Abstract
One of the most important achievements in the field of spintronics is the development of magnetic tunnel junctions (MTJs). MTJs exhibit a large tunneling magnetoresistance (TMR). However, TMR is strongly dependent on biasing voltage, generally, decreasing with applying bias. The rapid decay of TMR was a major deficiency of MTJs. Here we report a new phenomenon at room temperature, in which the tunneling magnetocapacitance (TMC) increases with biasing voltage in an MTJ system based on Co_{40}Fe_{40}B_{20}/MgO/Co_{40}Fe_{40}B_{20}. We have observed a maximum TMC value of 102% under appropriate biasing, which is the largest voltageinduced TMC effect ever reported for MTJs. We have found excellent agreement between theory and experiment for the bipolar biasing regions using DebyeFröhlich model combined with quartic barrier approximation and spindependent driftdiffusion model. Based on our calculation, we predict that the voltageinduced TMC ratio could reach 1100% in MTJs with a corresponding TMR value of 604%. Our work has provided a new understanding on the voltageinduced AC spindependent transport in MTJs. The results reported here may open a novel pathway for spintronics applications, e.g., nonvolatile memories and spin logic circuits.
Introduction
A new class of electronic devices based on the spin degrees of freedom has been extensively studied and it has given rise to the field of spintronics^{1,2,3,4,5,6,7}. One of the most important achievements in this field is the development of magnetic tunnel junctions (MTJs), which has enabled the observation of a large tunneling magnetoresistance (TMR)^{3,4,8,9,10,11,12}. Practical applications relying on this TMR effect range from data read heads in hard disk drives to highly sensitive magnetic sensors. The magnetic states of MTJs can also be manipulated by the current induced spintransfer torque (STT) effect^{13,14,15,16,17}. More applications have become possible, e.g., magnetic random access memories (MRAMs)^{18,19,20,21}, logicinmemory circuits^{22,23} and random number (RN) generators^{24,25}. More recently, neuromorphic computing using spintorque MTJ oscillators has been developed^{26}. Thus, MTJs can serve as one of the most crucial building blocks of spintronicsbased computing.
It is well known that TMR is strongly dependent on biasing voltage, generally, decreasing with applying bias^{27,28,29,30}. One major challenge in MTJ research is to find ways to increase the half biasing voltage, V_{1/2}, at which the total TMR ratio near zero biasing is halved. For MgObased MTJs^{3}, V_{1/2} is approximately 1 V. Normal electrical operations will always require certain applying biasing voltage, which renders TMR less optimal. Extensive efforts have been made to enhance V_{1/2}, but progress has come very slowly. Recently, it has been demonstrated that V_{1/2} depends strongly on the interfacial structures between the ferromagnetic (FM) and the insulator layer. A high V_{1/2} can be obtained in MTJs with i) few misfit dislocations near the interfaces, ii) a small lattice strain inside the insulator or iii) welldefined sharp interfaces^{31,32,33,34,35}. The value of V_{1/2} in latticematched Fe/spinel MgAl_{2}O_{4}/Fe(001) MTJs reaches +1.0 V (−1.3 V) for the positive (negative) bias^{31}. In other studies, relatively high V_{1/2} values, ranging from 0.5 to 1.0 V, have also been reported in fully epitaxial Fe/MgO/GaO_{x}/Fe^{32}, NiFe(111)/AlO_{x}/CoFe^{33}, Fe(211)/AlO_{x}/CoFe^{34} and epitaxialFe_{4}N/MgO/CoFeB MTJs^{35}.
Another approach to obtain a high V_{1/2} is to explore organic spin valves (OSVs), where carbonbased molecules are used between FM electrodes. The value of V_{1/2} for C_{60} and carbon nanotubesbased OSVs is approximately 0.2 V at 5 K^{36}. In OSVs using tris(8hydroxyquinolinate) aluminum (Alq_{3}) or rubrene, the V_{1/2} has been reported to be 0.1−0.2 V at room temperature^{37,38,39}. Interestingly, graphenebased OSVs show the robustness of spin polarization in a nonlocal scheme^{40}. The normalized spin polarization reaches up to 0.8 at 1.2 V, which is inferior to MgObased MTJs. Overall, efforts in increasing V_{1/2} have not been successful in MTJs or other spintronic devices.
