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Instruments / Technical Area	Publication Details	Published
I07-Surface & interface diffraction	Real-time monitoring the growth of strained off-stoichiometric Ni x Fe3−xO4 ultrathin films on MgO(001) J. Rodewald , J. Thien , T. Pohlmann , M. Hoppe , F. Bertram , K. Kuepper , J. Wollschlager Applied Physics Letters 117 DOI: 10.1063/5.0013925 Diamond Proposal Number(s): [16299] Show Abstract ▼ Abstract: NixFe3−x Ni x Fe 3 − x O4 thin films with varying Ni amount (0 ≤ ≤ x ≤ ≤ 1.5) were deposited on MgO(001) via reactive molecular beam epitaxy. The growth process was monitored during film deposition by means of X-ray diffraction. All prepared films exhibit a well-ordered structure with complete vertical crystallinity throughout the whole film growth and flat surfaces of the final films independent of the Ni amount. An enhancement of the vertical compression in the initial growth continuously decreases up to a film thickness of 8 nm. During further growth, all films exhibit residual and constant vertical compression with lateral adaption of the final films to the substrate lattice, as observed by high energy surface X-ray diffraction experiments. Hard X-ray photoelectron spectroscopy measurements of the final films reveal increasing Fe3+ Fe 3 + :Fe2+ Fe 2 + ratios for higher Ni content and point to additional NiO agglomerations within the films exceeding the stoichiometric Ni amount of x = 1.	Jul 2020
I07-Surface & interface diffraction	Real-time monitoring of strain accumulation and relief during epitaxy of ultrathin Co ferrite films with varied Co content Jannis Thien , Jari Rodewald , Tobias Pohlmann , Kevin Ruwisch , Florian Bertram , Karsten Kuepper , Joachim Wollschlager Materials 16 DOI: 10.3390/ma16237287 Diamond Proposal Number(s): [20857] Open Access Show Abstract ▼ Abstract: Ultrathin Co𝑥 Fe3−𝑥 O4 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 Co𝑥 Fe3−𝑥 O4 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 Fe3+ /Fe2+ cation ratios for a higher Co content.	Nov 2023
I07-Surface & interface diffraction I09-Surface and Interface Structural Analysis	Formation of ultrathin cobalt ferrite films by interdiffusion of Fe3O4 / CoO bilayers J. Rodewald , J. Thien , T. Pohlmann , M. Hoppe , F. Timmer , F. Bertram , K. Kuepper , J. Wollschlager Physical Review B 100 DOI: 10.1103/PhysRevB.100.155418 Diamond Proposal Number(s): [15205, 15334] Show Abstract ▼ Abstract: In this work an alternate pathway is demonstrated to form ultrathin cobalt ferrite ( Co x Fe 3 − x O 4 ) films by interdiffusion of Fe 3 O 4 /CoO bilayers. Bilayer samples with different Fe 3 O 4 /CoO thickness ratios have been prepared by reactive molecular beam epitaxy on Nb-doped SrTiO 3 (001) substrates to obtain cobalt ferrite films of varied stoichiometry. Subsequently, oxygen-assisted postdeposition annealing experiments for consecutive temperature steps between 300 ∘ C and 600 ∘ C have been conducted monitoring the interdiffusion process by means of high-resolution x-ray reflectivity, soft and angle-resolved hard x-ray photoelectron, and x-ray absorption spectroscopy. Magnetic properties were characterized using superconducting quantum interference device magnetometry. The interdiffusion process starts from 300 ∘ C annealing temperature and is completed for temperatures above 500 ∘ C . For completely interdiffused films with Co:Fe ratios larger than 0.84:2 a thin segregated CoO layer on top of the ferrite is formed. This CoO segregation is attributed to surface and interface effects. In addition, multiplet calculations of x-ray absorption spectra are performed to determine the occupancy of different sublattices. These results are correlated with the magnetic properties of the ferrite films. A stoichiometric CoFe 2 O 4 film with partial inversion has been formed exhibiting homogeneously distributed Co 2 + and mainly Fe 3 + valence states if the initial Co:Fe content is 1.09:2. Thus, for the formation of stoichiometric cobalt ferrite by the proposed postdeposition annealing technique an initial Co excess has to be provided as the formation of a top CoO layer is inevitable.	Oct 2019
NONE-No attached Diamond beamline	Inferface magnetization phenomena in epitaxial thin Fe3O4/COxFe3–xO4 bilayers K. Ruwisch , T. Pohlmann , M. Hoppe , F. Bertram , P. Shafer , J. Wollschlager , K. Küpper The Journal Of Physical Chemistry C DOI: 10.1021/acs.jpcc.1c05274 Show Abstract ▼ Abstract: In this work, we present a study about the chemical and magnetic properties of thin magnetite/cobalt ferrite bilayers deposited on MgO(001). Two series of samples with different CoxFe3–xO4 stoichiometries (x = 1 and x = 0.5) in combination with Fe3O4 layers of varying thickness were prepared by reactive molecular beam epitaxy. The quality of the respective films were controlled by means of in situ X-ray photelectron spectroscopy and low energy electron diffraction. Stoichiometry and electronic structure were carried out by hard X-ray photoelectron spectroscopy. To determine the cationic distribution and magnetic moments, X-ray magnetic circular dichroism measurements were performed and charge transfer multiplet and sum rule calculations were applied. Here we find an enhanced interface magnetization for the bilayers. Additionally, superconducting quantum interference device measurements showed characteristic exchange-spring behavior.	Oct 2021

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