University of Groningen
Controlling spins in nanodevices via spin-orbit interaction, magnons and heat
Das, Kumar Sourav
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Das, K. S. (2019). Controlling spins in nanodevices via spin-orbit interaction, magnons and heat. University of Groningen.
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1
Introduction
1.1
Spintronics
The electron, discovered in 1897 by J. J. Thomson [1], plays a central role in solid state physics. Electrons possess a finite charge and are responsible for carrying elec-tricity in metals and semiconductors. Utilizing and manipulating electrons through its intrinsic charge via the Coulomb and Lorentz forces form the basis of almost all electronic devices. Since the beginning of the Digital Revolution in the latter part of the 20th century, the roadmap of the semiconductor industry, in terms of the pro-cessing speed and transistor size, has been laid out by Moore’s Law [2]. According to this law, the number of transistors in an integrated circuit is expected to be dou-bled approximately every two years. Although this prediction has been remarkably accurate over the past decades, it is soon going to converge to a fundamental limit as the size of a transistor approaches the dimensions of atoms. Even before hitting this fundamental limit, the reduction of the channel length in the transistor to a few nanometres is expected to substantially affect its performance owing to undesirable quantum tunnelling effects, leakage currents and heat dissipation.
Arguably, the most promising alternative to the conventional charge-based elec-tronics is presented by the field of spin-based elecelec-tronics or spinelec-tronics [3]. In addition to the charge degree of freedom, the electron also possesses an intrinsic spin angular momentum, which is quantized [4–7]. Spintronics utilizes this spin degree of free-dom of the electron for realizing a new generation of non-volatile devices with faster data processing speeds, lower electrical power consumption and increased integra-tion densities [8].
The discovery of the giant magnetoresistance (GMR) effect in 1988 [9, 10] marks the beginning of intensive research and development activities in the field of spin-tronics. The GMR effect was demonstrated in artificial multilayers of alternating iron (Fe) and chromium (Cr) thin films. Depending on the relative magnetization orientation of the alternating Fe layers (aligned parallel or anti-parallel), the electri-cal resistance of the multilayer stack could be altered by several tens of percent. This
1
2 1. Introduction
effect could be explained with the two-spin channel conduction model, originally formulated by N. F. Mott [11, 12] and later verified experimentally in Ni alloys [13]. Since a larger resistance modulation could be achieved by utilizing the GMR effect as compared to the anisotropic magnetoresistance (AMR) in ferromagnets [14, 15], GMR read heads were implemented in magnetic hard disks within just a few years from its discovery. This revolutionized the information storage technology, resulting in increased storage capacities and faster operation times. Another magnetoresis-tance effect, similar to the GMR effect, is called the tunnelling magnetoresismagnetoresis-tance (TMR) [16], which utilizes a tunnel barrier instead of a non-magnetic metal as the spacer layer between the two ferromagnets. Although the TMR effect was discov-ered earlier than the GMR, poor quality of the tunnel barriers limited the resistance modulation in magnetic tunnel junctions (MTJs) to only a few percent. With the ad-vent of crystalline magnesium oxide (MgO) tunnel barriers, TMR ratios of greater than 200% [17, 18] were achieved and MTJs replaced GMR read heads in magnetic hard disks. The TMR effect has also been utilized in realizing non-volatile mag-netic random access memories (MRAMs), which are envisioned as the successor to the dynamic random access memories [19, 20]. MRAMs, based on the spin transfer torque (STT), can be read and written electrically [21] and are already available com-mercially. A new generation of MRAMs utilizing spin orbit torque (SOT) [22, 23] is currently under development [24] and at the forefront of spintronics research.
Apart from the memory applications, extensive research has also been conducted in the realization of a spin-transistor [25], the analogue of a field effect transistor (FET) in electronic circuits. The spin-FET would serve as the basic logic component of the spintronic circuit and together with spintronic memory elements and intercon-nects, a fully spin-based microprocessor can be realized. This is often considered as the holy grail of spintronics research [26, 27]. However, there are several challenges to be overcome before spintronics can fully replace charge-based electronics in the future. A major hurdle in the way is finding a material with a long spin relaxation time as well as a significant spin-orbit coupling which allows the manipulation of the spin current in the channel. However, these are contradictory requirements since a large spin-orbit coupling would inherently lead to a smaller spin relaxation time and cause the loss of spin information even before the manipulation can be performed [3]. Moreover, efficient ways of electrically generating and detecting spin currents are desirable for various spintronic applications. Therefore, the three main pillars of spintronics research are the generation, manipulation and detection of spin cur-rents in an efficient manner which are preferably compatible with the conventional semiconductor processing technologies. The research presented in this thesis aims at addressing these three important issues.
