Magnetic Racetrack Memory Storage

Derek M. Kita∗

MIT Department of Materials Science and EngineeringMIT Department of Physics

(Dated: February 3, 2014)

An overview of magnetic racetrack operation and components necessary to successfully realize thismemory technology is presented. Fundamental physical concepts of adiabatic and non-adiabaticspin-torque transfer is presented and used to explain how bits of information are read, written,and pushed through ferromagnetic racetracks. Recent challenges associated with this technologyare current densities required to move domain walls and the reliability of domain wall motion.Resonant amplification of spin-torque transfer via pulsed current operation has been proposed toovercome issues with current density. In addition, domain wall pinning via patterned racetrackshas demonstrated limited success in reliably controlling multiple domain walls in a single racetrack.Lastly, new material structures such as highly efficient anisotropic heavy-metal/ferromagnet/oxidelayers are promising candidates for new racetrack geometries. Future work in interfacial spintronicsand topological effects show promise in benefiting racetrack memory and the field of magneticmemory.

I. INTRODUCTION

Over the past decade, a rich new field combining mate-rials science and physics has developed around manipu-lating the spin of electrons in magnetic and semiconduct-ing devices. This field, called spintronics, has opened upthe potential for a myriad of previously unimaginablenew devices with applications in memory storage andlogic. Memory storage technologies in particular havereceived increased attention due to the high energy con-sumption and volatility of static and dynamic randomaccess memory (RAM), the slow read and write times ofhard disk drives (HDD), and the cost of flash memoryand solid-state drives (SSD). Among the most promis-ing of the spintronic memory solutions is the concept ofmagnetic race track memory, which stores bits of infor-mation in the magnetization orientation of regions in achannel-shaped ferromagnetic structure.

The operation of racetrack memory is analogous toa solid-state, non-volatile shift register. On one end ofthe racetrack, bits may be written into the racetrack viachanging the magnetization direction in the ferromag-netic material in the racetrack. From here, the bit ispushed along the racetrack and may be read by magne-toresistive tunnel junctions farther down the track, seeFigures 1 and 2. One of the true advantages to racetrackmemory lies in the ability to orient the racetrack upwardsand store bits in a larger 3-dimensional space. However,preliminary prototypes of this memory device operate intwo dimensions.

The proposed memory density, read/write speeds, andscalability of this device make it the strongest candidatefor a form of universal memory. However, the practical-ity is severely limited by high current densities requiredto move domain walls and the reliability of domain wall

∗ dkita@mit.edu

FIG. 1. Image borrowed from L. Thomas, et al. [1]. Concep-tual design of a magnetic racetrack memory structure. Blueand red colored regions indicate domains of opposite magne-tization to one another but in plane with the racetrack di-rection. This particular structure uses the stray-field from aneighboring domain wall to change the magnetization of a bitin the racetrack. Magnetization state is read from a magnetictunnel junction.

motion through the racetrack [2]. In addition, the exactcontributions to spin-induced magnetic flipping and do-main wall motion are highly dependent on the materialsused and structure of the device. Several solutions andmodels have been proposed to alleviate these issues, in-cluding patterned notches along the ferromagnetic race-track, pulsed current to move the domain walls, andexperimentation with different soft and hard magneticmaterials. In addition, anisotropic multilayer structureshave recently demonstrated very efficient movement ofdomain walls under the influence of a spin-polarized cur-rent. Examples of which are heavy metal-ferromagnet-oxide structures that form stable Neel-type domain wallsand exhibit higher than normal current-driven domain

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FIG. 2. Image borrowed from S. Parkin, et al. [2]. (A)Bending of ferromagnetic racetracks in the vertical directioncould allow for higher area density bit storage. Red and blueregions indicate areas of differing magnetization orientation.Pulsed current perpendicular to the structure pushes domainsinto or away from reading/writing elements. (B) Horizontalracetracks offer better initial prototypes due to ease of fabri-cation and accessibility of the device for characterization. (C)Magnetoresistive tunnel-junctions for sensing the resistance,corresponding to magnetization, across the domain. (D) Amethod of writing bits by moving the stray field from a neigh-boring domain wall into and out of range of the racetrack. (E)Concept design of a high-density magnetic racetrack array.

wall motion due to a number of physical phenomena andinterfacial effects. In particular, unique heterostructureswith metals, ferromagnets, and oxides have helped to un-cover interesting physical phenomena that will allow forthe design of practical, efficient racetrack memory.

