► Inducing uniaxial magnetic anisotropy

Magnetic materials that have a preferred direction, i.e. a direction in which the magnetic moments are preferentially arranged, have magnetic anisotropy. In the case of magnetic thin films, the shape usually ensures that the magnetic moments are preferably oriented in the plane of the magnetic thin film. In addition to this so-called shape anisotropy, other anisotropies can also be present. An example is magnetic uniaxial anisotropy in the film plane. This ensures that the magnetic moments not only lie preferably in the film plane but are also oriented along a certain direction within the film plane. Such uniaxial anisotropies are relevant for various applications, such as magnetic tunnel contacts, which in turn are used in read heads of hard disk drives or in magnetoresistive RAM (MRAM).

Uniaxial magnetic anisotropies can have various causes, but they can also be actively impressed into magnetic thin films. Two possibilities to be investigated in this work are the application of an external magnetic field during deposition and the use of a low angle of incidence of the atoms arriving at the substrate. The films are produced using magnetron sputter deposition and then examined for their magnetic properties using advanced magnetometry.

Contact: If you are interested, please contact Stephan Glamsch. → @

► Crystallization of orthoferrite thin films by post-annealing

Orthoferrites have interesting magnetic properties when prepared as a single crystal film. In single crystals, the atoms are arranged in a uniform, coherent lattice. However, when we deposit thin films of orthoferrite at room temperature, we obtain an amorphous material that is structurally disordered. However, such amorphous films can crystallize upon heating. In this work, amorphous orthoferrite films are grown by pulsed laser ablation and then brought into their crystalline form by post-annealing.

The post-tempering conditions (temperature, tempering time) should be varied and optimized. In addition, a so-called “rapid thermal annealing” system, which enables heating rates of up to 400 K/s, can also be used. The structural properties of the films treated in this way are analyzed by X-ray diffraction and scanning electron microscopy.

Contact: If you are interested, please contact Christian Holzmann. → @

► Synthesis and characterization of highly entropic perovskites as bulk material

Highly entropic materials are materials that still crystallize in a single phase due to their high entropy. Entropy dominates here and ensures a minimization of the Gibbs free energy

G = H – TS.

We focus on high-entropy perovskites and investigate the influence of different materials on the magnetic properties. The aim of the work is to produce various high-entropy perovksites using a solid-state reaction. The materials are examined for their structural properties using X-ray diffraction and then examined their magnetic properties at low temperatures. The properties will then be compared with other high-entropy materials.

Contact: If you are interested, please contact Maximilian Mihm. → @

► Growth and characterization of epitaxial Co₃O₄ thin films

Co₃O₄ is a spinel (AB₂X₄) with a cubic lattice. It is also an antiferromagnet (in an antiferromagnet the magentic spins align antiparallel). This antiferrimagnetic material system is of particular interest as it may be suitable for spintronic applications.

The aim of the work is to produce thin films of Co₃O₄ by pulsed laser deposition and to investigate their structural properties by X-ray diffraction (XRD) and atomic force microscopy (AFM).

Contact us: For further information, please contact  Maximilian Mihm. → @

► Influence of the seed layer on the properties of the ferrimagnetic Fe/Gd skyrmion system

One of our research topics are topologically protected, magnetic spin textures. These so-called skyrmions can, for example, be stabilized in the ferrimagnetic multilayer system [Fe/Gd] by dipolar interactions. When preparing these thin-film samples using magnetron sputter deposition, the magnetic layer is applied to a so-called seed layer on the substrate. As part of this work, several Fe/Gd multilayer films will be deposited on different seed layers, which will then be characterized using SQUID-VSM magnetometry, magnetic force microscopy (MFM), and Lorentz transmission electron microscopy (LTEM). The influence of the seed layer on the magnetic properties of the Fe/Gd system will be examined.

Contact: If you are interested, please contact Timo Schmidt. → @

► Interdigital Transducer (IDT) Design

Surface acoustic waves (SAWs) are an indispensable part of today’s communication technology. In electronic devices, elec­tric signals can be transformed into acoustic waves via the inverse piezoelectric effect and vice versa. For that, interdigital transducers (IDTs) are commonly used. IDTs can be described as metallic finger grating structures but there are many different possible shapes each with their own (dis-)advantages. The IDT geometry plays an important role in the effi­ciency of the IDT.

In our research, we use these IDTs as a tool to excite SAWs, which then, if the resonance condition is fulfilled, excite spin waves (SWs) in a magnetic thin film. However, the IDT design and optimization (e.g., impedance matching) is its own already quite well-established research field. The objective of this master thesis is the design and optimization of different IDT structures regarding large amplitude SAW output signals in experiment and simulation. For the fabrication of the optimized IDTs, e-beam lithography in our clean room facilities will be done. The SAW output signal will be experimentally characterized in terms of frequency-dependent elec­trical transmission measurements.

