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.

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.

We have performed a detailed study of the magnetic and electronic properties of highly manganese-doped β-NaGdF4 and β-NaEuF4 nanoparticles with a narrow size distribution. XPS as well as XRF experiments confirm the successful doping of 11 ± 2% Mn into β-NaGdF4 and β-NaEuF4 nanoparticles, i.e., a much higher Mn concentration than has been previously reported for the incorporation of bivalent transition metal ions into the β phase of NaREF4 (RE = rare earth) nanocrystals. Owing to the high manganese concentration, we observed quenching of the Mn2+ emission in both materials and even quenching of the red Eu3+ emission in β-NaEuF4:11% Mn particles. We have investigated the Mn–Mn magnetic interactions by means of EPR spectroscopy, and SQUID magnetometry further supports a significant increase of the magnetization of the Mn-doped β-NaEuF4 nanoparticles compared to pure β-NaEuF4. By using element specific X-ray absorption spectroscopy and X-ray magnetic circular dichroism, we show that the magnetism of Mn-doped β-NaEuF4 is dominated by the divalent Mn ions for Mn-doped β-NaEuF4. A parallel alignment of the gadolinium and manganese magnetic moments is found.

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.

No abstract