Chapter 6
Dynamical spin injection based on FMR heating
6.1 Introduction
There has been a great deal of interest in spintronics devices consisting of ferromag-net(FM) /non-magnet(NM) hybrid structures since the discovery of the spin-dependent transports[72, 73]. In operations of the spintronic devices, a spin current is a key ingredi-ent instead of electric curringredi-ent in charge-based electric devices because the magnetization can be controlled and detected by the spin current. Spin current is, in general, created by applying the electric field across the FM/NM interface, resulting in injecting the spin-polarized electrons from the FM into the NM[66–73]. This is known as electrical spin injection, which is a powerful method for generating the spin current because of its high selectivity and flexibility[66, 67, 145, 147]. Moreover, by extending the electrical spin injection to a multi-terminal FM/NM hybrid nano-structure, we are now able to
55
create pure spin current, a flow of spin angular momentum without accompanying the charge current. This pure spin current enables us to observe various intriguing spin-related phenomena sensitively because of the absence of the charge current and could be employed to transport the spin information with low power consumption[62, 65–71].
Apart from the electrical spin injection, the dynamical spin injection induced by the microwave irradiation is also an attractive means for generating and manipulating the spin current due to its simple experimental scheme. So far, the mechanism of the dynamical spin injection is attributed to the spin pumping induced by the magnetization precession of ferromagnetic resonance (FMR) in the FM/NM bilayer structure[23, 24, 145–147]. The magnitude of the spin current induced by the spin pumping is evaluated from the change of the line width of the FMR spectra[21, 23, 24, 56, 60, 120, 146, 148].
However, the line width of the FMR spectra is strongly affected by the crystal and geometrical inhomogeneity, namely inhomogeneous broadening[142–144], which makes it difficult to estimate the intrinsic and extrinsic contributions of the damping constant.
In addition, recent studies have pointed out the importance of the influence of the magnetic proximity effect[149–153] and the Dzyaloshinskii-Moriya interaction[154–158]
on the magnetization dynamics in the bilayer system. Therefore, the line width of the FMR is modified not only by the spin pumping but also by the other additional effects.
This complicates the quantitative estimation of the injected spin current due to the spin pumping. Moreover, the magnitude of the ac spin current with the precession frequency is much larger than the dc spin current in the spin pumping mechanism[159]. However, the contribution of the ac spin current has not discussed intensively because of the difficulty of the systematic analysis. Thus, the dynamical spin injection based on the
6.2. SAMPLE STRUCTURE AND EVALUATION METHOD 57 spin pumping effect still has various problematic issues.
Here, we propose another mechanism for the dynamical spin injection based on the FMR heating effect. As described in the previous chapter, the FMR heating effect in the FM/NM bilayer structure produces the temperature gradient across the FM/NM interface, which could be a driving force of the spin current, namely thermal spin injection[28, 74]. Since the CoFe-based ferromagnetic alloys showed the large thermal spin injection efficiency originating from its favorable band structure[30], we may develop a highly efficient dynamical spin injection technique by combining the FMR heating with the appropriate material. In the present chapter, we evaluate the performance of the dynamical thermal spin injection based on the ferromagnetic resonance in a FM/NM bilayer structure.
6.2 Sample structure and evaluation method
We experimentally investigate the dynamical thermal spin injection due to the FMR heating effect in a CoFeB/Pt bilayer system. In this present study, we adopt a CoFeB film as the FMR heating layer because the large electrical resistivity of the CoFeB film with a relatively small anisotropic magnetoresistance (AMR) effect suppresses the spurious signals for the spin current transport[140]. The electrical resistivity for CoFeB is about 100 µΩcm, indicating the amorphous structure. As schematically shown in Fig. 6.1, the Cu waveguide, which is a microwave generator (N5183B) for the FMR excitation, is prepared on the CoFeB/Pt bilayer film. Since the temperature of the amorphous ferromagnetic CoFeB increases during the FMR, the heat flow perpendicular to the interface between the CoFeB and the Pt is generated. This results in the thermal
spin injection from the CoFeB into the Pt film. A spin current flowing along the z-axis injected from the CoFeB into the Pt is converted to the electrical charge current along the Pt wire (x-axis) via the inverse spin Hall effect (ISHE) of the Pt[127–129] as schematically shown in inset of Fig. 6.1. Therefore, the relationship betweenJC andJS is given by
JC=DISHEJS×σ. (6.1)
Here, σ is the spin polarization vector, and DISHE is a coefficient representing the ISHE efficiency in a material, which is related to the spin Hall angle θSH. A CoFeB/Pt bilayer film has been prepared on a FZ-Si sub. by using an ultra-high-vacuum magnetron sputtering system. The lateral dimension of the bilayer film patterned by the stencil mask is 200µm in width and 1000µm in length. The thicknesses for CoFeB and Pt are, 40 nm and 10 nm, respectively. The waveguide and the bilayer are electrically separated by a 100-nm-thick SiO2 film. Here, the resistance of the bilayer film is as low as 33 Ω.
