research papers
Time-resolved high-energy X-ray diffraction studies of ultrathin Ni ferrite films on MgO(001)
aDepartment of Physics, Osnabrück University, Barbarastraße 7, 49076 Osnabrück, Germany, and bDeutsches Elektronen-Synchrotron DESY, Notkestraße 85, 22607 Hamburg, Germany
*Correspondence e-mail: aalexander@uni-osnabrueck.de, jwollsch@uni-osnabrueck.de
Time-resolved high-energy X-ray diffraction was used during growth of ultrathin NixFe3−xO4 films with varying Ni content (0 ≤ x ≤ 1.5) deposited on MgO(001) substrates by reactive molecular beam providing an insight into the growth dynamics of these films. In order to obtain structural information, reciprocal-space maps were recorded and the temporal evolution of the Bragg peaks specific to the octahedral and tetrahedral lattice sites of the inverse spinel structure of NixFe3−xO4 was observed during growth of the films. A time delay, corresponding to a coverage of 1.2–1.8 nm, between the appearance of the Bragg reflections originating from octahedral sites and reflections originating exclusively from tetrahedral sites indicates that the ferrite films grow in two stages. In the initial growth phase, a rock salt interface layer is formed. Afterwards, a occurs and the films grow in an inverse spinel structure. The thickness of the initial rock salt phase was found to increase with Ni content and to be responsible for atypical strain in the thin films. Films with Ni contents x > 1 do not show a These films remain in a (deficient) rock salt structure consisting of a mixed Ni–Fe oxide and do not form a spinel structure at all. They show an increased number of NiO clusters as detected by of the accompanied by a significant roughening of the films.
1. Introduction
Transition metal ferrites are the focus of current research as a result of their special properties. For instance, high Curie temperatures and significant magnetic moments make most of them promising candidates for applications in the fields of spintronics (Hoffmann & Bader, 2015; Moussy, 2013; Cibert et al., 2005), spincaloritronics (Bauer et al., 2012) or electrocatalysis (Alshehri et al., 2018; Kashyap & Kurungot, 2018), and as supercapacitors (Kumbhar et al., 2012) or as high-capacity materials in lithium ion batteries (Li et al., 2010; Chu et al., 2004). For insulating ferrite films, one of these applications in the field of spintronics is as a spin filter capable of generating highly spin-polarized currents (Moyer et al., 2015; Marnitz et al., 2015; Matzen et al., 2014, 2012; Moussy, 2013; Bibes & Barthelemy, 2007; Ramos et al., 2007; Lüders et al., 2006; Žutić et al., 2004; Coey & Chien, 2003).
One promising material for such applications is NiFe2O4, an insulating and metal ferrite with a high Curie temperature of TC = 865 K (Matzen et al., 2014; Lüders et al., 2006; Brabers, 1995).
As the spin filter efficiency and transmission of spin currents are highly dependent on the structural quality of the tunnelling barrier and its interfaces (Matzen et al., 2014), it is crucial to grow ultrathin films with low defect densities for the manufacture of high-quality devices. Therefore, MgO(001) is used as a substrate to minimize strain defects and disorder to allow pseudomorphic growth and obtain well ordered nickel ferrite structures, given its small lattice mismatch of 1% compared with NiFe2O4 and an O2− face-centred cubic (f.c.c.) common to both oxides (Rodewald et al., 2020; Chang et al., 2016; Bertram et al., 2013, 2012, 2011; Arora et al., 2008; Tobin et al., 2007; Tsymbal et al., 2003; Margulies et al., 1997).
