APPLIED PHYSICS LETTERS 89, 192504 共2006兲
Control of domain wall pinning by a switchable magnetic gate Masahiro Haraa兲 and Junya Shibata Frontier Research System, RIKEN, Wako, Saitama 351-0198, Japan
Takashi Kimura and Yoshichika Otani Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
共Received 29 August 2006; accepted 22 September 2006; published online 6 November 2006兲 Magnetically coupled domain wall pinning has been investigated by means of an attached pair of ferromagnetic wires. The magnetic configuration of the paired wires 共parallel or antiparallel兲 can be controlled by applying an external magnetic field along the wires. The strength of the pinning due to the magnetic interaction between the domain wall and the paired wires shows a significant difference between the parallel and antiparallel configurations, which is well reproduced by a micromagnetics simulation. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2385224兴 Recent progress in microfabrication techniques enables us to confine and manipulate a domain wall inside a submicron scale ferromagnet. Motion of the domain wall in a ferromagnetic wire behaves as a classical nanoparticle confined in a narrow capillary and is useful for a future domain wall logic circuit.1 In this sense it is important to investigate various methods for the control of the domain wall motion. An artificial constriction 共notch兲 in the wire acts as a pinning of the domain wall.2–12 An asymmetric triangular structure in the wire causes a unidirectional motion.13,14 However, the operation depends on the shape of the notch or the triangle and is not controllable after the sample preparation. In the present work, we demonstrate a controllable domain wall pinning by means of an attached pair of ferromagnetic wires. The strength of the pinning can be tuned by changing the magnetic configuration of the “magnetic gate.” We fabricated a NiFe wire 共width: 180 nm, thickness: 30 nm兲 for a domain wall propagation with two orthogonally contacted magnetic wires, which we hereafter call magnetic gate, on the surface of a GaAs/ AlGaAs two-dimensional electron gas 共2DEG兲 cross, as shown in Fig. 1. A domain wall is nucleated in the large pad in the left hand side and propagates in the ferromagnetic wire from the left to the right. The large pad in the upper wire of the magnetic gate makes a difference in the switching field between the upper and bottom wires so that we can control the magnetic configuration of the magnetic gate by sweeping the in-plane magnetic field along the vertical 共y兲 direction. The density n and mobility of the 2DEG before processing were 3.1 ⫻ 1015 m−2 and 67 m2 / V s, respectively. The depth of the 2DEG plane from the surface is 65 nm. The width of the active region in the 2DEG cross W is about 1.5 m. The stray field from the ferromagnetic wire and the magnetic gate is detectable by measuring a Hall resistance of the 2DEG cross.15,16 Hence we can monitor a domain wall pinning and a magnetic configuration of the magnetic gate. We can apply an in-plane magnetic field in the horizontal 共x兲 and the vertical 共y兲 directions independently by the use of a cross-coil magnet system. The measurement was carried out at 1.3 K. The magnetic configuration of the magnetic gate can be controlled by sweeping an in-plane magnetic field in the y direction. The switching field in the upper wire with a large a兲
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pad is smaller than the other since a domain wall is easily nucleated in the large pad under a smaller magnetic field. Figure 2 shows a change in the Hall resistance as a function of in-plane magnetic field By. We clearly observe a transition in the Hall resistance between the four types of the magnetic configurations of the gate, as schematically illustrated in the inset of Fig. 2. Hence, we can realize the parallel configuration, A 共↑↑兲 or C 共↓↓兲, when we apply the magnetic field above the switching field of the bottom wire ⬃50 mT and then turn off the magnetic field. In contrast, we can set the antiparallel configuration, B 共↓↑兲 or D 共↑↓兲, by reducing the magnetic field from the amplitude between the switching fields of both wires. The change in the Hall resistance corresponds to the net magnetic flux from the ferromagnetic wires in the 2DEG cross. The local Hall signal is roughly given by RH =
1 ⌽ , ne W2
共1兲
here ⌽ is the total flux of the stray field from the ferromagnets. The magnetic flux of the stray field generated by a single ferromagnetic wire in plane of the 2DEG is estimated to be ⬃2 ⫻ 10−15 Wb by a simple magnetostatic calculation. The change in the Hall resistance according to the estimation is about 2 ⍀, which is comparable to the experimental value. An in-plane magnetic field along the horizontal 共x兲 direction brings about a nucleation and a propagation of a domain wall in the horizontal ferromagnetic wire. We measured a Hall resistance as a function of in-plane field Bx for differ-
FIG. 1. 共a兲 Scanning electron microscopy 共SEM兲 picture of the device. A domain wall is nucleated in the left large pad and propagates towards the gap of the magnetic gate. The stray field from the domain wall and the magnetic gate in the 2DEG cross are detectable by measuring a local Hall resistance of the 2DEG. 共b兲 An enlarged SEM picture around the center of the 2DEG cross.