Complementary to TMR effect, the tunneling magnetocapacitance (TMC) is also inherent to the capacitive MTJ structure^{41,42,43,44}. Recently TMC has been increasingly studied due to their fascinating spin phenomena, such as spin capacitance^{45,46,47,48}, frequencydependent spin transport^{42,43,49} and potential applications^{41,50}. Though TMC and TMR share some similar origins and are correlated, TMC is unique in many aspects. For example, TMC is peaked at a specific frequency^{43}, but is weakly dependent on the temperature^{44}. Furthermore, V_{1/2} of TMC is higher than that of TMR^{44,47}. The highest V_{1/2} of TMC in MgObased MTJs is 0.7 V, which is almost twice as high as that of TMR in the same device^{47}. Therefore, TMC is more robust against biasing than TMR, and the mechanism of this robustness remains unclear. It is beneficial to understand this mechanism as the TMC effect may lead to more spintronics applications as well as insight into fundamental spintronic behavior.
In this work we report a new phenomenon at room temperature in which the TMC increases with applying biasing voltage in an MTJ system based on Co_{40}Fe_{40}B_{20}/MgO/Co_{40}Fe_{40}B_{20}. This result means an unprecedented enhancement of V_{1/2} with regard to TMC. We have observed a maximum TMC value of 102% under biasing, which is the largest voltageinduced TMC effect ever reported for MTJs. There is an excellent agreement between theory and experiment for the TMC in the bipolar bias regions using DebyeFröhlich (DF) model combined with quartic barrier approximation (QBA) and spindependent driftdiffusion (SDD) model. Based on our calculations, we predict that the voltageinduced TMC ratio could reach 1100% in MTJs with a corresponding TMR ratio of 604%.
Results and Discussion
Device structure and TMR and TMC under no bias voltage
Figure 1a shows the device structure prepared by a magnetron sputtering system with a base pressure of 2 × 10^{−8} Torr, with the following layer sequence: SiO_{2}/Ta(5)/Co_{50}Fe_{50}(2)/IrMn(15)/Co_{50}Fe_{50}(2)/Ru(0.8)/Co_{40}Fe_{40}B_{20}(3)/MgO(2)/Co_{40}Fe_{40}B_{20}(3)/contact layer (thickness in nm). Details of the device fabrication procedure are described in the Experimental Section. Using standard photolithography, we have patterned the multilayer MTJ stacks into a junction area of 1800 μm^{2} with an elliptical shape with Ar ionmilling and SiO_{2} insulation overlayer. The frequency characteristics and the bias voltage dependence of the TMR and TMC for MTJs were measured by an AC fourprobe method at room temperature. The AC voltage was set to 2.6 mV_{rms}. The magnetic field was applied along the magnetic easyaxis direction to 1.4 kOe.
