Controlled switching of ferromagnetic wire junctions by domain wall ...

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IEEE TRANSACTIONS ON MAGNETICS, VOL. 39, NO. 5, SEPTEMBER 2003

Controlled Switching of Ferromagnetic Wire Junctions by Domain Wall Injection Colm C. Faulkner, Dan A. Allwood, Michael D. Cooke, Gang Xiong, Del Atkinson, and Russell P. Cowburn

Abstract—The switching of submicrometer ferromagnetic wire junctions is investigated by controlled injection of domain walls (DWs). A three-terminal continuous Ni80 Fe20 structure is described, consisting of two input wires and one output wire. Separate structures were fabricated by focused ion beam (FIB) milling, to inject either zero, one, or two DWs to the junction inputs. Introduction of DWs to the junction was performed using one or two DW injection pads with low switching fields. Hysteresis loops measured by magnetooptical Kerr effect (MOKE) magnetometry on the device output wires showed the coercivity of the output is strongly dependent on the number of DWs incident at the junction. By injecting either one or two DWs into the junction, it is possible to switch the output wire at two distinct field values, each markedly lower than the nucleation field of the junction. Results presented are relevant to the future development of spintronics DW logic systems, and for fundamental studies into DW resistance and interaction between DWs and wire morphology. Index Terms—Domain walls (DWs), focused ion beam (FIB), magnetic wires, magnetization reversal.

I. INTRODUCTION

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ESEARCH into magnetism has long been driven by the demands of the data storage industry, where understanding the magnetization reversal mechanisms of single domain elements between two stable states is critical. However, due to advances in fabrication methods of nanoscale magnetic elements and more advanced magnetization measurement techniques, new spintronic devices that offer increased functionality are emerging [1]. For example, all-metallic submicrometer NOT gates and shift registers have recently been demonstrated [2]. These devices move domain walls (DWs) in a controlled manner at propagation fields below the nucleation field of the whole device. Due to wire shape anisotropy, magnetization lies parallel to the wire sides and magnetization switching in these wires is effected by domain wall motion. The devices rely on a defined direction of DW motion. Assigning a logical “1” to wire magnetization being in the same direction as DW motion and logical “0” to magnetization opposing DW motion means two Boolean logic states are clearly defined, as previously described [2]. However a full logic architecture requires further elements including three-terminal logic gates with either AND or OR functionality. Combination of these structures affords the possibly of processing, as well as simply storing magnetic information. Manuscript received January 3, 2003. This work was supported by Eastgate Investments Ltd. The authors are with the Department of Physics, Science Laboratories, University of Durham, Durham DH1 3LE, U.K. (e-mail: R.P.Cowburn@ durham.ac.uk). Digital Object Identifier 10.1109/TMAG.2003.816247

Fig. 1. Schematic of junction geometry. I and II are DW inputs, and III is the output wire. Tapering inputs to 125 nm wide at the junction was found to improve junction performance.

Much current work has focused on the properties and propagation of DWs in various nanostructures. Recently, propagation of individual DWs in submicrometer NiFe wires was reported on [3]–[5], with mobility and velocity measurements also being taken [3], [5]. The extraordinary Hall effect has been used to study the propagation of DWs through Pt–Co–Pt trilayer structures [6]. Magnetic force microscopy has elucidated DW behavior in constricted NiFe wires [7] and the domain configurations in mesoscopic ferromagnetic wire junctions [8]. In this paper, controlled propagation of 180 domain walls through three-terminal junctions, at low fields, is described. Introduction of DWs in these structures, and propagation of the walls at fields lower then the measured nucleation field, allows structures to be characterized for possible device applications. II. EXPERIMENT The structures of interest are three-terminal devices (Fig. 1) with two input wires meeting at a junction region, merging to form one output wire. 5-nm-thick thermally evaporated NiFe films were deposited on 5 mm Si(100) substrates. A focused beam of 30 keV Ga ions, at a beam current of 10 pA, was used to define the lateral dimensions of structures [9]. Higher beam currents, up to 20 nA, were used to mill away all magnetic material surrounding the structures. Wires were 5 nm thick, typically 200 nm wide, although the input wires are tapered to 125 nm wide at the junction region, which is submicrometer in dimension. A long output arm was fabricated for ease of magnetometry measurements [Fig. 2(a)–(c)], and the whole structures were 20–25 m long. Nanomagnetometry was performed using a magnetooptical Kerr effect (MOKE) magnetometer, which was operated in longitudinal mode with p-polarized laser light [2], [10]. The magnetometer can measure magnetization reversal events in a single nanowire. The laser spot m spot size on the surface of the sample. was focused to a Samples were mounted on a stepper-motor - stage, which allowed the structure to be positioned relative to the laser spot

