Detection of Domain Wall in a Permalloy Wire Using a Semiconductor and Ferromagnetic Hybrid Structure


Yoshiaki Sekine1, Tatsushi Akazaki1, and Junsaku Nitta2,3
1Physical Science Laboratory, 2Tohoku University, 3CREST-JST

 Using the local Hall effect (LHE), we have succeeded in detecting a magnetic domain wall (DW) trapped in a permalloy, NiFe, wire and in clearly distinguishing whether the DW structure is a tail-to-tail or a head-to-head DW [1]. 
 In the field of spintronics, devices utilizing DWs were proposed and many methods were applied to detect DWs.  Among them, the LHE method using a semiconductor and ferromagnetic hybrid structure has the advantage of large signal enough to investigate the DW dynamics.  An InGaAs two-dimensional electron gas (2DEG) that is 5 nm below the surface was used for a Hall sensor that detects the stray field from the NiFe wire.  A 60-nm-thick and 300-nm-wide NiFe wire consisting of a notch at the center, a taper end and a right-angle end was deposited on the surface.  These wire structures make it possible to trap a tail-to-tail or a head-to-head DW at the notch with changing the direction of the magnetic field, B.  Three Hall crosses were fabricated just below both ends and the center of the wire.  Figures 1(a) and (b) show a scanning electron microscopy (SEM) image and a cross-sectional view of the sample, respectively.  A tail-to-tail DW and a head-to-head DW are sketched in Fig. 1(c) and (d).  With sweeping B parallel both to the wire and the 2DEG, the DW nucleates at the right-angle end, then the DW displaces to and pins at the notch.  With changing B furthermore, the DW depins from the notch, then the DW moves to and annihilates at the right-angle end.  The B-dependence of the Hall resistivity, r yx, on three Hall crosses is shown in Fig. 2.  Here, r yx1, 2, 3 are defined as ryx on the right-angle end, notch, and taper end, respectively.  With increasing B, ryx1 shows the sharp drop at 22 mT, which corresponds to the DW nucleation at the right-angle end.  At 26 mT, rapid increase of ryx3 represents the DW annihilation at the taper end.  Between 22 and 26 mT, ryx2 shows the peak, which corresponds to the pinned tail-to-tail DW.  With decreasing B, the sharp changes in ryx1 and 3 represent the DW nucleation and annhilation.  Between -22 and -26 mT, the dip of ryx2 corresponds to the pinned head-to-head DW.  Note that the DW nucleates at the right-angle end with sweeping B.  The LHE method can make the distinction between the tail-to-tail and head-to-head DWs.  These results indicate the LHE method is attractive for investigating the DW movement that is a key to DW devices.

[1] Y. Sekine, et al., AIP Conference Proceedings 893 (2007) 1291.

Fig.1. (a) The SEM image of the sample.  (b) The cross-sectional view of the sample.  The schematic view of the (c) tail-to-tail and (d) head-to-head DWs.
Fig.2. The Hall resistivity, ryx, vs the magnetic field, B.  The peak and dip in ryx2 correspond to the trapped tail-to-tail and head-to-head DWs.

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