Domain Wall Propagation and Pinning Induced by Current Pulses in Cylindrical Modulated Nanowires

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Domain Wall Propagation and Pinning Induced by

Current Pulses in Cylindrical Modulated

Nanowires

C.Bran1, J.A. Fernandez-Roldan2, J. A. Moreno3, A. Fraile Rodríguez4,5, R. P. del Real1,

A. Asenjo1, E. Saugar1, J. Marqués-Marchán1, H. Mohammed3, M. Foerster6, L. Aballe6,

J. Kosel3,7, M. Vazquez1, O. Chubykalo-Fesenko1

1 Instituto de Ciencia de Materiales de Madrid, 28049 Madrid, Spain

2 Helmholtz-Zentrum Dresden-Rossendorf e.V., Institute of Ion Beam Physics and Materials

Research, Bautzner Landstrasse 400, 01328 Dresden, Germany

3 King Abdullah University of Science and Technology, Computer Electrical and Mathematical

Science and Engineering, Thuwal 23955-6900, Saudi Arabia.

5 Departament de Física de la Matèria Condensada, Universitat de Barcelona, Barcelona, 08028,

Spain

Institut de Nanociencia i Nanotecnologia (IN2UB), Universitat de Barcelona, Barcelona, 08028,

Spain

6 ALBA Synchrotron Light Facility, CELLS, Barcelona, 08290, Spain

7 Sensor Systems Division, Silicon Austria Labs, Villach 9524, Austria

Abstract

The future developments of three-dimensional magnetic nanotechnology require the

control of domain wall dynamics by means of current pulses. While this has been

extensively studied in planar magnetic strips (planar nanowires), few reports exist in

cylindrical geometry, where Bloch point domain walls are expected to have intriguing

properties. Here we report this investigation in cylindrical magnetic Ni nanowires with

geometrical notches. Experimental work based on synchrotron X-ray magnetic circular

dichroism (XMCD) combined with photoemission electron microscopy (PEEM) indicates

that large current densities induce domain wall nucleation while smaller currents move

domain walls preferably against the current direction. In the region where no pinning

centers are present we found domain wall velocity of about 1 km/s. The domain wall

motion along current was also detected in the vicinity of the notch region. Pinning of

domain walls has been observed not only at geometrical constrictions but also outside

of them. Thermal modelling indicates that large current densities temporarily raise the

temperature in the nanowire above the Curie temperature leading to nucleation of

domain walls during the system cooling. Micromagnetic modelling with spin-torque effect

shows that for intermediate current densities Bloch point domain walls with chirality

parallel to the Oersted field propagate antiparallel to the current direction. In other cases,

domain walls can be bounced from the notches and/or get pinned outside their positions.

We thus find that current is not only responsible for the domain wall propagation but is

also a source of pinning due to the Oersted field action.

Keywords: cylindrical magnetic nanowires; domain wall dynamics; 3D nanomagnetism;

XMCD-PEEM; micromagnetic modelling.

Introduction

Cylindrical magnetic nanowires provide versatile functionalities for data and energy

storage, sensing, magnetic nanocircuits or magneto-mechanical actuators.1 They are

promising candidates as building blocks for novel three-dimensional nanotechnology.2

The future implementation of such technology requires manipulation of magnetism in

cylindrical magnetic nanowires by means of low power consumption stimuli such as

electric currents. Spintronics is widely recognized within the scientific/technological

community as candidate for future energy-saving nano-applications.3 Comparatively to

the use of external fields, current-induced magnetization dynamics offer more energy

efficiency. However, unlike planar nanowires, spintronic-based manipulation of

magnetism in cylindrical magnetic nanowires has not yet been developed despite their

high potential for high storage density and other novel multifunctionalities.