0003-6951/2006/89共19兲/192504/3/$23.00 89, 192504-1 © 2006 American Institute of Physics Downloaded 06 Nov 2006 to 134.160.214.75. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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FIG. 2. 共Color online兲 Hall resistance as a function of in-plane field in the y direction. The offset due to an asymmetry of the 2DEG cross is subtracted. The magnetization of the horizontal NiFe wire was aligned along the +x direction. Inset: the four types of the magnetic configurations of the magnetic gate.
ent settings of the magnetic configuration of the magnetic gate. For the antiparallel configuration B 共↓↑兲, we find a clear step signal in the trace for both sweeping directions, as shown in Fig. 3共a兲, which indicates that a domain wall is trapped at the gap of the magnetic gate due to a magnetic interaction between the domain wall and the magnetic gate. The domain wall is nucleated and is pinned by the magnetic gate at ⬃17 mT, and then depinnes from the trap at ⬃30 mT. On the other hand, for the parallel case A 共↑↑兲, there is a very narrow step only for the one sweeping direction, as shown in Fig. 3共b兲. The depinning field of the domain wall for the parallel case is comparable to or smaller than the nucleation field. Table I shows depinning fields for the all magnetic configurations of the magnetic gate. The results reveal that the strength of the pinning by the magnetic gate is larger for the antiparallel configurations. We simulated a domain wall behavior in the ferromagnetic wire with the magnetic gate by the OOMMF package17 in order to analyze the difference in the pinning strength between the parallel configurations and the antiparallel configurations. The simulation is limited within 1.2⫻ 1.2 m2 region in the center of the device for a reduction of the simulation time. The mesh size of the simulation and the damping constant ␣ are 3 nm and 0.02, respectively. We considered the initial situation that there is a domain wall in the left half of the ferromagnetic wire. A vortex wall is more stable than a transverse wall for the geometry of the ferromagnetic wire in the experiment. Figure 4 shows a magnetization process for the parallel configuration A 共↑↑兲 when increasing a magnetic field along the wire. A propagation of the vortex wall driven by the magnetic field is blocked due to a magnetic interaction between the magnetic gate and the domain wall, as shown in Fig. 4共a兲. An increase of the magnetic field pushes the wall into the center of the device and then the vortex wall becomes a transverselike wall. Subsequently, the domain wall escapes from the trap by the magnetic gate and propagates towards the right edge of the wire. For the antiparallel configuration B 共↓↑兲, the domain wall is also trapped by the magnetic gate, as shown in Fig.
FIG. 3. 共Color online兲 Hall resistance as a function of in-plane field in the x direction. The magnetic configuration of the magnetic gate is set to be 共a兲 AP共B ↓ ↑ 兲 and 共b兲 P共A ↑ ↑ 兲. A domain wall pinning appears in a change in the Hall resistance.
5共a兲. The contribution of the magnetic gate is more effective than that for the parallel case so that the propagation of the domain wall is still blocked even above B = 20 mT. Under a larger magnetic field, another vortex is nucleated in the right side of the initial vortex, as shown in Fig. 5共b兲. An annihilation of the initial vortex and a propagation of the new vortex
TABLE I. Depinning fields for the all magnetic configurations of the magnetic gate. The pinning by the magnetic gate is stronger for the antiparallel 共AP兲 configurations. A 共P↑↑兲 共mT兲 B 共AP↓↑兲 共mT兲 C 共P↓↓兲 共mT兲 D 共AP↑↓兲 共mT兲 H-H DW T-T DW
18 ⬍17
32 30
22 ⬍17
31 27
FIG. 4. 共Color online兲 Micromagnetics simulation for the parallel configuration A 共↑↑兲. 共a兲 Bx = 10 mT. 关共b兲 and 共c兲兴 Bx = 16 mT. Downloaded 06 Nov 2006 to 134.160.214.75. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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In summary, we have investigated a controllable domain wall pinning by a magnetic gate attached with the ferromagnetic wire. The strength of the pinning exhibits a significant difference depending on the magnetic configuration of the magnetic gate. A micromagnetics simulation reveals the domain wall behavior interacting with the magnetic gate and well reproduces the change in the strength of the pinning. This method is applicable for a switch in a logic device based on domain wall motion. The authors would like to thank Y. Iye, S. Katsumoto, A. Endo, and K. Tsukagoshi for their kind help with sample preparation and measurements. 1
FIG. 5. 共Color online兲 Micromagnetics simulation for the antiparallel configuration B 共↓↑兲. 共a兲 Bx = 20 mT. 关共b兲 and 共c兲兴 Bx = 25 mT.
result in a full alignment of the magnetization in the ferromagnetic wire. The depinning field in the simulation 共25 mT兲 is larger than the value for the parallel case 共16 mT兲, which shows a good agreement with the experimental results. The vortex nucleation plays an essential role in the magnetization process for the antiparallel case. Therefore it is important that the contact between the ferromagnetic wire and the magnetic gate is narrow in order to make a significant difference in the depinning fields since a wider contact causes a nucleation of a vortex under a smaller magnetic field.
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