Enhanced magnetic moment in ultrathin Fe-doped CoFe${}_2$O${}_4$ films

The effect of film thickness on the magnetic properties of ultrathin Fe-doped cobalt ferrite (Co${}_{1-x}$Fe${}_{2+x}$O${}_4$) grown on MgO (001) substrates is investigated by superconducting quantum interference device magnetometry and x-ray magnetic linear dichroism, while the distribution of the Co${}^{2+}$ cations between the octahedral and tetrahedral lattice sites is studied with x-ray absorption spectroscopy. For films thinner than 10 nm, there is a large enhancement of the magnetic moment; conversely, the remanent magnetization and coercive fields both decrease, while the magnetic spin axes of all the cations become less aligned with the [001] crystal direction. In particular, at 300 K the coercive fields of the thinnest films vanish. The spectroscopy data show that no changes occur in the cation distribution as a function of film thickness, ruling this out as the origin of the enhanced magnetic moment. However, the magnetic measurements all support the possibility that these ultrathin Fe-doped CoFe${}_2$O${}_4$ films are transitioning into a superparamagnetic state, as has been seen in ultrathin Fe${}_3$O${}_4$. A weakening of the magnetic interactions at the antiphase boundaries, leading to magnetically independent domains within the film, could explain the enhanced magnetic moment in ultrathin Fe-doped CoFe${}_2$O${}_4$ and the onset of superparamagnetism at room temperature.

Figure 1

In-plane and out-of-plane $M$-$H$ loops at 100 K of Co${}_{1-x}$Fe${}_{2+x}$O${}_4$ films with stoichiometries of (a) and (b) $x$ = 0, (c) and (d) $x$ = 0.19, (e) and (f) $x$ = 0.60, and (g) and (h) $x$ = 0.78, respectively.

Figure 2

In-plane and out-of-plane saturation and remanent magnetic moments vs film thickness for Co${}_{1-x}$Fe${}_{2+x}$O${}_4$ films with stoichiometries of (a) $x$ = 0, (b) $x$ = 0.19, (c) $x$ = 0.60, and (d) $x$ = 0.78 measured at 100 K. The dotted lines correspond to the bulk magnetic moment for each stoichiometry.

Figure 3

Out-of-plane $M$-$H$ loops at 300 K of Co${}_{1-x}$Fe${}_{2+x}$O${}_4$ films with stoichiometries of (a) $x$ = 0, (b) $x$ = 0.19, (c) $x$ = 0.60, and (d) $x$ = 0.78.

Figure 4

(a) Experimental Co $L_{2,3}$ XAS spectra for Co${}_{0.40}$Fe${}_{2.60}$O${}_4$ thin films with thicknesses of 3.4, 6.7, 10.1, 13.4, and 20.0 nm (similar XAS spectra are seen for other Co${}_{1-x}$Fe${}_{2+x}$O${}_4$ samples). (b) $L_3$ edge for the spectra in (a). (c) Comparison of the Co $L_{2,3}$ XAS experimental and LFM-calculated spectra for 20.0 nm Co${}_{0.40}$Fe${}_{2.60}$O${}_4$ sample; shown below are the individual LFM spectra for Co${}^{2+}$ octahedral and tetrahedral cations.

Figure 5

Inversion parameter vs film thickness for Co${}_{1-x}$Fe${}_{2+x}$O${}_4$ films with stoichiometries of (a) $x$ = 0, (b) $x$ = 0.19, (c) $x$ = 0.60, and (d) $x$ = 0.78. Linear fits to the data provide a guide to the eye.

Figure 6

(a) Interpolated Co $L_{2,3}$ XAS spectra for a 20.0 nm Co${}_{0.40}$Fe${}_{2.60}$O${}_4$ film with the electric field of the x-rays polarized perpendicular and parallel to the [001] crystal direction, and the corresponding XMLD spectrum. Co $L_{2,3}$ XMLD spectra for Co${}_{1-x}$Fe${}_{2+x}$O${}_4$ films with stoichiometries of (b) $x$ = 0, (c) $x$ = 0.19, (d) $x$ = 0.60, and (e) $x$ = 0.78.

Figure 7

(a) Interpolated Fe $L_{2,3}$ XAS spectra for a 20.0 nm Co${}_{0.40}$Fe${}_{2.60}$O${}_4$ film with the electric field of the x-rays polarized perpendicular and parallel to the [001] crystal direction, and the corresponding XMLD spectrum. Fe $L_{2,3}$ XMLD spectra for Co${}_{1-x}$Fe${}_{2+x}$O${}_4$ films with stoichiometries of (b) $x$ = 0, (c) $x$ = 0.19, (d) $x$ = 0.60, and (e) $x$ = 0.78.