

We have investigated the influence of anisotropic exchange on the micromagnetic domain structure. Three-dimensional simulations based on the Landau-Lifshitz-Gilbert equation were performed incorporating a generalized tensor representation of the exchange following a phenomenological approach. In comparison to isotropic exchange, which is usually used in micromagnetic simulations, anisotropic exchange significantly affected the equilibrium distribution of the magnetization. The formation of slanted domain walls aligning in the direction of stiffest exchange and the deformation of edge domains were the most prominent consequences. In general, we found that anisotropic exchange may have profound effects on magnetic nanostructures. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.

Micromagnetic properties of epitaxial MnAs films on GaAs surfaces

physica status solidi (c) Wiley 4 (2007) 1585-1585

T Hesjedal, R Engel-Herbert, DM Schaadt, KH Ploog

High-aspect ratio patterning of MnAs films

Semiconductor Science and Technology 21 (2006) 1502-1506

W Seidel, KH Ploog, R Engel-Herbert, T Hesjedal

We report the high-aspect ratio patterning of epitaxial MnAs-on-GaAs(0 0 1) films. The control of strain is key since MnAs-on-GaAs(0 0 1) exhibits a strain-stabilized coexistence of two chemically, elastically and magnetically distinct phases forming a self-organized stripe structure over a temperature range of 10-40 °C. Anisotropic plasma etching allows for high-aspect ratios and good reproducibility. Using Ti films as an etch mask, arbitrarily oriented structures can be transferred into films of up to 300 nm thickness. The removal of the masking material is challenging as MnAs reacts with all common acids, alkalis and even water. Optimum results are obtained by etching the Ti mask in hydrofluoric acid at elevated temperatures (>50 °C), where MnAs is entirely in its β-phase. © 2006 IOP Publishing Ltd.

Micromagnetic properties of MnAs-on-GaAs(001) films

Physica Status Solidi (A) Applications and Materials Science 203 (2006) 3574-3580

KH Ploog, L Däweritz, R Engel-Herbert, T Hesjedal

Strained MnAs films on GaAs(001) grown by molecular beam epitaxy exhibit unique micro-magnetic properties due to the strain-mediated coexistence of hexagonal ferromagnetic a-MnAs and orthorhombic paramagnetic β-MnAs arranged in selforganized periodic stripe patterns. To explore the internal structure of the magnetization, which is not accessible by the magnetic imaging techniques AFM and XMCDPEEM, detailed micromagnetic simulations are needed. Otherwise, physically unreasonable models would be developed. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA.

The nature of charged zig-zag domains in MnAs thin films

Journal of Magnetism and Magnetic Materials 305 (2006) 457-463

R Engel-Herbert, DM Schaadt, S Cherifi, E Bauer, R Belkhou, A Locatelli, S Heun, A Pavlovska, J Mohanty, KH Ploog, T Hesjedal

We report on apparently charged domain walls in MnAs-on-GaAs(0 0 1) layers. For head-on domains, described as two domains facing each other with opposite magnetization, the domain walls of ≳ 200 nm thick films exhibit a zig-zag pattern. Depending on the width of the ferromagnetic stripes, which is a function of temperature and thus the strain in the easy axis direction, the zig-zag angle 2 θ increases from 90{ring operator} in the case of wide stripes to 180{ring operator} (i.e., to a straight wall) for narrow stripes. The underlying domain structure was calculated using a three-dimensional micromagnetic simulator. The calculations reveal a number of distinct domain patterns as a result of the system's attempt to reduce its energy through the formation of closure domain-like patterns in the easy plane. A diamond-like state consisting of two intersecting sub-surface domain walls is the underlying magnetic structure resulting in the observed, apparently charged domain walls. The zig-zag pattern of the domain boundary is explained by stray field minimization of the diamond state along the stripe. © 2006 Elsevier B.V. All rights reserved.

Analytical and numerical calculations of the magnetic force microscopy response: A comparison

Journal of Applied Physics 99 (2006)

R Engel-Herbert, DM Schaadt, T Hesjedal

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. © 2006 American Institute of Physics.

Investigation of magnetically coupled ferromagnetic stripe arrays

Applied Physics A: Materials Science and Processing 84 (2006) 231-236

R Engel-Herbert, A Locatelli, S Cherifi, DM Schaadt, J Mohanty, KH Ploog, E Bauer, R Belkhou, S Heun, A Pavlovska, T Leo, T Hesjedal

We studied the magnetic coupling of ferromagnetic, submicron-sized stripes in the material system MnAs on GaAs. A specific coupling state, determined by stripe period and stripe width, can be tuned via film thickness and temperature, respectively. Micromagnetic imaging - in combination with micromagnetic simulations - reveals two coupling regimes. Strong magnetic coupling between the stripes creates micromagnetic domains extending across several stripes, whereas weak coupling allows for demagnetization within individual stripes. This behavior is observed for all investigatedfilm thicknesses, since a stripe geometry leading to a given coupling scenario is a function of temperature.