Publications by Thorsten Hesjedal


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


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.


Nanoacoustics – High-Frequency Acoustic Wave Fields under the Microscope

in Science, Technology and Education of Microscopy: an Overview, Formatex 1 (2004) 9

T Hesjedal


High frequency surface acoustic waves under the microscope

Humboldt-Spektrum 3-4 (2004) 62-

R Koch, T Hesjedal, KH Ploog


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.


Evolution of stress and magnetism during the first-order phase transition of MnAs/GaAs(0 0 1)

Journal of Magnetism and Magnetic Materials 272-276 (2004) 1924-1925

A Ney, AK Das, C Pampuch, T Hesjedal, M Kästner, L Däweritz, R Koch, KH Ploog

We have investigated epitaxial MnAs/GaAs(0 0 1) films in the phase transition region between 10°C and 40°C where hexagonal, ferromagnetic α-MnAs and paramagnetic, orthorhomic β-MnAs coexist. Quantitative stress measurements with a cantilever beam magnetometer reveal that during the phase transition the atomic distances change only in the vertical direction providing insight into the nature of the exchange coupling mechanism. © 2003 Elsevier B.V. All rights reserved.


Nature of the magnetic and structural phase transition in MnAs/GaAs(001)

Physical Review B - Condensed Matter and Materials Physics 69 (2004)

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

In the bulk the structural and magnetic phase transitions between a- and β-MnAs are coupled and proceed abruptly by first order. In heteroepitaxially constrained MnAs/GaAs(001) films, on the other hand, the two phases coexist at 10-40°C and the magnetization decreases continuously with temperature. We present detailed magnetic measurements on MnAs/GaAs(001) in the phase transition region covering the ferromagnetic a phase, the paramagnetic β phase and the α + β coexistence region. The coexistence of two magnetically distinguishable types of a-MnAs around the onset of the coexistence region at 10°C, the occurrence of metastable magnetic properties, and the absence of a Curie-Weiss peak in the susceptibility point to a first order of the magnetic phase transition also in thin films. The two types of a-MnAs are characterized by different coercivity and magnetic anisotropy, the latter due to a change in shape anisotropy of the striped phase. © 2004 The American Physical Society.


Surface-acoustic-wave transducers for the extremely-high-frequency range using AlN/SiC(0001)

Semiconductor Science and Technology 19 (2004) 256-259

Y Takagaki, T Hesjedal, O Brandt, KH Ploog

The fast sound propagation in AlN layers is demonstrated to enable generation of surface acoustic waves (SAWs) by interdigital transducers at a frequency beyond 30 GHz. While scaling down the wavelength of the transducers fabricated on AlN/SiC structures and the thickness of the AlN top layer to raise the operation frequency, the excitation of SAWs becomes intricate due to the weak electromechanical coupling in SiC. We examine the dependence of the feasibility of SAW generation on the AlN layer thickness.


Multimode and multifrequency gigahertz surface acoustic wave sensors

Applied Physics Letters 84 (2004) 1407-1409

W Seidel, T Hesjedal

A simple surface acoustic wave sensor system was discussed. This system combines the advantages of multifrequency/multimode operation and GHz frequencies in a single acoustic device structure. The sensor system is based on floating electrode unidirectional transducers that allow efficient excitation of two modes and up to 48 harmonics. The basic operation of sensor system was tested by applying it to humidity sensing without a sensitive layer.


Understanding the submicron domain structure of MnAs thin films on GaAs(001): Magnetic force microscopy measurements and simulations

Applied Physics Letters 84 (2004) 1132-1134

R Engel-Herbert, J Mohanty, A Ney, T Hesjedal, L Däweritz, KH Ploog

The domain patterns of ferromagnetic MnAs wires on GaAs(001) were investigated by atomic force microscopy (AFM), magnetic force microscopy (MFM) and SQUID magnetometry. The MnAs films break up into ordered arrays of submicron-sized ferromagnetic α and paramagnetic β wires on GaAs(001) substrates. A variety of complex domain patterns were found in micromagnetic investigations. A strong uniaxial in-plane anisotropy with the easy and hard axis of magnetization being in plane and perpendicular to each other was exhibited by MnAs.


Distribution of the dynamic strain and stress components within a layered film of A SAW resonator on LiTaO<inf>3</inf>

Proceedings of the IEEE Ultrasonics Symposium 1 (2003) 312-315

Kubat, W Ruile, T Hesjedal, J Stotz, U Rösler, L Reindl

Based on recent reports about the acoustical power distribution in SAW resonators we present an analytical method to determine the distribution of the dynamic strain and stress components in SAW resonators on LiTaO3. This enables us to calculate the absolute strain and stress values for each point in the layer of a resonator for any driving condition and frequency. The SAW resonator is described by a P-Matrix based model, which gives us the distribution of the potential power and the resulting energy density. For calculation of the relative strain and stress values we used the Partial Wave Method. Using the correlation between the total acoustic power and the energy density distribution normal to the substrate surface, we can calculate the strain and stress values for a given input power. For the direct experimental verification of our calculations we measured the SAW induced displacements as a function of input power.