Magnetic DWs are expected to play a decisive role as information carriers in magnetic

circuits and thus manipulating their dynamics by means of electrical currents is important

for future developments.4,5 Cylindrical symmetry gives raise to new possible magnetic

configurations.6–10 Typical nanowires investigated experimentally, with diameters above

50 nm, present two types of domain walls (DWs)11–13: vortex-antivortex (VAV) and the

Bloch point (BP). The dynamics of both DWs is expected to be different from that of

planar nanowires14,15. For example, DWs in cylindrical geometry have been predicted

not to suffer from the Walker breakdown phenomenon, characteristic for the planar

geometry, and thus potentially very high velocities, above 1000 m/s, have been

theoretically predicted. 15,16 If these velocities can be achieved experimentally it is still an

open question.

Although the DW dynamics is well studied in planar magnetic nanowires (prepared by

lithography), in cylindrical geometry only a scarce number of articles have reported

experimental measurements. Ivanov et al17 measured the motion of 3D domain walls by

simultaneous application of field and current in bi-segmented Co/Ni nanowires,

estimating DW velocity as few hundreds meter per second. Schöbitz et al18 have

observed current-induced domain wall motion in Ni-based nanowires by Magnetic Force

Microscopy (MFM) and X-ray Magnetic Circular Dichroism (XMCD) combined with

Photo-Emission Electron Microscopy (PEEM) measurements18 estimating velocity up to

600 m/s. On the other hand, simulations show that during the current-induced dynamics

the BP DW may be converted into the VAV domain wall, limiting its velocity. Additionally,

the Oersted field was predicted to play an important role, being the source of DW

transformation and dynamics, even without the direct action of the spin-transfer torques

.15,16

Furthermore, if the BP DW velocities are found to be as high as theoretically predicted,

the control of DW pinning will be an important aspect towards implementation of

spintronics based on cylindrical nanowires. This may be achieved by creating special

notches designed to stop their propagation. While the use of notches to pin DWs is well

established in planar geometry19,20, an efficient control of DW pinning under applied field

in cylindrical nanowires has not yet been achieved .21–23

In this article we investigate the motion of DWs in Ni cylindrical nanowires with specially

designed notches. To compare their effect with straight nanowires, we produced them

only in one part of NW, leaving the other one free of defects. Significantly, while our

experiment is successful in terms of nucleation, motion, and pinning, we also

unexpectedly observe DW motion in the direction parallel to the current, i.e. against the

electron flow. Our simulations including the spin-torque effects and the Oersted field

assist in understanding the current-induced dynamics of DWs in the presence of

notches. They show that DWs can be scattered from the notches and propagate in the

opposite direction. Importantly, we identify a new DW pinning mechanism when the

Oersted field with rotational sense opposite to initial BP DW can be a source of its pining

outside the defect region.

Experimental. Cylindrical Ni nanowires with geometrical notches and high aspect ratio

were grown by electrodeposition into the pores of anodic alumina membranes (Figs.

S1(b)-Supplementary Information).24 They were removed from alumina membranes by

chemical etching and deposited onto a Si substrate. Finally, they were contacted with Au

electrodes to allow the injection of electric current (Figs. S1(c-d)-Supplementary

Information). More details about the fabrication and contacting procedures are given in

the Supplementary Information. The SEM images of a contacted nanowire with the main

diameter of about 100 nm and 13 m length is displayed in Fig. 1. The geometry is

schematically shown in the top panel. The close-up SEM images of Fig. 1 (b)-(c)

correspond to the marked green and orange areas in Fig. 1 (a). The nanowire shows

modulations/notches at the left side (area marked in green) and a uniform cylindrical

geometry at the right side (area marked in orange).

Figure 1. Schematic view of a contacted Ni nanowire (top panel). (a) SEM image of a contacted Ni

nanowire with notches, marked by arrows, at the left-side end, (b) close-up SEM image of the green

marked area in (a), (c) close-up SEM image of the orange marked area in (a).

Figure 2 presents XMCD-PEEM images of the same nanowire (NW) acquired at the Ni

L3-edge with the X-ray incidence at about 45o to the nanowire axis, i.e., sensitive to both

parallel and perpendicular magnetization components with respect to the NW axis. Fig.

2 (a) shows the NW in a single domain magnetic state as indicated by the uniform

dark/bright XMCD contrast in the NW and its shadow25, respectively. In the following, we

will refer to the contrast of the NW itself when we write “dark” or “bright” contrast.

