Publications by Thorsten Hesjedal

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.

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

Studies on acoustically-induced migration in thin layers at GHz frequencies

Sensoren und Messysteme 2006 (2006)

F Kubat, W Ruile, C Eberl, T Hesjedal, M Reindl

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.

Magnetization reversal in MnAs films: Magnetic force microscopy, SQUID magnetometry, and micromagnetic simulations

Physical Review B - Condensed Matter and Materials Physics 73 (2006)

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

The magnetization reversal of MnAs nanowires was studied by magnetic force microscopy (MFM) imaging in conjunction with superconducting quantum interference device magnetometry and micromagnetic simulations. MnAs films on GaAs(001) exhibit a submicron-sized regular array of ferromagnetic and nonmagnetic stripes, where the width of the stripes can be tuned by the temperature. The investigated thin samples show squarelike hysteresis loops, and the corresponding field-dependent MFM measurements confirm a collective flipping of the domains at the coercive field. Thicker samples, as well as thinner samples at higher temperatures, generally exhibit a rounded magnetization curve with a very low remanent magnetization. Based on three-dimensional micromagnetic simulations, the micromagnetic structure as well as the magnetic hysteresis of MnAs films on GaAs(001) is explained in a consistent way. © 2006 The American Physical Society.

Micromagnetic properties of MnAs(0001)/GaAs(111) epitaxial films

Applied Physics Letters 88 (2006) 1-3

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

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. © 2006 American Institute of Physics.

A microscopic view on acoustomigration.

IEEE Trans Ultrason Ferroelectr Freq Control 52 (2005) 1584-1593

T Hesjedal, J Mohanty, F Kubat, W Ruile, LM Reindl

Stress-induced material transport in surface acoustic wave devices, so-called acoustomigration, is a prominent failure mechanism, especially in high-power applications. We used scanning probe microscopy techniques to study acoustomigration of metal structures in-situ, i.e., during the high-power loading of the device. Scanning acoustic force microscopy (SAFM) allows for the simultaneous measurement of the acoustic wavefield and the topography with submicron lateral resolution. High-resolution microscopy is essential as acoustomigration is a phenomenon that not only results in the formation of more macroscopic voids and hillocks but also affects the microscopic grain structure of the film. We present acoustic wavefield and topographic image sequences giving a clear insight into the nature of the film damage on a submicron scale. The 900 MHz test structures were fabricated on 36 degrees YX-lithium tantalate (YX-LiTaO3) and incorporated 420-nm thick aluminium (Al) electrodes. By correlating the acoustic wavefield mapping and the local changes in topography, we confirmed model calculations that predict the correspondence of damage and stress (i.e., hillocks and voids) are preferentially formed in areas of high stress. The way the film is damaged does not significantly depend on the applied power (for typical power levels used in this study). Furthermore, acoustomigration leads to smoother surfaces via lateral grain growth. Another contribution to the grain dynamics comes from the apparent grain rotation in the highly anisotropic stress field of an acoustic wave. Thus, through in-situ scanning probe microscopy techniques, one can observe the initial changes of the grain structure in order to obtain a more detailed picture of the phenomenon of acoustomigration.

Competing magnetic interactions in MnAs studied via thin film domain pattern analysis

Physical Review B - Condensed Matter and Materials Physics 72 (2005)

A Ney, T Hesjedal, KH Ploog

Manganese arsenide is one of the few ferromagnetic metals that can be grown on semiconductor substrates as a thin film with high structural perfection. The coupled magnetic and structural phase transition around 40°C leads to a variety of different phenomena such as the self-organized stripe formation on GaAs(001) substrates or the anisotropic lattice shrinkage. By investigating the domain pattern in the phase coexistence region we provide experimental evidence that the magnetic order is due to competing ferromagnetic double-exchange and antiferromagnetic direct exchange interactions. This scenario corroborates recent theoretical calculations and may explain the frequently observed angle of 38° in the domain pattern of epitaxial MnAs films. © 2005 The American Physical Society.