Figure 1b,c show the TMR and TMC curves at 60 Hz. The DC applied voltage is 0 V. Clear TMR and TMC effects are observed, i.e., R_{P} < R_{AP} and C_{P} > C_{AP}. Large TMR and TMC ratios of approximately 100% are obtained at room temperature. Figure 1d,e shows the frequency characteristics of TMC, TMR and C_{P(AP)}. We calculated the frequency characteristics of the TMC and C_{P(AP)} using the DF model^{43,51}. Based on the model, the capacitance \({C}_{{\rm{P}}({\rm{AP}})}^{{\rm{DF}}}({f})\) as a function of frequency f for the P(AP) configuration in MTJs can be expressed by
where C_{∞,P(AP)} and C_{0,P(AP)} are the highfrequency and static capacitances, τ_{P(AP)} is the relaxation time of electric dipoles, consisting of electrons and holes, formed near the interface between the FM layer and insulator, and β_{P(AP)} is the exponent showing the distribution of the relaxation time, respectively, for the P(AP) configuration. After a straightforward calculation of Eq. (1), we can obtain
The relation between τ_{P} and τ_{AP} in FM/insulator/FM is given by \({\tau }_{{\rm{AP}}}={\tau }_{{\rm{P}}}(1+{P}_{{\rm{TMC}}}^{2})/(1{P}_{{\rm{TMC}}}^{2})\), where P_{TMC} is the spin polarization inside the FM layer^{43,51}. Using these formula, we can calculate the frequency characteristics of the TMC ratio, defined by \({\rm{TMC}}({f})=({C}_{{\rm{P}}}^{{\rm{DF}}}({f}){C}_{{\rm{AP}}}^{{\rm{DF}}}({f}))/{C}_{{\rm{AP}}}^{{\rm{DF}}}({f})\). From Fig. 1d,e, it is evident that the theoretical results of the TMC and C_{P(AP)} fit the experimental data very well. The calculation was based on the following parameters: C_{∞,P(AP)} = 1.45 (1.5) nF, C_{0,P(AP)} = 630 (590) nF, β_{P(AP)} = 0.998 (0.977), τ_{P} = 0.0071 s and P_{TMC} = 0.46. As for the magnetoresistance, we calculated the TMR ratio using the Julliere formula^{52}, assuming a spin polarization P_{TMR} of 0.59. The difference between P_{TMC} and P_{TMR} is attributed to the different penetration lengths of spindependent carriers (inside Co_{40}Fe_{40}B_{20}) contributing to TMC and TMR. Our previous paper reveals that the penetration length (λ_{TMC}) of TMC is longer than that (λ_{TMR}) of TMR^{43}. Here, as is well known, the spin polarization of surface atoms in the FM layer or the interfacial FM atoms between FM/insulator is higher than that of inner atoms from the surface/interface due to the twodimensional surface/interface effect^{53,54,55}. This picture can be applied to our MTJ system. Namely, the spin polarization (P_{inter}) of the first interfacial atoms in Co_{40}Fe_{40}B_{20} layers is considered to be higher than that (P_{inner}) of inner atoms from the Co_{40}Fe_{40}B_{20}/MgO interface (See Fig. S1). In fact, according to the firstprinciples calculation using the Vienna Ab Initio Simulation Package (VASP), the spin polarization at the Co terminated interface is higher than that of the inner Co and Fe atoms in CoFeB/MgO system^{56}. From these spinpolarization behavior, it is found that P_{TMC} is low for a long λ_{TMC}, and P_{TMR} is high for a short λ_{TMR}. According to the above fitting results, P_{TMC} is 0.46, and P_{TMR} is 0.59. The spin polarization P_{TMR} of 0.59 is in good agreement with the experimental value (0.57–0.65) of CoFeB alloy obtained by pointcontact Andreev reflection^{57}. Also, we herein note that a negative TMC, i.e., C_{P} < C_{AP} , is observed in the highfrequency region. This can be understood from the fitting results, i.e., C_{∞,P(AP)} = 1.45 (1.5) nF, as well as the experimental results shown in Fig. 1d. This is due to the appearance of the spin capacitance originated from the accumulation of spinpolarized carriers at the two FM/insulator interfaces. The detailed results and discussion are described in the Supplementary Information section. The excellent agreement between theory and experiment reveals that the TMC shows a maximum value of 96% at 60 Hz. Hence, we will use operation frequency of 60 Hz to study the bias dependence of TMC effect. Here, it is noted that we reported a high TMC of 155% in our previous paper^{43}. This result was obtained near zero bias voltage. In contrast, it is found that MTJs showing such high TMC tend to bring about the breakdown under the bias voltage. MTJs showing about 100% do not break down under the bias voltage, and the reproducibility is also good. From this reason, we have studied the bias dependence of TMC using MTJs with a TMC of about 100%.
