Breakdown of the electron-spin motion upon reflection at metal-organic or metal-carbon interfaces

Spin-polarized electron scattering experiments on different metal-organic and metal-carbon interfaces are performed. A completely unexpected behavior of the spin-motion angles as well as of related quantities as a function of the organic layer or carbon coverage is observed. In fact, by deposition of organic molecules or carbon onto ferromagnetic as well as nonmagnetic metal surfaces in the submonolayer thickness range, the electron reflection amplitude, i.e., both the reflectivity and the reflection phase, become spin independent. Our findings show that this behavior is a very general phenomenon which is independent of the electron energy and the choice of the metal as well as of the organic molecules and thus does not depend on the choice of the specific interface.

Figure 1

Illustration of the two types of motion of the spin-polarization vector. The initial spin polarization ${\mathbf{P}}_{\mathbf{0}}$ precesses around the magnetization $\mathbf{M}$ by an angle $\varepsilon$ and rotates in the plane $\mathbf{P}$-$\mathbf{M}$ by an angle $\varphi$.

Figure 2

The experimental setup consists of a laser diode that has a wavelength of 785 nm, a linear polarizer, a Pockels cell to create circularly polarized light and to change its helicity, an electrostatic 90${}^{\circ }$ deflector, coils to rotate the direction of the initial spin polarization, a GaAs crystal, two transfer electron optics, a sample, coils to remanently magnetize the sample, a retarding field grid as energy analyzer, and four Si detectors and a Au foil for spin detection.

Figure 3

Molecule of (a) H${}_2$Pc, (b) metal-substituted Pc (here CoPc), (c) PTCDA, (d) coronene, (e) C${}_{60}$, and (f) pentacontane.

Figure 4

Left: LEED image of 0.6 ML CoPc on Cu(001) taken at 14.5 eV. Right: LEED image of 2 ML CoPc on Cu(001) taken in an off-center position such that the (0,0) beam appears to the left. The electron energy is 16.5 eV. Crystallographic axes of the Cu(001) crystal are indicated.

Figure 5

Raman spectrum of a Au-covered 2 ML a-C film on Co(001). The presence of both peak G (“graphite”) and peak D (“disorder”) is typical of amorphous carbon films.

Figure 6

For the system CoPc/Co/Cu(001), the following quantities are shown as a function of the CoPc film thickness $d$ in monolayers (MLs): spin-averaged electron reflectivity $R$ (top), precession angle $\varepsilon$ (middle), and rotation angle $\varphi$ (bottom). The primary electron energy is 7 eV.

Figure 7

Spin-averaged electron reflectivity $R$, precession angle $\varepsilon$, and rotation angle $\varphi$ as a function of CoPc film thickness for four electron energies: 9 eV (top left), 11 eV (top right), 13 eV (bottom left), and 27 eV (bottom right).

Figure 8

For the system CoPc/Co/Cu(001), the following quantities as a function of electron energy for different CoPc coverages are shown: spin-averaged electron reflectivity $R$ (top), precession angle $\varepsilon$ (middle), and rotation angle $\varphi$ (bottom).

Figure 9

For the system CoPc/Co/Cu(001) the degree of spin polarization of the reflected electron beam as a function of the CoPc thickness is shown. The primary energy of the electrons is 8 eV.

Figure 10

Spin polarization of secondary electrons as a function of CoPc thickness. The energy of the unpolarized primary electrons is 132 eV. The solid line is a fit to the data based on an exponential decay. Note the logarithmic spin-polarization scale.

Figure 11

Work function changes as a function of CoPc thickness on Co(001) (top) and MnPc thickness on Cu(001) (bottom). Lines are exponential fits to the data.

Figure 12

For the systems CoPc/Fe/Ag(001) (left column) and CoPc/Fe/Pt(001) (right column), the following quantities as a function of CoPc film thickness are shown: spin-averaged electron reflectivity $R$ (top), precession angle $\varepsilon$ (middle), and rotation angle $\varphi$ (bottom). Primary energies are 16.5 and 23 eV, respectively.

Figure 13

For the system CoPc/Au(9ML)/Co/Cu(001), the spin-motion angles $\varepsilon$ (top) and $\varphi$ (bottom) are shown as a function of the CoPc coverage. The primary energy is 8 eV.

Figure 14

For the system CoPc/Au(15ML)/Co/Cu(001), the precession angle $\varepsilon$ as a function of the CoPc coverage is shown. The primary energy is 7.8 eV.

Figure 15

For the system C/Co/Cu(001) the rotation angle $\varphi$ as a function of the C coverage is shown. The primary energy is 11 eV.

Figure 16

For the system pentacontane/Co/Cu(001) the negative rotation angle $-\varphi$ as a function of the pentacontane thickness is shown. The primary energy is 9 eV. The three thickness regimes are indicated. Note the logarithmic $-\varphi$ scale. The line is an exponential fit to the data in thickness regime III.

