AlGaAs

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Electric Field Controlled Magnetization in GaAs/AlGaAs Heterostructures-chiral Organic Molecules Hybrids Eilam Z. B. Smolinsky, Avner Neubauer, Anup Kumar, Shira Yochelis, Eyal Capua, Raanan Carmieli, Yossi Paltiel, Ron Naaman, and Karen Michaeli J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00092 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 25, 2019

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The Journal of Physical Chemistry Letters

Electric field controlled magnetization in GaAs/AlGaAs heterostructureschiral organic molecules hybrids Eilam Z. B. Smolinsky,#a Avner Neubauer,#b Anup Kumar,a Shira Yochelis,b Eyal Capua,a Raanan Carmieli,c Yossi Paltiel,b Ron Naaman,a Karen Michaelid

a) Department of Chemical and Biological Physics, Weizmann Institute, Rehovot 76100, Israel b) Department of Applied Physics and Center for Nano Science and Nanotechnology, The Hebrew University, Jerusalem 91904, Israel c) Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel d) Department of Condensed Matter Physics, Weizmann Institute, Rehovot 76100, Israel Abstract We study GaAs/AlGaAs devices hosting a two-dimensional electron gas and coated with a monolayer of chiral organic molecules. We observe clear signatures of room temperature magnetism, which is induced in these systems by applying a gate voltage. We explain this phenomenon as a consequence of the spin-polarized charges that are injected into the semiconductor through the chiral molecules. The orientation of the magnetic moment can be manipulated by low gate voltages, with a switching rate in the MHz range. Thus, our devices implement an efficient, electric field controlled magnetization, which has long been desired for their technical prospects. TOC Graphic

# Contributed equally

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Control of magnetic memory devices is primarily based on spin-selective transport phenomena such as giant magnetoresistance.1-4 It is typically realized with ferromagnetic materials,5,6 in which the transition temperature can be enhanced by creating complex layered structures7-9 that utilize the proximity effect.10 The performance of giant magnetoresistance devices can be significantly enhanced by integrating organic molecules into them.11 This takes advantage of the long spin-coherence length that is characteristic of many organic molecules, and allows them to serve as buffers between magnetic components. Recently it has been suggested that organic molecules can also replace the magnetic elements.12 This is a consequence of the large spin-polarization that arises when electrons transfer through chiral organic molecules. The effect is known as the chiral induced spinselectivity (CISS).13 Numerous different chiral organic molecules are known to act as spin filters at room temperature, and a particularly strong CISS effect at room temperature has been observed in nucleic acids14-17 and peptides18,19. Importantly, the preferred spin direction in CISS is determined by the handedness of the molecule and by the direction of charge transfer. It has been demonstrated that CISS allows both writing and reading of a magnet’s orientation by driving current through a layer of chiral molecules.20-22 In fact, controlled alignment of magnetic domains has been achieved solely by the adsorption of organic molecules on top of a ferromagnet.23 In this case, the chemical potential mismatch between the two components leads to the injection of electrons or holes, which are spinpolarized due to CISS. One of the holy grails in designing memory devices is the ability to induce and locally manipulate magnetism solely through electric fields.24,25 So far, the major role of electric fields in such magnetic devices is restricted to reversing the orientation of an existing magnetic moments.26 In this paper we demonstrate that a robust magnetic moment can be switched on or turned off by applying a gate voltage on a GaAs/AlGaAs semiconductor-based heterostructure which has chiral molecules self-assembled on its surface and below the gate. Our GaAs/AlGaAs device hosts a layer of dopants and a two-dimensional electron gas (2DEG) separated by an insulating layer. Crucially, this device does not contain any ferromagnetic material. The spin injection is a manifestation of the recently observed CISS effect,27]which results in coupling between the electrons’ spins and their linear momenta due