Voltageinduced TMC and TMR
Figure 2 shows the bias dependence of TMC and TMR curves at 60 Hz. The TMC ratio under the applied bias voltage V_{DC} at the frequency f is defined by \({\rm{TMC}}({f},{V}_{{\rm{DC}}})\)\(=({C}_{{\rm{P}}}(f,{V}_{{\rm{DC}}}){C}_{{\rm{AP}}}(f,{V}_{{\rm{DC}}}))/{C}_{{\rm{AP}}}(f,{V}_{{\rm{DC}}})\). Interestingly, the TMC increases from 96% to 102% as the magnitude of bias voltage increases from 0 to 184 mV, whereas TMR rapidly decreases from 105% to 53%. As described earlier, the enhancement of \({V}_{1/2}^{{\rm{TMR}}}\) is an important issue in the development of highperformance TMR devices. The typical value of \({V}_{1/2}^{{\rm{TMR}}}\) is 0.1–1 V in MgObased MTJs^{3,30,58,59}. In our devices, \({V}_{1/2}^{{\rm{TMR}}}\) is 184 mV. In contrast, since the TMC does not decrease with the increasing bias voltage, the value of \({V}_{1/2}^{{\rm{TMC}}}\) cannot be defined. In fact, TMC remain robust against bias voltage which weakens the corresponding TMR quickly. Both the large TMC and its robustness are highly beneficial to spintronics applications associated with the TMC effect.
Modeling of voltageinduced TMC
Figure 3 shows the bias voltage dependence of R_{P(AP)}, C_{P(AP)} and the modeling of the voltageinduced TMC. According to Zhang’s theory^{27}, the tunnel conductance in FM/insulator/FM strongly depends on the bias voltage, especially within the order of a few hundred millivolts, due to hot electrons producing spin excitations. The conductance G_{P(AP)} (V_{DC}) at the bias voltage V_{DC} in the P(AP) configuration can be expressed by \({G}_{{\rm{P}}({\rm{AP}})}({V}_{{\rm{DC}}})={G}_{{\rm{P}}({\rm{AP}})}^{0}({\rm{1}}+{\gamma }_{{\rm{P}}({\rm{AP}})}{V}_{{\rm{DC}}})\), where \({G}_{{\rm{P}}({\rm{AP}})}^{0}\) is the conductance at zero bias in the P(AP) configuration and γ_{P(AP)} is a parameter determined by the Curie temperatures of FM, the density of states (DOS) of itinerant electrons in FM, direct and spindependent transfers and spin quantum number within the framework of the transfer Hamiltonian. The resistance R_{P(AP)}(V_{DC}) can be calculated from \({R}_{{\rm{P}}({\rm{AP}})}({V}_{{\rm{DC}}})=1/{G}_{{\rm{P}}({\rm{AP}})}({V}_{{\rm{DC}}})\). As shown in Fig. 3a, the experimental data of R_{P(AP)}(V_{DC}) are in good agreement with the calculation using Zhang’s model, where γ_{P(AP)} is set to 0.0188 (1.87). This means that Zhang’s model is effective for explaining the bias dependence of TMR under both AC and DC mode.
