A molecular spin valve (MSV) is a spintronic device mainly formed by three layers: a molecular semiconductor placed in between two ferromagnetic (FM) electrodes, named spin injector and spin detector. The MSC, or spin collector, has the function to transport the spin carriers pumped by the spin injector to the detector. Besides, both ferromagnets are chosen to have different coercive fields that will give rise to two magnetic alignments: the paralel state (P) and the antiparalel state (AP). In a current flow, the carriers find more or less scattering depending on their spin polarization when being injected into the second FM electrode.
Parallel (top panel) and antiparallel (bottom panel) states in a molecular spin valve. Carriers encounter high or low scattering depending on their spin orientation compared with the magnetization direction of the electrodes. On the right-hand side the equivalent circuit for each situation is depicted (R(r) stands for the high (low) equivalent resistance). White arrows point to the magnetization direction of the FM electrodes while black arrows and gray circles indicate the spin-up and spin-down charge carriers. Adapted from the work of Camarero et al.
Hence, various scenarios lead to different equivalent circuits based on Motts model, as illustrated abbove. In a first approximation, the equivalent circuit is composed by two channels, one for the spin-up and the other for the spindown carriers. A MSV with the two FM electrodes magnetically oriented in the same direction is shown in the top panel of the figure. This situation is known as the parallel (P) state. When traversing the device, a carrier oriented spin-up will find high scattering at both electrodes, unlike a carrier-oriented spin-down what is reflected in the magnitude of the resistances of the equivalent circuit. Consequently, the spin-up population is depleted.
The bottom panel of the figure depicts the antiparallel (AP) state where the electrodes have opposing magnetization directions. In this case both spin-up and spin-down carriers encounter many scattering processes and both spin-up and spin-down populations are attenuated.
Hence, various scenarios lead to different equivalent circuits based on Motts model, as illustrated abbove. In a first approximation, the equivalent circuit is composed by two channels, one for the spin-up and the other for the spindown carriers. A MSV with the two FM electrodes magnetically oriented in the same direction is shown in the top panel of the figure. This situation is known as the parallel (P) state. When traversing the device, a carrier oriented spin-up will find high scattering at both electrodes, unlike a carrier-oriented spin-down what is reflected in the magnitude of the resistances of the equivalent circuit. Consequently, the spin-up population is depleted.
The bottom panel of the figure depicts the antiparallel (AP) state where the electrodes have opposing magnetization directions. In this case both spin-up and spin-down carriers encounter many scattering processes and both spin-up and spin-down populations are attenuated.
Thus, two resistance states can be observed at a fixed voltage or at a constant current applied, in a magnetic field sweep, what gives rise to the magnetoresistance curves (see figure above). Two resistance states are established in the device due to the distinct values of the coercive fields (Hc). The measurement is usually initiated in a high negative field (stage 1 and red solid line in the figure) where bot electrodes are aligned. Then the field is decreased progressively until its direction is changed by making it positive. When the soft magnet coercive field is reached (Hc1) it flips its magnetization and the resistance switches to the resistance of the AP state. The spin valve remains in the high resistance state only until the hard magnet is oriented parallel, what happens at stage 3. At this point the resistance of the P state is recovered since both electrodes have again the same magnetization direction (stage 4). The dashed purple line shows the equivalent measurement in the backward sweep direction with the changes between P and AP states at the stages 5 (Hc of the soft magnet) and 6 (Hc of the hard magnet). The percentage of giant magnetoresistance or usually termed just magnetoresistance is calculated as: GMR = (RAP - RP) / RP.
In 2002 Dediu and coworkers14 designed a LSMO/T6/LSMO junction that varied the electrical resistance in a magnetic field. However, the identical LSMO electrodes showed equal coercive fields which precluded the MR measurement. Later in 2004, a magnetoresistance curve was measured for the first time in a vertical molecular spin valve designed by Xiong et al. They reported 40 % at 11 K in a LSMO/Alq3/Co junction.
Since then much progress has been made in this kind of molecular spintronics devices. In most cases, the device comprises inorganic electrodes that sandwich a molecular layer but a fully molecular spin valve was reported recently with the structure V[TCNE]x/rubrene/V[TCNE]x. Besides, MSVs are also evolving to multifunctionality and a photoresponsive spin valve with four different resistance states and based on F16CuPC was reported in 2016.
Most MSVs, apart from the two electrodes and the spin collector, include insulating barriers, being the most common the alumina barrier (Al2O3). These barriers contribute considerably to the overall resistance of the junction but they help with the high short circuit statistics, which constitutes an ongoing issue in the molecular spintronics field. Also, it has been demonstrated that the adhesion of Alq3 is improved when deposited on an oxide layer. Besides Al2O3, other type of barriers have been tested such as LiF.
Since then much progress has been made in this kind of molecular spintronics devices. In most cases, the device comprises inorganic electrodes that sandwich a molecular layer but a fully molecular spin valve was reported recently with the structure V[TCNE]x/rubrene/V[TCNE]x. Besides, MSVs are also evolving to multifunctionality and a photoresponsive spin valve with four different resistance states and based on F16CuPC was reported in 2016.
Most MSVs, apart from the two electrodes and the spin collector, include insulating barriers, being the most common the alumina barrier (Al2O3). These barriers contribute considerably to the overall resistance of the junction but they help with the high short circuit statistics, which constitutes an ongoing issue in the molecular spintronics field. Also, it has been demonstrated that the adhesion of Alq3 is improved when deposited on an oxide layer. Besides Al2O3, other type of barriers have been tested such as LiF.
REFERENCES
[1] J. Devkota, R. Geng, R. C. Subedi, and T. D. Nguyen. “Organic spin valves: a review”. Advanced Functional Materials 26.22 (2016), pp. 3881–3898.
[2] Z. Xiong, D.Wu, Z. V. Vardeny, and J. Shi. “Giant magnetoresistance in organic spin-valves”. Nature 427.6977 (2004), pp. 821–824.
[3] D. Sun, E. Ehrenfreund, and Z. V. Vardeny. “The first decade of organic spintronics research”. Chemical Communications 50.15 (2014), pp. 1781–1793.