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to the spin-orbit coupling that induced by the curvature of the electronic potential in chiral molecules.28 The appearance of a voltage-induced magnetic moment is confirmed by three independent experiments: microwave absorption, Hall effect, and magnetization measurements using a superconducting quantum interference device (SQUID). The magnetic moment persists as long as the voltage on the gate is maintained, but disappears rapidly once it is switched off. We interpret our observations as a magnetic phase transition at room temperatures, which is induced by an electric field. We discuss possible mechanisms behind this robust magnetic order and construct a phenomenological model that identifies its origin in exchange interactions mediated by the 2DEG. Since the orientation of the magnetic moment can be manipulated by low gate voltages, with a switching rate in the MHz range, our system holds great potential for realizing an efficient electric field-controlled magnetic device. All our devices are comprised of GaAs/AlGaAs heterostructures each hosts a shallow two-dimensional electron gas (2DEG), with a layer of organic molecules adsorbed on the surface (see Fig. 1 and the full structure is shown in the SI). We investigated four kinds of devices that differ by the type of organic molecules. Each device contained either only leftor only right-handed molecules of [SH]-C-(Ala-Ala-Ala-Ala-Lys)7-[COOH] (denoted by long AHPA-L and AHPA-D) or SHCH2CH2CO-{Ala-Aib}5-COOH (denoted by short AHPA-L and short AHPA-D), where Ala, Lys and Aib are Alanine, Lysine and 2Aminoisobutyric amino acids. Various experiments have shown that the spin filtering improves with the length of the molecule,13 therefore in general we preferred to use longer oligopeptides to maximize the observed effect. However, when we had to depost gate on top of the molecules (see below). We found that the devices obtained after the deposition are more stable and the data is more reproducible with the shorter oligopeptides. Therefore, the later were used with the gated devices. The instability with the long oligopeptides can be explained by large number of pinholes in the monolayers they form.

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(B)

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Figure 1: Schematics diagram for the GaAs/AlGaAs-chiral molecules hybrid devices studied here. Chiral poly-peptide molecules were adsorbed onto the surface of the shallow 2DEG (full structure is shown in the SI). GaAs/AlGaAs heterostructure. In one configuration (A), the structure used 2DEG located 36 nm under the surface (Rev. 7 in the SI Fig. S1). The chiral molecules monolayer is covered with a 30 nm thick MgO on which a gold gate electrode is deposited. In the second configuration (B), the structure used had the 2DEG 58 nm under the surface (Rev. 6 in the SI Fig. S1), the top gate is absent. Each device is patterned with four contacts, source (S), drain (D) and two transverse electrodes (H), to allow for Hall measurements. Upon applying a positive (negative) gate voltage, a short pulse of charge is transferred through the molecule to the semiconductor in order to compensate for the electric field. As a consequence of the CISS effect, this transferred charge should be spinpolarized. The injected electrons or holes can either enter the conduction band or valence band, respectively, or become trapped in impurity states (see SI). Typically, the life time of electron’s spin in GaAs/AlGaAs heterostructure is on the order of few hundreds of nanoseconds.29-31 The primary goal of the three experiments reported below is to measure the magnetic moment that results from the spin injection and to determine the parameters that affect its size and temporal behavior.

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Figure 2: Hall voltage measurements in the absence of an external magnetic field. A) The average Hall potential of a device coated with short AHPA-L poly-peptide as a function of time in response to external gate voltage of 0.3V (black line) in the configuration shown in Fig. 1A. The source drain current was 10µAmp in all measurements. The gate voltage is switched on at t=0 and off at t=125μsec. For comparison, a device with achiral molecules (3- mercaptopropionic acid) does not show any Hall effect (blue line). B) A similar experiment as conducted in A with the chiral molecule, only with three different values of gate voltages. Clearly, the signal depends on the sign of the gate voltage. The Hall response remains constant as long as voltage is applied. (C) Change in the Hall voltage as function of temperature in gate-less devices as shown in Fig 1B. Devices with both AHPA-L or D poly-peptide monolayers (only one enantiomer) shows a change in the hall voltage below 200°K even without an applied gate voltage. In contrast, no Hall signal is measured in devices with achiral molecules (dithiol). Hall effect without a magnetic field: In the first set of experiments, we used a Hall bar that was patterned on the GaAs/AlGaAs heterostructure, and had a monolayer of short AHPAL adsorbed on top of the source-drain current channel. Most significant results are achieved for shallow 2DEG. The data in Fig. 2, show that a Hall voltage is generated when an electric potential is applied between the monolayer and the 2DEG layer, even when no external magnetic field was applied (see Fig. 1A). The Hall signal displays a fast rise (about 1μsec) and remains constant as long as the gate voltage is applied. Once the voltage is turned off, the Hall signal decays within 1μsec. Note that the sign of the Hall response depends on the sign of the applied voltage (Fig. 2B). This observation agrees with the

