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

Advancement of growth and characteristics of ultrathin ferrite films

Within this thesis, (ultra)thin NiFe2O4 (NFO) and CoFe2O4 (CFO) films are prepared via reactive molecular beam epitaxy (RMBE) on MgO(001) and SrTiO3(001) substrates and are characterized in terms of their structural, electronic, and magnetic properties. In a first step, the structural properties of ultrathin off-stoichiometric NixFe(3-x)O4 films (0<x<1.5) deposited via RMBE on MgO(001) are investigated in situ during film deposition by means of synchrotron radiation-based x-ray diffraction (XRD) and ex situ after film growth by high energy surface x-ray diffraction (HESXRD). In the second major step of this work, a more extensive study on the dependence of the cationic ratio in NixFe(3-x)O4 thin films (0<x<2.07) grown on MgO(001) is conducted. The film surface structure and chemical composition is characterized in situ by low energy electron diffraction (LEED) and laboratory-based soft x-ray photoelectron spectroscopy (XPS), respectively. Film thicknesses are determined via analysis of x-ray reflectivity (XRR) data, while the film structure is analyzed by XRD measurements. Further, chemical properties and the electronic structure of the NFO films with focus on the cationic valencies of Ni and Fe cations with varying x is investigated by means of (angle-resolved) hard x-ray photoelectron spectroscopy [(AR-)HAXPES]. Complementary x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) investigations are conducted to obtain information on the cationic site occupancies and on the element-specific magnetic moments. The latter are compared to magnetic properties characterized via superconducting quantum interference device (SQUID) magnetometry. In a third step, the type of substrate is changed to SrTiO3(001) to investigate the influence of a larger strain applied by the substrate to NFO films with varying thicknesses. Structural characterization at the surfaces and in the films is conducted by means of LEED, XRR, and (grazing incidence) XRD, whereas XPS and HAXPES provide information on the chemical composition and electronic structure in the near-surface region and in deeper subsurface layers, respectively. Magnetic properties are characterized by SQUID magnetometry. In a fourth step, an alternative pathway for the formation of ferrite thin films is demonstrated exemplarily for CoFe2O4 films on SrTiO3(001), which are formed by interdiffusion of Fe3O4/CoO bilayers. The interdiffusion process was monitored via XRR, soft XPS and AR-HAXPES to determine the bilayer/film structure, stoichiometry, and chemical properties. Analysis of complementary XAS measurements provides additional information on the occupancies of Fe and Co cations during interdiffusion. Final SQUID magnetometry measurements are performed to gain information on the magnetic properties before and after complete interdiffusion. Overall, within this thesis, it was demonstrated that NFO and CFO thin films can be prepared in high structural quality with sharp interfaces and surfaces, which is crucial for the applicability in the fields of spintronics and spincaloritronics

Surface morphology of ultrathin hex−Pr2O−3hex-Pr_{2}O-{3} films on Si(1 1 1)

In this work, the morphology of the surface of hexagonal Pr2O3(0 0 0 1) films grown onSi(1 1 1) is studied by high-resolution low-energy electron diffraction combined with spotprofile analysis. For this purpose, praseodymia films prepared by molecular beam epitaxywere capped with protecting amorphous germanium films. After removal of the capping layersdue to heating in diluted oxygen atmosphere the surface properties of the oxide film wereinvestigated in situ with Auger electron spectroscopy and spot profile analysis low energyelectron diffraction. The removal of the capping layer has no impact on the hexagonalPr2O3(0 0 0 1) film structure which is shown by x-ray diffraction. Surface sensitive electrondiffraction confirms that the surface of the oxide film has hexagonal structure. Diffraction spotprofile analysis shows that the film surface has grain structure without any mosaic spread dueto the negligible lateral lattice mismatch between hexagonal Pr2O3(0 0 0 1) and Si(1 1 1). Inaddition, single atomic steps with complete bulk unit cell height are present at the surface. Thedensity of the atomic steps is small pointing again to the high quality of the surface ofhexagonal Pr2O3 films compared to cubic Pr2O3 films grown on Si(1 1 1)

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

Quadratic magnetooptic spectroscopy setup based on photoelastic light modulation

Silber R, Tomíčková M, Rodewald J, et al. Quadratic magnetooptic spectroscopy setup based on photoelastic light modulation. Photonics and Nanostructures - Fundamentals and Applications. 2018;31:60-65.In most of the cases the magnetooptic Kerr effect (MOKE) techniques rely solely on the effects linear in magnetization (M). Nevertheless, a higher-order term being proportional to M^2 and called quadratic MOKE (QMOKE) can additionally contribute to experimental data. Handling and understanding the underlying origin of QMOKE could be the key to utilize this effect for investigation of antiferromagnetic materials in the future due to their vanishing first order MOKE contribution. Also, better understanding of QMOKE and hence better understanding of magnetooptic (MO) effects in general is very valuable, as the MO effect is very much employed in research of ferro- and ferrimagnetic materials. Therefore, we present our QMOKE and longitudinal MOKE spectroscopy setup with a spectral range of 0.8–5.5 eV. The setup is based on light modulation through a photoelastic modulator and detection of second-harmonic intensity by a lock-in amplifier. To measure the Kerr ellipticity an achromatic compensator is used within the setup, whereas without it Kerr rotation is measured. The separation of QMOKE spectra directly from the measured data is based on measurements with multiple magnetization directions. So far the QMOKE separation algorithm is developed and tested for but not limited to cubic (001) oriented samples. The QMOKE spectra yielded by our setup arise from two quadratic MO parameters Gs and 2G44, being elements of quadratic MO tensor G, which describes perturbation of the permittivity tensor in the second order in M

