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Vacuum 80 (2006) 949–956

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Effect of annealing on characteristics of Ni48Fe12Cr40/Ni80Fe20 bilayerfilms deposited on Si(1 0 0)/SiO2 by electron beam evaporation

Ping Wu�, Hong Qiu, Yanqing Gao, Fengping Wang, Liqing Pan, Yue Tian

Department of Physics, School of Applied Science, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, China

Received 14 August 2005; received in revised form 31 December 2005; accepted 2 January 2006

Abstract

Ni48Fe12Cr40(7 nm)/Ni80Fe20(40 nm) bilayer films and Ni80Fe20(40 nm) monolayer films were deposited at ambient temperature on

Si(1 0 0)/SiO2 substrates by electron beam evaporation. The effect of annealing on the structure, composition, magnetization and

magnetoresistance of the Ni48Fe12Cr40/Ni80Fe20 bilayer films was investigated. The structure of the Ni48Fe12Cr40/Ni80Fe20 bilayer films

remains stable for annealing temperature up to 280 1C. For the as-deposited bilayer film the introducing of the Ni48Fe12Cr40 underlayer

promotes both the [1 1 1] texture and grain growth in the Ni80Fe20 layer. The annealing promotes the grain growth of the Ni48Fe12Cr40/

Ni80Fe20 bilayer films when the annealing temperature exceeds 280 1C. After annealing at a temperature over 280 1C, Cr atoms inside the

Ni48Fe12Cr40 layer diffuse into the Ni80Fe20 layer and segregate on the surface of the Ni80Fe20 layer. The Ni48Fe12Cr40 underlayer as a

seed layer can enhance the anisotropic magnetoresistance ratio of the Ni80Fe20 layer at a annealing temperature up to 280 1C compared

with Ni80Fe20 monolayer film. After annealing at a temperature over 280 1C, however, the anisotropic magnetoresistance ratio of the

Ni80Fe20 monolayer films exceeds that of the Ni48Fe12Cr40/Ni80Fe20 bilayer films. For all annealing temperatures, the coercivities of the

Ni48Fe12Cr40/Ni80Fe20 bilayer films are smaller than those of the Ni80Fe20 monolayer films.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Ni80Fe20 films; Ni48Fe12Cr40 seed layer; Annealing; Structure; Composition; Anisotropic magnetoresistance

1. Introduction

Ni80Fe20 films have superior soft magnetic properties,high anisotropic magnetoresistance (AMR) and lowmagnetostriction [1,2]. Therefore, they are widely used inmagnetic reading heads and magnetic sensors [3]. The fastspeed and high density of magnetic recording require themagnetoresistance/giant magnetoresistance sensors havinghigh output even as their film thickness is decreased. Muchattention has been paid to obtain the sensor materials withhigh AMR ratio ðDR=RÞ, e.g., Ni80Fe20 thin films. Therecent investigations have found that the microstructureand AMR property of thin Ni80Fe20 films can be improvedby depositing them on suitable buffer or seed layers, suchas Ta [4–7], Si [4], MgO [4], NiCr [6], Al2O3 [5,8] andNiFeCr [6,8–10]. Among these seed layers, Ni48Fe12Cr40seed layers exhibit outstanding effects on the AMR of the

ee front matter r 2006 Elsevier Ltd. All rights reserved.

cuum.2006.01.002

ing author. Tel.: +861062333786; fax: +86 10 62329145.

ess: pingwu@sas.ustb.edu.cn (P. Wu).

Ni80Fe20 films. The Ni80Fe20 films deposited on the suitableNi48Fe12Cr40 seed layers without high-temperature deposi-tion and post-annealing have a much higher AMR ratiocompared with those deposited on the more conventionallyused Ta seed layers [6,10].In practical applications, the thermal stability of

Ni48Fe12Cr40/Ni80Fe20 bilayer films is very important. Inthis paper, the effect of annealing on the characteristics ofthe Ni48Fe12Cr40/Ni80Fe20 bilayer films deposited onSi(1 0 0)/SiO2 by electron beam evaporation has beeninvestigated.

