Effect of sub-layer thickness on magnetic and giant magnetoresistance properties of Ni–Fe/Cu/Co/Cu multilayered nanowire arrays☆

Hongzhi Wang,Bo Huang,Huaquan Deng,Haochen Li,Weiguo Zhang*,Suwei Yao

Sugiyama Laboratory of Surface Technology,School of Chemical Engineering,Tianjin University,Tianjin 300072,China

Keywords:Electrochemistry Ni–Fe/Cu/Co/Cu multilayered nanowires Sub-layer thickness Magnetic property Giant magnetoresistance

ABSTRACT Ni–Fe/Cu/Co/Cu multilayered nanowire arrays were electrodeposited into anodic aluminum oxide template by using dual-bath method at room temperature.Scanning electron microscopy and transmission electron microscopy were used to characterize the morphology and structure of the multilayered nanowire arrays.Vibrating sample magnetometer and physical property measurement system were used to measure their magnetic and giant magnetoresistance(GMR)properties.The effect of sub-layer thickness on the magnetic and GMR properties was investigated.The results indicate that magnetic properties of electrodeposited nanowires are not affected obviously by Cu layer thickness,while magnetic layers(Ni–Fe and Co layers)have significant influence.In addition,GMR ratio presents an oscillatory behavior as Cu layer thickness changes.The magnetic and GMR properties of the multilayered nanowire arrays are optimum at room temperature for the material structure of Ni–Fe(25 nm)/Cu(15 nm)/Co(25 nm)/Cu(15 nm)with 30 deposition cycles.

1.Introduction

Since the discovery of giant magnetoresistance(GMR)effect[1],GMR materials have attracted increasing interest due to their unique magnetic properties and extensive applications[2],such as high density magnetic record[3],GMR sensors[4],and magnetic random access memory[5].Two GMR effect geometry models have been studied in recent years:current-in-plane(CIP)GMR and current-perpendicular-to-the-plane(CPP)GMR.Almost all metallic magnetic multilayer films belong to CIPGMR materials,which have been widely commercialized and reach the theoretical limits.It is found that multilayer nanowires of GMR materials exhibit a more significant GMR effect[6].Its structure makes it possible for CPP-GMR measurement with general techniques.Besides,the length of spin diffusion and parameters related to spin scattering of each sublayer metal can be obtained through measuring magnetization properties of multilayered nanowires[7].Therefore,multilayered nanowires are regarded as a promising GMR material.

Generally,GMR multilayered materials are made up of alternating ferromagnetic and nonmagnetic layers.Compared to sputtering and evaporation techniques[8,9],electrodeposition ofGMR multilayered materialis widely accepted due to its simplicity and low cost.Electrodeposited Co/Cu,Ni–Cu/Cu,and Ni–Fe/Cu bilayer films[10–13],and Co/Cu and Ni–Fe/Cu bilayer nanowires[14–17]are widely investigated.GMR materials with tri-layer structures[18]and four layer film structures[19,20]are also studied.However,electrodeposited GMR nanowire materials with four-layer nanowire structures are seldom reported and discussed.

In this study,we investigate the effect of sub-layer thickness on magnetic and giant magnetoresistance properties of Ni–Fe/Cu/Co/Cu multilayered nanowire arrays,which are electrodeposited into the pores of anodic aluminum oxide template with the dual-bath method.Transmission electron microscopy is used to characterize Ni–Fe/Cu/Co/Cu multilayered nanowires with different sub-layer thickness.

2.Experimental

2.1.Electrodeposition of multilayered nanowire arrays

In the experiment,anodic aluminum oxide(AAO)was used as a template for the electrodeposition of Ni–Fe/Cu/Co/Cu multilayered nanowire arrays.Preparation of ordered AAO template was prepared by a two-step anodization technique described elsewhere[21].A layer of Au film was sputtered onto one side of the through-hole template before electrodeposition.Different sub-layer thicknesses of Ni–Fe/Cu/Co/Cu multilayered nanowire arrays were obtained through the dual-bath electrodeposition method by controlling the deposition time at room temperature.The deposition experiment was carried out by CHI600B electrochemistry workstation under three-electrode system with AAO templates as working electrodes,a platinum plate as auxiliary electrode and a saturated calomel electrode as reference electrode.The compositions in the two baths are given in Table 1,with the pH of two baths controlled in a range from 3 to 5.Baths A and B were used to deposit Cu/Co/Cu layer and Ni–Fe layer,respectively.The deposition potential of Ni–Fe,Cu and Co was controlled at-1.0 V,-0.55 V and-0.95 V,respectively.The working electrode was dipped into bath B for deposition of Ni–Fe layer and then transferred to bath A for deposition of Cu/Co/Cu layer.The process was repeated several times.Before moving the electrode from one bath to another,the electrode must be carefully washed with deionized water in ultrasonic field.The alternating deposition cycles(n)in Baths A and B were 30 cycles.All the chemicals used in the experiment were analytical grade.