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known property of the spin-filtering due to CISS effect which depends on the current direction. In principle, measurements of transverse voltage could conceivably be falsely interpreted as Hall effect without a magnetic field for various reasons. For example, misalignment of the contacts can lead to mixing between the longitudinal and transverse resistances. In this scenario, our experimental observation would be attributed to changes in the longitudinal resistance induced by the large gate voltage. To rule out this possibility, the measured signal was anti-symmetrized as described in the supporting information (SI). In addition, we found that the change in Hall signal as a function of voltage is parametrically larger than that of the longitudinal resistance, Rxx, and does not show linear 𝛽

dependence on it. Specifically, the Hall resistance at room temperature scales as 𝑅𝑥𝑥 where 𝛽 = 3 ± 0.5, which is inconsistent with the misalignment explanation. This mixing between the longitudinal and transverse resistances does not fit also the difference in temperature dependence shown at figure 2C. To explicitly test for the role of chirality, we fabricated similar devices in which achiral molecules were adsorbed on the surface. We did not observe any Hall response in these devices (see Fig. 2A). As seen in Fig. 2B, the Hall potential for the negative biased Gate is larger than for the positively biased Gate. When applying negative potential on the gate, the channel is depleted, namely the number of the charge carriers is reduced. The Hall voltage measured is sensitive both to the change in Rxx (the resistance of the device), and the anomalous Hall Effect. When a negative potential is applied on the Gate and the number of charge carrier is reduced the ratio between the injected spins and the charge carrier concentration increases and as a result the Hall potential is larger. Interestingly, when long AHPA-L or D molecules are used, a permanent Hall response is measured at temperatures below 200°K even in the absence of an applied gate voltage (see Fig. 2C). An opposite sign of the Hall signal is observed for the left- and right-handed molecules. The magnitude of the Hall response, however, is unequal for the two types of molecules. We believe this is due to the lower purity of the D enantiomer that forms a less packed monolayer (see for example Ref. 23). As a control experiment, we repeated the Hall effect measurement on a bare semiconductor. To obtain a signal similar to the one achieved in the presence of the chiral molecules without magnetic field at room temperature and 0.3V, we had to apply an external magnetic field

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of the order of 100 Oe (see SI). This size of magnetic field can be produced by about 1012 to 1013 spins per cm2. Namely it means that each adsorb molecule injects between 0.1 to 1 spins. Microwave absorption: The set of Hall measurements strongly suggests that the adsorbed chiral molecules induce some magnetization in the semiconductor heterostructure, both below 200°K at zero gate voltage, and at room temperature upon applying a gate voltage. To further examine this phenomenon, we measured microwave (9.5 GHz) absorption by the AHPA-L device (in the configuration shown in Fig. 1A), when placed inside an EPR, without current between source and drain. We observed a microwave signal following the application of a gate voltage. Similar to the Hall response, the signal persisted as long as the gate potential was applied, and quickly decayed after it was switched off (Fig. 3). The sign of the voltage manifests itself in the phase of the microwave absorption signal. The difference between a positive and a negative gate voltage is a π-phase shift in the signal. An additional π-phase change was observed, in both cases, when the voltages was switched off.