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

Structural and magnetic investigation of the interfaces of Fe3_3O4_4/MgO(001) with and without NiO interlayer

We present an investigation on the structural and magnetic properties of the interfaces of $\mathrm{Fe_3O_4/MgO(001)}$ and $\mathrm{Fe_3O_4/NiO/MgO(001)}$ by extracting cation-selective magnetooptical depth profiles by means of x-ray magnetic reflectivity (XRMR) in combination with charge-transfer multiplet simulations of x-ray magnetic circular dichroism (XMCD) data.For $\mathrm{Fe_3O_4/MgO(001)}$, the magnetooptical depth profiles at the $\mathrm{Fe^{2+}_{oct}}$ and the $\mathrm{Fe^{3+}_{oct}}$ resonant energies follow exactly the structural profile, while the magnetooptical depth profile at the $\mathrm{Fe^{3+}_{tet}}$ resonance is offset by $3.2\pm1.3\,$\r{A} from the interface, consistent with a B-site interface termination of $\mathrm{Fe_3O_4}$ with fully intact magnetic order. In contrast, for $\mathrm{Fe_3O_4/NiO(001)}$, the magnetooptical depth profiles at the $\mathrm{Fe^{2+}_{oct}}$ and the $\mathrm{Ni^{2+}}$ resonances agree with the structural profile, but the interface positions of the magnetooptical depth profiles at the $\mathrm{Fe^{3+}_{oct}}$ and the $\mathrm{Fe^{3+}_{tet}}$ resonances are laterally shifted by $3.3\pm 1.4\,$\r{A} and $2.7\pm0.9\,$\r{A}, respectively, not consistent with a magnetically ordered stoichiometric interface. This may be related to an intermixed $\mathrm{(Ni,Fe)O}$ layer at the interface. The magnetooptical depth profiles at the Ni $L_3$ edge reveal uncompensated magnetic moments throughout the NiO film

Impact of Strain and Morphology on Magnetic Properties of Fe3O4/NiO\mathrm{Fe_{3}O_{4}/NiO} Bilayers Grown on Nb:SrTiO3\mathrm{Nb:SrTiO_{3}}(001) and MgO(001)

We present a comparative study of the morphology and structural as well as magnetic properties of crystalline Fe$_3$O$_4$/NiO bilayers grown on both MgO(001) and SrTiO$_3$(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 SrTiO$_3$. In the case of Fe$_3$O$_4$, the misfit strain on MgO(001) and SrTiO$_3$(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/SrTiO$_3$ 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 SrTiO$_3$ the NiO films showed strong strain relaxation. Fe$_3$O$_4$ films also exhibited strong relaxation, even for films of 5 nm thickness on both NiO/MgO and NiO/SrTiO$_3$. 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/SrTiO$_3$ due to the higher density of structural defects, compared to magnetite grown on NiO/MgO

Cation- and lattice-site-selective magnetic depth profiles of ultrathin Fe3O4Fe_{3}O_{4} (001) films

A detailed understanding of ultrathin film surface properties is crucial for the proper interpretation of spectroscopic, catalytic, and spin-transport data. We present x-ray magnetic circular dichroism (XMCD) and x-ray resonant magnetic reflectivity (XRMR) measurements on ultrathin $Fe_{3}O_{4}$ films to obtain magnetic depth profiles for the three resonant energies corresponding to the different cation species $Fe^{2+}_{oct}$, $Fe^{3+}_{tet}$, and $Fe^{3+}_{oct}$ located on octahedral and tetrahedral sites of the inverse spinel structure of $Fe_{3}O_{4}$. By analyzing the XMCD spectrum of $Fe_{3}O_{4}$ using multiplet calculations, the resonance energy of each cation species can be isolated. Performing XRMR on these three resonant energies yields magnetic depth profiles that each correspond to one specific cation species. The depth profiles of both kinds of $Fe^{3+}$ cations reveal a $(3.9±1.0)$−Å-thick surface layer of enhanced magnetization, which is likely due to an excess of these ions at the expense of the $Fe^{2+}_{oct}$ species in the surface region. The magnetically enhanced $Fe^{3+}_{tet}$ layer is additionally shifted about $2.9±0.4$Å farther from the surface than the $Fe^{3+}_{oct}$ layer