Ion-beam-induced magnetic transformation of CO-stabilized fcc Fe films on Cu(100)

We have grown 22-ML-thick Fe films on a Cu(100) single crystal. The films were stabilized in the face-centered-cubic (fcc) $\gamma$ phase by adsorption of carbon monoxide during growth, preventing the transformation to the body-centered-cubic (bcc) $\alpha$ phase. A structural transformation of these films from fcc to bcc can be induced by ${\text{Ar}}^{+}$ ion irradiation. Scanning-tunneling microscopy images show the nucleation of bcc crystallites, which grow with increasing ${\text{Ar}}^{+}$ ion dose and eventually result in complete transformation of the film to bcc. Surface magneto-optic Kerr effect measurements confirm the transformation of the Fe film from paramagnetic (fcc) to ferromagnetic (bcc) with an in-plane easy axis. The transformation can also be observed by low-energy electron diffraction. We find only very few nucleation sites of the bcc phase and argue that nucleation of the bcc phase happens under special circumstances during resolidification of the molten iron in the thermal spike after ion impact. Intermixing with the Cu substrate impedes the transformation. We also demonstrate the transformation of films coated with Au to protect them from oxidation at ambient conditions.

Figure 1

(Color online) Carbon and oxygen Auger signals for 22 ML Fe films as a function of CO pressure during evaporation. Normalized Auger peak-to-peak heights of carbon ${\text{C}}_{273}/\left({\text{Fe}}_{703}+{\text{Cu}}_{920}\right)$ and oxygen ${\text{O}}_{514}/\left({\text{Fe}}_{703}+{\text{Cu}}_{920}\right)$ show an increase in oxygen and decrease in carbon concentration with increasing CO pressure.

Figure 2

STM images of a 22 ML Fe film grown with unstable CO pressure during the experiment, leading to a surface with bcc crystallites (bright). Frames (a)–(d) show the same area with increasing magnification. (e) bcc area with an adjacent trough (image shown as if illuminated from the left). The inclined bcc wedges in the trough are indicated with arrows. The section profile (f) over the vertical frame in (e) shows the bcc trough next to the needle.

Figure 3

STM image of the as-grown 22 ML Fe film in the presence of CO. (a) The inset shows the oxygen $c\left(2\times 2\right)$ superstructure highlighted by a white square. The white spot in the inset is probably the missing oxygen atom (white circle). (b) The surface of an 8 ML Fe film grown without CO (shown for comparison).

Figure 4

Structure model for the Fe steps of the films grown in CO (top view). The $c\left(2\times 2\right)$ oxygen superstructure is shown only in the upper layer.

Figure 5

Fe film ($\approx 23$ to 24 ML) grown in CO. (a) STM image of the as-grown surface with partially transformed areas (bright). (b) After dosing atomic hydrogen to remove the surface oxygen.

Figure 6

Scanning tunneling microscopy images of a 22 ML Fe film prepared in $7.5\times {10}^{-10}\text{mbar}$ CO. (a) as-grown film, and after 500 eV ${\text{Ar}}^{+}$ ion irradiation with ion doses of (b) $1.3\times {10}^{15}{\text{cm}}^{-2}$, (c) $1.8\times {10}^{15}{\text{cm}}^{-2}$, and (d) $2.4\times {10}^{15}{\text{cm}}^{-2}$. Overview images (b)–(d) are high-pass-filtered to make the bcc needles more visible in spite of steps in the substrate.

Figure 7

(Color online) High-resolution STM image of the film in Fig. 2 after bombardment with 1 keV ${\text{Ar}}^{+}$ ions (dose: $3\times {10}^{14}{\text{cm}}^{-2}$). (a) The bcc area in the lower left shows to poorly ordered oxygen while the well-ordered $c\left(2\times 2\right)$-O structure with its $90°$ bond angle is clearly seen in the bcc areas. (b) Filtering in the Fourier domain (spots marked in the inset) clearly shows the bcc lattice with a bond angle of $71\pm 0.5°$.

Figure 8

(Color online) STM images of the 22 ML fcc Fe film irradiated with different ${\text{Ar}}^{+}$ energies: (a) 500, (b) 2000, and (c) 4000 eV. Height histograms are shown at the right.

Figure 9

(a) LEED image of the as-grown 22 ML Fe film, showing fcc(100) and oxygen $c\left(2\times 2\right)$ spots. (b) After ${\text{Ar}}^{+}$ irradiation of $2.4\times {10}^{15}{\text{cm}}^{-2}$ at 500 eV, diffuse bcc satellites are visible [the rectangular bcc(110) unit cell is indicated].

Figure 10

(Color online) Hysteresis loops of the 22-ML Fe films irradiated with 2 keV ${\text{Ar}}^{+}$ ions, showing longitudinal Kerr ellipticity $\epsilon$ as a function of magnetic field H. The transformation from paramagnetic (ion dose: 0) to ferromagnetic with increasing ion fluence can be seen.

Figure 11

(Color online) (a) Saturation value of longitudinal Kerr ellipticity ${\epsilon }_{\text{sat}}$ as a function of ${\text{Ar}}^{+}$ ion dose for different ion energies and (b) coercive field ${\text{H}}_{\text{c}}$ as a function of ${\text{Ar}}^{+}$ ion dose.

Figure 12

(Color online) (a) Calculated number of atoms $N_{\text{m}}$ in the molten volume for a 22-ML-thick fcc Fe film. The fraction of Cu atoms in the melt is negligible up to 2 keV and increases with increasing ion energy. (b) Schematic view of an arrangement leading to a sheared structure during recrystallization, a possible bcc nucleus.

Figure 13

STM images (strongly high-pass filtered) of the 22 ML fcc Fe film covered with 2.25 nm Pd. (a) Pd grows in (100) orientation in the regions indicated with black arrows and as (111) indicated with white arrows. (b) Most of this area shows (111) growth.

Figure 14

STM images of the 22 ML fcc Fe film (a) as prepared in $7.5\times {10}^{-10}\text{mbar}$ CO pressure and after deposition of (c) 0.52 nm and (b) 2 nm Au. (d) Surface covered with 2 nm Au after irradiation with $9\times {10}^{14}{\text{cm}}^{-2}$ ${\text{Ar}}^{+}$ ions at 3 keV.