II. OPERATING PRINCIPLES

Operation of magnetic racetrack memory requiresthree essential components: (1) reading elements suchas magnetoresistive tunnel junctions that detect the tun-neling resistance across a ferromagnetic region, (2) writ-ing elements such as nearby wires that produce a localoersted field or spin-torque transfer devices that changea bit by transferring angular momentum from spin-polarized currents, (3) a means by which domain wallsmay be pushed through the ferromagnet layer, typicallyvia pulsed spin-polarized currents [2], and (4) a methodof reliably restricting the position of domain walls to dis-crete distance intervals (ie. notched patterns along the

racetrack). With each of these requirements comes aunique set of constraints and limitations. Ideally, readingand writing elements will require low current densities towrite bits of information and not affect neighboring bits.The current parallel to the racetrack must also push allthe sequential domain walls along in a uniform, controlledmanner. Lastly, the domain walls must be pinned in dis-crete intervals by structures that do not significantly in-crease the current density and have sufficient tolerance asto prevent multiple domain walls from moving into thesame bit area. The exact geometry of proposed racetrackmemory models is visualized in Figure 2.

II.1. Reading Information from the Racetrack

To read the state of a domain in the magnetic race-track, a magnetoresistive tunnel junction senses the re-sistance of spin-polarized current across the bit. A mag-netoresistive tunnel junction is a device composed oftwo ferromagnetic layers separated by a thin oxide. Ineach ferromagnetic material, the density of states is spin-dependent because the internal magnetic field gives riseto a Zeeman splitting of electron energy levels alignedwith and against the field. As a result, a larger numberof states are available to electrons oriented along the fieldtherefore allowing for lower resistance. When the two fer-romagnetic materials are aligned in the same direction,the resistance may be described by two large resistors inseries and also in parallel with two small resistors in se-ries. When the two ferromagnetic materials are alignedantiparallel, the equivalent resistance may be describedby a small and large resistor in series and also in parallelwith another small and large resistor in series, see Figure3. The second configuration results in a much larger to-tal resistance than the first, and helps to account for thetotal change in resistance between the two orientations.The magnetic tunnel junction may be characterized byits tunneling magnetoresistance ratio (TMR) defined by

TMR ≡ Rap −Rp

Rp(1)

where Rap is the resistance in the anti-parallel configura-tion and Rp is the resistance in the parallel configuration.A high TMR makes it easier for on-chip electronics to ac-curately detect the state of the bit.

The oxide tunneling layer is also a critical componentof the tunnel junction. It both isolates the two ferromag-netic regions and restricts the movement of carriers toquantum mechanical tunneling. When a bias is appliedto the junction in the parallel configuration, the proba-bility of tunneling is much larger than when the ferro-magnets are perpendicularly configured, correspondingto a lower resistance state. Recent work has demon-strated that marked improvements in spin-polarizationefficiency have been possible by replacing aluminum ox-ide (AlOx) tunnel junctions with MgO junctions due tosymmetry effects and that majority charge carrier states

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FIG. 3. Image borrowed from Markus Meinert from the Creative Commons. On the left is a representation of two ferromagneticstructures that are oriented in parallel to one another. Spin down electrons move with low resistance (red) and spin upelectrons move with high resistance (blue). On the right, the ferromagnets are oriented in opposite directions, resulting inequal resistances for both spin electrons. The “eU” denotes the potential drop across the tunnel barrier from biasing.

