Vortex Induced by DC Current in a Circular Magnetic Spin Valve Nanopillar

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IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 4, APRIL 2012 1297

Vortex Induced by DC Current in a Circular

Magnetic Spin Valve Nanopillar

L. J. Chang

1;2

, Pang Lin

2

, and Shang Fan Lee

1

Institute of Physics, Academia Sinica, Taipei 115, Taiwan

Department of Materials Science & Engineering, National Chiao Tung University, Hsinchu 300, Taiwan

We investigate the characteristic of vortex states induced by a dc current in trilayer circular magnetic nanopillars experimentally. Our samples are designed such that both the soft and hard layers can contain a magnetic vortex and the interlayer is thick enough such that only dipolar interaction is important. The relative chiralities between the two layers’ vortex configurations as functions of external field and current are studied with various diameters. We also perform micromagnetic simulation on the magnetization reversals to compare with the magnetoresistance (MR) curves measured at low current and with current induced magnetization states. The current induced magnetization behaviors due to the spin transfer torque effect and the additional Oersted field are clearly identified on the resistance behavior.

Index Terms— Magnetic vortex state, spin transfer torque.

I. INTRODUCTION

T

HE magnetic vortex state in spin valve ferromagnetic disk has become the focus of investigation [1]–[3] due to both fundamental interests and industrial applications. Several of the promising applications on vortex based devices including spin-tronics memories [4] and spin torque nano oscillators [5], [6] have been proposed. Thus, understanding the characteristics of the magnetic vortex under different external conditions is one of the main requirements for application of nanotechnology on related industry. The most interesting properties of nano disk shape magnetic materials are the formation of the two distinct states, a vortex state and a uniform state, during magnetization reversal. The vortex state is a closed curling magnetic structure around the outer part with a core at the center in the ferromag-netic disk pattern with the core magnetization pointing out of the plane. The vortex chirality can be either clockwise or counter-clockwise and core polarity can be either direction out of plane. The vortex state has magnetic flux closure lines thus shows high stability and generates the smallest magneto static interaction between adjacent layers. Recently, the dynamics of vortex core magnetization excited by an electrical current has been intro-duced. The magnetic vortex core in nanoscale spin valve struc-ture can be excited into persistent microwave frequency oscil-lations by a spin polarized dc current [5]. The oscillation of magnetic vortex is corresponding to the core being set into a steady circular motion around the center of the disk. Locatelli

et al. [1] studied the dynamics of two coupled vortices driven

by spin currents. Above a threshold current, a highly coherent microwave signal can be observed, with dependence on the rela-tive polarities of the vortices. However, the detail of nucleation vortices configuration in response to spin transfer torque and

Manuscript received August 15, 2011; accepted October 13, 2011. Date of current version March 23, 2012. Corresponding author: S. F. Lee (e-mail: leesf@phys.sinica.edu.tw).

Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMAG.2011.2173670

current-induced Oersted field, which is a key for constructing a vortex based memory, has been lacking. Here, we investigate the characteristic of vortex states induced by a perpendicular dc current injection in trilayer circular magnetic nanopillars. The giant magnetoresistance (GMR) profile and the data of current induced magnetization switching have been measured. We have observed the behavior of nucleation of vortex state in disks with various diameters of samples. The micromagnetic simulations with the OOMMF code [7] were performed for analyzing the orientation of magnetizations.

II. EXPERIMENTALMETHODS

The samples of circular nanopillars having 90, 160, 380, and 450 nm diameters were fabricated by elec-tron-beam lithography and ion beam etching. The size of nanopillar was checked from the Scanning Electron Microscope (SEM) image. Multilayers with structure were first deposited onto oxidized silicon sub-strates at room temperature by magnetron sputtering as sketched in Fig. 1(a). The thickness of the sample was calibrated indepen-dently by an atomic force microscope (AFM). The estimated error on the thicknesses and lateral dimension was about 5 per-cent. The films were then patterned into desired shape. Top and bottom electrode leads were fabricated for four-probe resistance measurements. Fig. 1(b) shows an optical microscope image of one sample surrounded by insulation and the four leads. Fig. 1(c) shows a Scanning Electron Microscope (SEM) image of a typical sample with 160 nm diameter before the top electrode was put on. The electrodes cover the whole diameter of the samples. To understand the interplay between the vortex chiralities and the influence by vertical current injection, we used magnetoresistance (MR) measurement results to compare with the micromagnetic simulation, in which a unit cell of was used. The typical material parameters for

