Spin-current injection and detection in κ-(BEDT-TTF)₂Cu[N(CN)₂]Br

The field of spintronics has attracted great interest in the last decade because of an impact on the next generation magnetic memories and computing devices, where generation, detection and manipulation of a spin current play the key roles.¹ Here, as a method for direct conversion of a spin current into an electric signal, inverse spin Hall effect (ISHE) has been widely studied in various materials to investigate spin-current physics and realize large spin-charge conversion.^2–9 Among these materials, conjugated polymers have been suggested to work as a spin-charge converter recently,^10,11 and further investigation of the spin-charge conversion in different organic materials will open new possibilities for the application of organic materials in spintronic devices. On the other hand, in the organic system, all the energy scales: correlation, electron-lattice coupling, and kinetic energy are small enough to compete with spin currents at low temperatures. A study of their spin-charge conversion gives us a chance to understand the physics underlying the correlation between spins and other physical phenomena.

Organic semiconductor family κ-(BEDT-TTF)₂X with X being anions is an system with large anisotropic conductivity and correlated electron. κ-(BEDT-TTF)₂X can be an interesting system for the study of the spin-charge conversion for the following reasons. Firstly, it is a typical molecular salt and an organic band metal system, which is quite different from the polaron conductive conjugated polymers.^10,11 Secondly, since the physical and electrical properties of this organic family are well understood and highly controllable, it is easy to investigate and modulate the state of samples.

In the present work, the spin-charge conversion in κ-(BEDT-TTF)₂Cu[N(CN)₂]Br (called κ-Br) is studied. The κ-Br consists of alternating layers of conducting sheets (composed of BEDT-TTF dimers) and insulating sheets (composed of Cu[N(CN)₂]Br anions) (Fig. 1(a)). The ground state of bulk κ-Br is known to be superconducting with the transition temperature T_c ≈ 12 K,¹² which becomes antiferromagnetic and insulating by replacing Cu[N(CN)₂]Br with Cu[N(CN)₂]Cl (Fig. 1(d)).¹³ 

To inject a spin current into the κ-Br, we employed a spin-pumping method by using κ-Br/yttrium iron garnet (YIG) bilayer devices. In the κ-Br/YIG bilayer devices, magnetization precession motions driven by ferromagnetic resonance (FMR) and/or spin-wave resonance (SWR) in the YIG layer inject a spin current across the interface into the conducting κ-Br layer in the direction perpendicular to the interface.¹⁴ This injected spin current is converted into an electric field along the κ-Br film plane if κ-Br exhibits ISHE.

The preparation process for the κ-Br/YIG bilayer devices is as follows. A single-crystalline YIG film with the thickness of 5 μm was put on a gadolinium gallium garnet wafer by a liquid phase epitaxy method. The YIG film on the substrate was cut into a rectangular shape with the size of 3 × 1 mm². Two separated Cu electrodes with the thickness of 50 nm were then deposited near the ends of the YIG film. The distance between the two electrodes was 0.4 mm. Here, as shown in Fig. 1(b), 20-nm-thick SiO₂ films were inserted between the electrodes and the YIG film, by which the spin injection into the Cu electrodes is completely blocked.¹⁵ Finally, a laminated κ-Br single crystal, grown by an electrochemical method,¹⁶ was placed on the top of the YIG film between the two Cu electrodes (Fig. 1(b)). We prepared two κ-Br/YIG samples A and B to check reproducibility. The thicknesses of the κ-Br films for the samples A and B are around 100 nm but a little different from each other, resulting in the difference of the resistance of the κ-Br film (Fig. 1(c)). To observe the ISHE in κ-Br induced by the spin pumping, we measured the H dependence of the microwave absorption and DC electric voltage between the electrodes at various temperatures with applying a static magnetic field H and a microwave magnetic field with the frequency of 5 GHz to the device.