Distribution of the dynamic strain and stress components within a layered film of a SAW resonator on LiTaO<inf>3</inf>

Proceedings of the IEEE Ultrasonics Symposium 2 (2003) 1149-1152

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

Based on recent reports about the acoustical power distribution in SAW resonators we present an analytical method to determine the distribution of the dynamic strain and stress components in SAW resonators on LiTaO3. This enables us to calculate the absolute strain and stress values for each point in the layer of a resonator for any driving condition and frequency. The SAW resonator is described by a P-Matrix based model, which gives us the distribution of the potential power and the resulting energy density. For calculation of the relative strain and stress values we used the Partial Wave Method. Using the correlation between the total acoustic power and the energy density distribution normal to the substrate surface, we can calculate the strain and stress values for a given input power. For the direct experimental verification of our calculations we measured the SAW induced displacements as a function of input power.


Multi-frequency and multi-mode GHz surface acoustic wave sensor

Proceedings of the IEEE Ultrasonics Symposium 2 (2003) 1408-1411

W Seidel, T Hesjedal

We present a novel surface acoustic wave sensor, incorporating the advantages of multi-frequency and multi-mode operation in a single acoustic device structure. SAW sensors are commonly based on the change of the effective elastic constants or the mass loading of the device due to a different sensor environment. Usually, single acoustic modes at a fixed frequency are employed that are able to detect certain chemical species depending on the coating in the wave's propagation path. Analyzing multiple acoustic modes or multiple excitation frequencies that are each reacting differently on a particular load permits the multi-parameter analysis of gases and liquids. We present a sensor system that is based on floating electrode unidirectional transducers (FEUDTs) allowing for the excitation of a set of 48 equally spaced frequencies and at least two acoustic modes (depending on the material system) in a single transducer structure. FEUDTs are the basis for the efficient excitation of acoustic waves at GHz frequencies without the use of sophisticated nanofabrication techniques. Higher frequencies, on the other hand, are advantageous for the sensitivity of the device. We tested the basic operation of our sensor system by applying it to humidity sensing without a sensitive layer.


In-situ study of acoustomigration by scanning Acoustic Force Microscopy

Proceedings of the IEEE Ultrasonics Symposium 2 (2003) 1483-1486

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

High-power operation of surface acoustic wave devices may lead to stress induced material transport, so-called acoustomigration. We used Scanning Acoustic Force Microscopy (SAFM) to study acoustomigration of metal structures in-situ, i.e. during the high-power loading of the device. SAFM allows for the simultaneous measurement of the acoustic wavefield and the topography with submicron lateral resolution. 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°YX-LiTaO3 incorporating 420 nm thick Al electrodes. By correlating the acoustic wavefield mapping and the local changes in the topography point-by-point, already the initial changes of the granular structure that lead to acoustomigration can be visualized.


Variable-temperature micromagnetic study of epitaxially grown MnAs films on GaAs(001)

Applied Physics A: Materials Science and Processing 77 (2003) 739-742

J Mohanty, T Hesjedal, T Plake, M Kästner, L Däweritz, KH Ploog

We present variable-temperature magnetic force microscopy (VT-MFM) studies of epitaxially grown MnAs layers on GaAs(001). In MnAs, a structural and magnetic phase transition occurs at Tc ≈ 40 °C from the hexagonal, ferromagnetic α-phase below Tc to the orthorhombic, paramagnetic β-phase above Tc. In the investigated MnAs-GaAs system, both phases coexist over a temperature range of ≈ 30 °C below Tc due to the involved strain. Using MFM we are able to distinguish between the ferromagnetic and the paramagnetic phases by measuring topographic and magnetic contrast of the same sample area. For VT-MFM studies, we have employed a temperature stage that allows heating and cooling in a controlled atmosphere with small thermal drift (for this system, the temperature was varied from below 0 °C to above 45 °C). The ratio of the ferromagnetic to the paramagnetic phases shows a temperature hysteresis, i.e. the ratio is dependent on whether the sample was heated or cooled to reach the measurement temperature. Detailed studies of the domains and their arrangements over the hysteretic temperature cycle are shown and compared. Mainly three different domain types are found that are dominant in their respective temperature ranges.

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