Permalloy are saturation magnetization ,

exchange stiffness constant , damping

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1298 IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 4, APRIL 2012

Fig. 1. (a) Sketch of the disk spin valve nanopillar stack. (b) Optical Micro-scope image of our device circuit. The light blue circular isSiO and the sample is located at the center. (c) SEM image of a spin valve nanopillar sample with diameter 160 nm.

the Landau-Lifshitz-Gilbert equation with a spin momentum transfer term of Slonczewski’s theory [10].

III. RESULTS ANDDISCUSSIONS

In Fig. 2, we present the MR signal when the external field was applied in plane to the film of the circular nanopillar with different diameters. The magnetization configurations of thin and thick NiFe layer in the reversal process have been inves-tigation by micromagnetic simulation (see inset of Fig. 2). We observed that the variation of the MR profiles were markedly sensitive to the size of disk. For the circular nanopillar with 160 nm diameter, see Fig. 2(a), we observed the MR curve showing single transition between low resistance (saturation field), where the top and bottom magnetizations are parallel, to high resis-tance state (remanent field), where the magnetizations are an-tiparallel due to dipolar interaction. The MR curve is almost identical for both increasing and decreasing fields, suggesting a dominating dipolar interaction. Decreasing from saturation at 1000 Oe, the increase in the resistance occurs at 318 Oe, the plateau of maximum resistance is reached at magnetic field of 200 Oe with a MR ratio of 2.4%. At 240 Oe, the resistance shows sudden falling to saturation magnetization state. The data of 90 nm disk presented similar transition behavior with MR ratio of 1.5%. The remanent states of magnetic moments are favorable for the uniform, anti-parallel coupled single domains between the top and bottom magnetic layers with narrow diam-eter of disks. For the sample of 380 nm diamdiam-eter, in which the MR ratio is 1.6% as seen in Fig. 2(b), we found double transition between the switching processes. The first high resistance state is reached at 175 Oe when decreasing the magnetic field from saturation. The magnetization of the two NiFe layers undergoes a direct change from uniform parallel to anti-parallel alignment due to dipole interaction. As the field is swept to negative values, the MR is switched at 60 Oe to an intermediate resistance state corresponding to nucleation of c-shape configuration in both

Fig. 2. Magnetoresistance curves of the samples with diameter (a) 160 nm (b) 380 nm. Micromagnetic simulations show the states of magnetization for the indicated external fields.

magnetic layers with opposite chirality. The second transition of magnetic state occurs at 125 Oe and results in a high re-sistance value. The thick layer is in a vortex state at this region. Since the stray field from the thick layer vanishes after the nu-cleation of vortex state, the magnetization of the thin layer is no longer affected by the dipolar interaction from the thick layer. As a result, the orientation of magnetization in the thin layer be-comes parallel to the external field to decrease the Zeeman en-ergy. As the negative external field becomes stronger, the vortex core is moved from the center of the sample to the rim, then the resistance changes to a value corresponding to a uniform parallel alignment between the thin and thick layers. In this experiment, the vortex state is favorable for the large diameter (diameters 380 and 450 nm) and large thickness (NiFe 24 nm) layers of the samples in response to the magnetic field.

To study the characteristics of the samples in response to the spin transfer torque and the additional current-induced Oersted field, we performed the experiments of current perpendicular to structure plane induced switching processes by sweeping the dc current from 0 to 10 mA then back to 0 mA at zero external magnetic field. In our convention, a positive current corresponds to the electrons flowing from the thin layer to thick layer. The initial states were realized by external field sweeps from satura-tion to the remanent states. In Fig. 3, we show the variasatura-tion of the resistance versus current curve measured with nanopillar of 160

