Detection of the domain structure change using magnetotransport for a series of circular Permalloy dots

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Detection of the domain structure change using magnetotransport for a series of

circular Permalloy dots

T. Y. Chung and S. Y. Hsu

Citation: Journal of Applied Physics 99, 08B707 (2006); doi: 10.1063/1.2172888

View online: http://dx.doi.org/10.1063/1.2172888

View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/99/8?ver=pdfcov Published by the AIP Publishing

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Detection of the domain structure change using magnetotransport

for a series of circular Permalloy dots

T. Y. Chunga兲and S. Y. Hsu

Institute of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan 共Presented on 2 November 2005; published online 25 April 2006兲

The magnetic structure of a magnetic dot depends on its geometrical factors such as thickness and dot size and can be either a multidomain state or a vortex state. The magnetoresistance共MR兲 of a series of magnetic dots with diameters ranging from 0.3 to 5␮m was measured to investigate the correlation between domain-structure and magnetotransport properties. A dot with a diameter of less than 2␮m has a domain of vortex state at remanence and demonstrates similar reversible MR results. Moreover, the behaviors of MR show clear changes corresponding to the domain-structure change from vortex to multidomain states with an increasing dot diameter. Data can be qualitatively described by the anisotropic magnetoresistive effect. Hence, our results show that the magnetotransport can be a tool to detect the magnetic domain structure of a submicron magnetic dot. © 2006 American Institute of Physics.关DOI:10.1063/1.2172888兴

I. INTRODUCTION

Recent advances in the nanofabrication methods have made the possibility of studying the magnetism at a small length scale, in which can be potential applications in high density recording and modern magnetoelectronic devices. The magnetic reversal process in circular,1 square,2or other shape dots3 has been studied for a while by ␮-MOKE,1–4 magnetic force microscopy 共MFM兲,4–6 scanning tunneling microscopy 共STM兲,7 Lorentz microscopy,8 and electron holography.9 For circular dots of soft magnetic material, it has been found that the short range exchange energy is more important than the long range magnetostatic energy in deter-mining the magnetization configurations when the dimension is decreased. Between the multidomain and single-domain states the flux closure state is generated during the reversal process and is called the vortex state. From the practical viewpoint, the studies of magnetoresistance 共MR兲 are very important. Transport phenomena may occur for structures in sufficiently reduced dimensions. Up to now, almost all MR studies in nanostructures are focused on nanowires10 and rings.11The MR study of a single dot is very rare due to the probing difficulty. Recently, Vavassori et al.12 reported the MR measurement in a circular Permalloy dot with a diameter of 1␮m and a thickness of 25 nm. In their device, four 10 nm Au leads were arranged underneath the dot at four corners for electrical contacts resulting in a nonuniform cur-rent distribution and uncertain dot domain reversal pro-cesses. Here we used a simple design of electric contact con-figuration to obtain MR of a single submicron magnetic dot. In this work, the MR of a series of different size Permalloy dots was measured to investigate the correlation between domain-structure and magnetotransport properties.

II. EXPERIMENTAL DETAILS

Our samples were prepared by standard e-beam lithog-raphy, thermal evaporation, and lift-off techniques. The

cir-cular Permalloy dots have a thickness of 45 nm and diam-eters of 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, and 5␮m, respectively. In order to make an electrical measurement, several identical dots were distributed evenly atop a 30 nm thick Au strip with a width same as the diameter of the dots. Contact configuration was arranged for a four-terminal elec-trical measurement. Figure 1 shows a scanning electron mi-croscopy image of one of our samples. A device contains numerous areas of different dot sizes. The image exhibits that the dots keep a completely disklike shape and the sepa-ration between the neighboring dots in each area is about the same as the dot diameter.

