J. Cent. South Univ. Technol. (2009) 16: 0719-0724

DOI: 10.1007/s11771-009-0119-y

Composition design and properties of CoNbZr amorphous films

deposited by unbalanced magnetron sputtering

GUO Guang-wei(郭光伟)^{1, 2}, TANG Guang-ze(唐光泽)¹, WANG Ya-jun(王亚军)¹,

MA Xin-xin(马欣新)^{1, 3}, WANG Yu-jiang(王玉江)¹

(1. School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China;

2. School of Mechanical Science and Engineering, Daqing Petroleum Institute, Daqing 163318, China;

3. State Key Laboratory of Advanced Welding Production Technology, Harbin Institute of Technology,

Harbin 150001, China)

Abstract: Co₈₇Nb₁₀Zr₃, Co₇₆Nb₁₉Zr₅, Co₆₄Nb₂₆Zr₁₀ and Co₆₄Nb₁₆Zr₂₀ amorphous films were deposited on noncrystalline glass substrates by DC unbalanced magnetron sputtering. The compositions of amorphous films were tailored in the light of the individual deposition rate of Co, Nb and Zr. The amorphous films with the anticipated composition were prepared by means of co-sputtering Co, Nb and Zr targets simultaneously. It is indicated that there is interaction among three targets during co-sputtering. The morphology and composition of the films were observed by SEM, AFM and EDS. The structure and magnetic property were measured by XRD and physical property measurement system (PPMS). The coercivity changes with the composition, varying from 240 to 1 600 A/m. After vacuum isothermal annealing at temperatures of 475, 500, 525 and 550 ℃ for 15 and 30 min, respectively, it is found that high Nb content is beneficial to improving thermal stability of amorphous films. The crystallized films have the mean grain size of 2-19 nm.

Key words: CoNbZr amorphous film; magnetron co-sputtering; isothermal annealing

1 Introduction

CoNbZr amorphous film characterized by excellent soft magnetic performance is always of great interest from the viewpoint of fundamental investigation and application purpose in the past decade. It is feasible to employ this category of film as giant magnetoresistance (GMR) sensor and anti-faked material for the sake of high saturation magnetization, low coercivity, large magnetic permeability especially at high frequency, nearly zero magnetostriction and good wear resistance [1-4]. Co-based amorphous film can also be found in pesudo-spin-valve (PSV) due to its giant magneto- resistive effect [3]. Magnetic properties of amorphous alloy depend upon the electron surrounding state which is dramatically impacted by supplementary elements. The soft magnetic properties of amorphous films are influenced by alloy composition, film  thickness, microstructure, etc [4-5]. Since amorphous alloy phase is a thermal metastable state, heat treatment will influence the magnetic anisotropy in CoNbZr film. The thermal stability of amorphous films plays an important role in the performance of magnetic head and magnetic storage medium [2, 6]. Furthermore, there is interdiffusion during annealing that results in change in microstructure and mechanical properties of CoNbZr films [7].

With respect to Co-rich amorphous alloys, the magnetization reversal and inverting hysteresis loop are found to be dependent on the length of wire or the thickness of film. When Co₈₀Nb₁₀Zr₁₀ amorphous films are deposited with different thicknesses, test results on magnetic property reveal that magnetization reversal relies strongly on film thickness and the coercivity force reduces from 320 to 16 A/m along with the film thickness increasing from 35 to 840 nm [8].

Up to date, few studies have been presented on composition design and composition-dependent micro- structure that conducts evident effect on soft magnetic performance [9]. In this work, attention was paid on calibration of deposition rate of each component so as to determine appropriate parameter to control the amorphous film with the nominal composition. The practical composition of films deposited in terms of the determined sputtering current was tested in order to validate whether the real composition is consistent with the designed one. Interest was also absorbed in studying the performance of magnetic property and thermal stability of CoNbZr amorphous films at temperatures that the films will be used possibly.

