Spin Pumping and Spin Transfer

Spin pumping is the emission of a spin current by a magnetization dynamics while spin transfer stands for the excitation of magnetization by spin currents. Using Onsager's reciprocity relations we prove that spin pumping and spin-transfer torques are two fundamentally equivalent dynamic processes in magnetic structures with itinerant electrons. We review the theory of the coupled motion of the magnetization order parameter and electron for textured bulk ferromagnets (e.g. containing domain walls) and heterostructures (such as spin valves). We present first-principles calculations for the material-dependent damping parameters of magnetic alloys. Theoretical and experimental results agree in general well.Comment: To be published in "Spin Current", edited by S. Maekawa, E. Saitoh, S. Valenzuela and Y. Kimura, Oxford University Pres

DYNAMICS OF CURRENT-INDUCED SWITCHING IN SPIN-ORBIT TORQUE MAGNETIC DEVICES

Master'sMASTER OF ENGINEERIN

Shape-engineered ferromagnets and micromagnetic simulation techniques for spin-transfer-torque random access memory

Spin-transfer-torque random access memory (STTRAM) has received great attention as a prospective universal memory due to high speed read and write capabilities, scalability to smaller technology nodes and non-volatile data retention. Two major factors that could limit the performance of large scale STTRAM arrays are the high switching current and the stochastic switching behavior. In this work, possible routes to mitigate these issues have been explored and new techniques have been proposed to estimate the reliability of the write process. Large area of the selection transistor required to support high switching current impacts the bit storage density of an STTRAM memory array. To increase the bit storage density, a multi-state STTRAM cell employing a cross-shaped ferromagnet was proposed previously. Here, the spin-transfer-torque (STT) driven mag-netization dynamics of the cross-shaped ferromagnet is revisited. As a low power alternative, voltage controlled magnetic anisotropy (VCMA) based writing scheme is studied. Trade-offs and limitations of the VCMA-induced switching over STT are also discussed. In the next part of this dissertation, magnetic properties and magnetization process of epitaxial chromium telluride thin films have been studied. Presence of strong perpendicular magnetic anisotropy in this material makes it an attractive choice for device applications. In this work, anisotropy energies of chromium telluride thin films have been estimated from magnetization measurements. The magnetization reversal process is then studied using analytical models as well as micromagnetic simulations. The last part of this work focuses on the write error rates (WER) of STTRAM. The stochastic write process of STTRAM at finite temperatures gives rise to write errors when a bit fails to switch within the duration of the write pulse. Ultra-low WER on the scale of 10⁻⁹ or less are desired for practical applications. Micromagnetic simulations are required to capture spatially-incoherent magnetization dynamics inside a ferromagnet, which may effect the WER. In this work, using the techniques of rare event enhancement, reliable calculation of WERs to 10⁻⁹ is demonstrated while keeping the computational effort to a minimum. Employing rare-event-enhanced micromagnetic simulations, WERs of both perpendicular and in-plane STTRAM bits are calculated and effects of spatially-incoherent excitations on the WER slopes are discussed.Electrical and Computer Engineerin

Stochastic theory of spin-transfer oscillator linewidths

We present a stochastic theory of linewidths for magnetization oscillations in spin-valve structures driven by spin-polarized currents. Starting from a nonlinear oscillator model derived from spin-wave theory, we derive Langevin equations for amplitude and phase fluctuations due to the presence of thermal noise. We find that the spectral linewidths are inversely proportional to the spin-wave intensities with a lower bound that is determined purely by modulations in the oscillation frequencies. Reasonable quantitative agreement with recent experimental results from spin-valve nanopillars is demonstrated.Comment: Submitted to Physical Review

Spin transfer nano-oscillators

The use of spin transfer nano-oscillators (STNOs) to generate microwave signal in nanoscale devices have aroused tremendous and continuous research interest in recent years. Their key features are frequency tunability, nanoscale size, broad working temperature, and easy integration with standard silicon technology. In this feature article, we give an overview of recent developments and breakthroughs in the materials, geometry design and properties of STNOs. We focus in more depth on our latest advances in STNOs with perpendicular anisotropy showing a way to improve the output power of STNO towards the {\mu}W range. Challenges and perspectives of the STNOs that might be productive topics for future research were also briefly discussed.Comment: 11 pages, 10 figures, nanoscale 201

Field-free spin-orbit torque switching of a perpendicular ferromagnet with Dzyaloshinskii-Moriya interaction

Leveraging on interfacial Dzyaloshinskii-Moriya interaction (DMI) induced intrinsic magnetization tilting in nanostructures, a parametric window enabling field-free spin-orbit torque (SOT) magnetization switching in a perpendicular ferromagnet is established. The critical current density (Jc) bounds for SOT switching are highly dependent on the DMI, producing a distorted diamond-shaped region bounded by the Jc-DMI curves. The widest Jc interval is found for DMI values between 0.5 mJ/m2 and 0.8 mJ/m2. Geometrical modulation, of the ferromagnetic layer, reveals that the circular structure is optimum for minimizing the switching energy while maximizing the parametric window. For all the structures investigated, the SOT induced reversal process is via domain wall nucleation and propagation, and the switching is practical at room temperature