2. Experimental procedure

Ni48Fe12Cr40/Ni80Fe20 bilayer films and Ni80Fe20 mono-layer films were deposited at ambient temperature onSi(1 0 0)/SiO2 substrates by electron beam evaporation. ANi80Fe20 alloy target and a Ni48Fe12Cr40 alloy target wereused. The 400-nm-thick SiO2 layer was prepared bythermal oxidization of the Si(1 0 0) wafer surface. The

ARTICLE IN PRESSP. Wu et al. / Vacuum 80 (2006) 949–956950

Si(1 0 0)/SiO2 substrates with a size about 1.5 cm� 0.5 cmwere ultrasonically rinsed in acetone, in deionized water, inethanol, respectively. Prior to deposition, the workingchamber was evacuated to a pressure lower than5:0� 10�4 Pa. A 7-nm-thick Ni48Fe12Cr40 layer was firstdeposited on the Si(1 0 0)/SiO2 substrate at a rate of0.25 nm/s and then a 40-nm-thick Ni80Fe20 layer wassequentially deposited at a rate of 0.05 nm/s withoutbreaking down the vacuum and a Ni80Fe20 monolayer filmwas simultaneously deposited on another Si(1 0 0)/SiO2

substrate for comparison. During the depositions, thesubstrates were at room temperature. Then the Ni48Fe12Cr40/Ni80Fe20 bilayer films and the Ni80Fe20 mono-layer films were annealed at 150, 200, 280, 330, 400 and500 1C in a vacuum better than 3� 10�4 Pa for 1 h,respectively.

The sheet resistance and the AMR ratio of the films weremeasured using a four-point probe technique at roomtemperature. The structure of the films was determined byX-ray diffraction (XRD) and field emission scanningelectron microscopy (FE-SEM). Magnetic hysteresis loopsof the films were measured at room temperature using analternating gradient magnetometer (AGM). The composi-tions of the Ni48Fe12Cr40/Ni80Fe20 bilayer films wereanalyzed by Auger electron spectroscopy (AES).

(a)

(c)

Fig. 1. Auger electron spectra on the surface of the Ni80Fe20 layers of the Ni48annealed, (c) 330 1C annealed and (d) 500 1C annealed.

3. Results

3.1. Composition

The AES measurements were made on the surface of theNi80Fe20 layer, regions inside the Ni80Fe20 layer andregions inside the Ni48Fe12Cr40 layer for the Ni48Fe12Cr40/Ni80Fe20 bilayer films as-deposited and annealedat 280, 330 and 500 1C.Figs. 1–3 show the AES spectra of the surfaces of

Ni80Fe20 layers, inside the Ni80Fe20 layers and inside theNi48Fe12Cr40 layers for the Ni48Fe12Cr40/Ni80Fe20 bilayerfilms as-deposited and annealed at 280, 330 and 500 1C,respectively. As can be seen from Fig. 1, impurities, such ascarbon and oxygen, are detected on the surfaces of theNi80Fe20 layers for all the samples. For the as-depositedand 280 1C-annealed bilayer films, no chromium is detectedon the surface of the Ni80Fe20 layers. For the bilayer filmsannealed over 280 1C, however, chromium is detected. TheCr contents on the Ni80Fe20 layer surfaces are about 3.4and 5.2 at% for the bilayer films annealed at 330 and500 1C, respectively. As can be seen from Fig. 2, a fewimpurities, such as nitrogen and oxygen, are detected insidethe Ni80Fe20 layers for all as-deposited and annealedNi48Fe12Cr40/Ni80Fe20 bilayer films. However, chromium is

(b)

(d)

Fe12Cr40 (7 nm)/Ni80Fe20 (40 nm) bilayer films: (a) as-deposited, (b) 280 1C

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(a) (b)

(c) (d)

Fig. 2. Typical Auger electron spectra inside the Ni80Fe20 layers of the Ni48Fe12Cr40 (7 nm)/Ni80Fe20 (40 nm) bilayer films: (a) as-deposited, (b) 280 1C

annealed, (c) 330 1C annealed and (d) 500 1C annealed.

P. Wu et al. / Vacuum 80 (2006) 949–956 951

not detected. As can be seen from Fig. 3, for theNi48Fe12Cr40 layers chromium, iron, nickel, nitrogen andoxygen are detected. It can be considered that the oxygenpeaks in the Fig. 3 are mainly attributed to the SiO2 layersunder the Ni48Fe12Cr40 layers because the Ni48Fe12Cr40layers are only 7 nm thick.