6.2. SAMPLE STRUCTURE AND EVALUATION METHOD 59
Figure 6.1: Circuit diagram for the resistance measurement under RF current injection together with schematic illustration of FMR heating effect.
Figure 6.2: Representative result of the field dependence of the electrical voltage induced by the inverse spin Hall voltage under microwave irradiation. The inset shows the angular dependence of the overall signal change due to the inverse spin Hall signal.
6.3 Demonstration of dynamical thermal spin injection
Figure 6.2 shows the field dependence of the electrical voltage along the Pt wire under the microwave irradiation of the 6 GHz. A clear voltage peak was observed around the µ0H =20 mT, which is exactly the same as the resonance field under the microwave irradiation of the 6 GHz. Here, we define the ∆V as the maximum value of the electrical voltage change due to the FMR. The inset of Fig. 6.2 shows the angular dependence of the spin Hall voltages. We can clearly confirm the sinusoidal dependence, strongly supporting the inverse spin Hall effect nature of the voltage.
6.4. STRUCTURAL DEPENDENCE OF DYNAMICAL THERMAL SPIN ... 61
6.4 Structural dependence of dynamical thermal spin in-jection in the CoFeB/Pt bilayer structure
As mentioned before, our bilayer sample structure used in the present study is the same as that for the spin pumping experiments. Therefore, both the thermal spin injection and the spin pumping are the possible mechanism for the generation of the spin current in this structure. To distinguish the dominant mechanism of the spin current generation in the present experiment, we analyzed the experimental data more quantitatively and performed the additional experiments. First, to properly evaluate the performance of the present spin Hall device, we introduce the normalized spin Hall voltage, which is given by the overall change of the spin Hall voltage under the open circuit condition divided by the series resistance of the bilayer system for a unit widthRw, namelyV /(Rw). Here,R and ware the resistance and the width in bilayer film, respectively. Figure 6.3(a) shows the representative curve of the magnetic field dependence of the normalized spin Hall voltage under the microwave irradiation of the 6 GHz with the input power of about 280 mW. We note that the input power is 100 times larger than the absorption power, which is effective excitation power for the FMR, as mentioned later. The amplitude of the signal is as high as 20 mVΩ−1m−1, which is much larger than the other bilayer systems consisting of the Py/Pt[127, 128] or YIG/Pt[161, 162], where the spin pumping can be considered as the main mechanism. It also should be noted that the metallic ferromagnet produces an additional voltage with the resonant precessional motion due to the galvanomagnetic effects such as the anistropic magnetoresistance (AMR) and/ or the planar Hall effect (PHE) induced by the Radio Frequency (RF) current flowing in the
ferromagnet[163]. The voltage change due to the galvanomagnetic effects produces the dc component because to the homodyne detection principle and is known to show aniti-symmetric Lorentzian field dependence. Since the voltage change due to the inverse spin Hall effect shows the symmetric Lorentzian field dependence, we can separate the output voltage into the two components. Figure 6.3(b) is the symmetric and the anti-symmetric contribution of the signal separated from the original curve shown in Fig. 6.3(a). We find that the galvanomagnetic contribution is much smaller than the inverse spin Hall effect contribution in our sample. This is consistent with the fact that the AMR and PHE for the CoFeB film are relatively small compared to the conventional ferromagnetic metals such as the Py and Co. We have also investigated the inverse spin Hall effect under the dynamical spin injection in a Pt/CoFeB bilayer system, where the CoFeB and Pt film were deposited on the FZ-Si substrate with the configuration opposite to the previous experiment[164]. Since a partial heat from the CoFeB flows into the FZ-Si substrate directly without passing through the Pt film in this configuration, the magnitude of the thermally excited spin current injected into the Pt should decrease, resulting in the reduction of the inverse spin Hall voltage from the previous experiment using a CoFeB/Pt/FZ-Si sub. system. Moreover, we can clearly confirm that the significant reduction of the induced voltage. By separating the symmetric and anti-symmetric contributions, we can obtain more clear signature of the thermal spin injection. As can be seen in Fig. 6.3(d), the symmetric signal shows the significant reduction although the anti-symmetric signal does not change so much. This means the heat flow the CoFeB into the Pt is reduced, indicating that the origin of the dynamical spin injection in our CoFeB/Pt system is the thermally excited spin current associated with the large