In Fig. 2 (b) the NW is imaged after applying a current pulse of 1.1x1012 A/m2 for 8 ns,

with left polarity, that is large enough to break the magnetization into a multidomain state,

as seen from the XMCD contrast, dark/bright/dark/bright/dark. There are five magnetic

domains oriented antiparallel/parallel/antiparallel/parallel/antiparallel to the polarization

vector (i.e. along the NW axis with three DWs pinned at the notches labelled (3), (4) and

(6) and one DW pinned in the region of the NW with uniform diameter.

After the application of a second current pulse to the magnetic state shown in Fig. 2(b)

with the same polarity but smaller amplitude, 1.5x1011A/m2, the state in Fig. 2(c) is

obtained. The magnetic images suggest that the DW initially pinned at the notch (6) has

displaced towards the right, i.e., parallel to the electron flow (note that no pinning center

is present at that point). Concerning the DWs initially pinned at notches (3) and (4), while

the natural direction of motion is that the one pinned at the position (3) would displace to

the right, the imaging does not allow us to exclude the situation in which the DW (4)

would propagate to the left, i.e. opposite to the electron flow. The overall result is that

only one main domain wall is visible in Fig.2(c). A further current pulse of 2.3x1011A/m2

with the same polarity, applied to the magnetic state imaged in Fig. 2(c), pushes the DW

from left to the right, finally developing into a single domain state (Fig. 2(d)).

Although we have only one data set for the current density of 2.3x1011A/m2 where the

DW has propagated outside the NW, we can estimate the lower bound of the DW

velocity. Note that we have not seen additional DW nucleation at this current density.

Neither we expect a depinning of an additional DW from the right end of the NW since

the pinning there is strong and even in an ideal case cannot take place without an applied

field.26 Additionally the ends of the NW are kept at 300K and thus thermal depinning of

DWs can be excluded. An estimation of the DW velocity from Fig. 2 (d) for a current

pulse of 2.3x1011 A/m2, applied for 8 ns, gives rise to a value above 1000 m/s, in

agreement with theoretical predictions15,16,27 and higher than previously reported

experimental values.17,18

Domain walls were nucleated again by applying a high-amplitude current pulse,

1.35x1012A/m2 in the opposite sense. The resulting state, visible in Fig. 2 (e) has three

domains and two DWs, one pinned at notch (3) and another at the left of notch (6). Then

a sequence of low current pulses (1.5x1011A/m2) also with opposite sense to the initial

ones was applied to study DW propagation. In this case, both DWs seem to displace

along the NW, parallel to the applied current. In Fig. 2(f) the left-side DW is pinned at

notch (6) and the right-side one has stopped at the right of notch (6) - outside but close

to it. Upon application of a further identical current pulse, the NW saturates, as can be

seen from the uniform bright contrast in Fig. 2 (g).

Additional information for the behavior of the DW movement is presented in Figs. S2 (b)-

(c). In Fig. S2 (b) the NW is imaged before injecting a current pulse. Here we observed

three magnetic domains oriented parallel/antiparallel/parallel to the polarization vector

(i.e. along the NW axis). By injecting an intermediate current pulse of 5x1011 A/m2, the

DWs, one pinned at the notch (6) and the other, outside of the notch (right-side end of

the NW) move along the current, parallel to it, and the NW gets saturated, as can be

observed from the uniform dark contrast in Fig. S2(c). Note that at these intermediate

current densities, thermal depinning can be expected (see below).

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DomainWallPropagationandPinningInducedbyCurrentPulsesinCylindricalModulatedNanowiresC.Bran1,J.A.Fernandez-Roldan2,J.A.Moreno3,A.FraileRodríguez4,5,R.P.delReal1,A.Asenjo1,E.Saugar1,J.Marqués-Marchán1,H.Mohammed3,M.Foerster6,L.Aballe6,J.Kosel3,7,M.Vazquez1,O.Chubykalo-Fesenko11InstitutodeCienciadeMate...

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Domain Wall Propagation and Pinning Induced by Current Pulses in Cylindrical Modulated Nanowires

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