Enhanced magnetization of ultrathin ${\mathrm{NiFe}}_2{\mathrm{O}}_4$ films on ${\mathrm{SrTiO}}_3\left(001\right)$ related to cation disorder and anomalous strain

${\mathrm{NiFe}}_2{\mathrm{O}}_4$ thin films with varying thickness were grown on ${\mathrm{SrTiO}}_3\left(001\right)$ by reactive molecular beam epitaxy. Soft and hard x-ray photoelectron spectroscopy measurements reveal a homogeneous cation distribution throughout the whole film with stoichiometric Ni:Fe ratios of 1:2 independent of the film thickness. Low energy electron diffraction and high resolution (grazing incidence) x-ray diffraction in addition to x-ray reflectivity experiments were conducted to obtain information of the film surface and bulk structure, respectively. For ultrathin films up to 7.3 nm, lateral tensile and vertical compressive strain is observed, contradicting an adaption at the interface of ${\mathrm{NiFe}}_2{\mathrm{O}}_4$ film and substrate lattice. The applied strain is accompanied by an increased lateral defect density, which is decaying for relaxed thicker films and attributed to the growth of lateral grains. Determination of cationic site occupancies in the inverse spinel structure by analysis of site sensitive diffraction peaks reveals low tetrahedral occupancies for thin, strained ${\mathrm{NiFe}}_2{\mathrm{O}}_4$ films, resulting in the partial presence of deficient rocksaltlike structures. These structures are assumed to be responsible for the enhanced magnetization of up to $\sim 250$% of the ${\mathrm{NiFe}}_2{\mathrm{O}}_4$ bulk magnetization as observed by superconducting quantum interference device magnetometry for ultrathin films below 7.3 nm thickness.

Figure 1

(a) LEED images for the prepared substrate and NFO films at an electron energy of 98 eV. The square ($1\times 1$) reciprocal surface unit cell of the spinel type NFO films (green square) is half the size and rotated by ${45}^{\circ }$ with respect to the ($1\times 1$) surface structure of the perovskite type STO(001) substrate (red square). (b) $\mathrm{\Delta }\mathrm{FWHM}$ given as the FWHM of the (22)${}_{\text{S}}$ reflex for the different film thicknesses corrected by the instrumental broadening, which is estimated by the FWHM of the (20)${}_{\text{P}}$ reflex of the substrate. The unit of $\mathrm{\Delta }\mathrm{FWHM}$ is given in a percentage of the first Brillouin zone of STO(001) (100%BZ = $2\pi /{\mathrm{a}}_{\text{STO}}$). The (22)${}_{\text{S}}$ spot profiles (shown in the inset) were extracted from the 2D LEED images [marked in (a) by a blue line].

Figure 2

Ni $3p$ and Fe $3p$ spectra measured by XPS for all prepared NFO films with varying thickness. Both Ni $3p$ and Fe $3p$ spectra consist of the main $3p$ signal and two satellites at their higher binding energy sides. The resulting cationic Ni amount (shown in the inset) is constant for all film thicknesses and considering an experimental error of 5% includes the stoichiometric value of 33%.

Figure 3

XPS measurements of all prepared NFO films with varying film thickness. (a) Ni $2p$ core level spectra and a reference spectrum of a 12 nm thin NiO film. The Ni $2{\mathrm{p}}_{3/2}$ signals are shown in the inset on a smaller scale for better comparison. (b) Fe $2p$ core level spectra compared to ${\mathrm{Fe}}_2{\mathrm{O}}_3, {\mathrm{Fe}}_3{\mathrm{O}}_4$ and FeO reference spectra.

Figure 4

(a) Ni $2p$ and (b) Fe $2p$ core level HAXPES measurements of all prepared NFO films with varying film thickness.

Figure 5

X-ray reflectivity measurements for all NFO films with varying thickness. The calculated XRR curves are in excellent agreement with the measured data. The NFO surface roughness ${\sigma }_{\text{NFO}}$ is decreasing for thicker films (shown in the inset).

Figure 6

(a) X-ray diffraction mesaurements of the (00L) crystal truncation rod across the NFO(004) and STO(002) Bragg reflections. The vertical crystallite size $D_{\text{cryst}}^{\text{vert}}$ determined by the Scherrer formula $[D_{\text{cryst}}^{\text{vert}}\sim \mathrm{FWHM}{\left(004\right)}^{-1}]$ is clearly deviating from the film thickness to lower values for NFO films above 29 nm (shown in the inset). (b) Grazing incidence x-ray diffraction measurements along the (H00) direction across the NFO(400) and STO(200) reflexes. The FWHM of the NFO(400) reflex decreases meaning that the lateral crystallite size $[D_{\text{cryst}}^{\text{lat}}\sim \mathrm{FWHM}{\left(400\right)}^{-1}]$ increases for thicker films (shown in the inset).

Figure 7

Vertical and lateral layer distances for varying NFO film thicknesses determined by the NFO(004) and NFO(400) positions, respectively. For films up to 7.3 nm thickness, an unexpected behavior of lateral tensile and vertical compressive strain is observed, followed by complete relaxation for thicker films.

Figure 8

(a) GIXRD measurements along the NFO(22L) CTR across the NFO(222), NFO(224) and NFO(226) Bragg peaks. The increased intensity of the reflections for thicker films are due to the increased crystalline amount in the NFO films. Note the logarithmic scale on the intensity axis. (b) Calculated and experimental intensity ratios between the NFO(224) and NFO(222) reflexes as a function of the disorder parameter $\epsilon$. For increasing NFO film thicknesses, the occupancy of tetrahedral lattice sites and therefore the NFO(224)/NFO(222) intensity ratio increases, resulting in a decrease of $\epsilon$ (shown in the inset).

Figure 9

M vs H curves for all NFO films with varying film thickness measured by SQUID magnetometry at 5 K after zero field cooling. The magnetic saturation is significantly enhanced for film thicknesses up to 7.3 nm. Thicker films exhibit the NFO bulk magnetization of 2 ${\mu }_B$/f.u. (shown in the inset).