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CHANG et al.: VORTEX INDUCED BY DC CURRENT IN A CIRCULAR MAGNETIC SPIN VALVE NANOPILLAR 1299

nm diameter at zero fields, starting from the uniform anti-par-allel configuration between the two magnetic layers (resistance of 2.42 ). Interestingly, we found irreversible variation of re-sistance dependence on the sign of the apply current. When the current is increased in the positive direction, we observe the re-sistance is reversed at to a lower resistance state. This implies the injection of a perpendicular dc current produces an Oersted field that can lead to the nucleation of vortices con-figuration in both magnetic layers with parallel chirality. The transition from uniform anti-parallel to vortex state with parallel chiralities is associated with equal to about 65% of the full MR. This loss of MR is attributed to a loss of spin accumulation in the core region due to transverse spin diffusion [8]. When the current is swept back to zero value, the low resis-tance state is conserved until , then the resistance re-turn to the initial state. By resetting the configuration of magne-tization to uniform anti-parallel state with field sweep, followed by the injection of negative current, we observed two step re-versals in the switching processes. The resistance is decreased at to the first low resistance state corresponding to . According to the previous experiments with the switching of nanopillars [9], we have confirmed the electrons from the thicker NiFe layer produces a spin polarized current that can apply a torque to the thinner NiFe layer towards the par-allel alignment. Therefore, the first low resistance state at nega-tive current are majorly controllable by the current through the spin transfer torque effect. In this situation, the magnetization of top layer reverses to the opposite direction, simultaneously the bottom layer switches from the uniform state to the vortex state. As the current increases to , we observe the second sharp decrease in resistance of . The circumferential field tends to switch the thin NiFe layers into a vortex state, again the vortices configuration in both magnetic layers are with parallel chirality. When the current is swept back from 10 to 0 mA, the resistance jumps to the intermediate

re-sistance state of 2.39 at .

The configuration of magnetization is similar to the first step, the thick layer contained a vortex state and was stable even at zero current, the vortex in thin layer was annihilated at low neg-ative current. It indicates the vortex state is less favorable for thin NiFe layer than the uniform state. We clearly demonstrate the processes to change resistance state in spin valve circular nanopillars by perpendicular dc current injection in both direc-tions with significant dependence on the direction of current. The finial resistance state at zero current and field can be iden-tified from the sign of injection dc current. This behavior of re-sistance switching can provide an advantageous approach to the development of spintronic devices.

In Table I we list the critical fields and currents for the vortex nucleation and annihilation in the soft layer and hard layer. From the magnetization switching by applied magnetic field, we ob-served the formation of vortex states in the hard layer with larger diameters. From the hysteresis loop of current driven measure-ments, the vortex state exists with all the sample geometries in both soft and hard layers. It can be seen that the range of vortex state, , increased with increasing diameter in both layers. It indicated the stability of vortex state strongly depended on the diameter of the disk.

Fig. 3. Resistance versus current curves obtained with the uniform anti-parallel state at zero external field in the 160 nm diameter sample. Micromagnetic sim-ulations show the configurations for the indicated perpendicular d. c. current.

TABLE I

MAGNETIZATIONSTATES OF THELAYERS OFSAMPLESUNDEREXTERNAL

MAGNETICFIELDS ANDAPPLIEDCURRENTS. WHEN THEVORTEXSTATES

EXIST,THEVORTEXNUCLEATION ANDANNIHILATIONFIELDS(H ; H )AND

CURRENT(I ; I )ARELISTED

IV. SUMMARY

The characteristic of vortex states induced by a dc current in trilayer circular magnetic nanopillars was studied carefully. From magnetoresistance measurements, the relative chiralities of the top and bottom layer vortices can be revealed. When we compared our results with the micromagnetic simulation, the current induced magnetization behaviors due to the spin transfer torque effect and the additional Oersted field in samples with different diameters can be well understood. The vortex state is less favorable for thin NiFe layer than the uniform state. There-fore the current-induced switching is required to form vortex state in thin layers. We also found the finial resistance state at zero field depended on the original sign of injection dc current, which is a promising candidate for a memory cell for future non-volatile data storage devices.

ACKNOWLEDGMENT

This work was supported in part by the National Science Council and the Academia Sinica, Taiwan, Republic of China.

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1300 IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 4, APRIL 2012

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