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

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

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

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)

Here, we present the (element-specific) magnetic properties and cation ordering for ultrathin Co-rich cobalt ferrite films. Two Co-rich CoxFe3−xO4 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/Fe3O4 bilayers deposited before on Nb-doped SrTiO3(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 Co3+ cations in the low-spin state on octahedral sites for the Co1.4Fe1.6O4 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