FIG. 4. Illustration of an adiabatic spin-torque transfer process resulting in domain wall motion. Arrows in blue (bottom)show the changing magnetization from left to right across a domain wall. The arrows in purple (middle) show the orientationof an electron’s spin as it adiabatically follows the magnetization direction in the material. The loss of spin-angular momentumis transferred to the lattice, creating a torque shown by the red arrows (top). Image borrowed from A. Brataas, A. Kent, andH. Ohno [3]

decay more slowly across the insulator than the minoritystate [4]. In general, this effect is enhanced by choos-ing ferromagnetic materials whose symmetries for spin-up and spin-down electrons differ. The result is a smalloverlap between wavefunctions on either side of the ox-ide when configured anti-parallel and larger overlap whenconfigured parallel, leading to a very large TMRs thatincrease with widening MgO layers. Tunneling magne-toresistance ratios as large as 604% have been observedin magnetic tunnel junctions using MgO as the insulator[5]. Thus, with greatly changing values of resistance forboth magnetization orientations, it is easy to accuratelydetect the state of the domain on the racetrack.

II.2. Writing Information to the Racetrack

Spin-torque transfer and stray-fields from neighboringdomain walls are two methods that can be used to write

bits onto the racetrack. Using the induced magnetic fieldfrom current through a wire is another option, but willnot be discussed because it requires high current densitiesand can affect neighboring domains. First, for writingelements using spin-torque transfer effects it is crucial tomodel the magnetization of the material under differentcurrents. The standard Landau-Lifshitz-Gilbert (LLG)equation describes the behavior of a magnetic momentunder the influence of a field

∂ ~m

∂t= −γ ~m× ~Heff + α~m× ∂ ~m

∂t(2)

where γ is the gyromagnetic ratio, ~Heff is the effectivefield felt by the moments, and α is the damping param-

eter. The effective field ~Heff contains contributions ofthe magnetic field from the external field, the magneto-static field, the anisotropy field, the field from exchangeinteraction, and the field generated by the current. Thefirst term on the right describes the precessional motion

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FIG. 5. Image borrowed from A. Brataas, A. D. Kent, andH. Ohno [3]. Light blue arrow corresponds to an individualmagnetic moment under the influence of an applied field andspin-transfer torque. Gilbert damping (green arrow) repre-sents the torque aligning ~m with the field, while spin-transfertorque (red arrow) is the result of angular momentum transferfrom the current to the moment. An additional torque arisesfrom the field generated by the current (light blue arrow).

of the moment, and the left-most term phenomenologi-cally describes the damping of the moment. If we runcurrent through a pinned ferromagnetic layer, like NiFe,separated by a thin oxide above our racetrack, we candrive a highly spin-polarized current through the lat-tice and transfer some of this angular momentum to theatomic moments (if the direction of the electron spinsfrom the pinned layer ~mp, is in a different direction fromthe racetrack domain’s magnetization ~m, or equivalently~m · ~mp 6= 0). Ni rich materials like NiFe are typicallyused in spintronics since they have demonstrated up to70-90% efficiency for spin-polarizing applied currents [1].The NiFe layer’s magnetization is typically “pinned” byan adjacent antiferromagnetic material. The exchangeinteraction at the interface between the two materials in-creases the NiFe’s coercivity. The torque resulting fromthe spin-polarized current can be included in equation(2) resulting in the following modified equations of mo-tion for the moment

∂ ~m

∂t= −γ ~m× ~Heff+α~m×∂ ~m

∂t+γ~Jη(~m, ~mp)

2eMstF~m×(~m×~mp)

(3)

where η(~m, ~mp) represents the efficiency of the spin-torque transfer, J is the current density, and Ms is thesaturation magnetization [6]. Using this model, it is the-oretically possible to determine the current densities re-quired to both change the magnetization for perpendic-ular writing of bits or move domain walls along the race-track. For example, if the geometry of the ferromagneticlayer is assumed to be thin with uniaxial anisotropy in-plane with the film plane, J. Z. Sun linearized equation 3and determined the threshold current required for switch-ing to be

Ic = (2e

~)(α

η)m(H +Hk + 2πMs) (4)

where Hk is the anisotropy field and Ms is the saturationmagnetization (note that the above is in CGS units) [7].

Another method of changing the orientation of themagnet presented by S. Parkin is via the stray field froma domain wall adjacent to the racetrack [2], as shown inFigure 2D. A suitable material and distance between thenearby domain wall and racetrack must be chosen such

that the stray field increases the external field in ~Heff

from equation (2) enough to sufficiently rotate ~m by atleast an angle of π radians.