The calculation of C_{P}(f, V_{DC}) is performed using the DF model combined with QBA. In the QBA, the potential profile of the barrier is assumed to be a quartic function, which is considered to be a good approximation to describe an AC tunneling transport^{51}. The potential function \({\varphi }_{{\rm{P}}}(\eta )\) under the bias voltage V_{DC} in the P configuration can be expressed by \({\varphi }_{{\rm{P}}}(\eta )=16{\varphi }_{0,{\rm{P}}}{\eta }^{4}8{\varphi }_{0,{\rm{P}}}{\eta }^{2}+{\varphi }_{0,{\rm{P}}}+e{V}_{{\rm{D}}{\rm{C}}}(12\eta )/2\), where \({\rm{\eta }}=x/d\) is the reduced spatial variable, x is the distance from the center of the barrier, d and ϕ_{0,P} are the barrier thickness and height in the absence of the bias voltage, respectively, and e is the electron charge. The potential profile is depicted in Fig. 3b. The solution of \({\varphi }_{{\rm{P}}}(\eta )=e{{V}}_{{\rm{DC}}}\) is η_{1} = −1/2, η_{2}, η_{3} and η_{4} (for η_{2} < 0, η_{3} and η_{4} > 0, η_{3} < η_{4}). Since the values of η_{2}, η_{3} and η_{4} can be calculated by the Cardano’s method, the effective barrier thickness d_{eff} can be represented by d_{eff} = (1/2 + η_{3})d, where η_{3} depends on V_{DC} and ϕ_{0,P}. Therefore, C_{P}(f, V_{DC}) at the applied DC voltage in the P configuration can be written by
As shown in Fig. 3c, the measured capacitance C_{P}(f, V_{DC}) exhibiting a cusplike behavior, is very well described by Eq. (3), using parameters of C_{∞,P} = 1.45 nF, C_{0,P} = 630 nF, β_{P} = 0.998, τ_{P} = 0.0075 s, f = 60 Hz and ϕ_{0,P} = 2.15 eV. Hence, the QBA is a good approximation for the expression of the potentialbarrier profile in MTJs based on Co_{40}Fe_{40}B_{20}/MgO/Co_{40}Fe_{40}B_{20} in calculating the bias dependence of C_{P} under AC field.
The calculation of C_{AP}(f, V_{DC}) is performed using the DF model combined with QBA and SDD model. Based on the SDD model^{60}, the accumulation of minority spins and the depletion of majority spins take place at the interface between the Co_{40}Fe_{40}B_{20} and MgO layers in the AP configuration. The spin accumulation causes a difference in the chemical potential between majority and minority spins, resulting in a different diffusion length in each spin. The difference in the diffusion length gives rise to the creation of tiny screening charge dipoles, which act as an additional serial capacitance, i.e., spin capacitance. The screening charge density is given by \(e{n}_{{\rm{AP}}}({x}_{i})=e{n}_{0,\mathrm{AP}}\,\exp (\,\,{x}_{i}/\lambda )\), where x_{i} is the distance from the interface between the Co_{40}Fe_{40}B_{20} and MgO, λ is a characteristic screening length and en_{0,AP} is a screening charge density at the interface in the AP configuration. The spin capacitance can be expressed by \({{C}}_{{\rm{AP}}}^{{\rm{SDD}}}({V}_{{\rm{DC}}})={\rm{\Delta }}{Q}_{{\rm{AP}}}/{\rm{\Delta }}{V}_{{\rm{DC}}}\), where ΔQ_{AP} is the screening charge for the AP configuration and ΔV_{DC} is the electrical potential difference applied in the charging space. Therefore, the spin capacitance can be obtained as \({{C}}_{{\rm{AP}}}^{{\rm{SDD}}}({V}_{{\rm{DC}}})=eS{n}_{{\rm{AP}}}({x}_{i})d{x}_{i}/d{V}_{{\rm{DC}}}({x}_{i})\), where S is a junction area and V_{DC}(x_{i}) is an electrical potential as a function of x_{i} in the charging space, i.e., \({V}_{{\rm{DC}}}({x}_{i})={V}_{{\rm{eff}}}\,\exp (\,\,{x}_{i}/\lambda )\). V_{eff} is an effective applied voltage, which is expressed by κV, where κ is an adjustable positive parameter of much smaller than 1.0. Consequently, the spin capacitance can be represented by