Figure 17

Thicknesses $d_1$ and $d_2$ obtained from measurements of the precession angle $\varepsilon$.

Figure 18

Thicknesses $d_1$ and $d_2$ obtained from measurements of the rotation angle $\varphi$.

Figure 19

The ratio $R\left(d_2\right)/R\left(d_1\right)$ for (1) H${}_2$Pc/Co, (2) CoPc/Co, (3) CoPc/Au/Co, (4) CoPc/Fe/Ag, (5) CoPc/Fe/Pt, (6) FePc/Fe/Ag, (7) MnPc/Fe/Ag, (8) PTCDA/Co, (9) pentacontane/Co, (10) pentacontane/Au/Co, (11) coronene/Co, (12) C${}_{60}$/Co, (13) C${}_{60}$/Au/Co, and (14) C/Co. Note the logarithmic scale of the ordinate.

Figure 20

The product of $d_2$ and the average carbon surface density ${\sigma }_C^{\mathrm{ML}}$ of a completed ML is shown for all systems studied. In the case of C${}_{60}$ it is the projected average carbon density on the surface.

Figure 21

(a) STM image ($40\times 40$ nm${}^2$) taken at 4 K of 0.3 ML of MnPc deposited at room temperature onto a Co(001) film. (b–d) Kinetic Monte Carlo simulations of the deposition without any diffusion (b), with diffusion by one lattice step of particles which arrive at the surface during a deposition step (c), and with diffusion by one lattice step of all particles at each step of the deposition (d). Particles in (c) and (d) which become or are part of a cluster are not allowed to diffuse anymore.

Figure 22

Both the experimental obtained precession angle $\varepsilon$ (for the system CoPc/Au/Co; open circles) and the simulated percentage of nonpercolated sites (NP; lines) are shown as a function of the thickness. Note that the thickness scale of the experimental data is multiplied by a factor of 0.64 (see text). Simulations are shown for three situations: Situation 1 corresponds to the pure (square) particle. In situation 2 the influence zone is given by the particle plus a zone determined by the nearest-neighbor positions. In situation 3 the influence zone is given by the particle plus a zone determined by the nearest- and next-nearest-neighbor positions. Inset: Surface lattice and geometry of the three situations considered here.

Figure 23

For the system PTCDA/Co/Cu(001) the precession angle $\varepsilon$ as a function of the PTCDA thickness is shown for two orientations of the initial spin polarization ${\mathbf{P}}_{\mathbf{0}}$. The primary energy of the electrons is 8 eV.

Figure 24

For the system FePc/Fe(001) the exchange asymmetry $A_{\mathrm{ex}}$ as a function of the FePc thickness is shown. The primary energy of the electrons is 8 eV.

Figure 25

For the system C${}_{60}$/Co/Cu(001) the degree of spin polarization of the reflected electron beam as a function of the C${}_{60}$ thickness is shown. The initial spin polarization is 0. The primary energy of the electrons is 28 eV.

Figure 26

For the system CoPc/Co/Cu(001) the degree of spin polarization of inelastic electrons (both inelastically scattered primaries and secondaries), taken in off-specular geometry (by 5${}^{\circ }$), as a function of the CoPc thickness is shown. Primary energies of the unpolarized incident electrons are 26.5 and 132 eV.

Figure 27

For the system CoPc/Co/Cu(001) both the asymmetry of the (00) beam $A_{\left(00\right)}$ and the negative asymmetry of the absorbed sample current $-A_a$ as a function of the CoPc thickness are shown. The primary energy of the electrons is 13 eV. Inset: Asymmetry $A_a$ of an uncovered Co film as a function of the electron energy.

Figure 28

For the system CoPc/Co/Cu(001) both the asymmetry of the (00) beam $A_{\left(00\right)}$ and the asymmetry of the absorbed sample current $A_a$ as a function of the CoPc thickness are shown. The primary energy of the electrons is 132 eV.

Figure 29

For the system Au/0.7 ML C/Co(001), the precession angle $\varepsilon$ as a function of the Au coverage is shown. The primary energy is 9 eV. Inset: $\varepsilon$ as a function of Au coverage on top of Co(001).

Figure 30

Calculated $\varepsilon$ (top) and $\varphi$ (bottom) as a function of the electron energy for 9 ML of Fe(001) (thin line) and 1 ML of carbon on 9 ML of Fe(001) (thick line).

Figure 31

Top: For the system CoPc/Co(001) $A_{\mathrm{ex}}$ and $A_{\mathrm{so}}$ are shown as a function of the CoPc thickness. The primary energy of the electrons is 47 eV. Middle: For the system CoPc/10 ML Au/Co(001) $A_{\mathrm{ex}}$ and $A_{\mathrm{so}}$ are shown as a function of the CoPc thickness. The primary energy of the electrons is 15 eV. Bottom: For the system CoPc/Pt(001) $A_{\mathrm{so}}$ is shown as a function of the CoPc thickness. The primary energy of the electrons is 12 eV.