Nevertheless, defects might occur during growth due to the rock salt structure of the MgO competing with the inverse spinel structure of the film, which can be described as an f.c.c. 2− (as mentioned above) and two cation sublattices with doubled periodicity compared with MgO. The Ni2+ cations are on lattice sites octahedrally coordinated by O2− anions (B sites), while the Fe3+ cations are equally distributed on octahedrally (B sites) and tetrahedrally (A sites) coordinated sites. On the other hand, antiphase boundaries at the interface could be caused by the doubled lattice constant of the film compared with the substrate (Celotto et al., 2003; Eerenstein et al., 2003; Hibma et al., 1999).
of OTo steer the aforementioned properties, detailed knowledge of processes occurring during thin-film growth, especially in the very first growth stages, is necessary, since tunnelling barriers have a width of only a few nanometres. As the cationic ratio, i.e. the chemical composition, has a strong impact on structural, chemical, magnetic and electronic properties (de Biasi & dos Santos, 2017; Moyer et al., 2012, 2011; Lenglet et al., 1987), in this work, NixFe3−xO4 (NFO) thin films with varying Ni contents in the range 0 ≤ x ≤ 1.5 have been studied. The films were grown on MgO(001) by reactive molecular-beam (RMBE). Note that the ferrite with x = 0 is magnetite, which is a half-metallic ferrimagnet having a Curie temperature of TC = 850 K (Cornell & Schwertmann, 2003). The lattice constant of an Ni ferrite depends on its Ni content, as can be seen by comparing the bulk lattice constants of Fe3O4 (8.396 Å; Cornell & Schwertmann, 2003) and NiFe2O4 (8.339 Å; Liebermann, 1972) at room temperature. For instance, magnetite (x = 0) has a lattice mismatch of 0.3% compared with MgO.
To access the growth process directly, X-ray diffraction (XRD) experiments can be performed during deposition of the films (Rodewald et al., 2020; Kuschel et al., 2017). In this work, time-resolved high-energy X-ray diffraction (tr-HEXRD) was used to observe the temporal evolution of the Bragg peaks specific to the octahedral and tetrahedral lattice sites during growth of the NFO films to obtain structural information (Pohlmann et al., 2022).
2. Experimental details
The growth dynamics of NixFe3−xO4 thin films were studied by means of tr-HEXRD. The diffraction experiments were performed at the P07 beamline (EH2) of PETRA III at DESY in a specially designed ultra-high-vacuum (UHV) deposition chamber mounted on the heavy load diffractometer. Grazing-incidence diffraction was then performed during deposition (Pohlmann, 2021).
Before deposition, the MgO(001) substrates were annealed for 1 h at 673 K in a molecular oxygen atmosphere of 1 × 10−4 mbar to clean the substrate from adsorbates and other contaminants (Kuepper et al., 2016; Kuschel et al., 2016). Afterwards, NixFe3−xO4 thin films with varying Ni content (0 ≤ x ≤ 1.5) were deposited by RMBE at a substrate temperature of 523 K in an oxygen atmosphere of 5 × 10−6 mbar. With this technique, Fe and Ni are evaporated simultaneously by electron bombardment of the respective pure metal target. The resulting condense on the substrate surface and react to form an oxide film in the oxygen atmosphere. The growth rates and compositions of the ferrite films were steered by the fluxes of the individual evaporators. Table 1 summarizes the Ni content, final film thickness df and deposition rates of the six analysed samples.
|
For the diffraction experiment with a glancing angle of θ = 0.03°, a photon energy of 71.5 keV was used and the data were collected by a Perkin–Elmer XRD 1621 2D area detector.
Before deposition, the cleaned samples were azimuthally aligned to an angle ω(11L) to fulfil the Bragg condition for the (11L) crystal truncation rod (CTR) of the substrate. Here, L is defined by L = q⊥aMgO/2π, where q⊥ denotes the component of the scattering vector q perpendicular to the film/substrate surface [|q| = (4π/λ)sinθ, where θ is half the scattering angle and λ is the wavelength of the incident radiation] and aMgO = 421.2 pm is the bulk lattice constant of MgO at room temperature. During deposition, the samples were continually rotated between ω(11L) ± 7° with a rotation speed of 2° s−1. By this means, the diffraction signal was collected from the (22L)NFO CTR, which is close to (11L)MgO due to the small lattice mismatch. Afterwards, the detector images of these scans were summed in order to obtain a reciprocal-space map (RSM) every 10 s (Shipilin et al., 2014).
