Spin response in organic spin valves based on ${\mathrm{La}}_{2∕3}{\mathrm{Sr}}_{1∕3}\mathrm{Mn}{\mathrm{O}}_3$ electrodes

We fabricated spin-valve devices made of organic semiconductor thin films sandwiched between ferromagnetic half-metal ${\mathrm{La}}_{2∕3}{\mathrm{Sr}}_{1∕3}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) and cobalt electrodes, using three different organic molecules. Subsequently, we studied the spin injection and transport properties by measuring the device magnetoresistance (MR) response at various biasing voltages $V$ and temperatures $T$. We found that the spin-valve MR response in all devices monotonically decreases with $V$ and is asymmetric with respect to the voltage polarity. We also found a steep MR decrease with $T$, where it vanishes at $T\sim 220\mathrm{K}$, similar to other MR responses in inorganic tunneling junction devices based on LSMO and Co ferromagnetic electrodes. In contrast, the spin-$\frac{1}{2}$ photoluminescence detected magnetic resonance of the organic interlayer, which directly depends on the spin-lattice relaxation rate of polarons in the organic semiconductor, was found to be temperature independent. We thus conclude that the steep MR dependence on $T$ is due to the temperature dependence of the interfacial spin polarization of the LSMO electrode, which also drastically decreases up to $T\sim 220\mathrm{K}$. We thus conclude that (i) the spin-lattice relaxation time in organic semiconductors should not be the limiting factor in fabricating room temperature organic spin valves, and (ii) in order to achieve room temperature spin-valve operation with substantial MR value, spin-injection electrodes other than LSMO need to be involved, having large and less temperature dependent spin polarization.

Figure 1

(Color online) (a) The $I\text{‐}V$ characteristic response of $\mathrm{LSMO}∕\mathrm{CVB}\left(102\mathrm{nm}\right)∕\mathrm{Co}∕\mathrm{Al}$ diode at three temperatures: $14\mathrm{K}$ (blue (dark) circles), $140\mathrm{K}$ (red (dark gray) stars), and $200\mathrm{K}$ (black squares). The upper inset shows schematically the organic spin-valve device and magnetoresistance measurement configuration, where $I$ and $V$ are the injected current and biasing voltage across the device, and $H$ is the external in-plane magnetic field. The lower inset shows the chemical structure of the CVB molecule. (b) The magnetoresistance response of the LSMO∕CVB∕Co spin-valve device shown in (a) for an applied voltage of $10\mathrm{mV}$ at $14\mathrm{K}$; the blue squares [red (dark gray) circles] are for $H$ swept in the forward (backward) direction. The arrows show the relative magnetization directions of the FM electrodes at various $H$, in relation to the FM coercive fields.

Figure 2

(Color online) The spin-valve related MR value of the diode shown in Fig. 1 vs the applied biasing voltage $V$ at $14\mathrm{K}$ [red (dark gray) filled circles] and $140\mathrm{K}$ [blue (dark) empty circles]; the $140\mathrm{K}$ data were multiplied by a factor of 2.8 for normalization purpose.

Figure 3

(Color online) (a) The MR value of three different LSMO∕OSEC∕Co spin-valve devices vs temperature $T$ normalized at $T=14\mathrm{K}$; the MR absolute values of these devices are summarized in Table . The OSEC interlayers in these devices are $\mathrm{Al}{\mathrm{q}}_3$ [green (gray) squares], CVB [red (dark gray) circles], and NPD [blue (black) stars]. (b) The calculated LSMO spin polarization $P\left(\mathrm{LSMO}\right)$ using the Jullière model (Ref. 2) and the data in (a); the symbols and colors are the same as in (a) above. The line through the data points is the calculated polarized charge carrier density (PCCD) of the LSMO electrode normalized at $T=0$ using the model given in Ref. 31, with $T_C=325\mathrm{K}$ (see inset). The inset shows the magnetization $M$ (empty circles) of the LSMO film used as the bottom FM electrode for the OSV shown in (a); the Curie temperature $T_C=325\mathrm{K}$ is assigned.

Figure 4

(Color online) The temperature dependence of the spin-$\frac{1}{2}$ PLDMR $(\delta \mathrm{PL}∕\mathrm{PL})$ value at $950\mathrm{G}$ (i.e., spin-$\frac{1}{2}$) for an evaporated $\mathrm{Al}{\mathrm{q}}_3$ film. The inset shows the PLDMR value vs the applied field $H$ at $220\mathrm{K}$.