2 Experimental

CoNbZr films were prepared in high vacuum multi- functional magnetron sputtering system. In the first instance, metallic films of Co, Nb and Zr were deposited on noncrystalline glass substrates individually with series of sputtering currents, corresponding to different target powers and voltages. The purity of Co, Nb and Zr target is not less than 99.95% (mass fraction). The background pressure of sample chamber was less than 8×10^-4 Pa and the work pressure was about 1.5 Pa with pure Ar gas (99.99%). The distance between substrate and each target was about 150 mm separately and the deposition time was 1 h. The thickness of the metal films was measured by nanoindenter profile test on an MTS XP system. Deposition rate was obtained through the film thickness divided by the corresponding deposition time. The correlation between deposition rate and sputtering current was fitted. Then the sputtering currents were determined according to the modeling equations for the given films with nominal composition, as listed in Table 1.

Table l Compositions and processing parameters for deposition of CoNbZr amorphous films

All the substrates were cleaned in ultrasonic cleaner with acetone and ethanol subsequently for 10 min before deposition. During deposition the substrates were held without bias and special cooling. The primary magnetic property was tested on Quantum Design PPMS. Rigaku X-ray diffraction instrument was used to characterize phase structure of CoNbZr films. All the samples were sealed in vacuum tube before isothermal annealing at temperatures of 475, 500, 525 and 550 ℃ for 15 and  30 min.

3 Results and discussion

3.1 Calculation of deposition rate of single target

Soft magnetic characteristic of CoNbZr amorphous film, which is dependent upon composition and microstructure, is the key factor when the film is applied as pole head and magnetic record medium. The film composition is adjusted for the conventional magnetron sputtering process by sputtering either different alloy targets with certain composition or Co target attached various Nb and Zr chips on it [10]. In this work, CoNbZr films were deposited by co-sputtering Co, Nb and Zr targets simultaneously. By controlling the deposition rate of Co, Nb and Zr which was achieved from the functions between deposition rate and sputtering current, the film composition was designed as four groups listed in Table 1. Typical profile of Co film with a ditch to determine the film thickness is presented in Fig.1. The profile curve shows that the film is smooth and uniform in large scale except for a few points where the ordinate is abnormal. It is reasonable to attribute these points to the particles that result from preparing the ditches. The film thickness was calculated by means of the ordinate difference of the step ends.

Fig.1 Scratch profile for thickness of Co film deposited at sputtering current of 150 mA for 1 h

Deposition rate at different sputtering currents was adopted to fit the relationship between deposition rate (v_d) and sputtering current (I). The results are shown in Fig.2.

Fig.2 Deposition rate as function of sputtering current: (a) Co; (b) Nb; (c) Zr

It is found that Co deposition rate, which is characterized by quadratic equation, is different from those of Nb and Zr, which are linear correlation between v_d and I. This phenomenon may be resulted from the inherent magnetic nature of Co which brings the sputtering magnetic field to impose force on Co particles sputtered out of Co target. The impact of magnetic force decrease with increasing sputtering current, that is, Co particles sputtered at high current with enough kinetic energy restrain the influence of sputtering magnetic field.

Sputtering currents to deposit four groups of amorphous films are listed in Table 1. SEM image and EDS spectrum of Co₈₇Nb₁₀Zr₃ film as a typical one are displayed in Fig.3. The morphology of the films is smooth and uniform. The tested compositions of the four groups of films are also displayed in Table 1. Comparing the designed composition with the measured one, it is indicated that practical values are nearly consistent with the designed ones. More detailed, the content of Co in each group is a little more than the designed value. Meanwhile, the contents of Nb and Zr are a bit less than the anticipated value. For instance, Co content in Co₈₇Nb₁₀Zr₃ film is 86.55%, which is more than the designed value of 80%. As for Nb and Zr, the contents are lower than the planed value of 30.2% and 70.2% separately. This is ascribed to the deposition priority of certain component with higher sputtering current caused by higher target bias and sputtering power. It can be speculated that Co particles, having the highest energy among these three particles, play a stronger scattering effect on others than Zr particles and Nb particles during particle collision. At the same time, Co particles are partially scattered away from the substrate surface. The scattering interaction is also proved by film thickness of Co₈₇Nb₁₀Zr₃, about 301 nm, which is different from the

