Publications by Thorsten Hesjedal

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

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.

Nanofabrication for Surface-Acoustic-Wave Devices

in Nanotechnology focus, Nova Science Pub Inc (2005) 1

T Hesjedal, W Seidel

This book presents the latest research in this frontier field.

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.

Epitaxial MnAs films studied by ferromagnetic and spin wave resonance

Local-Moment Ferromagnets 678 (2005) 97-109

T Tolinski, K Lenz, J Lindner, K Baberschke, A Ney, T Hesjedal, C Pampuch, L Daweritz, R Koch, KH Ploog

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.

Selective etching of epitaxial MnAs films on GaAs(001): Influence of structure and strain

Journal of Applied Physics 98 (2005)

J Mohanty, Y Takagaki, T Hesjedal, L Däweritz, KH Ploog

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. © 2005 American Institute of Physics.

Calculation of the magnetic stray field of a uniaxial magnetic domain

Journal of Applied Physics 97 (2005)

R Engel-Herbert, T Hesjedal

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. © 2005 American Institute of Physics.

Extending the magnetic order of MnAs films on GaAs to higher temperatures

Journal of Magnetism and Magnetic Materials 288 (2005) 173-177

A Ney, T Hesjedal, L Däweritz, R Koch, KH Ploog

Manganese arsenide is a promising candidate for new spintronics applications since it is ferromagnetic at room temperature and can be grown with high epitaxial quality on semiconductors. However, the transition temperature of ∼40 °C is a limiting factor for device applications. Since the coupled magnetic and structural transition is of first order, it is in principle possible to shift the transition temperature by changing external parameters. Here we show that by either applying an external magnetic field or by growing the MnAs films on GaAs(1 1 1), i.e., by changing the epitaxial constraints which are equivalent to external pressure, ferromagnetic order can be stabilized well above the bulk-phase transition temperature. © 2004 Elsevier B.V. All rights reserved.

Calculation and experimental verification of the acoustic stress at GHZ frequencies in SAW resonators.

IEEE Trans Ultrason Ferroelectr Freq Control 51 (2004) 1437-1448

F Kubat, W Ruile, T Hesjedal, J Stotz, U Rösler, LM Reindl

High power applications of Surface Acoustic Wave (SAW) devices may lead to acoustomigration in their thin metal electrodes, which deteriorates the performance or may even destroy the SAW device. It is confirmed in this paper that the mechanism of acoustomigration is caused by the SAW-induced stress in the metal. The quantitative calculation of this stress will be shown in detail, starting from the widely used P-Matrix model as a standard analysis tool. The combination with the partial wave method (PWM) yields the stress distribution inside the metal. This approach provides the flexibility to determine the stresses for any given point in a SAW device, for any input power, frequency, wavetype, device geometry, or metal layer. In order to confirm the absolute values of the stress components, we calculated and measured displacements as a function of input power and frequency.

Magnetic coupling and exchange stiffness in striped MnAs films

Europhysics Letters 68 (2004) 726-732

T Toliński, J Lindner, K Lenz, K Baberschke, A Ney, T Hesjedal, C Pampuch, L Däweritz, R Koch, KH Ploog

We provide the first direct evidence of interactions in a self-organized pattern of coexisting ferromagnetic and paramagnetic MnAs stripe domains as a function of the coupling strength between the stripes (inter) and within one stripe (intra). The value of the intra exchange constant A = 17.7 × 1010 erg/cm is determined unambiguously from spin wave resonance. Additionally, in-plane ferromagnetic resonance measurements reveal the presence of a strong coupling between the neighboring stripes, which can dominate the weak intra-stripe interaction.

Crack-free and conductive Si-doped AlN/GaN distributed Bragg reflectors grown on 6H-SiC(0001)

Applied Physics Letters 85 (2004) 1970-1972

T Ive, O Brandt, H Kostial, T Hesjedal, M Ramsteiner, KH Ploog

The Si-doped n-type AIN/GaN distributed bragg reflectors grown on 6H-SiC(0001) were discussed. It was observed that the structures were crack-free and have a stopband centered around 450 nm with a full width at half maximum between 40 and 50 nm. It was also observed that the maximum measured reflectance was ≥99%. It was found that the vertical conductance measurements at room temperature on the samples shown on ohmic I-V behavior in the entire measurement range.

Semiautomatic wet chemical etching of an array of MnAs nanodots and their magnetic properties

Physica E: Low-Dimensional Systems and Nanostructures 24 (2004) 115-118

Y Takagaki, E Wiebicke, J Mohanty, T Hesjedal, L Däweritz, KH Ploog

Magnetic anisotropy of MnAs-films on GaAs(0 0 1) studied with ferromagnetic resonance

Journal of Magnetism and Magnetic Materials 277 (2004) 159-164

J Lindner, T Toliński, K Lenz, E Kosubek, H Wende, K Baberschke, A Ney, T Hesjedal, C Pampuch, R Koch, L Däweritz, KH Ploog

Thin films of MnAs grown on GaAs(001) show a self-organized structure of coexisting ferromagnetic α- and paramagnetic β-MnAs stripes in the temperature interval from 10 to 40°C. We quantify the magnetic anisotropies of the α-stripes via ferromagnetic resonance and superconducting quantum interference device magnetometry for samples with thicknesses of 57 and 165nm. The easy axis of magnetization is found to be located perpendicular to the stripe direction, whereas the direction parallel to the stripes is a hard one. While the intrinsic anisotropies show a bulk-like behavior and explain the direction of the hard axis, the key to understanding the direction of the easy axis is given by the demagnetizing fields due to the stripe formation. © 2003 Elsevier B.V. All rights reserved.