Since this screening charge acts as a serial capacitance, the capacitance C_{AP}(f, V_{DC}) under the applied DC voltage V_{DC} in the AP configuration,
where the capacitance \({C}_{{\rm{AP}}}^{\mathrm{DF}\mathrm{QBA}}\) (f, V_{DC}) based on the DF model combined with QBA in the AP configuration is represented by
The behavior of charge accumulation, contributing to \({{C}}_{{\rm{AP}}}^{\mathrm{DF}\mathrm{QBA}}\) and \({{C}}_{{\rm{AP}}}^{{\rm{SDD}}}\), is illustrated in Fig. 3d. The bias voltage dependence of the capacitance C_{AP} in the AP configuration is shown in Fig. 3e. The capacitance C_{AP} increases at around zero bias and then it decreases at higher voltages. This behavior is in good agreement with the results calculated by Eqs (4)−(6) with parameters of C_{∞,AP} = 1.5 nF, C_{0,AP} = 590 nF, β_{AP} = 0.977, τ_{P} = 0.0075 s, P_{TMC} = 0.46, f = 60 Hz, κ = 0.1, S = 1800 μm^{2} and λ = 0.1 nm. ϕ_{0,AP} is 0.144 (0.153) eV and n_{0,AP} is 0.94 (0.92) × 10^{23} cm^{−3} for the positive (negative) bias region. The increase of C_{AP} near zero bias is attributed to the decrease of the effective barrier thickness d_{eff}. As seen from the potential profile ϕ_{P}(x) of MTJs shown in Fig. 3b, d_{eff} decreases with increasing bias voltage V_{DC}. This corresponds to the decrease of η_{3}. According to our calculation, η_{3} is 0.5, 0.35 and 0.28 for V_{DC} of 0, 50 and 100 mV, respectively. From Eq. (6), it is found that \({{C}}_{{\rm{AP}}}^{{\rm{DF}}{\rm{QBA}}}(f,{V}_{{\rm{DC}}})\) increases with decreasing η_{3}. Therefore, C_{AP}(f, V_{DC}) increases with increasing V_{DC}. The reduction of C_{AP} at higher voltages is attributed to the spin capacitance, described in Eq. (4), i.e., the spin capacitance decreases with increasing V_{DC}.
Figure 4 shows the biasvoltage dependence of the TMC and TMR ratios. The calculation of the TMC is performed using Eqs (3–6) for setting the same parameters used in the calculation of C_{P(AP)} (f, V_{DC}) in Fig. 3c,e. The TMR is obtained from the Zhang formula. The calculation results of the TMC and TMR provide excellent fits to experimental data in bipolar bias regions. The reduction of TMR can be easily understood from the experimental results of both no significant change of R_{P} (V_{DC}) and the reduction of R_{AP} (V_{DC}) in higher voltages, as shown in Fig. 3a. In particular, the reduction of R_{AP} (V_{DC}) is due to the existence of hot electrons tunneling through the barrier, which brings about the decrease of TMR. The enhancement of TMC can also be clearly understood from the results of both the increase of C_{P} (f, V_{DC}) and the reduction of C_{AP} (f, V_{DC}) in higher voltages, as shown in Fig. 3c,e. The reduction of C_{AP} (f, V_{DC}) is attributed to the emergence of the spin capacitance, which promotes the increase of TMC. Thus, the spin capacitance gives a significant influence on TMC, and it will play an important role on future application.