Table 1 Dual-bath composition in the experiment

The surface morphology of the electrodeposited samples was observed by scanning electron microscopy(SEM,VEGATS-5140SB)and transmission electron microscopy(TEM,JEOL1000).Energy dispersive spectrometry(EDS)was used to determine the composition of the nanowires.The structure of the samples was characterized by X-ray diffraction(XRD,D/MAX-2500).

2.2.Magnetization measurement

The magnetic properties of nanowires were measured by a vibrating sample magnetometer(LDJ 9600)at room temperature,where the applied magnetic field was parallel(FPP)or perpendicular(FIP)to the surface of AAO template.The GMR of the nanowire arrays was measured through the physical property measurement system(Quantum Design PPMS-9)and was calculated by the following equation:GMR(%)=(RH-R0)×100%/R0,where R0is the resistance of the sample in zero magnetic field and RHis the resistance in an applied magnetic field H.

3.Results and Discussion

3.1.Characterization

To obtain the SEM image of the nanowire arrays,the sample was dipped into a solution containing 40 g·L-1NaOH for 2–3 h to dissolve the alumina matrix.SEM and TEM images of the prepared multilayered nanowires are shown in Fig.1.Fig.1(a)shows that all electrodeposited nanowire arrays are arranged in order with almost the same diameter.In Fig.1(b),single nanowire of the arrays presents an alternating light and dark multilayered structure.According to the mass-thickness contrast principle and the results of TEM and EDS,as selected area electron diffraction(SAED)of Sections A and B shown in Fig.1(c)and(d),it is confirmed that the brightest area is Cu layer,the deep dark Region A is Ni–Fe layer with polycrystalline morphology,and the rest dark gray Layer B is Co layer with hexagonal close-packed(hcp)(100)structure.With the deposition time of Ni–Fe,Cu,and Co layers as 300,50,and 100 s,respectively,their thicknesses in Fig.1(b)are 350,40,and 140 nm,respectively.Rates of growth for the Ni–Fe layer,Co layer and Cu layer are determined as 1.167,0.800,and 1.400 nm·s-1,respectively.Thus the specified thickness layer can be prepared by changing the deposition time.

Fig.1.SEM(a)and TEM(b)images of multilayered nanowire arrays with SAED images of Sections A(c)and B(d).

Fig.2 is the XRD pattern of multilayered nanowire array.The hcp(100)diffraction peek of Co is observed at2θ =41.728°.Two diffraction peeks at 2θ values of 43.309°and 74.230°correspond to face centered cubic(fcc)(111)and fcc(220)crystalline planes of Cu crystal,respectively.However,diffraction peek of Fe does not appear in the pattern while fcc(111)and fcc(200)peeks of Ni can be observed at 2θ values of 44.341°and 51.049°.It is inferred that Fe moves to the Ni lattice and forms Ni–Fe solid solution.The diffraction peeks at 38.182°,64.539°,77.557°and 81.734°can be indexed to fcc(111),fcc(220),fcc(311),and fcc(222)of Ag crystal,respectively,which comes from silver conductive adhesive in the sample.

Fig.2.XRD pattern of multilayered nanowire array.

