Kevin Ruwisch

Osnabrück University

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Kevin Ruwisch has a total of 43 co-authors. Here are the top co-authors by number of shared publications:

Joachim Wollschläger
Department of Physics, Osnabrück University, 49076 Osnabrück, Germany
9 shared publications
Karsten Küpper
Department of Physics, Osnabrück University, 49076 Osnabrück, Germany
8 shared publications
Tobias Pohlmann
Department of Physics, Osnabrück University, 49076 Osnabrück, Germany
6 shared publications
Florian Bertram
Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany
5 shared publications
Jannis Thien
Department of Physics, Osnabrück University, 49076 Osnabrück, Germany
4 shared publications
Jari Rodewald
Department of Physics, Osnabrück University, 49076 Osnabrück, Germany
3 shared publications

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Cited by 6
- 17 April 2024
- Published by MDPI AG in Nanomaterials
Structure-Related Electronic and Magnetic Properties in Ultrathin Epitaxial Ni_xFe_3−xO₄ Films on MgO(001)
by Jari Rodewald, Jannis Thien, Kevin Ruwisch, Tobias Pohlmann, Martin Hoppe, Jan Schmalhorst, Karsten Küpper, Joachim Wollschläger
Off-stoichiometric Ni_xFe_3−xO₄ ultrathin films (x < 2.1) with varying Ni content x and thickness 16 (±2) nm were grown on MgO(001) by reactive molecular beam epitaxy. Synchrotron-based high-resolution X-ray diffraction measurements reveal vertical compressive strain for all films, resulting from a lateral pseudomorphic adaption of the film to the substrate lattice without any strain relaxation. Complete crystallinity with smooth interfaces and surfaces is obtained independent of the Ni content x. For x < 1 an expected successive conversion from Fe₃O₄ to NiFe₂O₄ is observed, whereas local transformation into NiO structures is observed for films with Ni content x > 1. However, angle-resolved hard X-ray photoelectron spectroscopy measurements indicate homogeneous cationic distributions without strictly separated phases independent of the Ni content, while X-ray absorption spectroscopy shows that also for x > 1, not all ${Fe}^{2 +}$ cations are substituted by ${Ni}^{2 +}$ cations. The ferrimagnetic behavior, as observed by superconducting quantum interference device magnetometry, is characterized by decreasing saturation magnetization due to the formation of antiferromagnetic NiO parts.
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- 17 April 2024
- Published by MDPI AG in Nanomaterials
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Structure-Related Electronic and Magnetic Properties in Ultrathin Epitaxial Ni_xFe_3−xO₄ Films on MgO(001)
by Jari Rodewald, Jannis Thien, Kevin Ruwisch, Tobias Pohlmann, Martin Hoppe, Jan Schmalhorst, Karsten Küpper, Joachim Wollschläger
Off-stoichiometric Ni_xFe_3−xO₄ ultrathin films (x < 2.1) with varying Ni content x and thickness 16 (±2) nm were grown on MgO(001) by reactive molecular beam epitaxy. Synchrotron-based high-resolution X-ray diffraction measurements reveal vertical compressive strain for all films, resulting from a lateral pseudomorphic adaption of the film to the substrate lattice without any strain relaxation. Complete crystallinity with smooth interfaces and surfaces is obtained independent of the Ni content x. For x < 1 an expected successive conversion from Fe₃O₄ to NiFe₂O₄ is observed, whereas local transformation into NiO structures is observed for films with Ni content x > 1. However, angle-resolved hard X-ray photoelectron spectroscopy measurements indicate homogeneous cationic distributions without strictly separated phases independent of the Ni content, while X-ray absorption spectroscopy shows that also for x > 1, not all ${Fe}^{2 +}$ cations are substituted by ${Ni}^{2 +}$ cations. The ferrimagnetic behavior, as observed by superconducting quantum interference device magnetometry, is characterized by decreasing saturation magnetization due to the formation of antiferromagnetic NiO parts.
