Controlling the magnetic exchange coupling in hybrid heterojunctions via spacer layers of $\pi$-conjugated molecules

Mastering and understanding the magnetic couplings between magnetic electrodes separated by organic layers are crucial for developing new hybrid spintronic devices. We study the magnetic exchange interactions in organic-inorganic heterojunctions and unveil the possibility of controlling the strength of the magnetic exchange coupling between two ferromagnetic electrodes across $\pi$-conjugated molecules’ ($\alpha$-sexithiophene or para-sexiphenyl) ultrathin film. In $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4/\pi$-conjugated molecules/Co magnetic tunnel junctions, an antiferromagnetic interlayer exchange coupling with variable strength is observed according to the nature of the aromatic rings (thiophene or phenyl groups). The underlying physical mechanism is revealed by ab initio calculations relating the strength of magnetic coupling to the spin moment penetration into a molecular layer at the molecule/Co interface. The prospect that magnetic coupling between two ferromagnetic electrodes can be mediated and tuned by organic molecules opens different perspectives in the way magnetization of organic tunnel junctions or spin valves can be driven.

Figure 1

Hybrid magnetic heterojunction with a π-conjugated molecules spacer layer. (a) Chemical structure of α-sexithiophene and para-sexiphenyl molecules. (b) Schematic illustration of the stack used for probing the magnetic exchange interactions. (c) Zoom of the chemical adsorption of organic molecules on an oxide surface.

Figure 2

AFM images of α-sexithiophene and para-sexiphenyl thin films. (a) AFM topography images (contact mode) of P6P (1 ML) and (b) 6T (1 ML) ultrathin films vacuum deposited on epitaxial $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4\left(111\right)$. Area: 10 × 10 μ${\mathrm{m}}^2$. (c) AFM topography images of P6P (2.1 ML) and (d) 6T (2.1 ML) thin films on $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4\left(111\right)$. Area: 10 × 10 μ${\mathrm{m}}^2$. The color-scale bars reflect the topography height. Inset line profiles correspond to the topography recorded along the dashed lines. The dendritic island with the associated profile line observed in (c) and (d) corresponds to an island (4 ML).

Figure 3

Resistance map of 6T (1 ML) deposited onto a $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4\left(111\right)$ thin film recorded by current-sensing AFM at a tip voltage of 500 mV with the associated AFM topography in the inset. This image reveals a highly homogeneous and insulating behavior with resistances ranging from 500 GΩ to 1 TΩ.

Figure 4

Major hysteresis loop at $T=50\mathrm{K}$ for the P6P (2 ML) based heterojunction. The solid line is a calculation using the model described in the text.

Figure 5

Major and minor hysteresis loops near zero field at $T=50\mathrm{K}$ for the (a),(b) 6T (2 ML) based heterojunction and the (c),(d) P6P (2 ML) based heterojunction. Both minor loops show a small positive exchange field around 5–10 mT.

Figure 6

Major and minor hysteresis loops near zero field at $T=50\mathrm{K}$ for the (a),(b) 6T (1 ML) based heterojunction and the (c),(d) P6P (1 ML) based heterojunction. The solid lines are calculations using the model described in the text. For 6T (1 ML), (a) the major loop exhibits a crossover near zero field and (b) the minor loop shows a sizeable positive exchange, with a large exchange field of about 100 mT, together with an upwards flaring. For P6P (1 ML), (c) no crossover of the major loop is observed and (d) the minor loop exhibits a small exchange field of ≈10 mT.

Figure 7

Simulations of magnetic hysteresis loops: (a) minor loops for bilayers with AF and FM coupling, for $|J_{\mathrm{ex}}|=5\times {10}^{-4}\mathrm{J}/{\mathrm{m}}^2$, showing the opposite flarings for the two cases; (b) major loops for AF coupled bilayers for increasing values of the exchange coupling. At $J_{\mathrm{ex}}=4\times {10}^{-4}\mathrm{J}/{\mathrm{m}}^2$, the branches do not cross; at $J_{\mathrm{ex}}=4.75\times {10}^{-4}\mathrm{J}/{\mathrm{m}}^2$, the branches cross tangentially; and at $J_{\mathrm{ex}}=6\times {10}^{-4}\mathrm{J}/{\mathrm{m}}^2$, the two branches cross.

Figure 8

DFT results for hybrid Co/π-conjugated molecule interfaces. (a) Co/6T interface. (b) Co/P6P interface with a molecule in a model with flat configuration. (c) Co/P6P interface with a molecule in a realistic twisted configuration. Upper panels: Co/molecule adsorption configuration. Middle panels: three-dimensional plots of spin magnetization for positive (red) and negative (blue) isovalues. Lower panels: DOS projected onto all molecular orbitals; the HOMO and LUMO molecular orbitals are marked.

Figure 9

DFT results for hybrid π-conjugated molecule/$\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$ interfaces of different terminations. (a) 6T (and P6P) at $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$ terminated by octa. (b) 6T (and P6P) at $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$ terminated by tetra. Upper panels: $6\mathrm{T}/\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$ adsorption configurations (similar for P6P molecule); Fe layers in octa positions are labeled as O (the other Fe atoms are in tetra positions). Lower panels: in-plane averaged spin moment as a function of $z$ (perpendicular to the $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$ surface) for $6\mathrm{T}/\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$ (red) and $\mathrm{P}6\mathrm{P}/\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$ (blue). Insets: the same plot, but at a larger scale.