Though the application of the interesting TMC effect is not well understood at this stage, we venture to discuss the application potential of TMC devices. As can be seen from Fig. 4, TMR is larger than TMC in the low bias region. In considering the application, the important factor in sensing is an electric sensitivity. The electric sensitivity is generally expressed by the signaltonoise (S/N) ratio, which is defined as 20logV_{S}/V_{N}. Here V_{S} is a signal voltage and V_{N} is a noise voltage. In the case of TMR sensing under the applied voltage V_{DC}, since V_{S} is given by \(TMR\cdot {V}_{{\rm{DC}}}\) and V_{N} is proportional to \(\sqrt{{V}_{{\rm{DC}}}}\), V_{S}/V_{N} is proportional to \(TMR\cdot \sqrt{{V}_{{\rm{DC}}}}\). Therefore, the increase of V_{DC} is necessary for the improvement of the S/N ratio. In fact, magnetic read heads, sensors, or MRAMs are designed for operation under a bias of a few hundred mV^{61}. Additionally, the enhancement of TMR at a higher voltage is of importance. As shown in Fig. 4, the TMR decreases with increasing V_{DC}, whereas TMC increases at higher voltages. Since V_{S}/V_{N} is proportional to \(TMC\cdot \sqrt{{V}_{{\rm{DC}}}}\) in TMC sensing, the TMC is superior to TMR from the viewpoint of the S/N ratio. Furthermore, the impedance Z of TMC devices can be reduced in the high frequency region since the impedance of the capacitor is expressed as Z = 1/jωC, where ω is the angular frequency. This means that the noise voltage V_{N} could be reduced. The high V_{S} and low V_{N} lead to the enhancement of the S/N ratio. Also, the AC technique allows for the performance of various functions such as modulation/demodulation, filtering, oscillation and resonance. In fact, magnetoimpedance devices using the AC technique have enabled the application to geomagnetic sensors or positioning sensors of GPS^{62,63,64,65}. Also, a capacitive magnetic sensing, in which an RC parallel circuit module consisting semiconductor magnetoresistance device and capacitor was used in the feedback loop of a Hartley or Colpitts oscillator, has been proposed and demonstrated by our group^{66,67,68}. In short, TMC devices may pave a new way for various applications to the next generation of magnetic read heads, MRAMs, logic circuits, and highly sensitive magnetic sensors (including geomagnetic sensors, positioning sensors, etc.).
Prediction of an extremely large voltageinduced TMC
Finally, the spin polarization dependence of the voltageinduced TMC is calculated for the prediction of an extremely large TMC. Figure 5a,b shows the calculated frequency dependence of the TMC under no bias voltage and the bias dependence of the TMC with varying P, respectively. Here, P_{TMC} is assumed to be equal to P_{TMR}, which is denoted by P. The assumed maximum value of P is 0.87, which is estimated experimentally for highperformance MgObased MTJs at room temperature^{12}. The parameters used in the calculation of the TMC are C_{∞,P(AP)} = 1.45 (1.5) nF, C_{0, P(AP)} = 184 (172) nF, β _{P(AP)} = 0.998 (0.932) and τ _{P} = 0.0033 s. As can be seen from Fig. 5a, the TMC ratio at V_{DC} = 0 V shows the maximum value at f = 60 Hz for the P of 0.70, 0.79 and 0.87, respectively, and it increases from 273% to 995% with increasing P from 0.70 to 0.87. The maximum TMC of 273%, 505% and 995% are larger than TMR ratios of 192%, 325% and 604%, which are calculated from Julliere formula using P = 0.70, 0.79 and 0.87, respectively. The parameters used in the calculation of the bias dependence of TMC are f = 60 Hz, κ = 0.1, S = 1800 μm^{2} and λ = 0.1 nm. The barrier height ϕ_{0,P} is 2.15 eV, and ϕ_{0,AP} is 0.144 (0.153) eV in the positive (negative) bias voltage. The screening charge densities n_{0,AP} are 0.312 (0.306), 0.208 (0.204) and 0.125 (0.123) × 10^{23} cm^{−3} in the positive (negative) bias voltage for the spin polarizations P of 0.70, 0.79 and 0.87, respectively. The TMC increases from 995% to 1119% with increasing V_{DC} from 0 to 200 mV. Figure 5c shows the calculated frequency dependence of the TMC at V_{DC} = 200 mV with varying τ_{P}. The maximum peak of the TMC is shifted to a high frequency region on the order of MHz for a short τ_{P} in the subμs scale. According to the DF model, τ_{p} can be tuned by changing the oscillation speed of electric dipoles formed near the FM/insulator interfaces; τ_{p} is short for a high oscillation speed. From the viewpoint of MTJ device structure, τ_{p} can be tuned by changing the thickness of the insulating barrier; τ_{p} could shorten in MTJs with a thinner barrier. Although this realization in MTJs is future work, the recent study using FeCoMgF nanogranular films has demonstrated that the TMC peak is observed at a high frequency of 220 MHz, which corresponds to τ_{p} of 0.72 ns^{69}. Our calculations predict that a large TMC of over 1100% could be possibly observed using MTJs with a realistic P of 0.87. Furthermore, this large TMC can be tuned from low to high frequencies by shortening τ_{P}.
