Spin-polarized electron transfer in ${\text{ferromagnet}/C}_{60}$ interfaces

The contact between a molecule and a metallic electrode contributes to or even determines the characteristics of organic devices, such as their electronic properties. This is partly due to the charge transfer that takes place when two materials with different chemical potentials are put together. In the case of magnetic electrodes, the transfer can be accompanied by the transmission of a net spin polarization or spin doping. In nanocarbon systems, hybridization and spin doping can suppress the moment of a transition metal ferromagnet through the loss of majority spin electrons to the organic. Here, ${\mathrm{C}}_{60}$ is shown to become ferromagnetic as a result of spin doping from cobalt with an induced moment of 1.2 ${\mu }_{\mathrm{B}}$ per cage while suppressing the moment of the ferromagnet by up to 21%. Polarized neutron reflectivity and x-ray magnetic circular dichroism reveal the presence of an antiferromagnetic coupling of the interfacial layers of cobalt and ${\mathrm{C}}_{60}$, and weakly coupled induced magnetism propagating into the bulk organic. Thus, it is shown that the deposition of molecules with high electron affinity can be used to induce zero-voltage spin injection.

Figure 1

(a) Cross sectional TEM for a multilayer structure of Co/${\mathrm{C}}_{60}$ with 5 nm of Co and 10 nm of ${\mathrm{C}}_{60}$ using a Ta seed layer. Co layers remain continuous and show little intermixing despite the deposition of metal atoms onto the fullerenes. (b) shows the first layer of Co deposited onto a Ta seed layer and the formation of a crystalline structure. This crystallinity in the Co layers is maintained in subsequent repeats, as demonstrated by the fast Fourier transform (FFT) pattern in (c). The surface roughness for a 50 nm layer of ${\mathrm{C}}_{60}$ is shown in (d) The rms roughness for this film is 1–1.5 nm.

Figure 2

(a) shows the saturation magnetization is suppressed and coercivity enhancement for varying film thicknesses of ${\mathrm{C}}_{60}$ at 100 K. (b) shows the temperature dependence of these changes. The saturation magnetization appears independent of temperature from 10 to 300 K while the coercivity of the loop increases below 100 K. Though even a discontinuous 1 nm layer has a profound effect on the magnetization of the cobalt, the saturation magnetization continues to decrease logarithmically up to 200 nm [(c) black points]. A copper spacer layer [(a) inset] restores the cobalt layer to its pristine condition. DFT models of this system (d) show the distortion of the $d$ band as a result of ${\mathrm{C}}_{60}$ . The changes are most apparent in the polarized density of states (PDOS) for the $3d_z$ band. While hybridization is expected in other transition metals, the effect is considerably reduced in iron [(c) red points] (see Fig. S2.1 in the Supplemental Material [24]), with 200 nm of ${\mathrm{C}}_{60}$ causing a 13% change at 100 K as opposed to 21% in Co. This is due to the stronger $3s$-2π coupling in iron. The effect is entirely absent in nickel.

Figure 3

(a) Example of the various geometries used in simulating the ${\mathrm{Co}/C}_{60}$ contact. The indices p, hp, and h refer to which vertex is closest to the cobalt surface, a pentagonal-pentagonal vertex, hexagonal-pentagonal, or hexagonal-hexagonal, respectively. The optimal configuration, hp, is shown in more detail on the right. The atom denoted by an asterisk is displaced by 0.3 Å due to the ${\mathrm{C}}_{60}$ cage. The distribution of the transferred moment in the ${\mathrm{C}}_{60}$ cage is shown in (b). The moment appears to oscillate, beginning antialigned but varying as one moves away from the interface. The highest electron density is expected in the first angstrom of the cage. (c) Shows the PDOS for the adsorbed cage, demonstrating its asymmetrically distorted LUMO derived bands and metallic behavior.

Figure 4

XMCD of a bilayer sample with a 4 nm cobalt film covered with a 20 nm ${\mathrm{C}}_{60}$ film (a) and a discontinuous 5 nm film (b). The insets for both carbon edges track the asymmetry of the absorption spectra for opposite polarizations. The LUMO derived peak is significantly suppressed in the 5 nm sample, indicating a higher occupation. However, the normalized asymmetry of the LUMO peaks is the same for both films. The shoulder feature, however, is only present in the 5 nm film. Cobalt (c) is antiferromagnetically coupled to the hybridized interface state, observable in the shoulder feature of (b), as is clear from the polarization of the $L_3$ peak and LUMO shoulder. (d) shows the construction of each sample and the resultant different polarization observed in the carbon edge. The expected extent of the interfacial region is highlighted in red.

Figure 5

Magnetic profiles for the two multilayer samples: (a) Ta(5 nm)/[Co(4 nm)/C${}_{60}$ (21 nm)] × 10/Al(3 nm) and (b) Ta(5 nm)/[Co(2 nm)/C${}_{60}$ (13 nm)] × 5/Al(3 nm). In both cases, the moment of the C${}_{60}$ is multiplied by ten in order to make the differences between layers visible. Reflectivities [(c) and (d)] are fit using genx software. For higher layer numbers, both interfaces and Co layers become unstable due to the propagation of roughness through the stack.