6.5. FREQUENCY DEPENDENCE OF DYNAMICAL THERMAL SPIN ... 63 spin-dependent Seebeck effect.
6.5 Frequency dependence of dynamical thermal spin in-jection due to FMR heating effect
To obtain more define evidence of the thermal spin injection, we investigate the frequency dependence of the spin Hall voltage. According to [128], the relationship between the spin Hall voltage ∆VISHE induced by the spin pumping and the microwave frequency is given by the following equation.
VISHE∝ PABS r
1 +
2ω ωM
2. (6.2)
On the other hand, as mentioned before, in the case of the dynamical thermal spin injection, when the resonant frequencyω0 is sufficiently lower thanωM, the spin current excited by the FMR heating should increase with the microwave frequency because of the suppression of the inhomogeneous broadening [26, 142–144]. Indeed, in the VNA-FMR measurement for the present spin Hall devices, we can clearly see the frequency dependence of the absorption powerPABSand that of the linewidth ∆f, similarly to the experimental results of the FMR heating effect in the Chapter 5, as shown in Figs. 6.4(a) and (b). Moreover, in the case for the thermal spin injection, the injection efficiency is proportional to the longitudinal component of the magnetization at the resonant state.
Since the longitudinal component increases with increasing the microwave frequency because of the reduction of the precession angle, the thermal spin injection efficiency should increase with increasing the microwave frequency. Therefore, by measuring the frequency dependence of the output spin Hall voltage, we can extract the signature of the
Figure 6.3: (a) Field dependence of electric voltage induced in the CoFeB/Pt/FZ-Si sub. sample under the microwave irradiation. (b) Separation of symmetric and anti-symmetric contributions to the induced electric voltage in the CoFeB/Pt/FZ-Si sub.
sample. (c) Field dependence of electric voltage induced in Pt/CoFeB/FZ-Si sub. sam-ple under the microwave irradiation, (d) Separation of symmetric and anti-symmetric contributions to the induced electric voltage in the Pt/CoFeB/FZ-Si sub. sample.
6.5. FREQUENCY DEPENDENCE OF DYNAMICAL THERMAL SPIN ... 65 dominant mechanism for the thermal spin injection. Here we note that the absorption rate for the spin Hall device is relatively small compared with the conventional FMR measurement using the VNA. This is because the overlap area between the wave guide and the CoFeB is much smaller than that in the conventional FMR measurement. The frequency range in the measurement for the spin Hall device is relatively small compared with that for the FMR heating experiments. The difference is mainly caused by the dif-ference of the demagnetizing field between two ferromagnetic metals. Another reason is due to the experimental limitation for the maximum magnetic field in the setup for the spin Hall measurement. Figure 6.4(c) shows the spin Hall voltage for the various mi-crowave frequency. The maximum value of the spin Hall signal appears at the FMR field and its peak position changes systematically with increasing the microwave frequency.
We can clearly confirm that the output voltage which increases with increasing the mi-crowave frequency. Figure 6.4(d) is the summary of the output voltage increases with increasing the microwave frequency. The output voltage shows the significant increase with increasing the microwave frequency. These tendencies are quite different from the normal metal/YIG bilayer system[162], where the spin pumping is considered as the main mechanism for the spin current injection. Thus, these unique characteristics are the clear evidence that the thermal spin injection is the dominant mechanism for the spin current generation in our CoFeB/Pt bilayer structure.
Figure 6.4: (a) Frequency dependence of the absorption power ratio defined by the PABS/PIN due to the FMR excitation estimated from the VNA measurement for the spin Hall device. (b) FMR line width ∆f in the spectra of the PABS/ω as a function of the resonant frequency. (c) Field dependence of the inverse spin Hall spectra under the various microwave frequency. (d) Overall change due to the inverse spin Hall effect as a function of the resonance frequency.
6.6. SUMMARY 67
6.6 Summary
In summary, we demonstrated that the inverse spin Hall effect is induced by the thermal spin injection driven by the FMR heating effect in a CoFeB/Pt bilayer system. The obtained voltage due to the inverse spin Hall effect was relatively large compared with the similar bilayer system based on the spin pumping, indicating the efficient thermal spin injection from amorphous CoFeB. The structure and microwave-frequency depen-dences strongly support the fact that the spin current was generated by the thermal spin injection due to the FMR heating.