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FAULKNER et al.: CONTROLLED SWITCHING OF FERROMAGNETIC WIRE JUNCTIONS

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Fig. 2. FIB images of typical three-terminal structures fabricated, with (A) zero, (B) one, and (C) two DWs being injected into the junction in a horizontal field. Directly below each image is a hysteresis loop corresponding to the type of structure shown. Hysteresis loop (D) was taken at the junction to maximize signal—loops from padless structure inputs and outputs had the same switching field. Loops (E) and (F) were taken with the laser spot focused at the end of the output arm.

with submicrometer precision. A DW may be introduced into a planar nanowire at a field lower than the wire nucleation field, with a pad of magnetic material called a DW injection pad [11]. A DW injection pad has a lower coercivity than a nanowire due to its larger width and hence lower shape anisotropy, and injects a head-to-head DW into the nanowire when it switches. Structures with zero, one, and two injection pads [Fig. 2(a)–(c)], respectively, were used to inject zero, one, and two DWs into the junctions. Hysteresis loops of structures with zero, one, and two DWs injected were measured in a 27-Hz horizontal magnetic field. Loops were measured at the injection pads, at the structure junction, and on the output arms. Sets of structures were fabricated on the same substrates so that differences in their switching behavior could confidently be ascribed to the number of DWs incident upon the junction. III. RESULTS AND DISCUSSION First, the switching behavior of padless structures [Fig. 2(a)] . was studied to estimate the nucleation field of the junction represents the maximum field value at which these structures are suitable for controlled DW propagation device applications. Hysteresis loops from all places on padless structures Oe on structure inputs, juncwere square, with tion and outputs, and loops had the same squareness coefficient throughout the structure [Fig. 2(d)]. This indicates the reversal mechanism is likely to be DW nucleation followed by rapid DW propagation and reversal of the entire structure. Once a DW is nucleated at the part of the structure having the lowest coercivity, the whole structure reverses. No DW pinning effects are evident from these hysteresis loops. Secondly, structures with the same junction and output arm morphology as padless structures, but with 1 DW injection pad [Fig. 2(b)], were characterized. For a DW injection structure, a DW is typically injected from the pad into the input wire at 40 Oe or below. The propagation field of a DW has been measured at values as low as 11 Oe in focused ion beam (FIB) milled planar nanowires [5]. Therefore, the input wire magnetization reverses. The DW moves through the small kink in the input

wires close to the injection pads, as previously demonstrated [4], and propagates to the junction region. Output wire switching behavior in the cases of one or two DWs incident at the junction differs greatly. If one DW is injected, the DW waits at the junction, unable to switch the output arm at low field values. However, at higher fields, as the Zeeman applied field energy term increases, and for the structure under consideration at a field value Oe, the structure output arm switches [Fig. 2(e)]. The sharp transitions and high squareness of the loop suggest that the output is switched by smooth motion of one DW, and indicate that this is a reproducible switching event. Note Oe Oe , the output switching field is that markedly lower than in a padless device. The junction geometry is symmetrical, so a DW from either input arm has the same influence on output arm coercivity. Finally, for a two DW injector structure [Fig. 2(c)], the device Oe, just over the inoutput wire typically switches at jection field of the pads [Fig. 2(f)]. Once two DWs are waiting at the junction region together, they combine to switch the output at a markedly lower field than one DW. The junction region may be thought of as posing an energy , it is barrier to DW propagation. Because suggested that upon application of a sufficiently large field the DW at the junction expands outwards from the junction, eventually becoming depinned and reversing the output. A single DW input to the junction overcomes this energy barrier at a field , . It is assumed the DW expands from the much lower than input arm, right across the junction region before propagating further. The very low output wire switching field for the two , suggests that the two DWs link together, enDW case, abling propagation across the junction at a much lower field. The measurement of pinning of two DWs at the junction may be limited by the experimental method, and could be under the 40-Oe injection field of the pads. We would expect intuitively that two DW switching behavior with either parallel or antiparallel DW chirality would be different, but we are unable to image DWs or directly measure their chirality with the current magnetometer. Injection pads are used only to test the switching prop-