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Figure 3: Microwave absorbance as a function of time for two opposite gate voltages (±0.3V) manipulating out of plane spins. The voltages were applied for 20 microseconds, and a microwave absorption signal with a rise time of 10 microseconds was measured. The only substantial difference between a positive and a negative gate voltage is a π-phase shift in the signal. In both cases, a π-phase change also occurs when the voltage is turned off. The microwave absorption of 9.5 GHz corresponds to an energy splitting of around 10μeV in the spin states of the device. The change in the phase of the microwave signal relates to

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absorption-emission. Due to the low Q factor of the cavity, the microwave absorption is very broad and exceed 1 GHz. To test whether the measured signal can indeed be attributed to the presence of chiral molecules or to level splitting by the gate voltage, for example, several control experiments were conducted. We measured the microwave absorption of a gated semiconductor device (i) with a layer of achiral molecules adsorbed on its surface and (ii) in the absence of any molecule. In both cases no signal has been observed above the noise level (see SI). A

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Figure 4: Magnetic moment measurements using a SQUID as a function of an external out of plane perpendicular magnetic field. In (A) the magnetization of a short AHPA-L is measured in the presence of an applied gate voltage of -0.6V at 300°K and 200°K (presented with respect to the value at V=0). We see a clear hysteresis in both cases. In (B) the out pf plane magnetization of a long AHPA-L device is measured in the absence of an applied gate at 200°K (presented with respect to the value at T=300°K). Results around zero magnetic field are shown in the inset. The error in all measurements is about 0.02 µemu. The fact that Hall and microwave absorption signals are measured only in devices with chiral molecules is consistent with the CISS effect, which predicts that the charge transferred from the gate to the semiconductor is spin polarized. However, the charge transfer is expected to accompanied by spin polarization only over short time scale (less

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than a microsecond) after the gate voltage is switched on/off. Thus, a very surprising result of our experiment is that the Hall and microwave signal persist so long as the voltage is kept on, even after current flow has ceased, i.e., the lifetime of the injected spins is longer than tens of microseconds. Together with the observation of a Hall response (without a magnetic field) below 200°K in the ungated devices, our measurements suggest that the systems exhibit some form of magnetic order, for example ferromagnetism or superparamagnetism with high blocking temperature. In the third experiment we directly tested the magnetization scenario with a superconducting quantum interference device (SQUID). The magnetization of the short AHPA-L molecules-semiconductor hybrid is shown in Figure 4A. We observed a clear hysteresis, with a coercive field of about 100 Oe at 300°K that grows as the temperature decreases. Note that the data (obtained at -0.6V) are presented after subtracting the background magnetization measured at zero gate voltage. In the long AHPA-L devices (Fig. 1B), we measured a magnetic moment once the devices were cooled below 200°K (Fig. 4B). In contrast, no signature of magnetization was observed for achiral molecules (see SI). The SQUID measurements, thus, confirm the magnetic hypothesis. The magnetic moment measured in the first device (Fig. 1A) at saturation, corresponds to about 1012 spins. The coerciveness is about 100 Gauss and is consistent with the Hall measurements which indicate that about 0.1 to 1 spins are injected per adsorbed molecule. Former studies also indicated that each adsorbed molecule injects at most one electron into the semiconductor,32 which can explain the saturation we observed in all signals above a voltage of about ±0.5 V. To further investigate the origin of the magnetic phase, we studied the relation between the longitudinal (𝑅𝑥𝑥 ) and transverse (𝑅𝐻 ) resistance of the long AHPA devices that show spontaneous magnetization below 200°K. The Hall voltage measured in the absence of an external magnetic field is plotted in Fig. 5A as a function of Rxx. We found a clear linear dependence at low resistance. Since the magnetization is roughly constant in this regime, we may conclude that the Hall resistance scales with the longitudinal resistance as 𝑅𝐻 = (𝑅0 + 𝛼𝑅𝑥𝑥 )𝑀, where 𝑀 is the magnetization and 𝑅0 , 𝛼 are constants. Note that the two contributions to the resistance are of similar magnitude which suggests that a substantial part of the Hall effect is of the anomalous type.33,34 This observation implies strong exchange interactions between the

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magnetic moments and the 2DEG. A similar anomalous Hall effect is known to occur in ferromagnetic semiconductors such as Mn-doped GaAs.35 Finally, we added 6nm of Al2O3 tunnel barrier between the semiconductor surface and the organic molecules. As shown in Fig. 5B, the Hall effect disappears at all temperatures upon adding the tunnel barrier to the long AHPA device. Moreover, we did not observe a microwave signal in a semiconductor-chiral molecules hybrid device without the 2DEG (see SI). These two measurements indicate that the 2DEG is essential for establishing magnetization in our devices.