Contact us: For further information, please contact Matthias Küß. → @

► Nonreciprocal interactions of surface acoustic waves with spin waves

Surface acoustic waves (SAWs) are nano earthquakes with wavelengths and frequencies in the µm- and low GHz-range. These acoustic waves can be efficiently launched and detected on piezoelectric crystals due to the piezoelectric effect and are manifold employed in our cellphones as radio-frequency bandpass filters. Surface acoustic waves (SAWs) are inherently reciprocal, meaning the wave properties (amplitude, phase, or frequency) do not change under the inversion of their propagation direction. Spin waves (SWs) are propagating disturbances of the magnetic moments in the ordering of magnetic materials. In contrast to SAWs, SWs can show nonreciprocal behavior.

By coupling SAWs with SWs in piezoelectric/ferromagnetic heterostructures, the SAWs can inherit the nonreciprocal properties of SWs. This gives the perspective to build novel devices for radio-frequency signal processing, such as magnetoacoustic isolators and circulators. For large SAW-SW interaction, the resonance condition must be fulfilled (matching of frequency and wave vector), which strongly depends on the properties of the magnetic thin film, in which the SWs are excited. Therefore, the properties of magnetic thin films can be investigated via SAW-SW spectroscopy.

The goal of this study is to i) deposit various magnetic thin films via magnetron sputtering and ii) characterize the magnetic properties via SQUID-VSM magnetometry and SAW-SW spectroscopy.

Contact us: For further information, please contact Matthias Küß. → @

► Piezoelectric ZnO thin films for surface acoustic wave applications

Surface acoustic waves (SAWs) are earthquakes on a very small scale, and used for example as a part of circulators for RF signal processing, e.g. in your smartphone. The combination of SAWs with magnetic thin films is an interesting research topic, promising smaller sizes and energy consumption compared to existing technologies. Commonly, piezoelectric substrates are used to excite SAWs. However, for integration with different magnetic films, we cannot use a piezoelectric substrate. In this case, using a piezoelectric film layer is an alternative. A possible candidate is zinc oxide (ZnO), which possesses piezoelectric properties and can be grown by pulsed laser deposition.

In this thesis, the possibilities of growing ZnO thin films by pulsed laser deposition are analyzed. In this technique, we shoot with a powerful laser at a target to evaporate material, form a plasma, and finally grow a thin film on a substrate. In the first step, you will grow ZnO films on suitable substrates, measure and optimize their piezoelectric properties. For this, you will use x-ray diffraction, atomic force and scanning electron microscopy, as well as lithography and a custom SAW transmission setup. As soon as you are successful in growing ZnO thin films with the desired quality, the ZnO film can be combined with different magnetic thin films, or directly with magnetic substrates. One interesting material are garnets, as they are ideal to excite magnetic waves, but it is challenging to directly grow them on piezoelectric substrates. In your thesis, you can try to combine a garnet film and a piezoelectric film in order to study the interaction of SAWs and spin waves in garnets.

Contact us: For further information, please contact Matthias Küß. → @

► Fabrication and characterization of ferrimagnetic monolayers

Ferrimagnetic materials exhibit unique magnetic structures with opposing magnetic moments allowing them to host various magnetic objects, known as spin textures. Their controllable magnetization properties make them valuable for developing various advanced applications in the field of spintronics. Despite ferrimagnets hold great promise, there are still some challenges and limitations to overcome.

This project aims to investigate, create, and understand ferri­mag­netic monolayers and their unique magnetic properties. By using advanced techniques like magnetron sputter deposition, we will fabricate these single atomic layers of magnetic materials. Sub­se­quently, through the application of advanced characterization methods, including Magneto-Optical Kerr Effect measurements, SQUID-VSM magnetometry, Magnetic Force Microscopy, and Hall effect measurements, we seek to comprehensively analyze and interpret the distinctive behavior of these monolayers.

At the end of this project, you will be able to achieve great experience in material deposition and advanced characterization techniques. You will gain the skills and insights needed to make decisions in choosing and studying these materials by yourself. Ultimately advancing your understanding of the quantum world, basic magnetism and ferrimagnetic monolayers which will expand your horizon for your career.

Contact us: For further information, please contact Tamer Karaman. → @

► Investigation of topology and chirality in amorphous ferrimagnetic materials

Motivation: Skyrmions are tiny, vortex-like magnetic structures that exist at nanoscale with non-trivial topology. Skyrmions have attracted great attention because they have the potential to revolutionize spintronic information storage as well as neuromorphic and reservoir computing technologies. However, to realize this vision, new materials are required in which information-carrying topological magnetic spin textures must be small and fast simultaneously. Amorphous ferrimagnets with Dzyaloshinskii-Moriya interaction (DMI) are one of the most promising candidates, but have been little studied so far.