Cited by 6
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- 23 November 2023
- Published by MDPI AG in Materials
Real-Time Monitoring of Strain Accumulation and Relief during Epitaxy of Ultrathin Co Ferrite Films with Varied Co Content
by Jannis Thien, Jari Rodewald, Tobias Pohlmann, Kevin Ruwisch, Florian Bertram, Karsten Küpper, Joachim Wollschläger
Ultrathin Co $_{x}$ Fe $_{3 - x}$ O $_{4}$ films of high structural quality and with different Co content (x = 0.6–1.2) were prepared by reactive molecular beam epitaxy on MgO(001) substrates. Epitaxy of these ferrite films is extensively monitored by means of time-resolved (operando) X-ray diffraction recorded in out-of-plane geometry to characterize the temporal evolution of the film structure. The Co ferrite films show high crystalline ordering and smooth film interfaces independent of their Co content. All Co $_{x}$ Fe $_{3 - x}$ O $_{4}$ films exhibit enhanced compressive out-of-plane strain during the early stages of growth, which partly releases with increasing film thickness. When the Co content of the ferrite films increases, the vertical-layer distances increase, accompanied by slightly increasing film roughnesses. The latter result is supported by surface-sensitive low-energy electron diffraction as well as X-ray reflectivity measurements on the final films. In contrast, the substrate–film interface roughness decreases with increasing Co content, which is confirmed with X-ray reflectivity measurements. In addition, the composition and electronic structure of the ferrite films is characterized by means of hard X-ray photoelectron spectroscopy performed after film growth. The experiments reveal the expected increasing ${Fe}^{3 +}$ / ${Fe}^{2 +}$ cation ratios for a higher Co content.
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- 23 November 2023
- Published by MDPI AG in Materials
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Real-Time Monitoring of Strain Accumulation and Relief during Epitaxy of Ultrathin Co Ferrite Films with Varied Co Content
by Jannis Thien, Jari Rodewald, Tobias Pohlmann, Kevin Ruwisch, Florian Bertram, Karsten Küpper, Joachim Wollschläger
Ultrathin Co $_{x}$ Fe $_{3 - x}$ O $_{4}$ films of high structural quality and with different Co content (x = 0.6–1.2) were prepared by reactive molecular beam epitaxy on MgO(001) substrates. Epitaxy of these ferrite films is extensively monitored by means of time-resolved (operando) X-ray diffraction recorded in out-of-plane geometry to characterize the temporal evolution of the film structure. The Co ferrite films show high crystalline ordering and smooth film interfaces independent of their Co content. All Co $_{x}$ Fe $_{3 - x}$ O $_{4}$ films exhibit enhanced compressive out-of-plane strain during the early stages of growth, which partly releases with increasing film thickness. When the Co content of the ferrite films increases, the vertical-layer distances increase, accompanied by slightly increasing film roughnesses. The latter result is supported by surface-sensitive low-energy electron diffraction as well as X-ray reflectivity measurements on the final films. In contrast, the substrate–film interface roughness decreases with increasing Co content, which is confirmed with X-ray reflectivity measurements. In addition, the composition and electronic structure of the ferrite films is characterized by means of hard X-ray photoelectron spectroscopy performed after film growth. The experiments reveal the expected increasing ${Fe}^{3 +}$ / ${Fe}^{2 +}$ cation ratios for a higher Co content.
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Cited by 2
- 5 October 2022
- Published by MDPI AG in Materials
Influence of Oxygen Plasma on the Growth and Stability of Epitaxial NiCo₂O₄ Ultrathin Films on Various Substrates
by Kevin Ruwisch, Andreas Alexander, Tobias Pollenske, Karsten Küpper, Joachim Wollschläger
In this work, we investigated the influence of oxygen plasma on the growth of nickel cobaltite (NiCo₂O₄) thin films compared to growth in a molecular oxygen atmosphere. The films were grown on MgO(001), MgAl₂O₄(001) and SrTiO₃(001) substrates by oxygen plasma (atmosphere of activated oxygen)-assisted and reactive molecular beam epitaxy (molecular oxygen atmosphere). Soft X-ray photoelectron spectroscopy showed that only the use of oxygen plasma led to a spectrum characteristic of (NiCo₂O₄). Low energy electron diffraction measurements were conducted to obtain information on the structure of the film surfaces. The results proved the formation of a spinel surface structure for films grown with oxygen plasma, while the formation of a rock salt structure was observed for growth with molecular oxygen. To determine the film thickness, X-ray reflectivity measurements were performed. If oxygen plasma were used to grow (NiCo₂O₄) films, this would result in lower film thicknesses compared to growth using molecular oxygen although the cation flux was kept constant during deposition. Additional X-ray diffraction experiments delivered structural information about the bulk structure of the film. All films had a rock salt bulk structure after exposure to ambient conditions. Angle-resolved hard X-ray photoelectron spectroscopy revealed a homogeneous depth distribution of cations of the grown film, but no typical (NiCo₂O₄) spectrum anymore. Thus, on the one hand, (NiCo₂O₄) films with a spinel structure prepared using activated oxygen were not stable under ambient conditions. The structure of these films was transformed into NiCo oxide with a rock salt structure. On the other hand, it was not possible to form (NiCo₂O₄) films using molecular oxygen. These films had a rock salt structure that was stable under ambient conditions.