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Figure 5: A) The Hall resistance of a long AHPA-L device as a function of the longitudinal resistance. Both resistances were measured below 200°K. In the range that Rxx is between 6500 and 4500  the Hall potential follows a clear linear dependence. B) The change in the Hall voltage as a function of temperature for the same device as in (A) and the change in the Hall potential in a similar device in which the molecules are adsorbed on top of 6 nm of Al2O3 layer. The addition of the insulating alumina layer quenches the Hall signal. All our experimental findings imply that by applying a gate voltage we induced a room temperature magnetism in the GaAs/AlGaAs-chiral molecules hybrid devices. We now discuss possible origins of the observed magnetization. By eliminating scenarios that are inconsistent with the experimental observations, we arrive at a phenomenological model that captures the observed effect. All GaAs/AlGaAs heterostructures used in the experiment contain a layer of dopants (n-AlGaAs) and a spatially separated 2DEG. An approximately uniform distribution of impurity states is present in the former. The chiral molecules are deposited between the heterostructures and the top gate. These molecules

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control the spin of the injected charge carries upon applying a gate voltage by virtue of the CISS effect. The injected charge carriers, electrons or holes, may either join the 2DEG or occupy impurity states in the dopant layer. Importantly, it has been shown experimentally that electrons transferred through undoped GaAs can maintain their spin over a distance of several hundred angstroms.36-38 Thus, the charges injected through the molecules to the 2DEG or the dopant layer are spin-polarized. Moreover, we expect a significant hybridization between the molecules and the impurity states. At this point, we consider this hybridization and long-range spin transport as empirical facts; a direct measurement of the spin injection length as well as an ab-initio calculation of the molecule-semiconductor interactions are beyond the scope of this work. However, these two ingredients are not sufficient to establish long-range magnetic order. In the absence of a magnetic phase, spin-relaxation mechanisms should cause all the measured signals to decay quickly after the gate voltage is applied. To understand where the injected spins accumulate, we note that ferromagnetism has been predicted (but not seen) to arise in electron gases at ultralow densities.39 Consequently, increasing the density of the 2DEG by charge injection cannot induce the observed ferromagnetism. The scenario of charge injection into the impurity states occupying the dopant layer is, therefore, more plausible. These impurity states may either be localized (for example, in DX centers) or form a narrow band; either way these electrons are much less mobile than the ones in the 2DEG and more susceptible to interaction effects. Magnetic order in the impurity states can be induced by ferromagnetic exchange interactions that are mediated by the mobile 2DEG electrons. This scenario is consistent with the observation of a large anomalous Hall effect, as well as with the absence of a microwave absorption signal in devices without 2DEG. Exchange interactions mediated by the conduction electrons, known as the RKKY mechanism,40-43 give rise to ferromagnetism in three-dimensional magnetically doped semiconductors.44,45 The sign of the RKKY interaction oscillates with the distance 𝑟 between nearby impurities, as −sin(√8𝜋𝑛𝑟)/𝑟 2, where 𝑛 is the carrier density of the 2DEG.[40] In our devices the densities of injected electrons and 2DEG are comparable, and the RKKY interaction is always ferromagnetic. Note that within this scenario, increasing the voltage should strengthen the magnetization, as is indeed observed. Moreover, this exchange interaction is essential for both possible explanations, ferromagnetism or