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erties of junctions. For device applications, inputs would be directly linked to other gates or read-in units, with outputs linked to other gates or read-out units. Since the measurement of output switching behavior may be limited for this study by DW pad injection fields, other mechanisms of DW injection are currently being examined. Initial tolerance data for structures with zero, one, or two DW injectors, combined with analysis of scanning electron microscopy (SEM) micrographs, suggests that the magnetization switching properties reported in this paper are reproducible and are not due to defects or fabrication anomalies. We are currently investigating ways to adapt the three-terminal structure studied here to perform logical AND/OR operations. A biased alternating magnetic field may be required, similar to existing magnetic AND-like demonstrations for obtaining shifted hysteresis loops and memory effects in L-shaped wires [12]. Field values reported in this paper are similar to those needed for ferromagnetic NOT gate and shift register operation, which is encouraging for possible future integration of logic gates. Wire dimensions reported in this paper are also similar to ferromagnetic NOT gate wire dimensions. A biased rotating field may enable integration of all future gates. Recent DW velocity measurements in submicrometer structures, of similar dimensions to those reported here [5], indicate that for the field values reported in this paper, DW velocities are of the order of 1000 m/s. IV. CONCLUSION We have shown that by injecting either zero, one, or two DWs, via DW injection pads into identical ferromagnetic wire junctions, we can control the switching field of the output wire. Structures with no DW inputs switched at a high magnetic field, for the structure measured 155 Oe. Structure outputs with one DW input switched at an intermediate field, 65 Oe. A DW was pinned at the junction region until sufficient field was applied to switch the output. Structures output wires with two DW inputs switched at the DW injection field of 40 Oe. Square hys-

teresis loops with well-defined transitions indicate reproducible switching behavior. The ability to tailor devices with high nucleation fields and low DW propagation fields is important for future device applications. This switching behavior suggests the three-terminal structures may be suitable for integration into magnetoelectronic logic circuits in the future. In addition to DW logic devices, the ability to control DW propagation through junction regions of ferromagnetic nanowires is important for fundamental studies, such as DW resistance and interaction between DWs and wire morphology [13]. REFERENCES [1] S. A. Wolf et al., “Spintronics: A spin-based electronics vision for the future,” Science, vol. 294, p. 1488, 2001. [2] D. A. Allwood et al., “Submicrometer ferromagnetic NOT gate and shift register,” Science, vol. 296, p. 2003, 2002. [3] T. Ono et al., “Propagation of a magnetic domain wall in a submicrometer magnetic wire,” Science, vol. 284, p. 468, 1999. [4] R. P. Cowburn, D. A. Allwood, G. Xiong, and M. D. Cooke, “Domain wall injection and propagation in planar permalloy nanowires,” J. Appl. Phys., vol. 91, p. 6949, 2002. [5] D. Atkinson et al., “Magnetic domain wall dynamics in a submicrometer ferromagnetic structure,” Nature Mater., vol. 2, p. 85, 2003. [6] J. Wunderlich et al., “Influence of geometry on domain wall propagation in a mesoscopic wire,” IEEE Trans. Magn., vol. 37, p. 2104, July 2001. [7] K. Miyake, K. Shigeto, K. Mibu, T. Shinjo, and T. Ono, “Geometrical confinement of a domain wall in a nanocontact between two NiFe wires,” J. Appl. Phys., vol. 91, p. 3468, 2002. [8] A. Hirohata et al., “Influence of crystalline structure on the domain configurations in controlled mesoscopic ferromagnetic wire junctions,” J. Appl. Phys., vol. 91, p. 7308, 2002. [9] G. Xiong, D. A. Allwood, M. D. Cooke, and R. P. Cowburn, “Magnetic nanoelements for magnetoelectronics made by focused-ion-beam milling,” Appl. Phys. Lett., vol. 79, p. 3463, 2001. [10] R. P. Cowburn, D. K. Koltsov, A. O. Adeyeye, and M. E. Welland, “Probing submicron nanomagnets by magneto-optics,” Appl. Phys. Lett., vol. 73, p. 3947, 1998. [11] K. Shigeto, T. Shinjo, and T. Ono, “Injection of a magnetic domain wall into a submicron magnetic wire,” Appl. Phys. Lett., vol. 75, p. 2817, 1999. [12] D. A. Allwood et al., “Shifted hysteresis loops from magnetic nanowires,” Appl. Phys. Lett., vol. 81, p. 4005, 2002. [13] Y. Labaye, L. Berger, and J. M. D. Coey, “Domain walls in ferromagnetic nanoconstriction,” J. Appl. Phys., vol. 91, p. 5341, 2002.