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- 5 October 2022
- Published by MDPI AG in Materials
Open Access
Influence of Oxygen Plasma on the Growth and Stability of Epitaxial NiCo₂O₄ Ultrathin Films on Various Substrates
by Kevin Ruwisch, Andreas Alexander, Tobias Pollenske, Karsten Küpper, Joachim Wollschläger
In this work, we investigated the influence of oxygen plasma on the growth of nickel cobaltite (NiCo₂O₄) thin films compared to growth in a molecular oxygen atmosphere. The films were grown on MgO(001), MgAl₂O₄(001) and SrTiO₃(001) substrates by oxygen plasma (atmosphere of activated oxygen)-assisted and reactive molecular beam epitaxy (molecular oxygen atmosphere). Soft X-ray photoelectron spectroscopy showed that only the use of oxygen plasma led to a spectrum characteristic of (NiCo₂O₄). Low energy electron diffraction measurements were conducted to obtain information on the structure of the film surfaces. The results proved the formation of a spinel surface structure for films grown with oxygen plasma, while the formation of a rock salt structure was observed for growth with molecular oxygen. To determine the film thickness, X-ray reflectivity measurements were performed. If oxygen plasma were used to grow (NiCo₂O₄) films, this would result in lower film thicknesses compared to growth using molecular oxygen although the cation flux was kept constant during deposition. Additional X-ray diffraction experiments delivered structural information about the bulk structure of the film. All films had a rock salt bulk structure after exposure to ambient conditions. Angle-resolved hard X-ray photoelectron spectroscopy revealed a homogeneous depth distribution of cations of the grown film, but no typical (NiCo₂O₄) spectrum anymore. Thus, on the one hand, (NiCo₂O₄) films with a spinel structure prepared using activated oxygen were not stable under ambient conditions. The structure of these films was transformed into NiCo oxide with a rock salt structure. On the other hand, it was not possible to form (NiCo₂O₄) films using molecular oxygen. These films had a rock salt structure that was stable under ambient conditions.
Cited by 2
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Cited by 2
- 27 June 2022
- Published by American Physical Society (APS) in Physical Review B
Structural and magnetic investigation of the interfaces of ${Fe}_{3} O_{4} / MgO (001)$ with and without NiO interlayer
by Tobias Pohlmann, Florian Bertram, Jannis Thien, Jari Rodewald, Kevin Ruwisch, Timo Kuschel, Eugen Weschke, Karsten Küpper, Joachim Wollschläger
We present an investigation on the structural and magnetic properties of the interfaces of ${Fe}_{3} O_{4} / MgO (001)$ and ${Fe}_{3} O_{4} / NiO / MgO (001)$ by extracting cation-selective magneto-optical depth profiles by means of x-ray resonant magnetic reflectivity in combination with charge-transfer multiplet simulations of x-ray magnetic circular dichroism data. For ${Fe}_{3} O_{4} / MgO (001)$ , the magneto-optical depth profiles at the ${Fe}_{oct}^{2 +}$ and the ${Fe}_{oct}^{3 +}$ resonant energies follow exactly the structural profile, while the magneto-optical depth profile at the ${Fe}_{tet}^{3 +}$ resonance is offset by $3.2 \pm 1.3$ Å from the interface, consistent with a $B$ -site interface termination of ${Fe}_{3} O_{4}$ with fully intact magnetic order. In contrast, for ${Fe}_{3} O_{4} / NiO (001)$ , the magneto-optical depth profiles at the ${Fe}_{oct}^{2 +}$ and the ${Ni}^{2 +}$ resonances agree with the structural profile, but the interface positions of the magneto-optical depth profiles at the ${Fe}_{oct}^{3 +}$ and the ${Fe}_{tet}^{3 +}$ resonances are spatially shifted by $3.3 \pm 1.4$ and $2.7 \pm 0.9$ Å, respectively, not consistent with a magnetically ordered stoichiometric interface. This may be related to an intermixed $(Ni, Fe) O$ layer at the interface. The dichroic depth profile at the Ni $L_{3}$ edge might hint at uncompensated magnetic moments throughout the NiO film.