II.3. Shifting Domain Walls Down the Racetrack

Perhaps the most challenging aspect of the device’soperation is efficiently moving domain walls through theracetrack in a reliable and repeatable way. Ideally, do-main walls move down the racetrack at equal velocities.In addition, when the system is at equilibrium and nocurrent is applied, the domain walls should be confinedto discrete intervals so that thermal energy and neigh-boring magnetic fields can not move them.

Movement of the domain walls can be achieved bysending spin-polarized current through the racetrack.The magnetization orientation of the domains are eitheraligned with the direction of this current or in the oppo-site direction of this current which results in head-to-headdomain walls or tail-to-tail domain walls. Both head-to-head domain walls and tail-to-tail domain walls willhave moments perpendicular to the motion of the spin-polarized current and as a result experience a torque inthe same direction. Thus, spin-polarized currents makeit possible to move both types of domain walls in thesame direction. In fact, a good physical description ofdomain wall motion in nanowires or films with surfacepinning may be accounted for with only one- or two-dimensional Landau-Lifshitz-Gilbert equations (equation3) with spin-torque and accounting for thermal fluctua-tions [8]. These results, in addition to work done by S.Parkin [2], indicate efficient depinning and subsequentdomain wall motion through the use of short currentpulses.

Restricting the domain walls to discrete spatial inter-vals is key to preserving the digital nature of this memory

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storage device. One method of ensuring this is by fab-ricating pinning sites, or notches, along the racetrack.Several factors must be appropriately taken into accountthrough this approach. They are the current-densities re-quired to de-pin these domain walls and the exact typesof domain walls existing in the racetrack (which will de-termine the exact magnetization dynamics).

The operation of magnetic racetracks draws uponmany recent revelations in spintronics. High TMRs inrecent magnetic tunnel junctions allow for accurate read-ing of the state of domains in the racetrack, while spin-torque transfer effects and/or stray fields from adjacentdomain walls allow for accurate writing. The dynamics ofmoving domain walls along the racetrack and controllingthe position of the domains to a high degree of accuracyis under heavy research and will be further discussed inlater sections.

III. MATERIAL SELECTION

Material selection for the ferromagnetic racetrack ma-terial plays a large role in determining the dynamics ofdomain wall motion creation and movement. Both “soft”and “hard” magnetic materials have been explored forracetrack applications. “Soft” magnetic materials havegained increased interest due to the ability to manipu-late the type of domain walls and the resulting domainwall widths. Typical materials for this application areiron, cobalt, and/or nickel alloys due to their high satu-ration magnetization values. “Hard” magnetic materials,typically crystalline cobalt iron materials have also beenexplored. However, the magnetic properties tend to begoverned by intrinsic properties the material such as themagnetocrystalline anisotropy fields.

Material selection for future prototypes of racetrackmemory will be increasingly important, since interfa-cial effects have demonstrated the ability to reduce thethreshold current density for domain wall motion. Theexact nature of these effects is highly dependent bothon the ferromagnetic materials used, the materials sur-rounding the racetrack, the geometry of the channel,and the defects located at these interfaces. Recentwork has explored movement of domain walls throughTa/CoFe/MgO and Pt/CoFe/MgO, which will be dis-cussed in the Threshold Current section.

Materials for magnetic tunnel junctions and fixed fer-romagnetic layers (for spin-polarization) have receivedsignificant attention recently and are relatively well de-veloped. As such, magnetic tunnel junctions are typi-cally made from CoFeB or CoFe fixed layers separatedfrom the free layer (the racetrack in this case) by an in-sulator which is typically MgO or AlOx. Fixed layersfor spin-polarization also tend to consist of crystallinecobalt alloys due to their large coercivity, high saturationmagnetization, and strong uniaxial magnetocrystallineanisotropy.

FIG. 6. Image borrowed from S. Ladak et al. [9]. Top image(a) illustrates a typical transverse type of domain wall. At thebottom (b) is a vortex type of domain wall. Blue coloring cor-responds to a positive value of Div(~m), while red correspondsto negative values. Note that Div(~m) can be interpreted as amagnetic charge density.