After growth, the deposited film thickness was determined by means of X-ray reflectometry, and an RSM with a full 90° rotation at a speed of 0.25° s−1 was recorded to obtain a complete diffraction image of the film.
In order to probe the cation stoichiometry of the films, the electronic structure was characterized at Osnabrück University by Kα anode with a photon energy of Eph(Al Kα) = 1486.6 eV. The cation stoichiometry was determined by integrating the intensities of both Fe 3p and Ni 3p.
(XPS) after transport under ambient conditions. These studies were performed on a PHI VersaProbe III using a monochromated Al3. Results
Fig. 1 shows an RSM of the as-grown NiFe2O4 (x = 1) film with the expected Bragg positions for NFO and MgO.
One of the goals of this work was to record the temporal evolution of the intensities of the (222)NFO and (224)NFO reflections of the ferrite film to obtain information about the occupation of the tetrahedral and octahedral cation sites of the A and B sublattices, respectively. Due to the small lattice mismatch of 0.3–1% between MgO(001) and the halved lattice constant of NFO of varied Ni content, the Bragg reflections (HKL)MgO of MgO and (2H 2K 2L)NFO of NFO almost coincide and cannot be separated. In addition, the intensity of the substrate reflections compared with the weaker film reflections is too high to be recorded simultaneously. Therefore, the substrate reflections have to be blocked by beamstops on the detector.
Due to the overlap of the (222)NFO reflection of the film and the substrate (111)MgO reflection which has to be blocked, this film reflection cannot be observed directly. Hence, the intensity of the CTR at the position (2 2 2+ɛ)NFO with ɛ = 0.19 was analysed (Pohlmann et al., 2022). In contrast, (224)NFO is fully visible, since the corresponding Bragg reflection (112)MgO of the substrate is forbidden due to the rock salt structure of MgO.
Fig. 2 shows a colour map of the intensity evolution along the (22L)NFO CTR during the growth process of the NiFe2O4 film. Before deposition started, no diffraction intensity corresponding to the film could be detected. As soon as deposition is started, Laue oscillations close to the (222)NFO Bragg reflection can be observed, indicating well ordered crystalline film growth. The period of the Laue fringes changes with increasing film thickness. The Laue fringes were fitted with Gaussians and their distance ΔL was used to determine the time-resolved film thickness df according to
where aMgO is the bulk lattice constant of the substrate (see inset in Fig. 3). For all samples a linear relation between deposition time and film thickness could be found, as shown in Fig. 3 for growth of NiFe2O4 (x = 1). This result clearly emphasizes that deposition was performed at a constant rate and the films grow layer by layer.
Immediately after the start of deposition, the intensity in the (2 2 2+ɛ)NFO region starts to oscillate due to evolving Laue fringes (Fig. 4). In contrast, (224)NFO is absent at the beginning, but it appears, after some delay, with increasing film thickness. This means that the octahedral sites are occupied immediately, while the tetrahedral sites are occupied with a delay.
Fig. 5 shows the critical film thicknesses for the onset of the Laue fringes close to (222)NFO and of the (224)NFO Bragg peaks for different cation stoichiometries.
As mentioned before, for all samples the Laue fringes of (222)NFO related to (2 2 2+ɛ)NFO appear immediately after the start of deposition. The intensity rise in (224)NFO is delayed by 1.23 (4) nm for Fe3O4 (NFO with x = 0). This delay increases with Ni content up to x = 1. For higher Ni content (x > 1), (224)NFO was not visible at all, indicating that no spinel structure has formed and the oxide film has an apparent rock salt structure without occupation of A cation sites or with a strong disorder on the A cation sites.