Chapter 7
Conclusion
In this thesis, I intensively focus on the heating effect due to the magnetization dynam-ics. Since the heat dissipation due to the magnetization dynamics should increase with the motion of the magnetization, namely precession angle, I firstly developed the mi-crowave spin device that can excite a strong ac magnetic field. In this first experiment, it found the signature of the heating due to the magnetization dynamics. Therefore, I decided to develop the heating effect more quantitatively. So, secondly, I developed a reliable method for evaluating the FMR heating effect based on a simple resistance measurement. Surprisingly, the temperature change during the FMR is as high as 15 K although the system is in contact with a metallic Copper electrode, substrate and other heat conductors. This experimental fact clearly indicates the importance of the heat due to the magnetization dynamics. To clarify the influence of the heat in the dynamical spin injection experiments, I have systematically investigated the efficiency of the dynamical spin injection with changing the geometry and microwave frequency.
The obtained results can not be explained by only considering the spin pumping effect, 69
but can be understood only by the dynamical thermal spin injection. This indicates that the dominant mechanism for the dynamical spin injection in our device is the thermal spin injection.
The detailed conclusions for each experiment are as follows :
Magnetization dynamics under strong RF magnetic field.
I have developed a novel method, which can monitor the magnetization dynamics under the strong ac magnetic field irradiation. In this experiment, the signature of the heating due to the magnetization dynamics with large precession angle was found. (chapter 4)
Heating effect due to resonant magnetization motion
To evaluate the temperature increase due to the ferromagnetic resonance, I have developed a relatively simple way based on the resistance measurement. The tem-perature change during FMR is as high as 15K even though the ferromagnetic metal is in contact with the Copper electrode and the substrate. (Chapter 5) Dynamical spin injection based on FMR heating effect
In order to investigate the influence of the heating effect in the dynamical spin injection, I have systematically investigated the efficiency of the dynamical spin injection with changing the microwave frequency and the geometry. The results strongly suggested that the mechanism of the dynamical spin injection in our system is based on the thermal spin injection due to the FMR heating effect.
(Chapter 6)
Thus, the finding in this thesis provides the important message in this research field.
71 Especially, the explanation based on the spin pumping should be improved by tacking account of this heating effect. Based on the obtained knowledge, we expect to develop the wireless spintronic devices with higher performance and more flexible functions. I believe that these findings open a new avenue for future spintronics.
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Acknowledgement
This thesis could not be completed without the help of many people.
First of all, I would like to address my sincere gratitude to all the members in the defense committee, Prof. Takashi Kimura, Prof. Hirofumi Wada, Assoc. Prof. Akihiro Mitsuda and Assoc. Prof. Takuya Satoh for all their contributions, valuable suggestions and comments.
Particularly, I want to express to Prof. Takashi Kimura, who is my supervisor.
I would like to thank him for giving him a friendly research environment, advanced scientific instruments, a lot of discussion times, interesting ideas and financial support.
Moreover, I would like to thank you for supporting me with a lot of time to make presentations at domestic and international conferences. He also invites many excellent researchers to give us lectures or presentations for enriching our knowledge magnificently.
I would like to express my gratitude for teaching me so many things.
I would also like to thank Assistant Prof. Kazumasa Yamada and Kohei Ohnishi for their continuous support both on the experimental research and daily life for creating a qualify work environment for me. Especially, I appreciate Assistant Prof. Ohnishi for his supports about experiments and daily life.
I am also grateful to Prof. Satoshi Yakata for initiating me in high frequency mea-surement techniques. His expertise was of great help for the success of my project. I have learned a lot from him.
I want to thank Prof. Hiroyuki Awano for valuable advice on the FMR heating effect experiments. Thanks to his advice, research has made great progressed.
I would like to express my gratitude to Prof. Takashi Manago, a supervisor at the Master’s Course who always showed interest in the work I did. I am also thankful to him for not only teaching the fundamental knowledge about spin dynamics but also offering amazing suggestions and fruitful discussions. Furthermore, I always appreciate you for being concerned about me.
I am sincerely thankful to Atsushi Kenjo who is an excellent technician and helps to improve and maintain the measurement stage for all the equipment in the clean room and the measurement setup. I also thank Mika Ishima, who is secretary in our laboratory, a
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