New Speed Record for Magnetic Memories

August 19, 2008

An experiment carried out at the Physikalisch-Technische Bundesanstalt (PTB) has realized spin torque switching of a nanomagnet as fast as the fundamental speed limit allows. Using this so-called ballistic switching future non-volatile magnetic memories could operate as fast as the fastest non-volatile memories. The experiments are described in the next issue of Physical Review Letters (22 August, 2008.

Fast memory chips such as DRAMs and SRAMs (Dynamic and Static Random Access Memory) commonly used today have one decisive disadvantage: in case of power interruption, they lose their stored information. This problem could be solved by magnetic memory chips called MRAMs (Magnetic Random Access Memory). In MRAM the digital information is not stored by means of electric charge but by means of the orientation of the magnetization of a magnetic cell.

The latest generation of MRAM uses the so-called spin torque effect for programming the magnetic bits. Using spin torque the memory state of the cell can be programmed in a very simple way just by applying a current pulse. A positive current switches the magnetization to one direction (digital state "0") and a negative current to the other (digital state "1"). Spin torque MRAM further promise a high storage density comparable to DRAM and Flash. Most major semiconductor chip producers are developing spin torque memories and market introduction is expected, soon.

A spin torque current pulse excites a rotational motion of the magnetization of the memory cell - the so-called precession. Normally, the magnetization has to undergo several precessional turns before reliable magnetization reversal takes place. Therefore present spin torque MRAM prototypes must operate with rather long write pulses of about 10 nanoseconds duration which limits the MRAM clock speed.

In the experiment carried out at PTB Braunschweig spin torque magnetization reversal has now been realized by a single precessional turn, only. This so called "ballistic" spin torque magnetization reversal corresponds to the ultra short physical limit of spin torque magnetization reversal time. It was achieved by precise tailoring of the current pulse parameters in combination with a small magnetic bias field.

Using ballistic spin torque reversal future MRAM could be programmed by current pulses shorter than 1 nanosecond corresponding to write clock rates well above 1 GHz. It could thus enable a high-density and non-volatile memory operating at the clock rates of the fastest volatile memories.

