Enhanced magnetic moment and conductive behavior in $\mathrm{Ni}{\mathrm{Fe}}_2{\mathrm{O}}_4$ spinel ultrathin films

Bulk $\mathrm{Ni}{\mathrm{Fe}}_2{\mathrm{O}}_4$ is an insulating ferrimagnet. Here, we report on the epitaxial growth of spinel $\mathrm{Ni}{\mathrm{Fe}}_2{\mathrm{O}}_4$ ultrathin films onto $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ single crystals. We will show that—under appropriate growth conditions—epitaxial stabilization leads to the formation of a spinel phase with magnetic and electrical properties that radically differ from those of the bulk material: an enhanced magnetic moment $\left(M_S\right)$—about 250% larger—and a metallic character. A systematic study of the thickness dependence of $M_S$ allows us to conclude that its enhanced value is due to an anomalous distribution of the Fe and Ni cations among the $A$ and $B$ sites of the spinel structure resulting from the off-equilibrium growth conditions and to interface effects. The relevance of these findings for spinel- and, more generally, oxide-based heterostructures is discussed. We will argue that this novel material could be an alternative ferromagetic-metallic electrode in magnetic tunnel junctions.

Figure 1

RHEED patterns collected on the STO substrate and on the NFO films at the end of the growth for three different growth temperatures and along two azimuths corresponding to the [100] direction (left column) and [110] direction (right column) of STO.

Figure 2

$\theta \text{−}2\theta$ scans of three $12\text{−}\mathrm{nm}$-thick NFO films grown at different temperatures: (∗) and (+) signal reflections due to Cu $K\beta$ and to the plastiline holding the sample, respectively.

Figure 3

(a) $\varphi$ scans of the (404) reflection for three $12\text{−}\mathrm{nm}$-thick NFO films grown at different temperatures. (b) $\varphi$ scans of the (202) reflection for the STO substrate.

Figure 4

AFM images $(200\mathrm{nm}\times 200\mathrm{nm})$ of $12\text{−}\mathrm{nm}$ films grown at three different temperatures.

Figure 5

XPS spectrum of a $3\text{−}\mathrm{nm}$-thick NFO sample grown onto a LSMO/STO(001) bilayer in the region of Ni $3p$ and Fe $3p$. The Mn $3p$ peak arises from the LSMO underlayer.

Figure 6

Hysteresis cycles measured at $10\mathrm{K}$ and corrected from the high-field contribution of the substrate for three $12\text{−}\mathrm{nm}$ films grown at different temperatures.

Figure 7

Hysteresis cycles measured at $10\mathrm{K}$ and corrected from the high-field contribution of the substrate for three films with different thickness grown at $550°\mathrm{C}$. Inset: variation of the saturation magnetization with the inverse of the film thickness. The dotted line is a linear fit.

Figure 8

UPS spectra from the NFO sample already used for XPS investigation and from a Ta foil in electrical contact with the sample.