3.2. Orientation and structure

Fig. 4 shows the XRD spectra of the as-deposited andannealed Ni48Fe12Cr40/Ni80Fe20 bilayer films. As can beseen from Fig. 4, for the as-deposited film and the filmsannealed at temperatures lower than 500 1C, only theNiFe(1 1 1) diffraction peak appears. When the annealingtemperature reaches 500 1C, the NiFe(1 1 1) and NiFe(2 0 0)diffraction peaks are observed. Furthermore, for the as-deposited film and the films annealed at temperatures lowerthan 330 1C, the NiFe(1 1 1) diffraction peak is weak andalmost independent of the annealing temperature. Whenthe annealing temperature reaches or exceeds 330 1C, theNiFe(1 1 1) diffraction peak increases markedly andbecomes narrow with increasing annealing temperature.It means that the annealing promotes the grain growth of

the Ni48Fe12Cr40/Ni80Fe20 bilayer films when the annealingtemperature is equal to or higher than 330 1C.To clarify the effect of the Ni48Fe12Cr40 layers on the

structure of the Ni80Fe20 layers, XRD spectra of theNi80Fe20 monolayer films as-deposited and annealed underthe same conditions were taken. Fig. 5 shows thecomparison of the XRD spectra of the Ni48Fe12Cr40/Ni80Fe20 bilayer films with those of the Ni80Fe20 mono-layers. For the as-deposited film and the films annealed at atemperature up to 280 1C, the NiFe(1 1 1) diffraction peakof the Ni48Fe12Cr40/Ni80Fe20 bilayer films is higher thanthat of Ni80Fe20 monolayer films. As can be seen from Fig.5, the Ni48Fe12Cr40 layer has an inducement effect on the[1 1 1] orientation to the Ni80Fe20 layer, resulting in a better[1 1 1] textured structure of the Ni80Fe20 layer particularlyto the annealing temperature lower than 330 1C. Whenannealing temperature reaches 330 1C, the (1 1 1) diffrac-tion peak of the Ni48Fe12Cr40/Ni80Fe20 bilayer filmbecomes lower than that of the Ni80Fe20 monolayer film.After the annealing temperature further increases, XRDspectra of both Ni48Fe12Cr40/Ni80Fe20 bilayer film andNi80Fe20 monolayer film are almost the same.Fig. 6 shows FE-SEM microphotographs of the as-

deposited Ni80Fe20 monolayer film, the as-deposited and

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2θ(deg)

Fig. 4. XRD spectra of the Ni48Fe12Cr40 (7 nm)/Ni80Fe20 (40 nm) bilayer

films under the different annealing conditions.

(a) (b)

(c) (d)

Fig. 3. Typical Auger electron spectra inside the Ni48Fe12Cr40 layers of the Ni48Fe12Cr40 (7 nm)/Ni80Fe20 (40 nm) bilayer films: (a) as-deposited, (b) 280 1C

annealed, (c) 330 1C annealed and (d) 500 1C annealed.

P. Wu et al. / Vacuum 80 (2006) 949–956952

annealed Ni48Fe12Cr40/Ni80Fe20 bilayer films. ComparingFig. 6(a) with (b), it is obvious that the Ni48Fe12Cr40underlayer induces larger grains in the Ni80Fe20 layer. Ascan be seen from Fig. 6(b), the surface of as-depositedNi48Fe12Cr40/Ni80Fe20 bilayer film is compact without

voids and the grains are homogeneous and round-shapedwith an average diameter of about 10 nm. Fig. 6(c) showsthat the surface of the bilayer film annealed at 280 1C isslightly blurry with a few shallow voids. Furthermore, asshown in Figs. 6(d) and (e), it is observed that more voidsappear after 330 1C-annealing and then the voids becomebigger and deeper after 500 1C-annealing.

3.3. Sheet resistance, AMR and magnetization

Fig. 7 shows the influence of annealing temperature onthe sheet resistance of the Ni48Fe12Cr40/Ni80Fe20 bilayerfilms. The data of the Ni80Fe20 monolayer films are alsodrawn for comparison. As can be seen from Fig. 7, thesheet resistance of the Ni80Fe20 monolayer films decreasesmonotonously with increasing annealing temperature.Particularly, when annealing temperature increases from280 to 330 1C, the sheet resistance of the Ni80Fe20monolayer films decreases sharply. When annealing tem-perature is higher than 330 1C, the sheet resistance of theNi80Fe20 monolayer film is almost independent of theannealing temperature. For the Ni48Fe12Cr40/Ni80Fe20bilayer films as-deposited and annealed at temperaturesbelow 280 1C, the sheet resistance is almost unchangeablewith annealing temperature and is smaller than that of the

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2θ(deg) 2θ(deg)

2θ(deg)

2θ(deg)2θ(deg)

2θ(deg)

2θ(deg)

Fig. 5. Comparison of XRD spectra between Ni48Fe12Cr40 (7 nm)/Ni80Fe20 (40 nm) bilayer films and Ni80Fe20 (40 nm) bilayer films.