Micromagnetic Modeling of Magnetic Storage Devices

University of Minnesota Ph.D. dissertation. March 2021. Major: Electrical Engineering. Advisor: Randall Victora. 1 computer file (PDF); ix, 92 pages.Hard disk drives (HDDs) are the dominant mass storage devices for personal and cloud storage due to their low cost and high capacity. Heat-assisted magnetic recording (HAMR) is considered to be next-generation recording technology for HDDs. While HAMR shows the potential for areal density to go beyond one terabit per square inch, this new recording mechanism requires further understanding and optimization before commercialization. First, I examine the relationship between media noise power and linear density in HAMR. I observe that there is a noise plateau at intermediate recording density and show that the plateau can be shifted to different recording density regions depending on the temperature profile. This effect is argued to be a consequence of the competition between transition noise and remanence noise in HAMR. To extend the recording density limit, heat-assisted shingled magnetic recording is studied. The transitions are no longer symmetric about the track center after shingled writing, especially when the transitions are highly curved as a result of the temperature profile generated by the near-field transducer. I propose a new reading scheme by rotating the read head to match the curved transitions. For a single rotated head, more than 10% improvement in user density over that of a single non-rotated head is achieved. I found that the optimal rotation angle generally follows the transition shape. With an array of two rotated heads, a track pitch of 15 nm, and a minimum bit length of 6.0 nm, the user areal density reaches 6.2 terabits per square inch, more than 30% above previous projections for recording on granular media. Magnetoresistive random-access memory (MRAM) is another type of magnetic storage device that is mainly used as computer memory. As semiconductor-based memory begins to hit physical limits, spin-transfer torque (STT) MRAM and spin-orbit torque (SOT) MRAM appear to be strong candidates for future memory applications. I start first by studying SOT switching in magnetic insulators. Magnetic insulators (MIs), in particular rare-earth iron garnets, have low damping compared to metallic ferromagnetic materials due to lack of conduction electrons. Analogous to STT devices, their low-damping nature is presumed to be an advantage for SOT applications. I report that perpendicular magnetic anisotropy (PMA) material with low damping does not favor reliable SOT switching, but increased damping, interfacial Dzyaloshinskii–Moriya interactions, or field-like torques may help SOT switching in some cases. Notches in a nanometer-scale element, which is a more realistic size for practical applications, can also improve switching stability. To fully utilize low damping MIs with SOT, an in-plane exchange-coupled composite free layer SOT-MRAM is proposed. The free layer consists a low-damping soft MI and a high anisotropy material. The adoption of high anisotropy materials, such as L10 alloy, not only facilitates the achievement of ultra-high-density memory but also allows for the reduction of heavy metal layer volume and thus a reduction in write energy not seen in previous CoFeB-based SOT-MRAM. A write energy of 18 attojoules per bit for 1 ns switching is achieved which is only 72 times more than the theoretical limit of 60kBT. It also represents a factor of more than five hundred times improvement relative to state-of-the-art dynamic RAM

Modeling of magnetization dynamics and applications to spin-based logic and memory devices

The objective of this research is to develop models to better evaluate the performance and reliability of proposed spin-based boolean devices. This research will focus on a particular spin-based logic technology called Spin-Switch Logic. There are two primary reversal mechanisms that will be considered for a full evaluation of Spin-Switch technology. Firstly, nanomagnet reversal through the use of spin-transfer torque (STT) is studied. While switching through STT has been analytically solved for the uniaxial nanomagnet case, the biaxial case has yet to be studied on a sufficient scale and will be a focus of this research. Secondly, input-output isolation is achieved through dipolar coupling; hence, the performance and reliability of this type of reversal mechanism is extensively studied. It is shown that dipolar coupling strength is not only a function of geometric and material parameters, but also of reversal speed. If the reversal of a neighboring nanomagnet is very fast, the dipolar field reduces to a constant longitudinal field and can be analytically studied. However, if the reversal of the neighboring nanomagnet is slow, new models are needed to estimate the region of reliable coupling and delay. Lastly, a focal point of this research will be on the reliability of nanomagnet states in the presence of thermal noise and new models are proposed to estimate the reliability of complex spin-based systems. Not only does the thermal noise affect the probability of magnetization state consistency, it also alters nanomagnet precession during reversal, making the delay a random variable. Hence, models are developed for evaluating the variation in reversal delay through STT for both uniaxial and biaxial cases. Ultimately, these analytic models are combined to comprehensively evaluate the performance of Spin-Switch technology and identify possible improvements to this technology. While the end result of this research will be a thorough analysis of Spin-Switch logic, the models developed during this research are applicable to a variety of spin-based logic and memory technologies.Ph.D