Before studying the temperature dependence of the ISHE in the κ-Br, the state trajectory of our samples, which depends on the temperature, should be confirmed. This can be carried out by using the temperature dependence of resistance curve (Fig. 1(c)) and the phase digram (Fig. 1(d)). As shown in Fig. 1(c), the κ-Br films on YIG for the samples A and B exhibit no superconducting transition,^12,17 but do insulator-like behavior similar to a bulk κ-Cl.^13,17 Considering the sensitivity of the κ-Br to the external pressure,¹⁸ this result can be ascribed to tensile strain induced by the substrate due to the different thermal expansion coefficients of κ-Br and YIG. The similar phenomenon was reported in κ-Br on a SrTiO₃ substrate,¹⁶ of which the thermal expansion coefficient is close to that of YIG (∼ 10 ppm/K at room temperature^19,20). Thus, the ground state of the κ-Br film on YIG is expected to be slightly on the insulator side of the Mott transition. The red arrow in Fig. 1(d) schematically indicates a state trajectory of our κ-Br films with decreasing the temperature.^{16–18,21–26}

Figure 2(a) shows the FMR/SWR spectrum dI/dH for the κ-Br/YIG sample A at 300 K. Here, I denotes the microwave absorption intensity. The spectrum shows that the magnetization in the YIG film resonates with the applied microwave around the FMR field H_FMR ≈ 1110 Oe. As shown in Fig. 2(b), under the FMR/SWR condition, electric voltage with peak structure was observed between the ends of the κ-Br film at θ = ± 90^∘, where θ denotes the angle between the H direction and the direction across the electrodes (Fig. 2(b)). The voltage signal disappears when θ = 0^∘. This θ dependence of the peak voltage is consistent with the characteristic of the ISHE induced by the spin pumping. Because the SiO₂ film between the Cu electrode and the YIG film blocks the spin-current injection across the Cu/YIG interfaces, the observed voltage signal is irrelevant to the ISHE in the Cu electrodes. The magnitude of the electric voltage is one or two orders of magnitude smaller than that in conventional Pt/YIG devices,^27–30 but is close to that observed in polymer/YIG devices.¹⁰ 

To establish the ISHE in the κ-Br/YIG sample exclusively, it is important to separate the spin-pumping-induced signal from thermoelectric voltage induced by temperature gradients generated by nonreciprocal surface-spin-wave excitation,³¹ since thermoelectric voltage in conductors whose carrier density is low, such as κ-Br, may not be negligibly small. In order to estimate temperature gradient under the FMR/SWR condition, we excited surface spin waves in a 3-mm-length YIG sample by using a microwave of which the power is much higher than that used in the present voltage measurements, and measured temperature images of the YIG surface with an infrared camera (Figs. 3(a) and 3(b)). We found that a temperature gradient is created in the direction perpendicular to the H direction around the FMR field and its direction is reversed by reversing H, consistent with the behavior of the spin-wave heat conveyer effect (Fig. 3(c) and 3(d)).³¹ Figure 3(e) shows that the magnitude of the temperature gradient is proportional to the absorbed microwave power, as the dash line shows the linear least squares fitting of the data. This temperature gradient might induce an electric voltage due to the Seebeck effect in κ-Br with the similar symmetry as the ISHE voltage. However, the thermoelectric voltage is expected to be much smaller than the signal shown in Fig. 2(b); since all the measurements in this work were carried out with a low microwave-absorption power (marked with a green line in Fig. 3(e)), the magnitude of the temperature gradient on the surface of YIG is less than 0.015 K/mm. Considering the large volume ratio between the YIG substrate and κ-Br crystal, the temperature gradient in the κ-Br should be determined by the YIG substrate. Even when we use the Seebeck coefficient of κ-Br at the maximum value reported in previous literatures,^32–34 the electric voltage due to the Seebeck effect in the κ-Br film is estimated to be less than 0.01μV at 300 K, where the effective length of κ-Br is 0.4 mm. This is at least one order of magnitude less than the signals observed in our κ-Br/YIG sample. Therefore, we can conclude that the observed electric voltage with the peak structure is governed by ISHE.

From here on, the electric voltage signal observed at the FMR field is labeled as V_ISHE. Because the charge transport at high temperatures in the κ-Br is evidenced to be metallic transport at the Fermi level, the observation of the ISHE signal in the κ-Br means that the conjugated orbitals are not necessary for the ISHE in organic systems. However, reflecting the mainly carbon-based light-element composition, the spin Hall angle in such organic materials is expected to be small because of the weak spin-orbit interaction. The observable ISHE voltage may be attributed to the enhancement due to the anisotropic conductivity in the κ-Br, which is similar to the mechanism reported for the conjugated polymers.¹⁰ 