1
1.2
Motivation and outline
This thesis consists of research which is important not only for the fundamental un-derstanding of various spin-dependent phenomena in nanodevices but also highly relevant for realizing efficient spintronic circuitry for future applications. This PhD project started off with the goal of studying spin transport in curved nanoarchitec-tures. The role of the channel geometry was investigated as a way to control the spin and charge transport properties in the channel. While the research on the curved nanoarchitectures was in progress, an unprecedented feature was observed in the Hanle spin precession measurements in non-local spin valves with a flat metallic channel. A curiosity-driven approach to this observation led to the discovery and demonstration of the anisotropy in the thermoelectric coefficients of a ferromagnet. This effect, originating from the spin-orbit coupling in the ferromagnet, combines the charge, spin and heat transport, which can be highly relevant for future spin caloritronic applications [28]. The non-local spin valve measurement technique was then utilized to explore the temperature dependence of the spin-mixing conductance [29–32], a fundamentally important physical quantity governing the transfer of spin angular momentum across the interface of a normal metal and a magnetic insulator. The later part of the PhD project focussed on the spin-charge conversion utilizing the spin-orbit effects in a ferromagnet [33, 34]. With the rapidly developing interest in magnetic insulators for spintronic applications [35–37], electrical spin injection and detection techniques in such systems are highly desirable for their integration in solid state devices. An efficient and controllable way of electrical spin injection and detection in a magnetic insulator, using a common ferromagnetic metal, was demonstrated. Thereafter, the focus shifted on the efficient control of the magnon spins in a magnetic insulator via magnetic gating, which can, in principle, lead to a magnon transistor operation [38–41].
The research presented in this thesis has therefore explored new directions in spintronics utilizing different spin-orbit effects, the curved geometry of nanoarchi-tectures, magnon spin transport and magnetothermoelectrics for new spintronic func-tionalities encompassing the injection, detection and manipulation of spin informa-tion.
A brief overview of the chapters in this thesis is given below:
• Chapter 2 introduces the basic physical concepts behind spin injection and de-tection in non-magnetic metals using ferromagnets, one-dimensional model of diffusive spin transport in a homogeneous channel and the Hanle effect in the context of non-local spin valves.
1
4 1. Introduction
This chapter also describes briefly the different spin-orbit effects in non-magnetic heavy metals and in ferromagnetic metals, which are relevant for the research presented in this thesis.
This is followed by a description of the thermoelectric effects such as the Peltier effect, the Seebeck effect and the anomalous Nernst effect.
Finally, a brief introduction to magnon spintronics and non-local magnon trans-port in a magnetic insulator is presented.
• Chapter 3 describes the experimental techniques employed for the research presented in this thesis. It includes the description of the device fabrication methods, the experimental setup and the electrical measurements.
• Chapter 4 elucidates the role of magnetothermoelectric effects in lateral non-local spin valves leading to anisotropic line shapes in Hanle spin precession experiments. Such anisotropic line shapes typically correspond to anisotropic spin relaxation times in the spin transport channel. However, it is shown in this chapter that the anisotropic thermoelectric coefficients of the ferromagnetic electrodes can also lead to such anisotropic Hanle line shapes.
• Chapter 5 demonstrates the first non-local spin transport measurements in curved metallic nanochannels. It is shown that by controlling the channel ge-ometry, one can independently tune the charge and the spin transport prop-erties in the nanochannel. A theoretical model is developed for diffusive spin transport in channels with inhomogeneous charge and spin transport proper-ties and this model is validated by the experimental results.
• Chapter 6 investigates the temperature dependence of the effective spin-mixing conductance and the real part of the spin-mixing conductance for an interface between a normal metal and a magnetic insulator. This is done through non-local spin valve measurements using aluminium as the normal metal, which eliminates the spurious effects that might be present while extracting the spin-mixing conductance using normal metals with high spin-orbit coupling or close to the Stoner criterion (eg. platinum).
• Chapter 7 presents a novel mechanism of electrical spin injection and detec-tion via the anomalous Hall effect in a ferromagnetic metal (permalloy). This new mechanism, the anomalous spin Hall effect, is utilized to inject and detect magnon spin accumulation in a magnetic insulator (yttrium iron garnet). It is shown that the spin injection and detection efficiency of permalloy is compa-rable to that of platinum.
1
• Chapter 8 follows up on the work presented in chapter 7 by demonstrating effi-cient injection and detection of out-of-plane spins utilizing the anomalous spin Hall effect. Unlike the spin Hall effect, the anomalous spin Hall effect presents the advantage of controlling the spin direction by manipulating the magnetiza-tion orientamagnetiza-tion of the ferromagnetic metal. A second mechanism of detecting the out-of-plane spins is also discussed, which leads to an unexpected sign re-versal of the non-local signal in the first harmonic response.
• Chapter 9 presents an overview of the ongoing research activities and the initial results related to using a permalloy strip for the modulation of magnon spin transport in yttrium iron garnet (YIG). This magnetic gating effect essentially arises due to the transmission of magnons from YIG into Py depending on their relative magnetization orientations. A modulation in the magnon spin signal of up to 18% is achieved, opening up the possibility of using this prototype device for magnon transistor applications.
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