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- 27 June 2022
- Published by American Physical Society (APS) in Physical Review B
Structural and magnetic investigation of the interfaces of ${Fe}_{3} O_{4} / MgO (001)$ with and without NiO interlayer
by Tobias Pohlmann, Florian Bertram, Jannis Thien, Jari Rodewald, Kevin Ruwisch, Timo Kuschel, Eugen Weschke, Karsten Küpper, Joachim Wollschläger
We present an investigation on the structural and magnetic properties of the interfaces of ${Fe}_{3} O_{4} / MgO (001)$ and ${Fe}_{3} O_{4} / NiO / MgO (001)$ by extracting cation-selective magneto-optical depth profiles by means of x-ray resonant magnetic reflectivity in combination with charge-transfer multiplet simulations of x-ray magnetic circular dichroism data. For ${Fe}_{3} O_{4} / MgO (001)$ , the magneto-optical depth profiles at the ${Fe}_{oct}^{2 +}$ and the ${Fe}_{oct}^{3 +}$ resonant energies follow exactly the structural profile, while the magneto-optical depth profile at the ${Fe}_{tet}^{3 +}$ resonance is offset by $3.2 \pm 1.3$ Å from the interface, consistent with a $B$ -site interface termination of ${Fe}_{3} O_{4}$ with fully intact magnetic order. In contrast, for ${Fe}_{3} O_{4} / NiO (001)$ , the magneto-optical depth profiles at the ${Fe}_{oct}^{2 +}$ and the ${Ni}^{2 +}$ resonances agree with the structural profile, but the interface positions of the magneto-optical depth profiles at the ${Fe}_{oct}^{3 +}$ and the ${Fe}_{tet}^{3 +}$ resonances are spatially shifted by $3.3 \pm 1.4$ and $2.7 \pm 0.9$ Å, respectively, not consistent with a magnetically ordered stoichiometric interface. This may be related to an intermixed $(Ni, Fe) O$ layer at the interface. The dichroic depth profile at the Ni $L_{3}$ edge might hint at uncompensated magnetic moments throughout the NiO film.
Cited by 2
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Cited by 2
- 23 March 2022
- Published by MDPI AG in Materials
Real-Time Monitoring the Growth of Epitaxial Co_xFe_3−xO₄ Ultrathin Films on Nb-Doped SrTiO₃(001) via Reactive Molecular Beam Epitaxy by Means of Operando HAXPES
by Kevin Ruwisch, Tobias Pohlmann, Florian Bertram, Christoph Schlüter, Andrei Gloskovskii, Karsten Küpper, Joachim Wollschläger
In this work, we present a comprehensive study on real-time monitoring the growth of epitaxial Co_xFe_3−xO₄ thin films grown on SrTiO₃(001) substrates via reactive molecular beam epitaxy. The growth process was monitored during evaporation by means of time resolved operando hard X-ray photoelectron spectroscopy (HAXPES). We prepared ultrathin ferrite films using different oxygen partial pressures, showing pure metallic, light oxidic, and cobalt ferrite-like growth. Additional X-ray diffraction measurements confirm HAXPES results.
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- 23 March 2022
- Published by MDPI AG in Materials
Open Access
Real-Time Monitoring the Growth of Epitaxial Co_xFe_3−xO₄ Ultrathin Films on Nb-Doped SrTiO₃(001) via Reactive Molecular Beam Epitaxy by Means of Operando HAXPES
by Kevin Ruwisch, Tobias Pohlmann, Florian Bertram, Christoph Schlüter, Andrei Gloskovskii, Karsten Küpper, Joachim Wollschläger
In this work, we present a comprehensive study on real-time monitoring the growth of epitaxial Co_xFe_3−xO₄ thin films grown on SrTiO₃(001) substrates via reactive molecular beam epitaxy. The growth process was monitored during evaporation by means of time resolved operando hard X-ray photoelectron spectroscopy (HAXPES). We prepared ultrathin ferrite films using different oxygen partial pressures, showing pure metallic, light oxidic, and cobalt ferrite-like growth. Additional X-ray diffraction measurements confirm HAXPES results.