Qualitative and quantitative analysis of acoustomigration effects in SAW-devices

Microelectronic Engineering 82 (2005) 655-659

F Kubat, W Ruile, C Eberl, T Hesjedal, LM Reindl

Low-loss surface acoustic wave (SAW) filters are widely used in modern mobile phones, especially for front-end applications. Therefore high power durability of the SAW-structure is required. In order to develop an appropriate metallization the mechanism of acoustomigration has to be investigated in more detail. For this purpose, we designed a two-port test resonator which enables us to investigate acoustomigration in thin films. With a standard measuring procedure to quantify the power durability of metallization it is possible to investigate acoustomigration effects in thin layers systematically. For a better understanding of acoustomigration, we analysed the defect structure by different microscopy techniques as a function of the driving power, testing time and temperature. By in situ tests it was possible for the first time to investigate the growing of extrusions and cracks in real time on a submicron scale. We found a correlation between the loading and the measured number of extrusions as well as the frequency shift of the SAW device. © 2005 Elsevier B.V. All rights reserved.

From ferro- To antiferromagnetism via exchange-striction of MnAs/GaAs(001)

Europhysics Letters 72 (2005) 479-485

H Yamaguchi, AK Das, A Ney, T Hesjedal, C Pampuch, DM Schaadt, R Koch

We investigated the stress evolution in single-crystal MnAs films on GaAs(001) upon applying high external magnetic fields in the α/β phase transition regime (10-40 °C) and beyond. Our stress measurements reveal large field-induced lattice distortions at temperatures, where β-MnAs is present, even well above the phase transition (> 40 °C). A quantitative comparison with the field-induced increase of magnetization reveals that the changes in the lattice dimensions can be fully explained by the (reversible) back-transformation of β-MnAs to α-MnAs. Our direction-dependent experiments identify the structural distortions at the phase transition as a volume magnetostriction effect and - due to the persisting magnetocrystalline anisotropy above 40 °C - strongly support an antiferromagnetic state for β-MnAs. © EDP Sciences.

Field dependence of micromagnetic domain patterns in MnAs films

Journal of Applied Physics 98 (2005)

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

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. © 2005 American Institute of Physics.

Variable magnetic field and temperature magnetic force microscopy

Applied Physics A: Materials Science and Processing 81 (2005) 1359-1362

J Mohanty, R Engel-Herbert, T Hesjedal

Magnetic force microscopy (MFM) studies of epitaxial MnAs films on GaAs(001) have been performed as a function of the applied magnetic field and the sample temperature. For this purpose, we combined a stable variable-temperature sample stage with a compact magnet assembly to fit a commercial magnetic force microscope. In order to keep the thermal drift that affects MFM measurements low, we employed a permanent magnet that can be rotated in a yoke assembly guiding the magnetic flux to the sample. © Springer-Verlag 2005.

Tailoring of the structural and magnetic properties of MnAs films grown on GaAs-Strain and annealing effects

Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures 23 (2005) 1759-1768

L Däweritz, C Herrmann, J Mohanty, T Hesjedal, KH Ploog, E Bauer, A Locatelli, S Cherifi, R Belkhou, A Pavlovska, S Heun

MnAs films were deposited by molecular-beam epitaxy on GaAs(001) and GaAs(111)B surfaces. Imaging of the temperature-dependent magnetic structure by x-ray magnetic circular dichroism photoemission electron microscopy, and the comparison with magnetization measurements by superconducting quantum interference device (SQUID) magnetometry, is used to study the impact of the different strain state of MnAs/GaAs(001) and of MnAs/GaAs(111)B films on the phase transition between ferromagnetic α -MnAs and paramagnetic Β -MnAs, the spatial distribution of the two structural and magnetic phases, and the transition temperature. For the isotropically strained MnAs/GaAs(111)B films, the phase coexistence range is much wider than for the anisotropically strained MnAs/GaAs(001) films. The characteristic change of the saturation magnetization with film thickness is found to be a universal property of films with different epitaxial orientation, if at least one MnAs 〈11 2- 0〉 direction is in the film plane. For MnAs/GaAs(001) films this variation is related to the striped coexistence of α and Β MnAs and the changing intra- and inter-stripe magnetic interaction with film thickness and temperature. The magnetic structure of MnAs/GaAs(111)B is more complex due to the existence of three symmetry-equivalent α -phase domains superimposed by a honeycomb-like network of the coexisting Β phase. The magnetic properties (saturation magnetization, domain size) of thin MnAs/GaAs(001) films can be improved by postgrowth annealing. Above a certain film thickness this is inhibited by the complex magnetic structure of the film. © 2005 American Vacuum Society.