Measurements of Magnetic-Domain-Wall Velocity Using Local Hall Effect

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Yoshiaki Sekine1 and Junsaku Nitta2,3
Device Physics Laboratory1, Tohoku University2, CREST JST3

@The velocity of a magnetic domain wall (DW) in a NiFe wire has been obtained measuring the time dependence of the stray fields from both ends of the wire. Logic devices utilizing the motion of a DW were recently demonstrated and intensive investigations on the dynamics of a DW have been carried out. In this study, we focus on the local Hall effect (LHE) to deduce the velocity of a DW because the LHE method has the advantages of large signal, independency of the shapes of magnet, and flexibility of changing temperature. Using this LHE method, the DW velocity of 250 m/s at 77 K has been obtained and these results are useful for DW devices.
@In the magnetization reversal process of a NiFe wire, a DW nucleates at one end of the wire and displaces to the other end. Then, the DW and annihilates at the other end. The nucleation and annihilation of the DW result in the reverse of the direction of the stray field. Therefore, time in which the DW passes through the wire corresponds to a period between times when the directions of the stray fields from both ends of the wire change. Figures 1(a) and (b) shows a schematic cross-view and an optical microscopy image of the sample, respectively. The stray fields from both ends of the wires with different length were detected by the local Hall probes. We used an InGaAs two-dimensional electron gas and an external field was applied parallel both to the wire and the 2DEG. Figure 2(a) shows the time dependence of the Hall voltages, VH1 and - VH2, at 77 K. The rapid increase of VH corresponds to the reverse of the stray field direction caused by the nucleation and annihilation of the DW. With increasing the length of the wire, the period between the nucleation and annihilation times increases. The period (t) as a function of the wire length (L) is plotted in Fig. 2(b). The dotted line represents the velocity of 250 m/s. From this relation between t and L, the DW velocity of 250 m/s is deduced. These results show the LHE method is attractive for investigating the DW dynamics that is a key to DW devices.

[1] Y. Sekine and J. Nitta, in Narrow Gap Semiconductors, Inst. Phys. Conf. Ser. No 187, edited by J. Kono and J. Léotin (Taylor & Francis, New York, 2006), p. 461.

Fig. 1 (a) a schematic cross-view (b) an optical microscopy image of the sample. The stray field is detected with the InGaAs local Hall probe.
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Fig. 2 (a) the time dependence of VH. Rapid increases of VH correspond to the nucleation and annihilation of the DW. With increasing L, t increases. (b) t as a function of L. The dotted line represents the velocity of 250 m/s. From these results, the velocity of 250 m/s on NiFe is obtained.

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