P. Wu et al. / Vacuum 80 (2006) 949–956 953

Ni80Fe20 monolayer films. When annealing temperature ishigher than 280 1C, the sheet resistance of the Ni48Fe12Cr40/Ni80Fe20 bilayer films decreases with increasingannealing temperature and is larger than that of theNi80Fe20 monolayer films annealed at the same tempera-tures. Fig. 8 shows the AMR ratios of the Ni48Fe12Cr40/Ni80Fe20 bilayer and the Ni80Fe20 monolayer films as afunction of annealing temperature. For the films as-deposited and annealed at temperatures up to 280 1C, theAMR ratio of the Ni48Fe12Cr40/Ni80Fe20 bilayer films issomewhat higher than that of the Ni80Fe20 monolayer filmsand the both increase slightly with increasing annealingtemperature. After the annealing temperature increasesfrom 280 to 330 1C, the AMR ratio increases significantlyfor both the Ni48Fe12Cr40/Ni80Fe20 bilayer and Ni80Fe20monolayer films. However, the increase in the AMR ratiofor the Ni80Fe20 monolayer film is much larger than for theNi48Fe12Cr40/Ni80Fe20 bilayer film. Furthermore, theAMR ratio of the Ni80Fe20 monolayer films exceeds thatof the Ni48Fe12Cr40/Ni80Fe20 bilayer films after annealingat temperatures higher than 280 1C. The AMR ratio of theNi80Fe20 monolayer film annealed at 500 1C reaches about

4.1%, which is larger than the AMR ratio of 2.7% for theNi48Fe12Cr40/Ni80Fe20 bilayer film. Lee et al. [6,10]reported that Permalloy/NiFeCr bilayer films depositedby RF diode sputtering, ion beam and dc magnetronsputtering were thermally stable up to about 450 1C. In thepresent work, the results of the AMR ratios indicate thatthe Ni48Fe12Cr40/Ni80Fe20 bilayer films deposited byelectron beam evaporation can be thermally stable up toabout 500 1C because no degradation in the AMR ratiowas observed. In fact, the AMR ratio is raised up to about50% after 500 1C -annealing, compared with that of the as-deposited bilayer film. However, it should be noted that theAMR ratios of the Ni48Fe12Cr40/Ni80Fe20 bilayer films areless than those of the Ni80Fe20 monolayer films afterannealing at temperatures higher than 280 1C.Fig. 9 shows a typical magnetic hysteresis loop of the

Ni48Fe12Cr40/Ni80Fe20 bilayer film annealed at 280 1C for1 h. The magnetic hysteresis loop measurements indicatethat the annealing has little effect on the saturationmagnetizations of both the Ni48Fe12Cr40/Ni80Fe20 bilayerand Ni80Fe20 monolayer films. However, for all thesamples the coercivities of the Ni48Fe12Cr40/Ni80Fe20

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Fig. 6. The FE-SEM microphotographs of (a) the as-deposited Ni80Fe20 monolayer, (b) the as-deposited Ni48Fe12Cr40/Ni80Fe20 bilayer, (c) the 280 1C-

annealed Ni48Fe12Cr40/Ni80Fe20 bilayer, (d) the 330 1C-annealed Ni48Fe12Cr40/Ni80Fe20 bilayer and (e) the 500 1C-annealed Ni48Fe12Cr40/Ni80Fe20 bilayer.

The arrows represent the voids.

P. Wu et al. / Vacuum 80 (2006) 949–956954

bilayer films are smaller than those of the Ni80Fe20monolayer films, as shown in Table 1.

4. Discussion

According to the AES measurements results mentionedin Section 3.1, for the Ni48Fe12Cr40/Ni80Fe20 bilayer filmsas-deposited and annealed at temperatures below 280 1C,no Cr atoms are detected on the surface and inside theNi80Fe20 layer, indicating that no obvious Cr diffusionoccurs. For the bilayer films annealed at temperaturesabove 280 1C, Cr atoms are detected on the Ni80Fe20 layersurface whereas they are not detected inside the Ni80Fe20layer. Cr atoms on the surface come from the diffusion ofCr atoms of the Ni48Fe12Cr40 layers. Cr atoms diffusepreferentially through grain boundaries in the Ni80Fe20layer from the Ni48Fe12Cr40 layers to the surface, and