3.2.Magnetic property measurement

3.2.1.Effect of Cu layer thickness on magnetic properties

In order to study the effect of Cu layer thickness on magnetic properties,the thicknesses of Ni–Fe and Co layers(tNi–Fe,tCo)are kept at 25 nm.Fig.3 shows the magnetic hysteresis loops of[Ni–Fe/Cu/Co/Cu]n=30multilayered nanowires at different Cu layer thicknesses(tCu)and M represents magnetization(Oe,1Oe=79.5775 A·m-1).Magnetic parameters including saturated magnetization(Ms,Oe),coercivity(Hc,Oe),remanence(Mr,Oe),and remanence ratio(Mr/Ms)are calculated and listed in Table 2.It indicates that the magnetic properties of nanowires and magnetic anisotropy do not change obviously with Cu layer thickness.However,compared to CIP mode(FPP),the nanowires of CPP mode(FIP)present smaller magnetic coercivity and remanence ratio.

Table 2 Magnetic parameters of the samples with varied t Cu

3.2.2.Effect of magnetic layer thickness on magnetic properties

Fig.3.Magnetic hysteresis loops of samples with n=30 and t Ni–Fe=t Co=25 nm.(a)t Cu=5 nm;(b)t Cu=20 nm;(c)t Cu=30 nm;(d)t Cu=50 nm;— FPP;---FIP.

Fig.4.Magnetic hysteresis loops for samples with n=30 and t Cu=5 nm.(a)t Ni–Fe=t Co=10 nm;(b)t Ni–Fe=t Co=20 nm;(c)t Ni–Fe=t Co=50 nm; — FPP;---FIP.

The magnetic hysteresis loops of[Ni–Fe/Cu/Co/Cu]n=30(tCu=5 nm)multilayered nanowires with different magnetic layer thicknesses are shown in Fig.4.Data of tNi–Fe=tCo=25 nm are shown in Fig.3(a).The magnetic parameters are listed in Table 3.The coercivity increases with magnetic layer thickness and easy magnetization axis is parallel to the direction of nanowires.Large differences exist between FPP and FIP in the remanence ratio of materials,and the anisotropy is the most significant when thicknesses of Ni–Fe and Co layers are 50 nm.Coercivity is mainly caused by domain wall pinning effect[22]and that of multilayered nanowire arrays is in the direction of the easy magnetization axis of the free layer.With the increase of free layer thickness,the internal defects and impurities of materials accumulate,leading to more pinning sites and increasing the coercivity.In addition,we can find that remanence ratios and coercivity of nanowires are smaller with FPP than those with FIP.

Table 3 Magnetic parameters of the samples with varied t Ni–Fe/Co

3.3.Effect of Cu layer thickness on GMR properties

Fig.5 shows the variation in saturated magnetic field,GMR and magnetic sensitivity as the Cu layer thickness varies from 2 nm to 30 nm,when the thicknesses of Co layer and Ni–Fe layer are kept approximately the same(25 nm).The saturated magnetic field,GMR and magnetic sensitivity of nanowires present an oscillatory behavior with the increase of Cu layer thickness,with their maximum values achieving 3000 Oe,-45.2%,and 0.33%·Oe-1,respectively.With the increase of Cu layer thickness,strong scattering of conduction electrons takes place during the transmission process,which reduces the scattering opportunities of conduction electrons on the Ni–Fe and Co layer surface,partly declining GMR.However,the increased Cu layer thickness also weakens the coupling effect between Ni–Fe and Co layers and an appreciable GMR effect could be observed in this respect.Therefore,such dual mechanism may lead to the oscillatory GMR behavior.From the above results,we can reach the conclusion that the best material structure is[Ni–Fe(25 nm)/Cu(15 nm)/Co(25 nm)/Cu(15 nm)]n=30.

4.Conclusions

Highly uniform Ni–Fe/Cu/Co/Cu multilayered nanowire arrays were prepared by dual-bath electrodeposition method.The magnetic properties ofNi–Fe/Cu/Co/Cu multilayered nanowires were determined by the thicknesses of magnetic layers(Ni–Fe and Co layers).However,GMRofthe multilayered nanowire arrays presents an oscillatory behavior with varying thicknesses of Cu layer.The bestmultilayered nanowire array structure obtained is[Ni–Fe(25 nm)/Cu(15 nm)/Co(25 nm)/Cu(15 nm)]n=30.Its maximum GMR value can achieve-45.2%.The saturated magnetic field and magnetic sensitivity value are 3000 Oe and 0.33%·Oe-1,respectively.It is confirmed that the magnetic properties and giant magnetoresistance properties of multilayered nanowire arrays can be improved through changing magnetic layer thickness and non-magnetic layer thickness.Further researches are needed to explain such GMR behavior.