Real-space imaging of the Verwey transition at the (100) surface of magnetite

Effects of the Verwey transition on the (100) surface of magnetite were studied using scanning tunneling microscopy and spin polarized low-energy electron microscopy. On cooling through the transition temperature $T_{\mathrm{V}}$, the initially flat surface undergoes a rooflike distortion with a periodicity of $\sim 0.5$ $\mu$m due to ferroelastic twinning within monoclinic domains of the low-temperature monoclinic structure. The monoclinic $c$ axis orients in the surface plane, along the ${\left[001\right]}_c$ directions. At the atomic scale, the charge-ordered ($\sqrt{2}\times \sqrt{2}$)$R{45}^{\circ }$ reconstruction of the (100) surface is unperturbed by the bulk transition, and is continuous over the twin boundaries. Time resolved low-energy electron microscopy movies reveal the structural transition to be first order at the surface, indicating that the bulk transition is not an extension of the Verwey-like ($\sqrt{2}\times \sqrt{2}$)$R{45}^{\circ }$ reconstruction. Although conceptually similar, the charge-ordered phases of the (100) surface and sub-$T_{\mathrm{V}}$ bulk of magnetite are unrelated phenomena.

Figure 1

(a) High-resolution STM topograph (3.5$\times$3.5 nm${}^2$, $V_{\mathrm{sample}}=+1\mathrm{V}$, $I_{\mathrm{tunnel}}=0.3$ nA) of the Fe${}_3$O${}_4$(100) surface acquired at RT; the ($\sqrt{2}\times \sqrt{2}$)$R{45}^{\circ }$unit cell is indicated by the cyan square. (b) High-resolution STM topograph ($3.5\times 3.5$ nm${}^2$, $V_{\mathrm{sample}}=+1$ V, $I_{\mathrm{tunnel}}=$ 0.12 nA) of the Fe${}_3$O${}_4$(100) surface acquired at 78 K. The image displays the same ($\sqrt{2}\times \sqrt{2}$)$R{45}^{\circ }$ periodicity observed at RT (cyan square). (c) $500\times 300$ nm${}^2$ STM image of the Fe${}_3$O${}_4$(100) surface acquired at 78 K ($V_{\mathrm{sample}}=+1$ V, $I_{\mathrm{tunnel}}=0.1$ nA). The surface exhibits a roof-like structure with a periodicity of $\sim 0.5$ $\mu$m. (d) Profile along the line marked in red in (c). The angle at the ridge is 0.346${}^{\circ }$ ($+/-0.{018}^{\circ }$). (e) $30\times 16.3$ nm${}^2$ STM image ($V_{\left(\mathrm{sample}\right)}=+1$ V, $I_{\mathrm{tunnel}}=0.1$ nA) showing a valley of the roof structure. No disruption of the atomic structure occurs at the twin boundary.

Figure 2

Imaging the Verwey transition at the Fe${}_3$O${}_4$(100) surface using LEEM. The size of all the images is $8.6\times 8.6$ $\mu$m${}^2$, and the electron energy is 9.4 eV. (a) LEEM image of the Fe${}_3$O${}_4$(100) surface acquired at room temperature. Dark lines in the image correspond to step bunches. (b) LEEM image of the sub $T_{\mathrm{V}}$ Fe${}_3$O${}_4$(100) surface. Light-dark stripes with an average periodicity of $\sim 0.5$ $\mu$m run perpendicular to the local ${\left[001\right]}_m$ direction of the monoclinic sub-$T_{\mathrm{V}}$ structure. (c)–(h) Selected LEEM images from a LEEM movie (Ref. 35) acquired while slowly heating the sample from 121 K. Over the course of the movie the striped phase (m) recedes from the top and bottom of the imaged area and is replaced by the RT (cubic) phase (c). The boundaries between both phases are marked by green lines.

Figure 3

Schematic illustrating the lattice distortions that occur in the Verwey transition in magnetite. (a) The cubic unit cell of magnetite in the high-temperature phase drawn in perspective view. (b) The low-temperature monoclinic unit cell of magnetite. A lattice distortion causes the lattice vectors a and b to become inequivalent. The periodicity in the c direction doubles to the equivalent of two cubic unit cells. The angle between the a and c vectors distorts from ${90}^{\circ }$ to $89.{764}^{\circ }$, causing the top (100) surface to tilt with respect to the high-temperature cubic cell. (c) Two mirrored monoclinic cells with opposite monoclinic c axis joined at a twin boundary (indicated by the red lines). The top (100) surfaces meet at a peak at the twin boundary.

Figure 4

(a) LEEM image of the Fe${}_3$O${}_4$(100) surface at RT. (b), (c) SPLEEM images showing the magnetization along ${\left[011\right]}_c$ and ${\left[01\overline{1}\right]}_c$, respectively. (d) LEEM image of the Fe${}_3$O${}_4$(100) surface below $T_{\mathrm{V}}$. Light/dark stripes due to differing electron reflectivity from microtwins run parallel to the ${\left[001\right]}_c$ directions. (e), (f) SPLEEM images showing the magnetization along ${\left[001\right]}_c$ and ${\left[010\right]}_c$ directions. No out-of-plane magnetization is observed, either at RT or below $T_{\mathrm{V}}$. All the images have a field of view of 13 $\mu$m, and are acquired at a start voltage of 9.3 eV.