Magnetic structure and magnetotransport measurements were performed. The former was obtained using a magnetic force microscope 共Nanoscope Dimension 3100兲 in the tapping/lift mode. The magnetic configurations were imaged at a lift height of 100 nm by commercial CoCr coated Si cantilever tips. The latter was performed in a pumped 4He cryostat and at the center of a superconducting magnet sole-noid. For the electrical measurement, several dots in series were used to increase the signal to noise ratio instead of a single dot. Single dot behavior can be extracted simply by using Kirchhoff’s circuit theorem. A chain of N dots can be treated as N + 1 Au square sheets共dot spacing兲 in series with N combinative resistors of a dot and Au sheet in parallel. An independent experiment of a sequence of N confirmed such circuit analysis.13 Hence, electrical transport of a single dot is easily obtained using such contact configuration.

a兲_{Electronic mail: siky.ep86@nctu.edu.tw}

FIG. 1. Scanning electron microscopy共SEM兲 image of one sample. Mea-suring current is applied along the long Au strip and resistance is measured between two neighboring vertical Au contacts. In this device, four sections are arranged for studies of bare Au and Permalloy dots with diameters of 1, 0.6, and 0.3␮m共from left to right兲.

JOURNAL OF APPLIED PHYSICS 99, 08B707共2006兲

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III. EXPERIMENTAL RESULTS AND DISCUSSION It has been known that the magnetic structure of a mag-netic dot depends on its geometrical factors such as thickness and dot size. To check the magnetization state of our 45 nm thick Permalloy dots we investigated the domain structure using MFM at room temperature. Figure 2共a兲 shows the MFM images of two samples with diameters of 5 and 0.8␮m, respectively, at remanence. Prior to image scanning, dots were magnetized in an opposite out-of-plane field to prevent magnetic configuration of the dots from being dis-torted by the stray field of the tip.6As seen from Fig. 2共a兲, a contrast spot is at the center for a 0.8␮m dot corresponding to the turned-down vortex core, while a complicated combi-nation of dark and bright areas is present for a 5␮m dot. In Fig. 2共b兲 the arrow lines for local spin are sketched to show the difference between these two domain structures. These results can also be confirmed by the magnetic moment be-haviors. Corresponding MH curves of the same size dot ar-rays are plotted in Fig. 2共c兲. The sudden loss of magnetiza-tion close to zero field for the smaller dot array is very characteristic of the formation of the vortex state. MFM and magnetization moment investigations show that dots with a diameter of less than 2␮m are in the vortex states and a dot with a diameter of 5␮m is in multidomain state, at rema-nence.

Before discussing the magnetotransport results of the single dot, we would like to point out that the MR of the

bottom Au layer served as electrical contacts has a very rare effect on the top magnetic dots. The characteristics of the MR of the Au layer follow the B2law due to deflection of the moving carrier by Lorentz force. In our measuring field range, −50 kOe艋H艋50 kOe, the MR ratio of Au is about 0.002%/square and is relatively small compared with that of the magnetic dot.

We measured the MR of a series of different diameter Permalloy dots, from 0.3 to 5 ␮m, in different field orienta-tions. The MRs of three dots with diameters of 0.6, 1, and 5 ␮m are plotted in Fig. 3. Here, ⌬R is defined as R共H兲−R共Hsaturation兲 and MR is defined as ⌬R/R共Hsaturation兲. Figures 3共a兲, 3共c兲, and 3共e兲 are ⌬R_⬜curves with a magnetic field applied perpendicular to the dot plane. The signs of MR_⬜of all samples are negative independent of the dot di-ameter. However, there are systematic changes in the MR_⬜ curves with the dot diameter. A clear hysteretic loop appears for the 5␮m dot while reversible loops for the two other samples. This is evidence that magnetotransport is sensitive to the domain structure. From MFM and magnetization mea-surements, a sample with a diameter of less than 2␮m has a vortex state at remanence. At saturation field all local mo-ments are aligned with a field resulting in a 90° angle relative to measuring current and a lowest net resistance. Let␣ rep-resent the angle between magnetization and measuring cur-rent. As the field is reduced, some moments start to lie in the dot plane resulting in different␣values共␣⫽90°兲. Based on the anisotropic magnetoresistive 共AMR兲 effect, resistance

FIG. 2.共a兲 MFM images of two dots with diameters of 5 共left兲 and 0.8␮m 共right兲, respectively. 共b兲 The arrow line sketches to show two domain struc-tures in共a兲. 共c兲 Normalized magnetization moments as a function of applied field H of two dot arrays with diameters of 5共left兲 and 0.8 共right兲␮m.