IV. DOMAIN WALL MOVEMENT

Different low-energy domain wall configurations havebeen shown to exist in ferromagnetic racetracks. Bothmicromagnetic simulations and results from experimentshave shown that transverse and vortex domain wallstructures are stable and common in nanowires withanisotropy along the direction of the wire. A transversedomain wall occurs when the magnetic moments grad-ually begin to change direction, but the changing angleoccurs faster along one edge than the other. This domainwall can be seen in Figure 6 (a) and has a triangle-shapeddomain wall region. Vortex domain walls, on the otherhand, occur when the gradual change in magnetic mo-ments happens on both walls. As a result, when thetwo resulting transverse-like domain walls meet, a vortexshaped pattern is formed. It is worth noting that bothstructures are chiral and exhibit correspondingly inter-esting symmetry properties. Both transverse and vortexstructures are stable domain walls, but vortex domainwalls tend to be lower in energy. Vortex domain walls aretypically favored in wider nanowire ferromagnets sincethey minimize the amount of stray field emerging fromthe surfaces and minimize the magnetic charge density.In practice, vortex domain walls can vary based on theirchirality (the clockwise or anti-clockwise orientation ofthe moments Fig 6) and also the vortex core can bepointing in- or out-of-plane. The dynamics of domainwall motion, such as the pinning strength, are highly de-pendent on the type of domain wall that forms [2]. Achallenge associated with magnetic racetrack memory isthe creation of specific types of domain walls in the race-tracks to ensure uniform motion and behavior of bits.

Another important topic in the field of spintronics hasbeen the relative importance and magnitude of adiabaticand non-adiabatic spin-torque transfer. When an elec-tron passes through a magnetic material and changes its

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FIG. 7. Image borrowed from L. Thomas et al. [1]. Top imageshows the notched pattern in a ferromagnetic nanowire usedto pin domain walls. Bottom image shows three vortex shapeddomain walls (black and white triangular shapes) pinned atthe notched locations.

moment direction, there is an equivalent amount of an-gular momentum transferred to the magnetic material.This spin-torque is typically described as a combinationof both adiabatic and non-adiabatic torques. The adi-abatic torque is in-plane with the electron’s initial di-rection and the material’s magnetization, as shown bythe red arrow in Figure 5. The non-adiabatic torque isthe term perpendicular to this plane (light blue arrowin Figure 5) and typically arises from a gradient magne-tization, such as in the presence of a domain wall. Therelative magnitude of these two torques depends stronglyon the geometry of the device and materials chosen. Theadiabatic term is given by the third term in equation 3and is historically well-understood. However, the LLGequations are typically further modified to include theless-well understood non-adiabatic torque

τβ =γ~

2eMsβP ~m× ( ~J · ~∇)~m (5)

where β characterizes the non-adiabicity, P is the spin-

polarization of the electric current, and ~J is the electriccurrent density [3].

In a completely adiabatic system, spin-polarized cur-rent would cause the magnet moments of the material toprecess such that the adiabatic torque and Gilbert damp-ing are equal and opposite. In this situation, there wouldbe no domain wall movement or magnetization reversaluntil the critical current density (Ic in equation 4) isreached. However, many experiments have demonstratedscenarios where Ic is much smaller than predicted, sug-gesting scenarios where the non-adiabatic term is domi-nant. In fact, the non-adiabatic torque turns out to be animportant parameter in describing systems with wide do-main walls and extrinsic defects. In these cases, domainwall movement is proportional to the non-adiabicity βand decreases with increasing Gilbert damping (α) [3].The critical current density also tends to scale with theextrinsic pinning strength in alloys of cobalt, iron, nickelferromagnets, which both indicates the non-adiabatic be-havior of the interaction and provides motivation for thepurposeful fabrication of pinning sites along the racetrackto control domain wall motion.

