Recent developments in the controlled movement of domain walls in magnetic nanowires by short pulses of spin-polarized current give promise of a nonvolatile memory device with the high performance and reliability of conventional solid-state memory but at the low cost of conventional magnetic disk drive storage. The racetrack memory described in this review comprises an array of magnetic nanowires arranged horizontally or vertically on a silicon chip. Individual spintronic reading and writing nanodevices are used to modify or read a train of ∼10 to 100 domain walls, which store a series of data bits in each nanowire. This racetrack memory is an example of the move toward innately three-dimensional microelectronic devices.

http://science.sciencemag.org/content/sci/320/5873/190.full.pdf

Magnetic Domain-Wall Racetrack Memory

Electrons have a charge and a spin, but until recently these were considered separately. In classical electronics, charges are moved by electric fields to transmit information and are stored in a capacitor to save it. In magnetic recording, magnetic fields have been used to read or write the information stored on the magnetization, which 'measures' the local orientation of spins in ferromagnets. The picture started to change in 1988, when the discovery of giant magnetoresistance opened the way to efficient control of charge transport through magnetization. The recent expansion of hard-disk recording owes much to this development. We are starting to see a new paradigm where magnetization dynamics and charge currents act on each other in nanostructured artificial materials. Ultimately, 'spin currents' could even replace charge currents for the transfer and treatment of information, allowing faster, low-energy operations: spin electronics is on its way.

/pdf/the-emergence-of-spin-electronics-in-data-storage-1vrnm5549k.pdf

The emergence of spin electronics in data storage

“Spintronics,” in which both the spin and charge of electrons are used for logic and memory operations, promises an alternate route to traditional semiconductor electronics. A complete logic architecture can be constructed, which uses planar magnetic wires that are less than a micrometer in width. Logical NOT, logical AND, signal fan-out, and signal cross-over elements each have a simple geometric design, and they can be integrated together into one circuit. An additional element for data input allows information to be written to domain-wall logic circuits.

http://science.sciencemag.org/content/sci/309/5741/1688.full.pdf

Magnetic Domain-Wall Logic

The authors of the International Technology Roadmap for Semiconductors-the industry consensus set of goals established for advancing silicon integrated circuit technology-have challenged the computing research community to find new physical state variables (other than charge or voltage), new devices, and new architectures that offer memory and logic functions beyond those available with standard transistors. Recently, ultra-dense resistive memory arrays built from various two-terminal semiconductor or insulator thin film devices have been demonstrated. Among these, bipolar voltage-actuated switches have been identified as physical realizations of 'memristors' or memristive devices, combining the electrical properties of a memory element and a resistor. Such devices were first hypothesized by Chua in 1971 (ref. 15), and are characterized by one or more state variables that define the resistance of the switch depending upon its voltage history. Here we show that this family of nonlinear dynamical memory devices can also be used for logic operations: we demonstrate that they can execute material implication (IMP), which is a fundamental Boolean logic operation on two variables p and q such that pIMPq is equivalent to (NOTp)ORq. Incorporated within an appropriate circuit, memristive switches can thus perform 'stateful' logic operations for which the same devices serve simultaneously as gates (logic) and latches (memory) that use resistance instead of voltage or charge as the physical state variable.

‘Memristive’ switches enable ‘stateful’ logic operations via material implication

In most ferromagnets the magnetization rotates from one domain to the next with no preferred handedness. However, broken inversion symmetry can lift the chiral degeneracy, leading to topologically rich spin textures such as spin spirals and skyrmions through the Dzyaloshinskii-Moriya interaction (DMI). Here we show that in ultrathin metallic ferromagnets sandwiched between a heavy metal and an oxide, the DMI stabilizes chiral domain walls (DWs) whose spin texture enables extremely efficient current-driven motion. We show that spin torque from the spin Hall effect drives DWs in opposite directions in Pt/CoFe/MgO and Ta/CoFe/MgO, which can be explained only if the DWs assume a Neel configuration with left-handed chirality. We directly confirm the DW chirality and rigidity by examining current-driven DW dynamics with magnetic fields applied perpendicular and parallel to the spin spiral. This work resolves the origin of controversial experimental results and highlights a new path towards interfacial design of spintronic devices.

/pdf/current-driven-dynamics-of-chiral-ferromagnetic-domain-walls-1g9i94jrpa.pdf

Current-driven dynamics of chiral ferromagnetic domain walls

An all-metallic submicrometer device is demonstrated experimentally at room temperature that performs logical NOT operations on magnetic logic signals. When this two-terminal ferromagnetic structure is incorporated into a magnetic feedback loop, the junction performs a frequency division operation on an applied oscillating magnetic field. Up to 11 of these junctions are then directly linked together to create a magnetic shift register.

https://science.sciencemag.org/content/sci/296/5575/2003.full.pdf

Submicrometer ferromagnetic NOT gate and shift register.

As fabrication technology pushes the dimensions of ferromagnetic structures into the nanoscale, understanding the magnetization processes of these structures is of fundamental interest, and key to future applications in hard disk drives, magnetic random access memory and other 'spintronic' devices1,2,3,4. Measurements on elongated magnetic nanostructures5,6 highlighted the importance of nucleation and propagation of a magnetic boundary, or domain wall, between opposing magnetic domains in the magnetization reversal process. Domain-wall propagation in confined structures is of basic interest7,8 and critical to the performance of a recently demonstrated magnetic logic scheme for spintronics9. A previous study of a 500-nm-wide NiFe structure obtained very low domain-wall mobility in a three-layer device10. Here we report room-temperature measurements of the propagation velocity of a domain wall in a single-layer planar Ni80Fe20 ferromagnetic nanowire 200 nm wide. The wall velocities are extremely high and, importantly, the intrinsic wall mobility is close to that in continuous films11, indicating that lateral confinement does not significantly affect the gyromagnetic spin damping parameter to the extreme extent previously suggested10. Consequently the prospects for high-speed domain-wall motion in future nanoscale spintronic devices are excellent.

Magnetic domain-wall dynamics in a submicrometre ferromagnetic structure.

Unique surface imperfections serve as an easily identifiable feature in the fight against fraud.

Forgery: 'fingerprinting' documents and packaging.

We present a treatment of polarized light analysis and noise sources relevant to magneto-optical Kerr effect (MOKE) measurements of magnetic nanostructures and give performance details of our state-of-the-art MOKE magnetometer in these terms. By considering the various signal contributions, we are able to determine experimental conditions for optimal MOKE signal dynamic range and signal-to-noise ratio. A description of our MOKE instrument includes a novel facility to locate magnetic nanostructures by producing a susceptibility map of the sample surface. The performance of the magnetometer is demonstrated by hysteresis loops of single 100–200 nm wide magnetic nanowires, including a loop obtained with no time averaging during a single magnetic field cycle.

https://iopscience.iop.org/article/10.1088/0022-3727/36/18/001/pdf

Gang Xiong

Papers

Magnetic Domain-Wall Logic

Submicrometer ferromagnetic NOT gate and shift register.

Magnetic domain-wall dynamics in a submicrometre ferromagnetic structure.

Forgery: 'fingerprinting' documents and packaging.

Magneto-optical Kerr effect analysis of magnetic nanostructures