Cited by 2
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Cited by 9
- 18 January 2022
- Published by American Physical Society (APS) in Physical Review B
Time-resolved x-ray diffraction and photoelectron spectroscopy investigation of the reactive molecular beam epitaxy of ${Fe}_{3} O_{4}$ ultrathin films
by Tobias Pohlmann, Martin Hoppe, Jannis Thien, Arka Bikash Dey, Andreas Alexander, Kevin Ruwisch, Olof Gutowski, Jan Röh, Andrei Gloskovskii, Christoph Schlueter, et al.
We present time-resolved high-energy x-ray diffraction, time-resolved hard-x-ray photoelectron spectroscopy (tr-HAXPES), and time-resolved grazing incidence small-angle x-ray scattering (tr-GISAXS) data of the reactive molecular beam epitaxy of ${Fe}_{3} O_{4}$ ultrathin films on various substrates. Reciprocal space maps are recorded during the deposition of ${Fe}_{3} O_{4}$ on ${SrTiO}_{3} (001)$ , MgO(001), and NiO/MgO(001) in order to observe the thickness-dependent evolution of Bragg reflections sensitive to the octahedral and tetrahedral sublattices of the inverse spinel structure of ${Fe}_{3} O_{4}$ . Rock salt and spinel-exclusive reflections appear at different thicknesses, revealing that first, the iron oxide film grows with ${Fe}_{1 - δ} O$ rock salt structure with exclusive occupation of octahedral lattice sites. After reaching a film thickness of $1.1 nm$ , further growth of the iron oxide film proceeds in the inverse spinel structure, with both octahedral and tetrahedral lattice sites being occupied. In addition, iron oxide on ${SrTiO}_{3} (001)$ initially grows with none of these structures. Here, the formation of the rock salt structure starts when reaching a film thickness of $1.5 nm$ . This is confirmed by tr-HAXPES data obtained during growth of iron oxide on ${SrTiO}_{3} (001)$ , which demonstrate an excess of ${Fe}^{2 +}$ cations compared to ${Fe}_{3} O_{4}$ in growing films thinner than $3.2 nm$ . This rock salt phase only appears during growth and vanishes after the supply of the Fe molecular beam is stopped. Thus, it can be concluded that the rock salt structure of the interlayer is a property of the dynamic growth process while the film is still exposed to oxygen. The tr-GISAXS data link these structural results to an island growth mode of the first 2–3 nm on both MgO(001) and ${SrTiO}_{3} (001)$ substrates.
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- 18 January 2022
- Published by American Physical Society (APS) in Physical Review B
Time-resolved x-ray diffraction and photoelectron spectroscopy investigation of the reactive molecular beam epitaxy of ${Fe}_{3} O_{4}$ ultrathin films
by Tobias Pohlmann, Martin Hoppe, Jannis Thien, Arka Bikash Dey, Andreas Alexander, Kevin Ruwisch, Olof Gutowski, Jan Röh, Andrei Gloskovskii, Christoph Schlueter, et al.
We present time-resolved high-energy x-ray diffraction, time-resolved hard-x-ray photoelectron spectroscopy (tr-HAXPES), and time-resolved grazing incidence small-angle x-ray scattering (tr-GISAXS) data of the reactive molecular beam epitaxy of ${Fe}_{3} O_{4}$ ultrathin films on various substrates. Reciprocal space maps are recorded during the deposition of ${Fe}_{3} O_{4}$ on ${SrTiO}_{3} (001)$ , MgO(001), and NiO/MgO(001) in order to observe the thickness-dependent evolution of Bragg reflections sensitive to the octahedral and tetrahedral sublattices of the inverse spinel structure of ${Fe}_{3} O_{4}$ . Rock salt and spinel-exclusive reflections appear at different thicknesses, revealing that first, the iron oxide film grows with ${Fe}_{1 - δ} O$ rock salt structure with exclusive occupation of octahedral lattice sites. After reaching a film thickness of $1.1 nm$ , further growth of the iron oxide film proceeds in the inverse spinel structure, with both octahedral and tetrahedral lattice sites being occupied. In addition, iron oxide on ${SrTiO}_{3} (001)$ initially grows with none of these structures. Here, the formation of the rock salt structure starts when reaching a film thickness of $1.5 nm$ . This is confirmed by tr-HAXPES data obtained during growth of iron oxide on ${SrTiO}_{3} (001)$ , which demonstrate an excess of ${Fe}^{2 +}$ cations compared to ${Fe}_{3} O_{4}$ in growing films thinner than $3.2 nm$ . This rock salt phase only appears during growth and vanishes after the supply of the Fe molecular beam is stopped. Thus, it can be concluded that the rock salt structure of the interlayer is a property of the dynamic growth process while the film is still exposed to oxygen. The tr-GISAXS data link these structural results to an island growth mode of the first 2–3 nm on both MgO(001) and ${SrTiO}_{3} (001)$ substrates.