Discretely spaced pinning sites allow for the control ofthe individual bit locations. Typical pinning sites con-sist of horizontal notches patterned into the side of theracetrack, see Figure 7. A pinning site between domains

allows the magnetization from one side of the site to theother to change more abruptly. The result is a decreasein domain wall energy due to a reduction in the totalexchange energy [10]. Thus, the notch effectively createsa low-energy potential well that the domain wall prefersto settle into. Movement of the domain wall from thisposition requires both a current density larger enough tomove the domain wall (i > Ic) and a current density largeenough to excite the wall out of the potential.

V. THRESHOLD CURRENT

To effectively integrate magnetic racetrack memorywith CMOS ICs, it is important to make sure the cur-rent densities required to move domain walls are suffi-ciently attainable. In addition, high current densities cancause Joule heating and result in domain wall instability.Recent racetrack memory prototypes developed at IBMhave included heat sinks to account for this. In addition,the energy barrier created by domain wall pinning sites(both the purposefully created sites discussed previouslyand the pinning sites from racetrack imperfections) in-creases the complexity of this challenge.

One proposed solution to the problem of exciting thedomain wall over the pinning potentials is using reso-nantly tuned current-pulses. Each pulse, if the frequencyis tuned to the resonant frequency of the pinning poten-tial, will increase the energy of the domain wall until it isno longer bound to the potential. The total current den-sity will still need to exceed the critical current density,but pulsed operation allows for the movement of domainwalls without increasing the current density proportionalto the depth of the potential. An illustration of this prin-ciple can be seen in Figure 8, courtesy of S. Parkin [2].

Several challenges exist with the use of pulsed currentsto excite domain walls from the pinning sites. First, itis important for the pinning sites to be fabricated con-sistently enough that the resonant frequency is not sub-stantially altered between pinning sites. If even a sin-gle domain wall is unable to be resonantly excited andmoved to the next position, the performance of the entireracetrack is compromised. Also, the depth of the pin-ning potential should be uniform such that the numberof current pulses required to excite the wall is consistentand that there is no unnecessary delay in bit movement.If two 180o domain walls become sufficiently close, theycan form a 360o domain wall. Such domain walls haveentirely different motional dynamics since they interactdifferently with spin-polarized currents and pinning po-tentials.

Alternative structures have been receiving increasedattention for the purpose of threshold current reduction.Several methods of obtaining highly spin-polarized cur-rents using anisotropic structures and interesting surfacephenomena are under investigation due to their abilityto produce larger spin-transfer torques than with tradi-

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FIG. 8. Image borrowed from S. Parkin [2]. At the top is a graphical representation of the current profile and the correspondingdomain wall motion is visualized below it. As the current reverses polarity at the potential well’s resonant frequency, the domainwall becomes increasingly excited until it is finally capable of exiting the potential and moving in space.

tional devices utilizing spin valves and tunnel junctions.In structures with inversion asymmetry (i.e. the layers

above and below the ferromagnet have different physicalproperties resulting in interfacial asymmetry) the inter-nal electric field of the material can enhance spin-orbitcoupling and give rise to additional spin-orbit torques.This phenomena is known as the Rashba effect, and isbelieved to stabilize Bloch-type domain walls in additionto providing additional torque on the materials [11] [12].It has also been suggested that in structures where a fer-romagnet is adjacent to a metal layer, the Rashba effectcan enhance spin-torque transferred to the ferromagnetfrom an injection of spin-polarized carriers. The spin-polarized carriers arise from the spin-Hall effect, which isthe accumulation of spin-up and spin-down charge carri-ers on opposite surfaces of metallic and semiconductingstructures [11].

Another interesting phenomena, the Dzyaloshinskii-Moriya interaction (DMI), has also been under investiga-tion due to its ability to stabilize chiral domain walls [13].These type of domain walls have been demonstrated tomove very efficiently under the influence of spin-polarizedcurrent. Several heavy-metal/ferromagnet/oxide struc-tures such as Pt/CoFe/MgO and Ta/CoFe/MgO stackshave been fabricated to study these interfacial magneticeffects. Interestingly, these structures exhibit domainwall motion against the flow of electrons (or along theflow of current), which may be accounted for if the do-main walls are of Neel-type. To distinguish between effi-cient domain wall movement as a result of the Rashbaeffect or the DMI, G. Beach et al. applied magneticfields in-plane with domain walls and along the flow ofcurrent [11] [13]. The results confirmed the existenceof Neel-type domain walls which are efficiently movedby spin-Hall currents injected from the heavy-metal re-

gion. This work provided evidence that the DMI is ul-timately responsible for efficient domain wall movementin these asymmetric heavy-metal/ferromagnetic stacks.The high current densities associated with domain wallmovement in racetrack memory could be significantly re-duced through engineering asymmetric structures whereinterfacial effects such as the DMI are dominant.