Figure 4(a) and 4(b) shows the microwave power and electric voltage spectra in the κ-Br/YIG sample A for various values of the temperature T. Clear voltage signals were observed to appear at the FMR fields when T > 80 K. We found that the sign of the voltage signals is also reversed when H is reversed, which is consistent with the ISHE as discussed above. Surprisingly, the peak voltage signals V_ISHE at the FMR fields decrease steeply with decreasing T and merge into noise around 80 K, while the temperature dependence of the microwave absorption power is very small. This anomalous suppression of the voltage signals cannot be explained by the resistance R change of the κ-Br film because no remarkable R change was observed in the same temperatures (Fig. 1(c)). At temperatures lower than 60 K, large voltage signals appear around the FMR fields as shown in Fig. 4, but its origin is not confirmed because of the big noise and poor reproducibility. Therefore, hereafter we focus on the temperature dependence of the voltage signals above 80 K.

In Fig. 5, we plot the T dependence of V_ISHE/R for the κ-Br/YIG samples A and B, where $V ISHE = V ISHE ( − H ) − V ISHE ( + H ) /2$ with V_ISHE(±H) being the electric voltage at the FMR fields, to take into account the resistance difference of the κ-Br films. The V_ISHE/R dat for both the samples exhibit almost same T dependence, indicating that the observed voltage suppression is an intrinsic phenomenon in the κ-Br/YIG samples.

Here we discuss a possible origin of the observed temperature dependence of the voltage in the κ-Br/YIG systems. ISHE voltage is determined by two factors. One is spin-to-charge conversion efficiency, i.e. the spin-Hall angle, in the κ-Br film. The mechanism of ISHE consists of intrinsic contribution due to spin-orbit coupling in the band structure and extrinsic contribution due to the impurity scattering.³⁵ In organic systems such as κ-Br, the extrinsic contribution seem to govern the ISHE since intrinsic contribution is expected to be weak because of their carbon-based light-element composition. Judging from the predicted rather weak temperature dependence of impurity scattering, the temperature dependence of the spin-Hall angle can not be the origin of the sharp suppression of the voltage signal in the κ-Br/YIG systems (Fig. 5). The other factor is the spin-current injection efficiency across the κ-Br/YIG interface, which can be affected by spin susceptibility³⁶ in κ-Br. Importantly, the temperature dependence of the spin susceptibility for the κ-X family was shown to exhibit a minimum at temperatures similar to those at which the anomalous suppression of the spin-pumping-induced ISHE voltage was observed,¹⁷ suggesting an importance of the temperature dependence of the spin-current injection efficiency in the κ-Br/YIG systems. We also mention that the temperature at which the ISHE suppression was observed coincides with a glass transition temperature of κ-Br films.^37,38 However, at the present stage, there is no framework to discuss the relation between the spin-current injection efficiency and such lattice fluctuations. To obtain the full understanding of the temperature dependence of the spin-pumping-induced ISHE voltage in the κ-Br/YIG systems, more detailed experimental and theoretical studies are necessary.

In summary, we have investigated the spin pumping into organic semiconductor κ-(BEDT-TTF)₂Cu[N(CN)₂]Br (κ-Br) films from adjacent yttrium iron garnet (YIG) films. The experimental results show that an electric voltage is generated in the κ-Br film when ferromagnetic or spin-wave resonance is excited in the YIG film. Since this voltage signal was confirmed to be irrelevant to extrinsic temperature gradients generated by spin-wave excitation and the resultant thermoelectric effects, we attribute it to the inverse spin Hall effect in the κ-Br film. The temperature-dependent measurements reveal that the voltage signal in the κ-Br/YIG systems is critically suppressed around 80 K, implying that this suppression is associated with the spin and/or lattice fluctuations in κ-Br. This result offers an alternative way for the spin current manipulation.

This work was supported by PRESTO “Phase Interfaces for Highly Efficient Energy Utilization,” Strategic International Cooperative Program ASPIMATT from JST, Japan, Grant-in-Aid for Young Scientists (A) (25707029), Grant-in-Aid for Young Scientists (B) (26790038), Grant-in-Aid for Challenging Exploratory Research (26600067), Grant-in-Aid for Scientific Research (A) (24244051), Grant-in-Aid for Scientific Research on Innovative Areas “Nano Spin Conversion Science” (26103005) from MEXT, Japan, and NEC Corporation.