Luc Thomas

Magnetic Domain-Wall Racetrack Memory

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.

Current-Controlled Magnetic Domain-Wall Nanowire Shift Register

The controlled motion of a series of domain walls along magnetic nanowires using spin-polarized current pulses is the essential ingredient of the proposed magnetic racetrack memory, a new class of potential non-volatile storage-class memories. Using permalloy nanowires, we achieved the successive creation, motion, and detection of domain walls by using sequences of properly timed, nanosecond-long, spin-polarized current pulses. The cycle time for the writing and shifting of the domain walls was a few tens of nanoseconds. Our results illustrate the basic concept of a magnetic shift register that relies on the phenomenon of spin-momentum transfer to move series of closely spaced domain walls.

Oscillatory dependence of current-driven magnetic domain wall motion on current pulse length

Magnetic domain walls, in which the magnetization direction varies continuously from one direction to another, have long been objects of considerable interest. New concepts for devices based on such domain walls are made possible by the direct manipulation of the walls using spin-polarized electrical current through the phenomenon of spin momentum transfer. Most experiments to date have considered the current-driven motion of domain walls under quasi-static conditions, whereas for technological applications, the walls must be moved on much shorter timescales. Here we show that the motion of domain walls under nanosecond-long current pulses is surprisingly sensitive to the pulse length. In particular, we find that the probability of dislodging a domain wall, confined to a pinning site in a permalloy nanowire, oscillates with the length of the current pulse, with a period of just a few nanoseconds. Using an analytical model and micromagnetic simulations, we show that this behaviour is connected to a current-induced oscillatory motion of the domain wall. The period is determined by the walls mass and the slope of the confining potential. When the current is turned off during phases of the domain wall motion when it has enough momentum, the domain wall is driven out of the confining potential in the opposite direction to the flow of spin angular momentum. This dynamic amplification effect could be exploited in magnetic nanodevices based on domain wall motion.

Dynamics of Magnetic Domain Walls Under Their Own Inertia

Moving Walls The current-induced movement of magnetic domain walls in magnetic nanowires is a candidate for a new architecture in logic processing and memory. Controlling the motion and position of the domain walls as they move along the wires in excess of 100 meters per second requires an understanding of the processes involved. Thomas et al. (p. 1810) investigated the dynamics of magnetic domain wall motion, looking at the acceleration, constant motion, and deceleration processes in detail. The whole process could be described in terms of the inertia of the domain wall. The distance traveled was simply proportional to the length of the current pulse used to move the wall, which should simplify implementation in a circuit or network architecture. The current-induced motion of magnetic domain walls is controlled by the length of the current pulse. The motion of magnetic domain walls induced by spin-polarized current has considerable potential for use in magnetic memory and logic devices. Key to the success of these devices is the precise positioning of individual domain walls along magnetic nanowires, using current pulses. We show that domain walls move surprisingly long distances of several micrometers and relax over several tens of nanoseconds, under their own inertia, when the current stimulus is removed. We also show that the net distance traveled by the domain wall is exactly proportional to the current pulse length because of the lag derived from its acceleration at the onset of the pulse. Thus, independent of its inertia, a domain wall can be accurately positioned using properly timed current pulses.

Perpendicular spin transfer torque magnetic random access memories with high spin torque efficiency and thermal stability for embedded applications (invited)

Magnetic random access memories based on the spin transfer torque phenomenon (STT-MRAMs) have become one of the leading candidates for next generation memory applications. Among the many attractive features of this technology are its potential for high speed and endurance, read signal margin, low power consumption, scalability, and non-volatility. In this paper, we discuss our recent results on perpendicular STT-MRAM stack designs that show STT efficiency higher than 5u2009kBT/μA, energy barriers higher than 100u2009kBT at room temperature for sub-40u2009nm diameter devices, and tunnel magnetoresistance higher than 150%. We use both single device data and results from 8u2009Mb array to demonstrate data retention sufficient for automotive applications. Moreover, we also demonstrate for the first time thermal stability up to 400u2009°C exceeding the requirement of Si CMOS back-end processing, thus opening the realm of non-volatile embedded memory to STT-MRAM technology.

Resonant amplification of magnetic domain-wall motion by a train of current pulses.