Although these examples are highly specific to ferro-magnetic films and nanowires, the underlying physicalmechanisms behind these interfacial magnetic interac-tions are typically dependent on the materials used, thestructure of the device, the types of symmetries that ex-ist, spin-orbit coupling and torque, internal electric andmagnetic fields, domain wall type and chirality, and evenrelativistic effects. The parameter space open for scien-tific exploration is large and suggests that improvementsto the threshold current for magnetic racetrack memorywill be found in the realm of interfacial spintronics.

VI. FUTURE OUTLOOK

Magnetic racetrack memory technology has recentlyexperienced a large push from academia and industrydue to its promise of energy efficient, non-volatile, highdensity memory and the potential to advance our sci-entific understanding of spin-dependent electronics andmagnetism. The future of magnetic racetrack memoryis bright, but there are several issues that need atten-tion before this promising technology becomes a reality.Either one of three situations is likely to occur in thenear future: (1) advances in engineering will improve do-main wall reliability and the high current density willbe alleviated of with heatsinks and robust materials, (2)advances in interfacial spintronics will enable controlled,

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FIG. 9. Image borrowed from A. J. Annunziata [14]. Thetop-most image (a) depicts the schematic for a racetrack pro-totype recently fabricated at IBM using CMOS processes, (b)shows a top-view of the ferromagnetic racetrack, (c) shows asingle memory cell, and (d) and (e) depict zoomed-out imagesof an array of 256 cells with contact pins for testing

efficient domain wall motion with novel heterostructures,or (3) an improved understanding of magnetic interac-tions will motivate the discovery of more efficient, reli-able, and practical memory architectures.

Pioneering work has been done at IBM, led by S.Parkin, to develop several prototypes of small but func-tional racetrack memory cells. For example, fully CMOSintegrated prototypes of this memory were reported andfunctioning with the assistance of magnetic fields [14].Magnetic tunnel junctions were also incorporated to readthe state of the bit. Images of these fabricated devicescan be seen in Figure 9. This effort demonstrates boththe feasibility of the current racetrack memory designand the need for engineering developments to optimizethe processing and repeatability issues associated withfabrication.

Racetrack memory could also see a significant increasein performance if interfacial spintronic effects may beproperly utilized to decrease current densities. For ex-ample, depositing ferromagnetic racetracks onto an ox-ide material and then using a heavy-metal such as Ptor Ta for the other side could allow next-generation race-tracks to take advantage of the DMI-assisted domain wallmovement. Other near-future advances in this field arelikely to contribute to the materials selection and devicegeometry for magnetic racetrack memory.

Lastly, it is very likely that new developments in in-terfacial spintronics will reveal other, more interesting

physical phenomena that are not immediately applica-ble to this technology. The study of these phenomenaare likely to result in the development of better, morepractical memory solutions. For example, the study oftopological effects in ferromagnetic materials, which hasgained an especially large interest recently, has theoreti-cally and experimentally demonstrated that dissipation-less pure spin-polarized currents can be found at thesurface of certain materials [15]. These materials, typ-ically topological insulators (materials with conductivesurface states but an insulating bulk) are of interest tounderstanding fundamental concepts in condensed mat-ter physics. Their ability to generate pure spin-polarizedcurrents through the quantum spin-Hall effect, as de-scribed previously, has also attracted interest from thefield of spintronics. In these cases, a deep understandingof the underlying physics will enable materials scientistsand physicists to design novel devices for efficient mem-ory storage.