Physikalisch-Technische Bundesanstalt

---

---

	Next Generation MRAM Development: A 4kb MRAM Array for Spin Torque Transfer Switching Measurement by Masood Qazi (Author) Only recently has the possibility of a universal memory, a fast random access memory that retains its state during complete power-down, turned into a realizable opportunity. Such a memory can eliminate static power, improve system reliability in the face of power interruption, and eliminate the need for a separate FLASH memory module, reducing system component count. One candidate in the race for a universal memory is magnetoresistive random access memory (MRAM). In the development of MRAM, design challenges related to isolating memory elements, obtaining a compatible operating point with CMOS technology, and sensing data reliably have emerged. Therefore, there still exists a barrier to achieving the cost and performance characteristics of traditional volatile solid state memories---SRAM...
	Ferromagnetic resonance study of the Half-Heusler alloy NiMnSb: The benefit of using NiMnSb as a ferromagnetic layer in pseudo-spin-valve based spin-torque oscillators by Andreas Riegler (Author) Since the discovery of spin torque in 1996, independently by Berger and Slonczewski, and given its potential impact on information storage and communication technologies, (e.g. through the possibility of switching the magnetic configuration of a bit by current instead of a magnetic field, or the realization of high frequency spin torque oscillators (STO)), this effect has been an important field of spintronics research. One aspect of this research focuses on ferromagnets with low damping. The lower the damping in a ferromagnet, the lower the critical current that is needed to induce switching of a spin valve or induce precession of its magnetization. NiMnSb with its good properties, namely low magnetic damping in combination with a high spin-polarization, is a promising candidate for...
	Exploration and development of novel rutile oxide materials for next generation spin torque transfer heterostructures. by Kevin G West (Author) As the scaling of traditional CMOS goes beyond the 32 nm node, spin torque transfer random access memory (STT-RAM) has the potential to replace all other memory technologies. However, the switching current density needed to program a bit is still an order of magnitude too high to be drive by a conventional CMOS transistor and needs to be lowered to 105 A/cm 2. In this thesis a materials tool-kit was developed for an all oxide heterostructure that could deliver superior performance to the current state-of-art STT-RAM device. Utilizing the reactive bias target ion beam deposition (RBTIBD) growth technique, the growth and characterization of three transition metal oxides was investigated. These include vanadium dioxide (VO2), substitutionally alloyed chromium vanadium dioxide (Cr:VO2) and...
	Magnetoresistive devices with perpendicular magnetic anisotropy for spin-torque driven applications. by Jeong-Heon Park (Author) The spin-torque-transfer (STT) effect present in nano-magnetoresistive devices has attracted extensive interest, fueled by the advances it has provided for future Magnetoresistive Random Access Memory (MRAM) and spin-torque oscillators. In this thesis, we implement perpendicular magnetic anisotropy (PMA) into magnetoresistive devices to enhance the promises of spin-torque driven applications. For this work, we have designed two different types of magnetoresistive devices, which include magnetic tunnel junctions (MTJs) and giant magnetoresistance (GMR) spin-valves, composed of Co/Pt multilayers with PMA. The Co/Pt multilayer films are optimized in terms of growth orientation, surface roughness, and switching field distribution. To focus on obtaining highly spin-polarized current, thick Co...
	Electron Spin Resonance and Related Phenomena in Low-Dimensional Structures (Topics in Applied Physics) by Marco Fanciulli (Editor) This book is devoted to the discussion of the state of the art of spin resonance in low dimensional structures, such as two -dimensional electron systems, quantum wires, and quantum dots. World leading scientists in the field report on recent advances and discuss open issues and perspectives. Frontiers and opportunities for spin resonance techniques, with particular emphasis on fundamental physics, nanoelectronics, spintronics and quantum information processing, are discussed.
	Handbook of Spin Transport and Magnetism by Evgeny Y. Tsymbal (Editor), Igor Zutic (Editor) In the past several decades, the research on spin transport and magnetism has led to remarkable scientific and technological breakthroughs, including Albert Fert and Peter Grünberg’s Nobel Prize-winning discovery of giant magnetoresistance (GMR) in magnetic metallic multilayers. Handbook of Spin Transport and Magnetism provides a comprehensive, balanced account of the state of the art in the field known as spin electronics or spintronics. It reveals how key phenomena first discovered in one class of materials, such as spin injection in metals, have been revisited decades later in other materials systems, including silicon, organic semiconductors, carbon nanotubes, graphene, and carefully engineered nanostructures. The first section of the book offers a historical and personal...
	Field-driven and Spin-transfer-torque-driven Domain-Wall Dynamics: in Ferromagnetic Micro-/Nano- structures by Shuqiang Yang (Author) The main topic of this book is to explore magnetic-field- and electric-current-driven domain-wall motion in thin-film-based magnetic microstructures. Conventional thin-film growth and microstructure fabrication techniques including electron-beam lithography and focused ion beam milling are used to fabricate nanometer-scale one-dimensional and two-dimensional magnetic structures that support magnetic domains (regions of different magnetization orientation separated by domain walls). A high-spatial resolution, high-temporal resolution technique for measuring the field- or current- driven dynamics of the domain walls, based on the magneto-optic Kerr effect, is developed and used to study the wall dynamics. Domain-wall motion driven by (spin-polarized) electric current is...
	Spin Wave Technology: Initial Release by George J Bugh (Author) This is the paperback version of the original multimedia CD introducing Spin Wave Technology. It has many minor updates and corrections. References to scientific research support information on the internet has been updated to remove dead links and add new links. In addition, this book contains 4 more preliminary chapters introducing magnetic levitation designs that utilize spin wave technology. This paperback version includes a table of contents, footnotes, bibliography and an index. This is a book of informal research papers written by George J Bugh while investigating claims by many inventors and researchers who have built unusual electromagnetic devices said to produce anomalous energy output and even electrogravity effects. Mr. Bugh is a senior staff aerospace electronics...
	Spin Waves: Theory and Applications by Daniel D. Stancil (Author), Anil Prabhakar (Author) This book begins by introducing magnetism and discusses magnetic properties of materials, magnetic moments of atoms and ions, and the elements important to magnetism. It covers magnetic susceptibilities and electromagnetic waves in anisotropic dispersive media among other topics. There are problems at the end of each chapter, many of which serve to expand or explain the material in the text. The bibliographies for each chapter give an entry to the research literature.
	Time-resolved x-ray imaging of spin-torque-induced magnetic vortex oscillation. by Xiaowei Yu (Author) The spin transfer phenomenon provides a new method to manipulate magnetization without applying an external magnetic field and a new playground to study the spin degree of freedom of electrons. Two types of magnetic dynamics excited by the spin transfer torque from a direct current were predicted in 1996: magnetization reversal and steady-state precession. The physics of spin-torque-induced magnetization reversal of a single magnetic domain is now relatively well understood, but study of spin-torque-induced high frequency oscillation is still at an early stage. The electronic transport properties of this type of oscillation have been the subject of a lot of recent work, but no direct imaging has been reported. Another trend in the research of spin-torque dynamics is the focus shifting...