Developing Variation Aware Simulation Tools, Models, and Designs for STT-RAM

DEVELOPING VARIATION AWARE SIMULATION TOOLS, MODELS, AND DESIGNS FOR STT-RAM Enes Eken, PhD University of Pittsburgh, 2017 In recent years, we have been witnessing the rise of spin-transfer torque random access memory (STT-RAM) technology. There are a couple of reasons which explain why STT-RAM has attracted a great deal of attention. Although conventional memory technologies like SRAM, DRAM and Flash memories are commonly used in the modern computer industry, they have major shortcomings, such as high leakage current, high power consumption and volatility. Although these drawbacks could have been overlooked in the past, they have become major concerns. Its characteristics, including low-power consumption, fast read-write access time and non-volatility make STT-RAM a promising candidate to solve the problems of other memory technologies. However, like all other memory technologies, STT-RAM has some problems such as long switching time and large programming energy of Magnetic Tunneling Junction (MTJ) which are waiting to be solved. In order to solve these long switching time and large programming energy problems, Spin-Hall Effect (SHE) assisted STT-RAM structure (SHE-RAM) has been recently invented. In this work, I propose two possible SHE-RAM designs from the aspects of two different write access operations, namely, High Density SHE-RAM and Disturbance Free SHE-RAM, respectively. In addition to the SHE-RAM designs, I will also propose a simulation tool for STT-RAMs. As an early-stage modeling tool, NVSim has been widely adopted for simulations of emerging nonvolatile memory technologies in computer architecture research, including STT-RAM, ReRAM, PCM, etc. I will introduce a new member of NVSim family – NVSim-VXs, which enables statistical simulation of STT-RAM for write performance, errors, and energy consumption

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

Fabrication and Simulation of Nanomagnetic Devices for Information Processing

Nanomagnetic devices are highly energy efficient and non-volatile. Because of these two attributes, they are potential replacements to many currently used information processing technologies, and they have already been implemented in many different applications. This dissertation covers a study of nanomagnetic devices and their applications in various technologies for information processing – from simulating and analyzing the mechanisms behind the operation of the devices, to experimental investigations encompassing magnetic film growth for device components to nanomagnetic device fabrication and measurement of their performance. Theoretical sections of this dissertation include simulation-based modeling of perpendicular magnetic anisotropy magnetic tunnel junctions (p-MTJ) and low energy barrier nanomagnets (LBM) – both important devices for magnetic device-based information processing. First, we propose and analyze a precessionally switched p-MTJ based memory cell where data is written without any on-chip magnetic field that dissipates energy as low as 7.1 fJ. Next, probabilistic (p-) bits implemented with low energy barrier nanomagnets (LBMs) are also analyzed through simulations, and plots show that the probability curves are not affected much by reasonable variations in either thickness or lateral dimensions of the magnetic layers. Experimental sections of this dissertation comprise device fabrication aspects from the basics of material deposition to the application-based demonstration of an extreme sub-wavelength electromagnetic antenna. Magnetic tunnel junctions for memory cells and low barrier nanomagnets for probabilistic computing, in particular, require ultrathin ferromagnetic layers of uniform thickness, and non-uniform growth or variations in layer thickness can cause failures or other problems. Considerable attention was focused on developing methodologies for uniform thin film growth. Lastly, micro- and nano-fabrication methods are used to build an extreme sub-wavelength electromagnetic antenna implemented with an array of magnetostrictive nanomagnets elastically coupled to a piezoelectric substrate. The 50 pW signal measured from the approximately 250,000-nanomagnet antenna sample was 10 dB above the noise floor

Study of spin-dependent transport phenomena in magnetic tunneling systems?

Ph.DDOCTOR OF PHILOSOPH

Strain control of ferromagnetic thin films and devices

Magnetic memory and logic technologies promise greater energy efficiency and speed than conventional, semiconductor-based electronics. To date, electrical current has been used to operate such devices, although voltage-control may be a more efficient way to control magnetisation. One route to achieving voltage control of magnetisation is to use a hybrid piezoelectric/ferromagnetic device in which a voltage applied to the piezoelectric induces a strain in the ferromagnetic layer, which in turn induces a magnetic anisotropy. In this thesis such hybrid devices are used to investigate the control of magnetisation by inducing uniaxial anisotropy in the ferromagnetic layer. One material that shows promise for use as the ferromagnetic layer is Fe81Ga19. This material is attractive since it contains no rare earth elements, and in bulk crystals has been shown to be highly magnetically responsive to strain. This thesis investigates the magnetic properties of epitaxial Fe81Ga19 thin films grown by molecular beam epitaxy and it is demonstrated that these thin films retain the attractive magnetostrictive properties previously observed in bulk crystals. The presence of strong cubic magnetocrystalline anisotropy in the layers is exploited to demonstrate the non-volatile switching of magnetisation using strain-induced anisotropy in the absence of an applied magnetic field. This thesis shows also the manipulation of magnetic anisotropies and control of the configuration of magnetic domains and domain walls in Fe81Ga19 at a range of different lateral dimensions, from50 μm to 1 μm. It is shown that as the lateral dimensions of the device structures studied are reduced the domain configuration appears more regular, and that strain-induced anisotropy is more able to control these domains. In wires around 1 μm in width it is shown that growth strain relaxation by lithographic patterning induces sufficient anisotropy to cause a change in the domain configuration of the wire studied. Finally, this thesis begins to investigate how inverse magnetostriction can be used to tune the behaviour of domain walls in wires 1 μm wide and narrower. Experimental control of the field required to depin a vortex domain wall from a notch in a 1 μm wide Co wire is demonstrated. Using micromagnetic simulations it is shown that a large degree of control over the depinning of domain walls from notches in wires 1 μm wide and narrower is possible. The influence of in plane uniaxial magnetic anisotropy on the domain wall velocity in wires supporting in plane transverse domain walls driven by an external magnetic field is also investigated. Work previously done on the effect of uniaxial anisotropy on domain wall velocities close to Walker breakdown is extended in this thesis and in investigating the velocity and structure at driving magnetic fields far above walker Breakdown a second peak in domain wall velocity is observed, a phenomenon previously observed in wide wires, and wires under the influence of a transverse magnetic field

