There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

/pdf/spintronics-fundamentals-and-applications-2dx38n6phu.pdf

Spintronics: Fundamentals and applications

“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 recent discovery that a spin-polarized electrical current can apply a large torque to a ferromagnet, through direct transfer of spin angular momentum, offers the possibility of manipulating magnetic-device elements without applying cumbersome magnetic fields. However, a central question remains unresolved: what type of magnetic motions can be generated by this torque? Theory predicts that spin transfer may be able to drive a nanomagnet into types of oscillatory magnetic modes not attainable with magnetic fields alone, but existing measurement techniques have provided only indirect evidence for dynamical states. The nature of the possible motions has not been determined. Here we demonstrate a technique that allows direct electrical measurements of microwave-frequency dynamics in individual nanomagnets, propelled by a d.c. spin-polarized current. We show that spin transfer can produce several different types of magnetic excitation. Although there is no mechanical motion, a simple magnetic-multilayer structure acts like a nanoscale motor; it converts energy from a d.c. electrical current into high-frequency magnetic rotations that might be applied in new devices including microwave sources and resonators.

Microwave oscillations of a nanomagnet driven by a spin-polarized current

https://ralphgroup.lassp.cornell.edu/papers/JMMM-320-1190-2008.pdf

Spin transfer torques

The spin torque effect that occurs in nanometre-scale magnetic multilayer devices can be used to generate steady-state microwave signals in response to a d.c. electrical current. This establishes a new functionality for magneto-electronic structures that are more commonly used as magnetic field sensors and magnetic memory elements. The microwave power emitted from a single spin torque nano-oscillator (STNO) is at present typically less than 1 nW. To achieve a more useful power level (on the order of microwatts), a device could consist of an array of phase coherent STNOs, in a manner analogous to arrays of Josephson junctions and larger semiconductor oscillators. Here we show that two STNOs in close proximity mutually phase-lock-that is, they synchronize, which is a general tendency of interacting nonlinear oscillator systems. The phase-locked state is distinct, characterized by a sudden narrowing of signal linewidth and an increase in power due to the coherence of the individual oscillators. Arrays of phase-locked STNOs could be used as nanometre-scale reference oscillators. Furthermore, phase control of array elements (phased array) could lead to nanometre-scale directional transmitters and receivers for wireless communications.

/pdf/mutual-phase-locking-of-microwave-spin-torque-nano-4bwvd9mra3.pdf

Mutual phase-locking of microwave spin torque nano-oscillators

We have directly measured coherent high-frequency magnetization dynamics in ferromagnetic films induced by a spin-polarized dc current The precession frequency can be tuned over a range of several gigahertz by varying the applied current The frequencies of excitation also vary with applied field, resulting in a microwave oscillator that can be tuned from below 5 to above 40 GHz This novel method of inducing high-frequency dynamics yields oscillations having quality factors from 200 to 800 We compare our results with those from single-domain simulations of current-induced dynamics

/pdf/direct-current-induced-dynamics-in-co90-fe10-ni80-fe20-point-1zxq2i4dwa.pdf

Direct-current induced dynamics in Co90 Fe10/Ni80 Fe20 point contacts.

We have directly measured phase locking of spin transfer oscillators to an injected ac current. The oscillators lock to signals up to several hundred megahertz away from their natural oscillation frequencies, depending on the relative strength of the input. As the dc current varies over the locking range, time-domain measurements show that the phase of the spin transfer oscillations varies over a range of approximately $\ifmmode\pm\else\textpm\fi{}90\ifmmode^\circ\else\textdegree\fi{}$ relative to the input. This is in good agreement with general theoretical analysis of injection locking of nonlinear oscillators.

/pdf/injection-locking-and-phase-control-of-spin-transfer-nano-1kmk3ob2oc.pdf

Injection locking and phase control of spin transfer nano-oscillators.

Combining superconducting and magnetic layers offers a route to high-density and ultra-low power memory. Here, the authors extended this idea to more complicated structures by combining superconducting Josephson junctions and magnetic spin valves.

/pdf/hybrid-superconducting-magnetic-memory-device-using-1rezdr0ka8.pdf

Hybrid superconducting-magnetic memory device using competing order parameters.

https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=32791

William H. Rippard

Papers

Mutual phase-locking of microwave spin torque nano-oscillators

Direct-current induced dynamics in Co90 Fe10/Ni80 Fe20 point contacts.

Injection locking and phase control of spin transfer nano-oscillators.

Hybrid superconducting-magnetic memory device using competing order parameters.

Developments in nano-oscillators based upon spin-transfer point-contact devices