Highly unquenched orbital moment in textured Fe-phthalocyanine thin films

X-ray magnetic circular dichroism (XMCD) measured at $T=6\text{K}$ and ${\mu }_{0}H=5\text{T}$ on the $\alpha$-phase Fe-phthalocyanine (FePc) textured thin films shows that the ${\text{Fe}}^{2+}$ ions present an unusually large, highly unquenched $m_L=0.53\pm 0.04{\mu }_B$ orbital component, with planar anisotropy. The spin $m_S=0.64\pm 0.05{\mu }_B$ and the intra-atomic magnetic dipolar $m_T$ components were also obtained. The $m_L/m_S=0.83$ ratio is the largest measured in $3d$ complexes and compounds. The origin of this unusually high orbital moment is the incompletely filled $e_g$ level lying close to the Fermi energy. This explains the unusually large and positive hyperfine field detected by Mössbauer spectroscopy in FePc. The FePc film strong planar anisotropy inferred from XMCD experiments is fully confirmed by magnetization measurements.

Figure 1

(Color online) $\theta \text{−}2\theta$ diffraction patterns of FePc film grown on a gold surface. The oscillations at low diffraction angles correspond to finite-size oscillations from the thin film. The inset shows an expanded view of the oscillations and the FePc (313) peak. The small peak at $26.6°$ is the same FePc (313) peak due to a tungsten impurity on the copper x-ray source.

Figure 2

(Color online) XLPA spectra recorded at the $N$ K-edge with grazing angle incidence $\left(\gamma =75°\right)$ at $T=6\text{K}$. The two spectra correspond to the $H$ and $V$ polarization modes: $\vec{E}$ is either parallel to the FePc plane ($V$, dashed line) or forming an angle $15°$ with the normal to the substrate, i.e., to the FePc plane ($H$, full line). In accordance with Ref. 15, peaks $A$, $B$, and $C$ are assigned to ${\pi }^{\ast }$ resonances, and $D$, $E$, and $F$ to ${\sigma }^{\ast }$ ones. Inset: schematic view of sample FePc deposited on a Au film, previously deposited on sapphire. The incoming beam at grazing incidence is shown with the $\vec{E}$ in the $H$ and $V$ polarization modes.

Figure 3

(Color online) Hysteresis loop of the FePc film deposited on Au covered sapphire measured at $T=1.8\text{K}$ in the direction perpendicular $\left(M_z\right)$ and parallel $\left(M_{xy}\right)$ to the substrate. Insets: (a) upper left; initial magnetization at $T=1.8\text{K}$, in the two directions; (b) lower right; remanent moment $M_r$ as a function of temperature.

Figure 4

(Color online) (a) normalized XLPA, (b) XMCD spectra, and (c) integrated XMCD area at the Fe $L_{2,3}$ edges on FePc at $T=6\text{K}$ for incidence angle $\gamma =0°$, $45°$, and $75°$. In the XMCD experiment, the applied field $\left({\mu }_{0}H=5\text{T}\right)$ and the helicity of the beam are parallel for every $\gamma$. The arrows indicate the higher limit of the $p$ and $q$ integrals.

Figure 5

Spin split molecular orbital energy level scheme for FePc (Ref. 17). For clarity, only the states with a relevant component of $3d$ weight $\left(w_{3d}>0.05\right)$ have been included. Note also the global change in spin sign with respect to Ref. 17.

Figure 6

(Color online) $m_L$ (◻) and $m_S^{\text{eff}}$ (●), as determined from the XMCD spectra at $T=6\text{K}$ and ${\mu }_{0}H=5\text{T}$. The lines show the fits to Eqs. (5, 6).

Figure 7

(Color online) $m^{xy}$ (◆), $m_L^{xy}$ (◻), and $m_S$ (●) as a function of field of the FePc film, as determined from XMCD measurements (see text). Magnetization versus field at $T=6\text{K}$ (◼), measured parallel to the substrate plane and scaled to $m^{xy}$, deduced from XMCD (see text and Table for details). Dotted lines are guides to the eye calculated by scaling the magnetization curve.

Figure 8

(Color online) Experimental absorption spectra (red full lines) compared to theoretical predictions from Ref. 17 (dotted blue lines) of FePc at the $L_{2,3\mathrm{Fe}}$ edges, for normal incidence, $\gamma =0$. (a) XAS spectra; (b) XMCD spectra. Both panels share the same (arbitrary) units.