Contact: For further information, please contact Tamer Karaman. → @

► Preparation and characterization of L1₀-Fe(NiPt) alloy thin films

L1₀ FePt is a very useful material because of its hard magnetic properties. It is considered a pro­mising candidate for future digital storage media based on Heat Assisted Magnetic Recording (HAMR) technology. Such hard drives with considerable storage density are already available on the open market. A structurally similar material is the ferromagnetic L1₀-FeNi, which is considered a potential replacement for the rare earth-based permanent magnets currently in use. However, their fabrication turns out to be extremely difficult. However, gradually replacing Pt with Ni in L1₀-FePt could stabilize a higher Ni content alloy that exhibits similarly promising properties, which is the topic of this study. To start, you will grow these thin films on standard substrates by magnetron sputter deposition and characterize the structural and magnetic properties of the grown thin films. You will use different methods including advanced x-ray diffraction, atomic force and scanning electron microscopy, as well as SQUID-VSM magnetometry.

Contact us: For further information, please contact Johannes Seyd. → @

► High-entropy garnets

Garnets are very interesting magnetic materials, as they are the best-known materials to host magnetic waves, so-called magnons. Using magnons in thin garnet films for example for future (neuromorphic) computing applications promises very low energy consumption and small device sizes. However, we must be able to tune their magnetic properties and to grow them as high quality, single-crys­talline films. Generally, garnets have the formula R₃Fe₅O₁₂, with 3 rare earth ions, 5 iron ions, and 12 oxygen ions per formula unit. Using a high entropy approach, we use a mixture of different rare earths (e.g. Gd, Y ,Bi, Tm, Sm), or substitute some iron ions (for example with Ni or Co), and try to grow a new garnet with tuneable magnetic properties.

This thesis will start by thinking of a suitable atom mixture to achieve the desired magnetic pro­perties. Your will then try to grow these new garnets using our pulsed laser deposition system. Here we shoot with a powerful laser at a target to evaporate material, form a plasma, and finally grow thin film on a substrate. To start, you will try to grow a thin film on standard substrates, and characterize the structural and magnetic properties of the grown thin films. You will use different methods including advanced x-ray diffraction, atomic force and scanning electron microscopy, as well as SQUID-VSM magnetometry. Finally, the goal is to use different substrates, which are more com­monly used in semiconductor technology, like silicon or LiNbO₃. You will try to grow the high entropy garnet on these substrates, which is challenging using standard garnets.

Contact us: For further information, please contact Christian Holzmann. → @

► Comparison of the properties of a high entropy perovskite as a bulk material and as a thin film

High entropy materials are materials which crystallizes single phase due to their high entropy. The Entropy is the dominating factor to minimize the Gibbs free energy (G = H – TS). We are dealing with high entropy perovskites and investigate the influence of the different materials on the magnetic properties.

The aim of the work is to produce different high entropy perovskites via solid state reaction. After that these high entropy perovskites will be grown as thin film via pulsed laser deposition. These thin films will be investigated structural with atomic force microscopy and x-ray diffraction and magneti­cally with SQUID-VSM magnetometry. After that, the material will be compared to other high entropy perovskites.

Contact us: For further information, please contact Maximilian Mihm. → @

► Growth and characterization of epitaxial Co₃O₄ thin films

Co₃O₄ is a spinel (AB₂X₄) with a cubic lattice. It is also an antiferromagnet (in an antiferromagnet the magentic spins align antiparallel). This antiferrimagnetic material system is of particular interest as it may be suitable for spintronic applications.

The aim of the thesis is to prepare epitaxial Co₃O₄ thin films via pulsed laser deposition (PLD). These films will be characterized structurally via x-ray diffraction (XRD) and atomic force microscopy (AFM). Also the magnetic properties of the films are also of interest and will be probed by SQUID-VSM magnetometry. The first step will be to produce a suitable target for the PLD process. The next step will be to test different substrate materials to grow single-crystalline films.

Contact us: For further information, please contact  Maximilian Mihm. → @

► Skyrmion formation in FeGd-alloy wedge samples

Our research focuses on the study of topologically protected, magnetic spin textures. These so-called skyrmions can, for ex­ample, be stabilized in ferrimagnetic FeGd-alloy thin films through dipo­lar interactions. For this purpose, the samples are produced using magnetron sputter deposition and characterized using SQUID-VSM magnetometry, magnetic force microscopy (MFM), and Lorentz transmission electron microscopy (LTEM). In addi­tion, the thickness of the alloys will be varied via wedge sputtering and their influence will be investigated via local measurements on the wedge sample using LTEM and MFM.

Contact us: For further information, please contact Timo Schmidt. → @