Spin transport and magnetoresistance in magnetic thin films with inversion broken crystals and multi-layers

This thesis firstly investigates spin-orbit torques (SOTs) generated by microwave current and detected by ferromagnetic resonance (FMR) spectroscopy in a uniform ferromagnetic system, namely NiMnSb, a half-Heusler ferromagnet with broken inversion symmetry at room temperature. The spin-orbit coupling (SOC) and the noncentrosymmetric structure of the epitaxial NiMnSb allow microwave current to induce oscillating SOTs on the magnetisation, and by analysing the measured FMR lineshape, the direction and magnitude of the effective spin-orbit fields (SOFs) are determined to characterise the SOTs. Furthermore, the observed current-induced SOFs have the symmetries of both Dresselhaus and Rashba SOC. The second part of this thesis explores the temperature-dependent behaviour of the current-induced SOFs in NiMnSb from 10 K to room temperature. We found that both Dresselhaus and Rashba types of SOFs are substantially enhanced at low temperature. This work is the first reported temperature-dependent SOFs in epitaxial NiMnSb system which provides more insights into the underlying mechanism that governs the SOFs in the noncentrosymmetric ferromagnets. Finally, the nonlinear magnetoresistance (MR) effect – unidirectional magnetoresistance (UMR) has been studied in CoFeB/Pt bilayer and epitaxial NiMnSb. In the CoFeB/Pt system, the close correspondence between the asymmetry in resistance and reduction in magnetisation confirmed by the current-induced spin-torque FMR (CI-FMR) measurements demonstrate that observed MR change is due to the creation or annihilation of magnons through spin-flip process and GHz magnetisation excitation by spin-torque effect. Motivated by the experimental approach established in the UMR study of CoFeB/Pt bilayer system, we investigate the resistance asymmetry in NiMnSb extracted from the non-resonating background voltage. The observed resistance asymmetry scales linearly with current density and has a crystallographic dependence. We interpret this resistance asymmetry in NiMnSb as the relative orientation between the magnetisation and the current-induced SOFs

Strain control of ferromagnetic thin films and devices

Magnetic memory and logic technologies promise greater energy efficiency and speed than conventional, semiconductor-based electronics. To date, electrical current has been used to operate such devices, although voltage-control may be a more efficient way to control magnetisation. One route to achieving voltage control of magnetisation is to use a hybrid piezoelectric/ferromagnetic device in which a voltage applied to the piezoelectric induces a strain in the ferromagnetic layer, which in turn induces a magnetic anisotropy. In this thesis such hybrid devices are used to investigate the control of magnetisation by inducing uniaxial anisotropy in the ferromagnetic layer. One material that shows promise for use as the ferromagnetic layer is Fe81Ga19. This material is attractive since it contains no rare earth elements, and in bulk crystals has been shown to be highly magnetically responsive to strain. This thesis investigates the magnetic properties of epitaxial Fe81Ga19 thin films grown by molecular beam epitaxy and it is demonstrated that these thin films retain the attractive magnetostrictive properties previously observed in bulk crystals. The presence of strong cubic magnetocrystalline anisotropy in the layers is exploited to demonstrate the non-volatile switching of magnetisation using strain-induced anisotropy in the absence of an applied magnetic field. This thesis shows also the manipulation of magnetic anisotropies and control of the configuration of magnetic domains and domain walls in Fe81Ga19 at a range of different lateral dimensions, from50 μm to 1 μm. It is shown that as the lateral dimensions of the device structures studied are reduced the domain configuration appears more regular, and that strain-induced anisotropy is more able to control these domains. In wires around 1 μm in width it is shown that growth strain relaxation by lithographic patterning induces sufficient anisotropy to cause a change in the domain configuration of the wire studied. Finally, this thesis begins to investigate how inverse magnetostriction can be used to tune the behaviour of domain walls in wires 1 μm wide and narrower. Experimental control of the field required to depin a vortex domain wall from a notch in a 1 μm wide Co wire is demonstrated. Using micromagnetic simulations it is shown that a large degree of control over the depinning of domain walls from notches in wires 1 μm wide and narrower is possible. The influence of in plane uniaxial magnetic anisotropy on the domain wall velocity in wires supporting in plane transverse domain walls driven by an external magnetic field is also investigated. Work previously done on the effect of uniaxial anisotropy on domain wall velocities close to Walker breakdown is extended in this thesis and in investigating the velocity and structure at driving magnetic fields far above walker Breakdown a second peak in domain wall velocity is observed, a phenomenon previously observed in wide wires, and wires under the influence of a transverse magnetic field