In order to obtain qualitative information about the roughening of the film, the evolution in the intensity of one specific Laue fringe (second order) was analysed [inset in Fig. 6(a)]. Assuming an ideal smooth surface, the intensity of the Laue oscillations can be described by the N-slit function,
with the out-of-plane scattering vector component q⊥, the layer distance c and the number of layers N (Robinson & Tweet, 1992). With df = Nc being the film thickness and assuming that a maximum is approximately in the centre between two minima, the position of the nth order maximum can be approximated by
Thus, the intensity for an nth-order maximum is given as
Hence, the quotient of the intensity and the square of the film thickness,
should remain constant for an ideal smooth surface/interface. Any deviation from a constant behaviour for the growing film with evolving film thickness is due to a change in roughness of the surface or interface of the film during growth. Thus, a variation in this quotient points to roughening of the film. As seen in Fig. 6(a), this variation can be approximated by a linear function,
The slope m of this approximation can be used as a measure of the roughness.
Fig. 6(b) shows the slope m of this quotient for different cation stoichiometries. For films with an Ni content of x ≤ 1, there is no significant variation in the quotient [equation (5)] of the intensity and the square of the film thickness, indicating no increase in roughness with increasing film thickness. Only the film with x = 0.2 shows a slope related to an increase in roughening, which can be explained by structural disorder as a result of a significantly higher deposition rate (Table 1). For overstoichiometric NFO films (x > 1) the slope decreases with increasing Ni content, which means more roughening of the layer with increasing film thickness. This could be explained by the fact that no ordered spinel structure could be formed, either due to a disorder of the A or due to intermixing rock salt phases.
To obtain information about the temporal evolution of the vertical and lateral layer distances during growth, the position in NFO reflection of the film was determined, since it is the measured reflection with the highest intensity and does not coincide with a Bragg reflection of the substrate. Therefore, the Bragg peak was fitted with a 2D Gaussian to obtain both the out-of-plane and in-plane positions. Overstoichiometric films (x > 1) did not show the (113)NFO reflection due to their apparent rock salt structure (see above). Therefore, we used the interpolated position of (222)NFO to determine the temporal evolution of the lattice constants of these growing films. The lateral position was determined by fitting the (2 2 1.88)NFO CTR and the vertical position by interpolation of the corresponding Laue fringes (inset in Fig. 7). Fig. 7 shows a comparison of these two fitting methods for the NiFe2O4 film where both Bragg peaks (113)NFO and (222)NFO are present. Despite a worse signal-to-noise-ratio for (222)NFO, the two methods provide the same values for the layer distances, confirming the validity of the approach.
of the (113)Fig. 8(a) shows the temporal evolution of the lateral and vertical layer distances for different Ni contents obtained from this analysis. For films with an Ni content up to x = 1 with evolving spinel structure, both the lateral and the vertical layer distances move from higher to lower values, corresponding to a decreasing expansion in the lateral direction and an increasing compression in the vertical direction with increasing film thickness. The relaxation of the lateral layer distance is much smaller than that of the vertical layer distance. For overstoichiometric films (x > 1) with the apparent rock salt structure, there is a slight decrease in the lateral direction and an increase in the vertical layer distance with increasing film thickness, corresponding to a decreasing compression and slightly decreasing expansion in the vertical and lateral directions, respectively. The dashed lines in Fig. 8 correspond to the layer distances expected for bulk FeO, MgO, Fe3O4, NiFe2O4 and NiO at the deposition temperature of 523 K, determined by their respective coefficients (Nelson et al., 2014; Levy et al., 2004; Touloukian et al., 1977).
The lateral layer distances of the NFO films are slightly higher than the expected value for pseudomorphic growth on bulk MgO(001), while the vertical distances tend to be smaller than the expected layer distance for bulk Fe3O4 and NiFe2O4/NiO, which is expected for tetragonally distorted films (Hashimoto et al., 1985). Fig. 8(b) shows the layer distances c for different cation stoichiometries at a film thickness of 8.25 (7) nm. It can be seen that, for the films with an Ni content up to x = 1, the lateral layer distances, starting at the bulk value for MgO, increase with increasing Ni content x. The lateral layer distance of the overstoichiometric films, however, returns to the lattice distance of MgO. The vertical lattice distances decrease from slightly below the expected bulk value for Fe3O4 to below the bulk value of NiFe2O4 and/or NiO. Thus, the films are vertically compressed.
