Verwey transition in ${\mathrm{Fe}}_3{\mathrm{O}}_4$ thin films: Influence of oxygen stoichiometry and substrate-induced microstructure

We have carried out a systematic experimental investigation to address the question why thin films of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ (magnetite) generally have a very broad Verwey transition with lower transition temperatures as compared to the bulk. We observed using x-ray photoelectron spectroscopy, x-ray diffraction, and resistivity measurements that the Verwey transition in thin films is drastically influenced not only by the oxygen stoichiometry but especially also by the substrate-induced microstructure. In particular, we found (1) that the transition temperature, the resistivity jump, and the conductivity gap of fully stoichiometric films greatly depends on the domain size, which increases gradually with increasing film thickness, (2) that the broadness of the transition scales with the width of the domain size distribution, and (3) that the hysteresis width is affected strongly by the presence of antiphase boundaries. Films grown on MgO (001) substrates showed the highest and sharpest transitions, with a 200 nm film having a ${\mathit{T}}_V$ of 122 K, which is close to the bulk value. Films grown on substrates with large lattice constant mismatch revealed very broad transitions, and yet all films show a transition with a hysteresis behavior, indicating that the transition is still first order rather than higher order.

Figure 1

RHEED and LEED electron diffraction patterns of the following: the clean MgO (001) substrate (a) and (e); 40 nm ${\mathrm{Fe}}_3{\mathrm{O}}_4$ films grown at ${\Phi }_{\text{Fe}}=1\AA /\min$, ${\mathrm{T}}_{\text{substrate}}=250{}^{\circ }\mathrm{C}$, and ${\mathrm{P}}_{O_2}=5\times {10}^{-8}$ mbar (b) and (f); $1.0\times {10}^{-6}$ mbar (c) and (g); $1.0\times {10}^{-5}$ mbar (d) and (h).

Figure 2

RHEED intensity oscillations of the specularly reflected electron beam recorded during deposition of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ on MgO (001) substrate at ${\Phi }_{\text{Fe}}=1\AA /\min$, ${\mathrm{P}}_{O_2}=1.0\times {10}^{-6}$ mbar, and ${\mathrm{T}}_{\text{substrate}}=250{}^{\circ }\mathrm{C}$.

Figure 3

XPS Fe $2p$ core-level spectra of the following. (a) 40 nm epitaxial ${\mathrm{Fe}}_3{\mathrm{O}}_4$ films grown on MgO (001) by keeping the Fe flux constant and varying the oxygen pressure from $5.0\times {10}^{-8}$ to $1.0\times {10}^{-5}$ mbar; (b) bulk ${\mathrm{Fe}}_3{\mathrm{O}}_4$ and bulk FeO, reproduced from Ref. [33], and Fe metal film; (c) bulk $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$, reproduced from Ref. [33].

Figure 4

Resistivity as a function of temperature for 40 nm ${\mathrm{Fe}}_3{\mathrm{O}}_4$ films grown under various ${\mathrm{O}}_2$ partial pressures ${\mathrm{P}}_{O_2}$. Inset: the Verwey transition temperatures (${\mathit{T}}_{V+}$ and ${\mathit{T}}_{V-}$) (see text for their definition) are plotted as a function of ${\mathrm{P}}_{O_2}$.

Figure 5

Room temperature values of the resistivity of 40 nm ${\mathrm{Fe}}_3{\mathrm{O}}_4$ films as a function of ${\mathrm{P}}_{O_2}$.

Figure 6

Fe $2p$ XPS spectra of 5-, 10-, 20-, 40-, 80-, and 200-nm-thick epitaxial ${\mathrm{Fe}}_3{\mathrm{O}}_4$ films on MgO (001).

Figure 7

Resistivity as a function of temperature for ${\mathrm{Fe}}_3{\mathrm{O}}_4$ films with different thicknesses. Inset: the Verwey transition temperature (${\mathit{T}}_{V+}$ and ${\mathit{T}}_{V-}$) as a function of film thickness.

Figure 8

Resistivity of the ${\mathrm{Fe}}_3{\mathrm{O}}_4$ films at 90 K (blue) and 140 K (red) as a function of thickness.

Figure 9

Structural average domain size obtained from x-ray diffraction versus thickness. Inset: high resolution rocking curves of the (115) peak of 10-, 20-, 40-, 80-, and 200-nm-thick ${\mathrm{Fe}}_3{\mathrm{O}}_4$ films.

Figure 10

${\mathit{T}}_{V+}$ as a function of average domain size of the 10-, 20-, 40-, 80-, and 200-nm-thick ${\mathrm{Fe}}_3{\mathrm{O}}_4$ films.

Figure 11

Resistivity as a function of temperature of 200-nm-thick ${\mathrm{Fe}}_3{\mathrm{O}}_4$ films grown on MgO (001), MAO (001), and STO (001) and of the single crystal bulk ${\mathrm{Fe}}_3{\mathrm{O}}_4$. Inset: rocking curves of the (115) peak of 200 nm ${\mathrm{Fe}}_3{\mathrm{O}}_4$ films grown on MgO (001), MAO (001), and STO (001) substrates.

Figure 12

Fe $2p$ XPS spectra of 200-nm-thick epitaxial ${\mathrm{Fe}}_3{\mathrm{O}}_4$ films on MgO(001), MAO(001), and STO(001).