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superparamagnetism. The latter may occur if the density of impurity states is nonhomogenous and hence, the ferromagnetic exchange is large in areas of high concentration, which are separated by regions with vanishing (or even antiferromagnetic) exchange.46 Since both the ferromagnetic and antiferromagnetic states originate from the same impurities, we expect them to have similar transition temperatures. For the superparamagnet an additional scale, the blocking temperature, becomes important. Below the blocking temperature, global measurements such as those reported here cannot distinguish between the two alternative magnetic states. Thus, in the following we focus on the ferromagnetic transition. The data shown in Fig. 2C provide another support to the model presented above. It is important to realize that upon adsorption of molecules on the device the surface potential is modified and as a result spins are injected. Upon cooling the device, the electrons density in the 2DEG layer drops rapidly. According to the model, the density of the injected spins needs to be of the order of 10% of the number of electrons in the 2DEG or higher to obtain the magnetic properties. At room temperature, the spontaneously injected spins density is too small relative to the electrons concentration in the 2DEG. Therefore, by cooling and the decrease of the density in 2DEG, the system is in the right regime for observing the magnetic properties. Additional contribution to the Hall signal may be due to the increase in the electrons’ mobility in the 2DEG, as a result of the cooling. Our proposed phenomenological explanation for the observed ferromagnetism does not yet reflect the importance of the chiral molecules. According to our scenario, the chirality of the molecules only enters as an initial condition in determining the orientation of the ferromagnetic moment. This is analogous to cooling a conventional ferromagnet in the presence of an external magnetic field. Consequently, in the SQUID measurement, no distinction between devices with chiral and achiral molecules after one magnetic-field sweep should be measured. However, while magnetization is observed in the chiral molecule/semiconductor devices, it does not show up in the latter case. This implies that the molecules have more than just a transient effect. The difference between devices with chiral and achiral molecules can be understood from the following reason: Recall that electrons injected through chiral molecules have a fixed spin polarization, which is parallel or antiparallel to their central axis. We expect that their hybridization with impurity states

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introduces an Ising anisotropy along this axis (perpendicular to the 2DEG). Such an anisotropy can significantly enhance the transition temperature, especially in lowdimensional systems. In the limit of two dimensions, such an anisotropy would in fact be required to form any magnetically ordered phase at finite temperature.47 We believe that in our quasi two-dimensional system, this explains the difference between the chiral and achiral cases. Notice also that the mechanism proposed here relies on the strength of the exchange interaction 𝐽0 between the impurity states and the 2DEG. Unfortunately, its value is not directly accessible through the measurements conducted here. In the SI we provide an estimate of the exchange interaction needed to observe room-temperature ferromagnetism. Up to now we focused on the mechanism that leads to room temperature magnetism upon applying a gate voltage. The origin of magnetism in the ungated devices, which occurs at lower temperatures, is similar. Here the spin-polarized charges are injected to the semiconductor when the temperature is lowered. To conclude, we presented here clear evidence for electric-field induced ferromagnetism in GaAs/AlGaAs heterostructures coated with self-assembled monolayers of chiral molecules. This observation is manifestation of the chiral induced spin selectivity, and its potential for magnetic-field-free memory and spintronic devices. In particular, our device allows creation of localized magnetic fields on a sub-micron scale at room temperature solely by applying a small gate voltage (~0.1V). From the measured Hall response, we estimate these magnetic fields to be of order 100 Oe. Furthermore, the response time of the induced magnetization is very fast and could exceed 1MHz without special optimization. Our devices are a new addition to the family of magnets where spinorbit coupling lies at the core of the observed phenomenology. Other well-established examples include helimagnets48,49 and Kitaev magnets50 which have recently drawn the attention of the physics community for exhibiting new forms of magnetism and hosting unusual collective excitations.51-5455

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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxxxxxxxxx. It provides information regarding the experimental procedures, the production of the devices and data obtained from various control experiments. AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] [email protected] [email protected] ORCID Yossi Paltiel 0000-0002-8739-9952 Karen Michael 0000-0003-0331-3355. Ron Naaman 0000-0003-1910-366X Author Contributions EZBS and AN contributed equally and performed the electronic measurements. AK and RC performed the microwave measurements. AK and EC designed the devices. SY prepared the organic monolayers on the gate less devices. YP and RN commenced the experiments and wrote the manuscript. KM formalized the theory and participated in writing the manuscript. Notes The authors declare no competing financial interest. Acknowledgments YP and RN acknowledge the partial support of the Volkswagen Foundation, the Israel Ministry of Science, and the John Templeton Foundation; KM acknowledges the support of the Israel Science Foundation Grant No. 1889/16. References (1) Baibich, M. N., Broto, J. M., Fert, A., Nguyen Van Dau F., Petroff, F., Etienne, P., Creuzet, G., Friederich, A., Chazelas, J., Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic Superlattices. Phys. Rev. Lett. 1988, 61, 2472–2475.

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