Cited by 9
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Cited by 6
- 22 December 2021
- Published by MDPI AG in Materials
Cationic Ordering and Its Influence on the Magnetic Properties of Co-Rich Cobalt Ferrite Thin Films Prepared by Reactive Solid Phase Epitaxy on Nb-Doped SrTiO₃(001)
by Jannis Thien, Jascha Bahlmann, Andreas Alexander, Kevin Ruwisch, Jari Rodewald, Tobias Pohlmann, Martin Hoppe, Fatih Alarslan, Martin Steinhart, Baki Altuncevahir, et al.
Here, we present the (element-specific) magnetic properties and cation ordering for ultrathin Co-rich cobalt ferrite films. Two Co-rich ${Co}_{x} {Fe}_{3 - x} O_{4}$ films with different stoichiometry ( $x = 1.1$ and $x = 1.4$ ) have been formed by reactive solid phase epitaxy due to post-deposition annealing from epitaxial CoO/Fe $_{3}$ O $_{4}$ bilayers deposited before on Nb-doped SrTiO $_{3}$ (001). The electronic structure, stoichiometry and homogeneity of the cation distribution of the resulting cobalt ferrite films were verified by angle-resolved hard X-ray photoelectron spectroscopy. From X-ray magnetic circular dichroism measurements, the occupancies of the different sublattices were determined using charge-transfer multiplet calculations. For both ferrite films, a partially inverse spinel structure is found with increased amount of ${Co}^{3 +}$ cations in the low-spin state on octahedral sites for the ${Co}_{1.4} {Fe}_{1.6} O_{4}$ film. These findings concur with the results obtained by superconducting quantum interference device measurements. Further, the latter measurements revealed the presence of an additional soft magnetic phase probably due to cobalt ferrite islands emerging from the surface, as suggested by atomic force microscope measurements.
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- 22 December 2021
- Published by MDPI AG in Materials
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Cationic Ordering and Its Influence on the Magnetic Properties of Co-Rich Cobalt Ferrite Thin Films Prepared by Reactive Solid Phase Epitaxy on Nb-Doped SrTiO₃(001)
by Jannis Thien, Jascha Bahlmann, Andreas Alexander, Kevin Ruwisch, Jari Rodewald, Tobias Pohlmann, Martin Hoppe, Fatih Alarslan, Martin Steinhart, Baki Altuncevahir, et al.
Here, we present the (element-specific) magnetic properties and cation ordering for ultrathin Co-rich cobalt ferrite films. Two Co-rich ${Co}_{x} {Fe}_{3 - x} O_{4}$ films with different stoichiometry ( $x = 1.1$ and $x = 1.4$ ) have been formed by reactive solid phase epitaxy due to post-deposition annealing from epitaxial CoO/Fe $_{3}$ O $_{4}$ bilayers deposited before on Nb-doped SrTiO $_{3}$ (001). The electronic structure, stoichiometry and homogeneity of the cation distribution of the resulting cobalt ferrite films were verified by angle-resolved hard X-ray photoelectron spectroscopy. From X-ray magnetic circular dichroism measurements, the occupancies of the different sublattices were determined using charge-transfer multiplet calculations. For both ferrite films, a partially inverse spinel structure is found with increased amount of ${Co}^{3 +}$ cations in the low-spin state on octahedral sites for the ${Co}_{1.4} {Fe}_{1.6} O_{4}$ film. These findings concur with the results obtained by superconducting quantum interference device measurements. Further, the latter measurements revealed the presence of an additional soft magnetic phase probably due to cobalt ferrite islands emerging from the surface, as suggested by atomic force microscope measurements.