To probe the electronic structure, the a posteriori and compared with reference XPS spectra for FeO, Fe3O4, NiFe2O4 and NiO [Fig. 9(a)]. Since none of the measured spectra show similarities with the reference spectrum of FeO, this indicates that no wustite was formed in large amounts. As expected, the spectrum of the NFO film with x = 0 (magnetite) matches very well with the reference spectrum for Fe3O4. In particular, the feature labelled (I) at about 0.2 eV binding energy is characteristic of magnetite and comes from Fe2+ ions on octahedrally coordinated cation sites (Paul, 2010). The spectra for the unstoichiometric NFO films (0 < x < 1) are a superposition of the spectra of both magnetite and NiFe2O4. The disappearance of the characteristic shoulder (I) with increasing Ni content x indicates that the Fe2+ cations are replaced by Ni2+ ions. Obviously, electronic states at the Fermi edge are increasingly suppressed, pointing to a transition from half-metallic Fe3O4 to insulating NiFe2O4. The spectrum of the NiFe2O4 film (NFO with x = 1) shows strong similarities to the reference spectrum but also shows feature (I) originating from Fe2+ ions on B sites. This means that not all Fe2+ cations are replaced by Ni2+ ions or that there is no perfect inverse spinel structure of the film (`inversion defect').
of the NFO films was analysed by means of XPS performedSince the overstoichiometric films show the characteristic features (II), (III) and (V) of NiO, it is evident that NiO, or an intermixing of NiO and a nickel–iron oxide rock salt structure, has formed in these films. The energy shift of the Ni peak [feature (II)] from the expected position for NiFe2O4 to the expected values for NiO with increasing Ni content [Fig. 9(b)] agrees with the result that, for Ni contents of x > 1, clusters of NiO are formed embedded in a (deficient) rock salt matrix (Bertram et al., 2013).
4. Discussion
While the intensity of the (222)NFO reflection has contributions from both oxygen and cations on the B (octahedral sites), occurring also in the rock salt structure, the (224)NFO reflection only has a contribution from the A cation (tetrahedral), which is exclusive to the spinel structure (Bertram et al., 2013). The (222)NFO reflection appears immediately after the start of deposition, while the formation of the (224)NFO reflection follows with a delay for Ni contents x ≤ 1. This result indicates that in the very first growth stages either the film grows in a rock salt structure NiyFe1−yO (with y = x/3) with only the octahedral sites occupied, instead of an (inverse) spinel structure, or the A is heavily disordered. It is also possible that, initially, a deficient rock salt structure is formed by the NiFe oxide, where A sites are unoccupied and only 3/4 of the B sites are occupied by cations (Bertram et al., 2013). After this initial growth stage, a occurs and the spinel structure starts to form, as already observed for Fe3O4 (Pohlmann et al., 2022; Bertram et al., 2012). The thickness of this initial growth phase was determined to be 1.2–1.8 nm, corresponding to about three or four (rock salt) unit cells or about six to eight atomic layers of FeO or NiO.
For higher Ni contents x > 1, the formation of a spinel structure is not observed at all and the films have the rock salt structure NiyFe1−yO or a deficient rock salt structure. Thus, for 1 < x < 1.3 there is a critical Ni content where no spinel structure can be formed. This result is also supported by the XPS measurements of the showing that NiO is formed for x > 1, which crystallizes in a rock salt structure. Hence, one can assume that the films consist of different phases as clusters of NiO embedded in an NiyFe1−yO rock salt matrix or in a deficient rock salt structure.