In summary, we observed a new phenomenon in which the TMC increases with the bias voltage in MgObased MTJs at room temperature. The enhancement of TMC is attributed to the emergence of the spin capacitance in the AP configuration of MTJs. The voltageinduced TMC increases to 102%, which is the largest value ever reported for MTJs. We also found the voltageinduced TMC can be well explained by a newly proposed theoretical calculation using DF model combined with QBA and SDD model. This calculation predicts that the voltageinduced TMC could potentially reach 1100% in MTJs with a corresponding TMR value of 604%. These theoretical and experimental findings provide a deep insight into the voltageinduced AC spin transport in MTJs. We demonstrated that the complementary TMR and TMC effects must be treated on equal footing. The large TMC effect and the associated robustness against biasing may open up new avenues for spintronics applications and electrical modeling.
Methods
Preparation of the samples
The MTJs were prepared by using a magnetron sputtering system with a base pressure of 2 × 10^{−8} Torr. The MTJs have the following layer sequence: SiO_{2}/Ta(5)/Co_{50}Fe_{50}(2)/IrMn(15)/Co_{50}Fe_{50}(2)/Ru(0.8)/Co_{40}Fe_{40}B_{20}(3)/MgO(2)/Co_{40}Fe_{40}B_{20}(3)/contact layer (thickness in nm). We deposited all the metallic layers in DC mode under a sputtering Ar gas pressure of 1.5 mTorr. The MgO layer was deposited with radio frequency (RF) magnetron sputtering at an Ar gas pressure of 1.1 mTorr. Using standard photolithography, we have patterned the multilayer MTJ stacks into a junction area of 1800 μm^{2} with an elliptical shape with Ar ionmilling and SiO_{2} insulation overlayer. Finally, we annealed the MTJs at 310 °C for 4 h in vacuum of 10^{−6} Torr under a uniform magnetic field of 4.5 kOe to define the pinning axis for the Co_{40}Fe_{40}B_{20} bottom electrode.
Measurements of the voltageinduced TMC
The frequency characteristics and the bias voltage dependence of the TMR and TMC for MTJs were measured by an AC fourprobe method using an Agilent Technologies 4284 A LCR meter at room temperature. The frequency ranged from 50 Hz to 1 MHz and the bipolar bias voltage was applied up to 200 mV. The AC voltage was set to 2.6 mV_{rms}. The magnetic field was applied along the magnetic easyaxis direction to 1.4 kOe.
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Acknowledgements
This research was supported by the GrantinAid for Scientific Research (B) (under Grant No. 15H03981) and Challenging Exploratory Research Program (No. 17K19019) funded by the Japan Society for the Promotion of Science (JSPS), the Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials and the Cooperative Research Program of “Network Joint Research Center for Materials and Devices” funded by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Research Project funded by the Center for Spintronics Research Network (CSRN) at Tohoku University, and the US National Science Foundation at Brown University (under Grant No. DMR1307056).
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H.K. and G.X. conceived and designed the experiments using the help of other authors. G.X. performed the sample preparation and structural analysis. H.K., T.M. and T.N. performed dc measurements. H.K., M.F. and J.N. performed ac impedance measurements. H.K., G.X., T.K. and O.K. performed the theoretical analysis and calculation. All the authors contributed to analyzing and interpreting the data, and to writing the manuscript.
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Kaiju, H., Misawa, T., Nagahama, T. et al. Robustness of Voltageinduced Magnetocapacitance. Sci Rep 8, 14709 (2018). https://doi.org/10.1038/s4159801833065y
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Keywords
 Magnetic Tunnel Junctions (MTJs)
 Large TMR
 Biased Regions
 Bias Voltage Dependence
 Spinning Capacity
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