Cited by 6
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Cited by 1
- 19 October 2021
- Published by American Chemical Society (ACS) in The Journal of Physical Chemistry C
Inferface Magnetization Phenomena in Epitaxial Thin Fe₃O₄/Co_xFe_3–xO₄ Bilayers
by K. Ruwisch, T. Pohlmann, M. Hoppe, F. Bertram, P. Shafer, J. Wollschläger, K. Küpper
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- 19 October 2021
- Published by American Chemical Society (ACS) in The Journal of Physical Chemistry C
Inferface Magnetization Phenomena in Epitaxial Thin Fe₃O₄/Co_xFe_3–xO₄ Bilayers
by K. Ruwisch, T. Pohlmann, M. Hoppe, F. Bertram, P. Shafer, J. Wollschläger, K. Küpper
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Cited by 23
- 24 September 2021
- Published by MDPI AG in Sensors
Magnetoelectric Response of Laminated Cantilevers Comprising a Magnetoactive Elastomer and a Piezoelectric Polymer, in Pulsed Uniform Magnetic Fields
by Gašper Glavan, Inna A. Belyaeva, Kevin Ruwisch, Joachim Wollschläger, Mikhail Shamonin
The voltage response to pulsed uniform magnetic fields and the accompanying bending deformations of laminated cantilever structures are investigated experimentally in detail. The structures comprise a magnetoactive elastomer (MAE) slab and a commercially available piezoelectric polymer multilayer. The magnetic field is applied vertically and the laminated structures are customarily fixed in the horizontal plane or, alternatively, slightly tilted upwards or downwards. Six different MAE compositions incorporating three concentrations of carbonyl iron particles (70 wt%, 75 wt% and 80 wt%) and two elastomer matrices of different stiffness are used. The dependences of the generated voltage and the cantilever’s deflection on the composition of the MAE layer and its thickness are obtained. The appearance of the voltage between the electrodes of a piezoelectric material upon application of a magnetic field is considered as a manifestation of the direct magnetoelectric (ME) effect in a composite laminated structure. The ME voltage response increases with the increasing total quantity of the soft-magnetic filler in the MAE layer. The relationship between the generated voltage and the cantilever’s deflection is established. The highest observed peak voltage around 5.5 V is about 8.5-fold higher than previously reported values. The quasi-static ME voltage coefficient for this type of ME heterostructures is about 50 V/A in the magnetic field of ≈100 kA/m, obtained for the first time. The results could be useful for the development of magnetic field sensors and energy harvesting devices relying on these novel polymer composites.
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- 24 September 2021
- Published by MDPI AG in Sensors
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Magnetoelectric Response of Laminated Cantilevers Comprising a Magnetoactive Elastomer and a Piezoelectric Polymer, in Pulsed Uniform Magnetic Fields
by Gašper Glavan, Inna A. Belyaeva, Kevin Ruwisch, Joachim Wollschläger, Mikhail Shamonin
The voltage response to pulsed uniform magnetic fields and the accompanying bending deformations of laminated cantilever structures are investigated experimentally in detail. The structures comprise a magnetoactive elastomer (MAE) slab and a commercially available piezoelectric polymer multilayer. The magnetic field is applied vertically and the laminated structures are customarily fixed in the horizontal plane or, alternatively, slightly tilted upwards or downwards. Six different MAE compositions incorporating three concentrations of carbonyl iron particles (70 wt%, 75 wt% and 80 wt%) and two elastomer matrices of different stiffness are used. The dependences of the generated voltage and the cantilever’s deflection on the composition of the MAE layer and its thickness are obtained. The appearance of the voltage between the electrodes of a piezoelectric material upon application of a magnetic field is considered as a manifestation of the direct magnetoelectric (ME) effect in a composite laminated structure. The ME voltage response increases with the increasing total quantity of the soft-magnetic filler in the MAE layer. The relationship between the generated voltage and the cantilever’s deflection is established. The highest observed peak voltage around 5.5 V is about 8.5-fold higher than previously reported values. The quasi-static ME voltage coefficient for this type of ME heterostructures is about 50 V/A in the magnetic field of ≈100 kA/m, obtained for the first time. The results could be useful for the development of magnetic field sensors and energy harvesting devices relying on these novel polymer composites.