FIG. 3. The MR behaviors of three different diameter Permalloy dots. 0.6 关共a兲 and 共b兲兴, 1 关共c兲 and 共d兲兴, and 5␮m 关共e兲 and 共f兲兴, respectively, at T = 5 K. The left figures关共a兲, 共c兲, and 共e兲兴 are ⌬R_⬜where the field is perpen-dicular to the film plane and the right figures关共b兲, 共d兲, and 共f兲兴 are ⌬R储where

the field is parallel to both film plane and measuring current.

08B707-2 T. Y. Chung and S. Y. Hsu J. Appl. Phys. 99, 08B707共2006兲

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change is proportional to cos2_␣ _{and, hence, resistance is a} maximum at remanence where all moments lie in the dot plane forming a vortex state. The magnetization curve is re-versible and so is the MR_⬜ curve. The scenario was still observed in a 0.3␮m Py dot. A sample with a diameter larger than 2 ␮m has a multidomain structure at remanence. The hysteretic MR curve corresponds to a hysteretic MH curve.

We also checked the evolution of MR储 where the

mag-netic field is applied along the measuring current in the dot plane. As shown in Figs. 3共b兲, 3共d兲, and 3共f兲, ⌬R储is positive.

At saturation field, all local moments are aligned with cur-rent resulting in ␣= 0° and a maximum net resistance. The systematic correlation between magnetization moment and magnetotransport is similar to MR_⬜. There is slightly hyster-esis in the MR储 loops in Figs. 3共b兲 and 3共d兲 due to the

dif-ferent entrances of the vortex core when H is parallel to the dot plane. Recent work on MR储of a 1␮m Permalloy dot12is

in consistence with our results. A slight difference in shape may be caused by the contact configuration.

The reversible MR behaviors observed in submicron dots with a vortex state can be qualitatively attributed to the ordinary AMR effect.12 MR is reversible corresponding to the reversible change between two stable states, the single-domain state at saturation field and the vortex state at rema-nence. Quantitative analysis is still in process. When the dot size is increased and approaches to the critical length, where the magnetostatic energy overcomes the domain wall energy, the multidomain state becomes a preferably stable configu-ration at remanence and the clear hysteretic loops appear in the MR. This is a very clear evidence for the occurrence of transition from the vortex state to the multidomain state by the electrical transport investigation.

In summary, a special electrical contact configuration was designed for the MR study of any single submicron magnetic dot. The MFM images and magnetization

measure-ments confirm that our 45 nm thick Permalloy dots can have the vortex state for a diameter of less than 2␮m and the multidomain state for larger dots. The behaviors of MR de-pend on the domain structures. The clear change in the MR shape occurs when the domain structure changes from vortex to multidomain states. Hence, our results imply that the mag-netotransport can be a tool to detect the magnetic domain structure of a dot.

ACKNOWLEDGMENTS

MFM images were taken in the advance storage thin film laboratory of Dr. C. H. Lai. Magnetic moment measurements were made in the superconducting and magnetism laboratory of Dr. J. Y. Lin. This work was supported by the NSC of Taiwan grant under Project Nos. NSC94-2112-M-009-030 and NSC94-2120-M-009-002.

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Resis-tance across two voltage leads is equal to 共N+1兲RAu+ N共1/RAu

+ 1 / R_dot兲−1_{where R}

Aucan be obtained by measuring a bare Au strip.

08B707-3 T. Y. Chung and S. Y. Hsu J. Appl. Phys. 99, 08B707共2006兲