Impact of Strain and Morphology on Magnetic Properties of Fe3O4/NiO Bilayers Grown on Nb:SrTiO3(001) and MgO(001)

Kuschel O, Pathé N, Schemme T, et al. Impact of Strain and Morphology on Magnetic Properties of Fe3O4/NiO Bilayers Grown on Nb:SrTiO3(001) and MgO(001). Materials. 2018;11(7): 1122.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 5nm 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 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

Influence of growth conditions on magnetic, electronic and structural properties of ultrathin Co-based oxide films

This work deals with the growth of ultrathin magnetite (Fe3O4), cobalt ferrite (CoFe2O4) and nickel cobaltite (NiCo2O4) films and their characterization in terms of magnetic, electronic, and structural properties. The films are grown on different substrates using reactive molecular beam epitaxy (RMBE). First, a Fe3O4/CoxFe3-xO4 (x=0.5 and x=1) bilayer film system is investigated in terms of interfacial magnetization. Here, the Fe3O4 layer thickness varies from 0nm to 13nm, while the CoxFe3-xO4 layer thickness is kept constant. The quality and stoichiometry of the films are checked in-situ using X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). X-ray reflectivity (XRR) measurements are used to determine the film thicknesses of the respective films. To determine the cationic distribution and magnetic moments, X-ray magnetic circular dichroism (XMCD) measurements are performed and afterwards multiplet calculations and sum rules are applied, resulting in increased magnetization at the interface of the bilayers. Finally, magnetic hysteresis measurements are carried out using a superconducting quantum interference device (SQUID) showing typical exchange-spring behavior. In the second step, the growth behavior of ultrathin CoFe2O4 films in the early stages is investigated using operando hard x-ray photoelectron spectroscopy (HAXPES). For this purpose, the CoFe2O4 films are grown on Nb doped SrTiO3(001) by RMBE and simultaneously HAXPES measurements are performed during deposition. The varying parameter is the oxygen partial pressure, which is changed from 5E-7 mbar to 5E-6 mbar. This results in a classification of the samples into metallic, light oxide and cobalt ferrite-like growth. Final X-ray diffraction measurements confirm the findings that emerge from the HAXPES measurements. Finally, the material system is changed and NiCo2O4, also an Co-based oxide, is investigated. In particular, the influence of the use of oxygen plasma on the growth and stability is investigated here. For this purpose, two series of NiCo2O4 films on the substrates MgO(001), SrTiO3(001) and MgAl2O4(001) are investigated. Molecular oxygen is used for the first series and oxygen plasma is used for the second series. The in-situ XPS and LEED measurements show that a rock salt structure forms when molecular oxygen is used and the expected spinel structure forms when oxygen plasma is used. After transport under ambient conditions, XRR measurements are performed in order to determine the film thicknesses. Moreover, XRD and HAXPES measurements for structural and electronic properties are performed. The results of XRD measurements show that no spinel structure is present, regardless of whether molecular oxygen or oxygen plasma is used. The HAXPES measurements also show no spinel structure anymore, not even in deeper layers

Influence of Oxygen Plasma on the Growth and Stability of Epitaxial NiCo2_2O4_4 Ultrathin Films on Various Substrates

In this work, we investigated the influence of oxygen plasma on the growth of nickel cobaltite (NiCo$_2$O$_4$) thin films compared to growth in a molecular oxygen atmosphere. The films were grown on MgO(001), MgAl$_2$O$_4$(001) and SrTiO$_3$(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 (NiCo$_2$O$_4$). 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 (NiCo$_2$O$_4$) 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 (NiCo$_2$O$_4$) spectrum anymore. Thus, on the one hand, (NiCo$_2$O$_4$) 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 (NiCo$_2$O$_4$) films using molecular oxygen. These films had a rock salt structure that was stable under ambient conditions

Magnetoelectric Response of Laminated Cantilevers Comprising a Magnetoactive Elastomer and a Piezoelectric Polymer, in Pulsed Uniform Magnetic Fields

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

Real-Time Monitoring of Strain Accumulation and Relief during Epitaxy of Ultrathin Co Ferrite Films with Varied Co Content

Ultrathin CoxFe3−xO4 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 CoxFe3−xO4 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

Real-Time Monitoring the Growth of Epitaxial Cox_xFe3−x_{3−x}O4_4 Ultrathin Films on Nb-Doped SrTiO3_3(001) via Reactive Molecular Beam Epitaxy by Means of Operando HAXPES

In this work, we present a comprehensive study on real-time monitoring the growth of epitaxial Co$_x$Fe$_{3−x}$O$_4$ thin films grown on SrTiO$_3$(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

Inferface Magnetization Phenomena in Epitaxial Thin Fe3_3O4_4/Cox_xFe3–x_{3– x}O4_4 Bilayers

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 CoxFe$_{3–x}$O$_4$ stoichiometries (x = 1 and x = 0.5) in combination with Fe$_3$O$_4$ 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

Real-Time Monitoring the Growth of Epitaxial CoxFe3−xO4 Ultrathin Films on Nb-Doped SrTiO3(001) via Reactive Molecular Beam Epitaxy by Means of Operando HAXPES

In this work, we present a comprehensive study on real-time monitoring the growth of epitaxial CoxFe3−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

Structural and magnetic investigation of the interfaces of with and without NiO interlayer

Pohlmann T, Bertram F, Thien J, et al. Structural and magnetic investigation of the interfaces of with and without NiO interlayer. Physical Review B. 2022;105(23): 235436.We present an investigation on the structural and magnetic properties of the interfaces of Fe3O4/MgO(001) and Fe3O4/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 Fe3O4/MgO(001), the magneto-optical depth profiles at the Fe2+oct and the Fe3+oct resonant energies follow exactly the structural profile, while the magneto-optical depth profile at the Fe3+tet resonance is offset by 3.2±1.3 Å from the interface, consistent with a B-site interface termination of Fe3O4 with fully intact magnetic order. In contrast, for Fe3O4/NiO(001), the magneto-optical depth profiles at the Fe2+oct and the Ni2+ resonances agree with the structural profile, but the interface positions of the magneto-optical depth profiles at the Fe3+oct and the Fe3+tet resonances are spatially shifted by 3.3±1.4 and 2.7±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 L3 edge might hint at uncompensated magnetic moments throughout the NiO film