Cited by 23
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Cited by 5
- 30 June 2018
- Published by MDPI AG in Materials
Impact of Strain and Morphology on Magnetic Properties of Fe₃O₄/NiO Bilayers Grown on Nb:SrTiO₃(001) and MgO(001)
by Olga Kuschel, Nico Pathé, Tobias Schemme, Kevin Ruwisch, Jari Rodewald, Ralph Buss, Florian Bertram, Timo Kuschel, Karsten Kuepper, Joachim Wollschläger
We present a comparative study of the morphology and structural as well as magnetic properties of crystalline Fe₃O₄/NiO bilayers grown on both MgO(001) and SrTiO₃(001) substrates by reactive molecular beam epitaxy. These structures were investigated by means of X-ray photoelectron spectroscopy, low-energy electron diffraction, X-ray reflectivity and diffraction, as well as vibrating sample magnetometry. While the lattice mismatch of NiO grown on MgO(001) was only 0.8%, it was exposed to a lateral lattice mismatch of −6.9% if grown on SrTiO₃. In the case of Fe₃O₄, the misfit strain on MgO(001) and SrTiO₃(001) amounted to 0.3% and −7.5%, respectively. To clarify the relaxation process of the bilayer system, the film thicknesses of the magnetite and nickel oxide films were varied between 5 and 20 nm. While NiO films were well ordered on both substrates, Fe₃O₄ films grown on NiO/SrTiO₃ exhibited a higher surface roughness as well as lower structural ordering compared to films grown on NiO/MgO. Further, NiO films grew pseudomorphic in the investigated thickness range on MgO substrates without any indication of relaxation, whereas on SrTiO₃ the NiO films showed strong strain relaxation. Fe₃O₄ films also exhibited strong relaxation, even for films of 5 nm thickness on both NiO/MgO and NiO/SrTiO₃. The magnetite layers on both substrates showed a fourfold magnetic in-plane anisotropy with magnetic easy axes pointing in $⟨100⟩$ directions. The coercive field was strongly enhanced for magnetite grown on NiO/SrTiO₃ due to the higher density of structural defects, compared to magnetite grown on NiO/MgO.
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- 30 June 2018
- Published by MDPI AG in Materials
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Impact of Strain and Morphology on Magnetic Properties of Fe₃O₄/NiO Bilayers Grown on Nb:SrTiO₃(001) and MgO(001)
by Olga Kuschel, Nico Pathé, Tobias Schemme, Kevin Ruwisch, Jari Rodewald, Ralph Buss, Florian Bertram, Timo Kuschel, Karsten Kuepper, Joachim Wollschläger
We present a comparative study of the morphology and structural as well as magnetic properties of crystalline Fe₃O₄/NiO bilayers grown on both MgO(001) and SrTiO₃(001) substrates by reactive molecular beam epitaxy. These structures were investigated by means of X-ray photoelectron spectroscopy, low-energy electron diffraction, X-ray reflectivity and diffraction, as well as vibrating sample magnetometry. While the lattice mismatch of NiO grown on MgO(001) was only 0.8%, it was exposed to a lateral lattice mismatch of −6.9% if grown on SrTiO₃. In the case of Fe₃O₄, the misfit strain on MgO(001) and SrTiO₃(001) amounted to 0.3% and −7.5%, respectively. To clarify the relaxation process of the bilayer system, the film thicknesses of the magnetite and nickel oxide films were varied between 5 and 20 nm. While NiO films were well ordered on both substrates, Fe₃O₄ films grown on NiO/SrTiO₃ exhibited a higher surface roughness as well as lower structural ordering compared to films grown on NiO/MgO. Further, NiO films grew pseudomorphic in the investigated thickness range on MgO substrates without any indication of relaxation, whereas on SrTiO₃ the NiO films showed strong strain relaxation. Fe₃O₄ films also exhibited strong relaxation, even for films of 5 nm thickness on both NiO/MgO and NiO/SrTiO₃. The magnetite layers on both substrates showed a fourfold magnetic in-plane anisotropy with magnetic easy axes pointing in $⟨100⟩$ directions. The coercive field was strongly enhanced for magnetite grown on NiO/SrTiO₃ due to the higher density of structural defects, compared to magnetite grown on NiO/MgO.
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