The roughening of the surface increases with increasing film thickness and Ni content for x > 1. This behaviour is also observed for the film with x = 0.2, which was grown with a significantly higher deposition rate. This critical stoichiometry for the film roughening correlates with the transition from spinel to apparent rock salt structure. This could be due to competition between the spinel NFO structure and the (deficient) rock salt NiyFe1−yO structure, or to a roughness related to disorder of the A which also affects the interface.
While these overstoichiometric films (x > 1) show a lateral film-thickness-independent expansion towards the layer distance of MgO and a vertical compression compared with the expected bulk lattice constant, as is expected for pseudomorphic growth, films with Ni contents of x ≤ 1 show behaviour not expected from classical growth theory. As the obtained lateral layer distance is larger than that of MgO, the tensile strain on the NFO films has to be much higher. This effect was already observed by Kuschel et al. (2018) for the growth of magnetite on an NiO interlayer pseudomorphic to the MgO(001) substrate and could be attributed to the formation of antiphase boundaries (APBs). Thus, we conclude that this effect also appears here at the interface of NFO and MgO, since the lattice constant of NFO is roughly twice as large as that of the substrate. Accordingly, the additional tensile strain induced by APBs increases at a higher Ni content, caused by the higher lattice mismatch of NiFe2O4 to MgO versus Fe3O4 to MgO [Fig. 8(b)]. Assuming the rock salt structure for overstoichiometric films, it is evident that no APBs are formed and pseudomorphic growth occurs.
A similar observation can be found in the relaxation behaviour of the NFO films. The overstoichiometric films with apparent rock salt structure show decreasing compression in the vertical direction and slightly decreasing expansion in the lateral direction with increasing film thickness, which is expected for (quasi-)pseudomorphic growth of NFO or NiO on MgO and supports the assumption of the deficient rock salt structure. In contrast, the NFO films with Ni contents of x ≤ 1 show decreasing expansion and increasing compression for the lateral and vertical directions, respectively, so pseudomorphic growth of NFO on MgO cannot be assumed for these films. A similar behaviour was observed by Bertram et al. (2012) for Fe3O4 grown on MgO at room temperature. This effect is assumed to be due to the initial rock salt interface layer, which could be induced by the rock salt structure of the substrate. Another explanation could be a deficit of oxygen in the initial growth phase. Oxygen vacancies in the substrate due to the preparation regime could bind oxygen from the film, resulting in a deficient rock salt structure at the interface consisting of an intermixing wustite and NiO film (Bertram et al., 2013).
5. Summary
In conclusion, ultrathin NixFe3−xO4 films with varying Ni content (0 ≤ x ≤ 1.5) were grown by RMBE. Using tr-HEXRD during deposition provides insight into the growth dynamics of these films. It was found that NFO films grow in two stages. At first, the films start to grow in a rock salt phase. Then, after a coverage of 1.2–1.8 nm, a occurs and the films grow in an inverse spinel structure for Ni contents x ≤ 1. The thickness of the interface layer formed during the initial growth phase, responsible for atypical strain in the NFO film, increases with Ni content. However, this transition could only be observed for films with a maximum Ni content of x = 1. Overstoichiometric films did not form a spinel structure and remained in a rock salt structure. spectra show that the densities of states at the Fermi edge are reduced with increasing Ni content. Thus, one needs high Ni content to obtain the insulating Ni ferrite films that are necessary for application as spin filters.
Acknowledgements
We acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. Parts of this research were carried out on PETRA III beamline P07 EH2. Beamtime was allocated for proposal II-20170008. We would also like to acknowledge support by the PETRA III sample environment group. Open access funding enabled and organized by Projekt DEAL.
Funding information
Financial support from the Bundesministerium für Bildung und Forschung (grant No. FKZ 05K16MP1) is gratefully acknowledged. We are also grateful for the kind support of the Deutsche Forschungsgemeinschaft (DFG; grant Nos. KU2321/6-1 and WO533/20-1).
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