The future of magnetic racetracks for non-volatilememory is contingent upon the success of materials pro-cessing and the understanding of physical concepts thatdescribe the interactions between electron spin and thelattice. Assuming a sufficient understanding of spin in-teractions is attained, numerical methods may be usedto design novel structures with optimized efficiency andgood processing bias. Already progress has been madein computation micromagnetics, with the development ofopen-source software such as the Object Oriented Micro-Magnetic Framework (OOMMF) developed at the Na-tional Institute of Standards and Technology (NIST).Such software enables scientists and engineers to modelthe behavior magnetic moments under the influence ofdifferent fields by numerically propagating the LLG equa-tions of motion. As such, when new spin-torque correc-tions must be made, the torque can be converted to aneffective field and directly calculated/visualized for a par-ticular structure. Such software allows for the effectiveoptimization and design and is of critical importance tofinding applications for interesting new physical phenom-ena.

The ultimate success of this specific magnetic race-track architecture, as proposed in this report, is cur-rently unclear. Pioneering work is currently uncover-ing the mechanisms by which domain wall movement viaspin-torque interactions occur and will ultimately decideif this technology is practical. In addition, the mar-ket success of racetrack memory depends on how wellit fares against competing memory solutions like spin-torque transfer magnetic random access memory (STT-MRAM), shown in Figure 10. STT-MRAM suffers froma lower overall information density and is more expensiveto scale, but does not depend upon precision domain wallmovement. Both technologies require high current den-sities for bit manipulation, which is a disadvantage forspintronic memory competing against conventional mem-ories. Regardless, new discoveries in the field of spin-tronics are bound to result in technologies that will have

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FIG. 10. Image of a basic STT-MRAM cell borrowed fromS. A. Wolf et al. [16]. In the above, “BL” corresponds tothe bit line, “SL” corresponds to the signal line, and “WL”corresponds to the write line.

lasting impacts on the field of computer hardware.

VII. CONCLUSION

Magnetic racetrack memory is an exciting new tech-nology that has many fundamental advantages over cur-rent RAM, HDDs, and SSDs. Its non-volatility, highread/write speeds, and potential for scalable ultra densememory make it an attractive type of memory. If effec-tively implemented, racetrack memory has the potentialto become a universal memory (i.e. a memory devicethat is fast enough to compete with RAM, cheap andnon-volatile enough to compete with HDD, and can serveas the sole unit of memory in a computer). Such memory

has the potential to greatly simplify computer architec-ture, effectively eliminating the need to cache data.

The potential of magnetic racetrack memory also hasbroad implications for society, prominently in cuttingdown the worlds consumption of energy for computing.In 2006, data centers in the US, excluding personal andbusiness computers, required 6.9 Gigawatts of power, orthe equivalent of 98,000 barrels of oil per day [17]. As thetotal power consumption from computing is increasingyearly, it is important to develop greener technologies. Asignificant percentage of used power is from RAM, whichneeds continuous power to maintain the charge on ca-pacitors that store bits. Magnetic random access mem-ory technologies are fundamentally able to retain datawhile unpowered and could pave the way towards faster,cleaner computing in the future.

The operation of racetrack memory relies on efficientread elements (such as magnetoresistive tunnel junc-tions), low current density write elements (such as spin-torque transfer elements from spin-polarized currents),and a means by which domains storing the bit informa-tion, or magnetization state, can be moved along thelong, thin ferromagnetic racetrack. Read and write el-ements are relatively well developed, but there is work tobe done in researching domain wall dynamics. Specif-ically, reliable control of domain walls through longerracetracks and current densities associated with movingdomain walls and writing bits are important issues facingthis device.

Due to broad interest from both academia and indus-try, it is of critical importance to identify the under-lying physics behind the physics of new ferromagneticheterostructures so that materials may be engineered forimproved memory storage applications. Development ofpractical applications using this novel structure is ex-pected to advance the computing industry in a direc-tion unimagined by CMOS roadmaps. Future work isinvaluable for improving the operation and feasibility ofracetrack memory and understanding other applicationsof this exciting new technology.

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ACKNOWLEDGMENTS

The author gratefully acknowledges Professor Ross forguidance and interesting discussions of future magneticmemory technologies.