Electronic and magnetic structure of epitaxial ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(001\right)/\mathrm{NiO}$ heterostructures grown on MgO(001) and Nb-doped ${\mathrm{SrTiO}}_3\left(001\right)$

We study the underlying chemical, electronic, and magnetic properties of a number of magnetite-based thin films. The main focus is placed onto ${\mathrm{Fe}}_3{\mathrm{O}}_4$(001)/NiO bilayers grown on MgO(001) and Nb-${\mathrm{SrTiO}}_3$(001) substrates. We compare the results with those obtained on pure ${\mathrm{Fe}}_3{\mathrm{O}}_4$(001) thin films. It is found that the magnetite layers are oxidized and ${\mathrm{Fe}}^{3+}$ dominates at the surfaces due to maghemite ($\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$) formation, which decreases with increasing magnetite layer thickness. For layer thicknesses of around 20 nm and above, the cationic distribution is close to that of stoichiometric ${\mathrm{Fe}}_3{\mathrm{O}}_4$. At the interface between NiO and ${\mathrm{Fe}}_3{\mathrm{O}}_4$ we find the Ni to be in a divalent valence state, with unambiguous spectral features in the Ni $2p$ core level x-ray photoelectron spectra typical for NiO. The formation of a significant ${\mathrm{NiFe}}_2{\mathrm{O}}_4$ interlayer can be excluded by means of x-ray magnetic circular dichroism. Magneto-optical Kerr effect measurements reveal significant higher coercive fields compared to magnetite thin films grown on MgO(001), and an altered in-plane easy axis pointing in the $\langle 100\rangle$ direction. We discuss the spin magnetic moments of the magnetite layers and find that a thickness of 20 nm or above leads to spin magnetic moments close to that of bulk magnetite.

Figure 1

Fe $2p$ HAXPES of samples S1 (a) and S2 (b) recorded at excitation energies between 2200 and 9600 eV. Reference spectra of FeO, ${\mathrm{Fe}}_3{\mathrm{O}}_4$, ${\mathrm{NiFe}}_2{\mathrm{O}}_4$, and ${\mathrm{Fe}}_2{\mathrm{O}}_3$ [42] are also shown for comparison. (c) Fe $2{\mathrm{p}}_{3/2}$ core level of samples S1, S2, SN1, and SN2 recorded at $E_{\mathrm{exc}}=2200$ eV.

Figure 2

HAXPE spectra of samples SN1 and SN2 recorded at excitation energies between 2200 and 8100 eV. (a) and (b) Fe $2p$ and Ni $2p$ HAXPES of sample SN1. (c) and (d) Fe $2p$ and Ni $2p$ HAXPES of sample SN2, reference spectra of NiO [47] and ${\mathrm{NiFe}}_2{\mathrm{O}}_4$ [42] are also shown for comparison.

Figure 3

HAXPES valence bands of samples S1, S2, SN1, and SN2 recorded at excitation energies of 2200 eV (a) and 3600 eV (b). Reference spectra of ${\mathrm{SrTiO}}_3$, NiO [47], and ${\mathrm{Fe}}_3{\mathrm{O}}_4$ are also shown for comparison.

Figure 4

Magnetization curves along the substrates [100] and [110] directions of samples MN3 (top), SN3 (center), and S2 (bottom).

Figure 5

Polarization-dependent x-ray absorption spectra performed at the Ni $L_{2,3}$ edges of samples MN1 along with the resulting XMCD difference spectrum. The latter has been also multiplied by a factor of five for better visibility.

Figure 6

(a) Polarization-dependent XA spectra performed at the Fe ${\mathrm{L}}_{2,3}$ edges of sample MN1 along with the resulting XMCD and its integral. (b) XMCD difference spectra and corresponding simulated fit as a result from summarizing the suitable amounts of the charge-transfer multiplet simulations of ${\mathrm{Fe}}^{2+}$ ions in octahedral coordination, and ${\mathrm{Fe}}^{3+}$ ions in octahedral and tetrahedral coordination, respectively.

Figure 7

(a) Fe ${\mathrm{L}}_{2,3}$-XMCD spectra and the corresponding fits from CTM simulations for all samples investigated in this work. The average number of holes ($n_h$) per Fe atom as derived from the CTM simulations is also given in parentheses. (b) Cationic distribution of the divalent and trivalent iron ions in octahedral and tetrahedral coordinations as derived from the CTM simulations.

Figure 8

Calculated magnetic spin moments in ${\mu }_B$/f.u. from the XMCD spin sum rule.