Unidirectional Spin-Dependent Molecule-Ferromagnet Hybridized States Anisotropy in Cobalt Phthalocyanine Based Magnetic Tunnel Junctions

Organic or molecular spintronics is a rising field of research at the frontier between condensed matter physics and chemistry. It aims to mix spin physics and the richness of chemistry towards designing new properties for spin electronics devices through engineering at the molecular scale. Beyond the expectation of a long spin lifetime, molecules can be also used to tailor the spin polarization of the injected current through the spin-dependent hybridization between molecules and ferromagnetic electrodes. In this Letter, we provide direct evidence of a hybrid interface spin polarization reversal due to the differing hybridization between phthalocyanine molecules and each cobalt electrode in $\mathrm{Co}/\mathrm{CoPc}/\mathrm{Co}$ magnetic tunnel junctions. Tunnel magnetoresistance and anisotropic tunnel magnetoresistance experiments show that interfacial hybridized electronic states have a unidirectional anisotropy that can be controlled by an electric field and that spin hybridization at the bottom and top interfaces differ, leading to an inverse tunnel magnetoresistance.

Figure 1

(a) Schematic of the $\mathrm{Co}/\mathrm{CoPc}/\mathrm{Co}$ nanocontact. (b) $I(V)$ curve of a $\mathrm{Co}/\mathrm{CoPc}/\mathrm{Co}$ nanocontact recorded at $T=2\mathrm{K}$. (c),(d) $R(H)$ curves recorded at 0° and 180°. The magnetic field is first swept from positive to negative values (black or red curve) and then from negative to positive values (gray or orange curve).

Figure 2

Magnetoresistance curves recorded at $T=2\mathrm{K}$, $V=-200\mathrm{mV}$ at 0° and 90°. The gray area corresponds to TAMR effects. The orange, green, and red arrows and dashed lines indicate the resistance levels corresponding to magnetizations saturated in, respectively, the 0°, $\pm 90°$, 180° directions (relative to field cooled). The green, orange, and red symbols in the $R(H)$ curves represent the magnetization directions of the two cobalt electrodes.

Figure 3

Variation of the resistance as a function of the direction of the magnetic field at 2 T (red dots). The expected overall behavior (guide for the eyes) of the TAMR for uniaxial (gray dashed line) and unidirectional anisotropies (black dashed line) is also shown.

Figure 4

(a) Variation of the TAMR and TMR as a function of bias voltage. (b) Magnetoresistance curves at 90° and 2 K recorded at different applied bias voltages. (c) Variation of the TAMR coercive field as a function of bias voltage.