Review
Current-driven magnetization switching and domain wall motion in nanostructures—Survey of recent experiments

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Abstract

An overview of recent experimental studies and new routes in the field of current-driven magnetization dynamics in nanostructured materials is given. The review introduces the basic concepts (Landau–Lifshitz phenomenology, critical current, spin currents in relation to spin accumulation, adiabatic/non-adiabatic spin-torque) and describes the main results of recent experiments on current-driven magnetization reversal within vertical pillar-like nanostructures and current-driven domain wall motion within laterally confined specimens. While for the pillar systems a discussion is provided of how the introduction of layers with perpendicular magnetic anisotropy, tunnel barriers and exchange bias and(or) oxide layers can be used to reduce the critical current densities for current-induced switching, the role of perpendicular anisotropy, use of spin valve structures, diluted magnetic semiconductors and epitaxial materials to increase the domain wall velocities are reviewed in the case of current-driven domain wall movement within lateral systems.

Introduction

The discovery of interlayer exchange coupling [1], [2] that describes the coupling between two ferromagnetic thin films separated by a non-ferromagnetic spacer layer (trilayer or spin-valve) and the giant magneto-resistance (GMR) effect that is given by the change of electric resistance observed when the relative orientation of the two magnetizations of the ferromagnetic films is changed [3], [4], led to a reactivation of interest in magnetic thin film devices. While in [3] a monotonic decrease of the interlayer coupling as function of the spacer thickness was reported, it was found later by Parkin et al. [5] that the coupling exhibits an oscillatory character. It is interesting to note that this oscillatory nature was predicted theoretically already 25 years earlier by Bardasis et al. [6] and that the first experimental work on spin-valves dates back to the work of Néel who already reported the coupling of ferromagnetic films over a non-ferromagnetic spacer layer, however without detecting the GMR response [7].

At first the GMR effect was mostly studied within current in-plane (CIP) systems, for which the electric current flows in the plane of the magnetic trilayer or multilayer sample, but later also vertical structures for which the current flows perpendicular to the plane (CPP geometry) of the stack of layers were employed. The routine fabrication of structured samples for GMR investigations founded a basis for exploring further effects of nanomagnetism.

The first prediction and experimental verification of spin-torque-driven magnetization dynamics dates back to the work of Berger in the 70s and 80s. He predicted current-driven domain wall movement [8] and later provided the experimental proof [9]. However, the current pulses used to drive the domain walls within thin films had magnitudes of 45 A being impractical to a direct application, and so the effect did not attract much attention. The starting signal to again study the effects of electric currents on the magnetization dynamics was given by a theoretical work of Slonczewski [10] and, independently, by Berger [11], this time considering nanostructured ferromagnets. They predicted that spin-polarized currents may act on a magnetic layer, exerting a spin-torque. After the first experimental verification of current-induced magnetization reversal by Katine et al. [12], many investigations followed. One should note that the original argumentation of Slonczewski and Berger is quite general: as angular momentum is conserved in a system containing itinerant electrons and local magnetic moments, every divergence of a spin-polarized current (spin current) of itinerant electrons will be accompanied by an opposite and equal change of angular momentum of the local moments (spin-torque). In contrast to the GMR effect, for which the orientation of the magnetization influences the electric properties of the sample, spin-torque describes the effect of a spin-polarized current on magnetic moments. Nowadays, experimental verification of the spin-torque has been carried out in magnetic nanowires (lateral geometry) as well as in vertical spin valve pillar structures.

The present review does not aim to give a complete overview about theory and experimental results on spin-torque-driven dynamics. There exist excellent overview papers discussing the topic on a detailed basis (see e.g. the paper by Stiles and Zangwill [13], the paper by Stiles and Miltat [14] or the review by Ralph and Stiles [15]). The aim of the present review is rather to provide a summary of recent work and new approaches with a focus on experimental studies. While the first part is dedicated to discuss the general aspects of current-driven magnetization dynamics, the following sections discuss recent progress in employing spin-torque within vertical pillar-like structures (Section 3, see [16] for a detailed discussion of such structures) and lateral structures (Section 4, see [17] for a detailed overview of domain wall motion).

Section snippets

Influence of spin-polarized currents on magnetic moments: general aspects

The use of spin-polarized currents to influence or even switch the orientation of magnetic layers provides a tool to manipulate the magnetization without external magnetic fields. Although many investigations still use applied fields, the aim is to completely avoid them in application to realize purely current-driven devices [18], [19].

Critical current

For the application of current-induced switching, integration with existing electronic technology would require switching current densities of less than 105–106 A/cm2[19], [36]. From a stability analysis of the LLG equation including the spin-torque term it is possible to derive the critical current threshold Ic for which the free layer’s orientation becomes instable. Assuming that the polarizing layer has an in-plane easy axis of magnetization (for other situations, see Sections 3.2 Polarizers

Spin-torque in lateral structures: current-induced domain wall motion

The prediction that it should be possible to move domain walls within ferromagnets using a spin-polarized current was made by Berger [62]. The principal possibility of driving domain walls using currents was confirmed experimentally for L-shaped elements [63], [64], for ring-shaped [65], [66] and straight [67] structures with constrictions, and in multilayer wires [68]. While most studies were performed on Py, also magnetic semiconductors [69], [70], [71] and materials with out-of-plane

Conclusion

Recent results on spin-torque-driven magnetization reversal in vertical pillar systems as well as in lateral structures were discussed. The most important quantities which determine the use of spin-torque devices within application are (i) the critical current density needed and (ii) the velocity of the magnetization dynamics. The latter is given by the time for magnetization reversal for the vertical structures, while it is given by the domain wall velocity for the lateral ones. The summary of

Acknowledgements

The author acknowledges support by the Deutsche Forschungsgemeinschaft, Sfb 491 and by the Alexander von Humboldt Foundation through the Feodor Lynen program. Fruitful discussions with C. Hassel and O. Posth, P. Landeros, D. L. Mills and S. Mangin are acknowledged.

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