segregate on the surface. Since the Ni48Fe12Cr40 layer isonly 7 nm thick, the amount of Cr atoms is small and theCr atoms on the surface of the Ni80Fe20 layer are notsaturated. Therefore less Cr atoms diffuse into the Ni80Fe20layer, resulting in no Cr atoms are detected inside theNi80Fe20 layer.According to the XRD spectra shown in Fig. 5, for the

as-deposited film, the height of the NiFe(1 1 1) peak isenhanced by introducing the Ni48Fe12Cr40 underlayer whilethe 2y position and the width of the peak have no obviouschanges, indicating the strength of the [1 1 1] texture and noobvious change in the residual stress state in the Ni80Fe20layer. Compared Fig. 6(a) with (b), it can be seen that thegrain size of the Ni80Fe20 layer with the Ni48Fe12Cr40underlayer is slightly larger than that without theNi48Fe12Cr40 underlayer. Therefore, it can be said thatfor the as-deposited bilayer film the introducing of the

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Fig. 9. The magnetic hysteresis loop of Ni48Fe12Cr40 (7 nm)/Ni80Fe20(40 nm) bilayer film annealed at 280 1C for 1 h.

Fig. 7. Influence of annealing temperature on the sheet resistances of the

Ni48Fe12Cr40 (7 nm)/Ni80Fe20 (40 nm) bilayer and Ni80Fe20 (40 nm)

monolayer films.

Fig. 8. Influence of annealing temperature on the AMR ratios of the

Ni48Fe12Cr40 (7 nm)/Ni80Fe20 (40 nm) bilayer and Ni80Fe20 (40 nm)

monolayer films.

Table 1

Comparison of the coercivities of the Ni48Fe12Cr40 (7 nm)/Ni80Fe20(40 nm) bilayer films with that of the Ni80Fe20 (40 nm) monolayer films

Annealing temperature (1C) Hc (Oe)

Ni48Fe12Cr40/

Ni80Fe20 bilayer

Ni80Fe20 monolayer

As-deposited 0.50 0.86

150 0.52 0.96

280 0.57 0.87

330 0.45 0.72

500 0.56 0.84

P. Wu et al. / Vacuum 80 (2006) 949–956 955

Ni48Fe12Cr40 underlayer promotes both the [1 1 1] textureand grain growth in the Ni80Fe20 layer. The measurementof the AMR ratios shown in Fig. 8 indicates that the AMRratio of the as-deposited Ni48Fe12Cr40/Ni80Fe20 bilayer filmis slight larger than that of the as-deposited Ni80Fe20monolayer film. It is consistent with the changes in thestructure of the Ni80Fe20 layer with the Ni48Fe12Cr40underlayer. However, for the Ni48Fe12Cr40/Ni80Fe20 bi-layer films the enhancement in the AMR ratio withannealing temperature is limited. When the annealingtemperature reaches or exceeds 330 1C, Cr atoms in theNi48Fe12Cr40 layer diffuse into the Ni80Fe20 layer andsegregate on the surface. It is considered that the Crdiffusion results in lowering the AMR ratios of theNi48Fe12Cr40/Ni80Fe20 bilayer films compared with thoseof the Ni80Fe20 monolayer films.

5. Summary

(1)

For the as-deposited Ni48Fe12Cr40/Ni80Fe20 bilayerfilm, the introducing of the Ni48Fe12Cr40 underlayerpromotes both the [1 1 1] texture and grain growth inthe Ni80Fe20 layer.

(2)

The structure of Ni48Fe12Cr40/Ni80Fe20 bilayer filmsremains stable for annealing temperature up to 280 1C.The annealing promotes the grain growth of theNi48Fe12Cr40/Ni80Fe20 bilayer films when the annealingtemperature exceeds 280 1C.

(3)

After annealing at a temperature over 280 1C, Cr atomsinside the Ni48Fe12Cr40 layer diffuse into the Ni80Fe20layer and segregate on the surface of the Ni80Fe20 layer.

(4)

The Ni48Fe12Cr40 underlayer as a seed layer canenhance the AMR ratio of the Ni80Fe20 layer at aannealing temperature up to 280 1C compared withNi80Fe20 monolayer film. After annealing at a tem-perature over 280 1C, the AMR ratio of the Ni80Fe20monolayer films exceeds that of the Ni48Fe12Cr40/Ni80Fe20 bilayer films.

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(5)

For all the annealing temperatures, the coercivities ofthe Ni48Fe12Cr40/Ni80Fe20 bilayer films are smaller thanthose of the Ni80Fe20 monolayer films.

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