From ${\mathrm{Fe}}_3{\mathrm{O}}_4$/NiO bilayers to ${\mathrm{NiFe}}_2{\mathrm{O}}_4$-like thin films through Ni interdiffusion

Ferrites with (inverse) spinel structure display a large variety of electronic and magnetic properties, making some of them interesting for potential applications in spintronics. We investigate the thermally induced interdiffusion of ${\mathrm{Ni}}^{2+}$ ions out of NiO into ${\mathrm{Fe}}_3{\mathrm{O}}_4$ ultrathin films, resulting in off-stoichiometric nickel ferrite–like thin layers. We synthesized epitaxial ${\mathrm{Fe}}_3{\mathrm{O}}_4$/NiO bilayers on Nb-doped ${\mathrm{SrTiO}}_3$(001) substrates by means of reactive molecular beam epitaxy. Subsequently, we performed an annealing cycle comprising three steps at temperatures of 400 ${}^{\circ }\mathrm{C}$, 600 ${}^{\circ }\mathrm{C}$, and 800 ${}^{\circ }\mathrm{C}$ under an oxygen background atmosphere. We studied the changes of the chemical and electronic properties as result of each annealing step with help of hard x-ray photoelectron spectroscopy and found a rather homogeneous distribution of Ni and Fe cations throughout the entire film after the overall annealing cycle. For one sample we observed a cationic distribution close to that of the spinel ferrite ${\mathrm{NiFe}}_2{\mathrm{O}}_4$. Further evidence comes from low-energy electron diffraction patterns indicating a spinel-type structure at the surface after annealing. Site- and element-specific hysteresis loops performed by x-ray magnetic circular dichroism uncovered the antiferrimagnetic alignment between the octahedral coordinated ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ ions and the ${\mathrm{Fe}}^{3+}$ ion in tetrahedral coordination. We find a quite low coercive field of 0.02 T, indicating a rather low defect concentration within the thin ferrite films.

Figure 1

LEED patterns for sample A recorded directly after (a) preparation of the ${\mathrm{SrTiO}}_3$ substrate, (b) deposition of $\mathrm{NiO}$, (c) deposition of ${\mathrm{Fe}}_3{\mathrm{O}}_4$, and (d) the last annealing step of $800{}^{\circ }\mathrm{C}$. For comparison, (e) shows the LEED pattern for sample B after the final annealing step. Marked with red squares are the respective $(1\times 1)$ surface unit cells in reciprocal space. The blue square indicates the $(\sqrt{2}\times \sqrt{2})\mathrm{R}{45}^{\circ }$ superstructure typical for magnetite.

Figure 2

Soft XPS spectra of (a) Fe $2p$ and (b) Ni $2p$ regions of sample A and (c) Fe $2p$ and (d) Ni $2p$ regions of sample B after each annealing step. For sample A the spectra of the untreated sample are shown exemplarily.

Figure 3

Reflectivity curves and calculations from XRR measurements before and after the annealing experiments (a) for sample A and (b) for sample B. The insets show the underlying models.

Figure 4

HAXPES spectra of Fe $2p$ core level at $10{}^{\circ }$ off-normal photoelectron emission after annealing at different temperatures (a) for sample A and (b) for sample B.

Figure 5

HAXPES spectra of Ni $2p$ core level at $10{}^{\circ }$ off-normal photoelectron emission after annealing at different temperatures (a) for sample A and (b) for sample B.

Figure 6

Relative photoelectron yield at different off-normal emission angles (a) for sample A and (b) for sample B. The dashed lines show the calculated intensities using the models obtained from XRR analysis.

Figure 7

SR-XRD measurements along the ($00L)$ direction and calculated intensities. The insets show the layer structures used in the calculation. The model is similar to that obtained from the analysis of the XRR (see inset in Fig. 3).

Figure 8

(a) Ni ${\mathrm{L}}_{2,3}$-XMCD spectra of samples A and B. (b) Fe ${\mathrm{L}}_{2,3}$-XMCD spectra of samples A and B. (c) Experimental Fe ${\mathrm{L}}_{2,3}$ edge XMCD of samples A and B and the corresponding XTM4XAS fits with and without considering octahedral coordinated ${\mathrm{Fe}}^{2+}$ ions.

Figure 9

(a) Element- and site-specific hysteresis loops of the Ni $L_3$ and Fe $L_3$ intensities of sample B. (b) Expanded view of the loops near $H=0$ T. Insets show the Ni hysteresis loop measured in perpendicular (out-of-plane) geometry.