The current-induced motion of magnetic domain walls confined to nanostructures is of interest for applications in magnetoelectronic devices in which the domain wall serves as the logic gate or memory element. The injection of spin-polarized current below a threshold value through a domain wall confined to a pinning potential results in its precessional motion within the potential well. We show that by using a short train of current pulses, whose length and spacing are tuned to this precession frequency, the domain walls oscillations can be resonantly amplified. This makes possible the motion of domain walls with much reduced currents, more than five times smaller than in the absence of resonant amplification.

Racetrack memory cell array with integrated magnetic tunnel junction readout

In this paper, we report the first demonstration of CMOS-integrated racetrack memory. The devices measured are complete memory cells integrated into the back end of line of IBM 90 nm CMOS. We show good integration yield across 200 mm wafers. With magnetic field-assist, we demonstrate current-driven read and write operations on cells within a 256-cell CMOS-integrated array.

Magnetoresistance, micromagnetism, and domain-wall scattering in epitaxial hcp Co films

Large negative magnetoresistance ~MR! observed in transport measurements of hcp Co films with stripe domains were recently reported and interpreted in terms of domain-wall ~DW! scattering mechanism. Here detailed MR measurements, magnetic force microscopy, and micromagnetic calculations are combined to elucidate the origin of MR in this material. The large negative room-temperature MR reported previously is shown to be due to ferromagnetic resistivity anisotropy. Measurements of the resistivity for currents parallel ~CIW! and perpendicular to DW’s ~CPW! have been conducted as a function of temperature. Low-temperature results show that any intrinsic effect of DW’s scattering on MR of this material is very small compared to the anisotropic MR. The effect of magnetic domain walls ~DW’s! on the transport properties of thin films and nanostructures is a topic of great current interest. Recent experimental research has extended early studies of iron single crystals 1,2 to nanofabricated thin-film structures of 3d transition metals 3‐5 and transition-metal alloys. 6,7 This topic has been approached from a number of viewpoints. In nanowires an experimental goal has been to use magnetoresistance ~MR! to investigate DW nucleation and dynamics in search of evidence for macroscopic quantum phenomena. Conductance fluctuations and MR hysteresis observed at low temperature in nanowires of Ni, Fe, and Co ~Refs. 8 and 9! have stimulated theoretical work on the effect of DW’s on quantum transport in mesoscopic ferromagnetic conductors. 10,11 In thin films and microstructures with stripe domains, experiments have focused on understanding the basic mechanisms of DW scattering of conduction electrons. Specifically, large negative MR observed at room temperature in hcp Co thin films with stripe domains were recently reported and interpreted in terms of a giant DW scattering contribution to the resistivity. 4 Independently, and to understand this result, a mechanism of DW scattering was proposed which invokes the two channel model of conduction in ferromagnets and spin dependent electron scattering—a starting point for understanding the phenomena of giant MR ~GMR!. 12 Within this model DW’s increase resistivity because they mix the minority and majority spin channels and thus partially eliminate the short circuit provided by the lower resistivity spin channel in the magnetically homogeneous ferromagnet.

On the exchange biasing through a nonmagnetic spacer layer

We present results on the magnetic coupling between a ferromagnetic (F) thin film and an antiferromagnetic (AF) thin film through a nonmagnetic metallic spacer (S) layer. Multilayered structures have been grown on silicon substrates using dc magnetron sputtering. We have studied the structure AF/S/F for different materials. AF is Ir22Mn78, F is Fe16Co16, and the spacer S is Al, Ag, Au, Si, Pd, Ru, and Ti. In most cases, both the exchange-bias and the coercive fields decrease exponentially with the spacer thickness with a decay length of a few angstroms, depending slightly on the material. Similar decay lengths are observed for the reversed structure F/S/AF. In some specific cases, we observe a nonmonotonic variation of the exchange-bias field with the spacer thickness. For example, the exchange-bias field increases when a thin Ag layer is inserted between the F film and an AF film grown at high sputter pressure.

Micromagnetics of mesoscopic epitaxial (110) Fe elements with nanoshaped ends

The magnetization reversal and magnetic domain configurations of 0.5-μm-wide epitaxial (110) Fe particles with rectangular and needle-shaped ends and competing magnetic anisotropies have been investigated. Magnetic force microscopy imaging and longitudinal Kerr hysteresis loop measurements in conjunction with micromagnetic simulations have been used to elucidate the basic micromagnetic behavior. End shape is shown to be a determining factor for the nucleation of magnetization reversal and the resulting magnetic domain configurations.