Fig.3 Morphology and composition of Co₈₇Nb₁₀Zr₃ film:     (a) SEM image; (b) EDS spectrum

calculated value of 375 nm. It should be proposed that reducing the sputtering current or prolonging the deposition time correspondingly is a preferred approach to minimize the composition deviation at cost of lower efficiency. Optimizing the technology parameters to control composition more accurately should bring in an interaction coefficient to the deposition rate equations which describe the relationship between v_d and I during co-sputtering. Moreover, the practical composition deviation may be attributed to other factors, such as the film density which is different from bulk material, fitting error and test accuracy.

Fig.4 shows the morphologies of Co₈₇Nb₁₀Zr₃ and Co₇₆Nb₁₉Zr₅ films with the roughness of 2.1 and 1.5 nm, which agree with the topography achieved by SEM and profile test. It is suggested that the amorphous films grow on substrates in the island mode. Morphology and roughness of Co₆₄Nb₂₆Zr₁₀ and Co₆₄Nb₁₆Zr₂₀ films are similar to Fig.4(b). This implies that decreasing Co content and increasing Nb and Zr may decrease the roughness of amorphous films.

3.2 Effect of film composition on magnetic property

The magnetic hysteresis loops of Co₈₇Nb₁₀Zr₃ and Co₇₆Nb₁₉Zr₅ films were measured by PPMS at ambient

Fig.4 AFM morphologies of CoNbZr amorphous film:       (a) Co₈₇Nb₁₀Zr₃; (b) Co₇₆Nb₁₉Zr₅

temperature in magnetic field intensity (H_c) up to    800 kA/m in the parallel direction to the film plane and the important parts nearby zero point are presented in Fig.5. The coercivity of the Co₈₇Nb₁₀Zr₃ film is about   1 600 A/m, which is higher than 800 A/m (critical value for soft magnetic material). At the same mole ratio of Nb to Zr, 3?1, the coercivity of Co₇₆Nb₁₉Zr₅ film is lowered to about 240 A/m. This is implied that the Co film plays a significant role in the magnetic domain reversal in CoNbZr amorphous films. It can be believed that the coercivity decrease along with reducing Co content further. There is an interesting phenomenon that the hysteresis loop of Co₇₆Nb₁₉Zr₅ film as deposited state exhibits asymmetry, which shows different coercivities along the whole circle of hysteresis loop. This may be attributed to the amorphous structure of Co₇₆Nb₁₉Zr₅ film, which is characterized by the metastable state that results in parts of magnetic domains converting when the outer magnetic field is reversed from positive to negative.

3.3 Microstructure evolution of CoNbZr amorphous films during vacuum isothermal annealing

DSC test results depict that the crystallization temperature of CoNbZr amorphous films is 425-450 ℃

Fig. 5 Magnetic hysteresis loops of CoNbZr amorphous films: (a) Co₈₇Nb₁₀Zr₃; (b) Co₇₆Nb₁₉Zr₅

[11-12]. The four groups of specimens were annealed at 475 and 500 ℃ for 15 and 30 min, meanwhile, at   525 and 550 ℃ for 15 min, respectively. The XRD patterns are shown in Fig.6. It can be found that the four groups of films as deposited are all amorphous completely without diffraction peak. This is identical to the results reported previously [13]. Crystallization is nearly unconscious for Co₆₄Nb₂₆Zr₁₀ film with the highest Nb content, as shown in Fig.6(c). Adding Nb into noncrystalline system may increase the crystallization activation energy and enhance thermal stability [14-15]. The results demonstrate that Nb enhances the thermal stability of CoNbZr amorphous films.

Crystallization of Co₇₆Nb₁₉Zr₅ is not apparent because the real composition of Nb is still 19.28% (mole fraction). On the other hand, Co₈₇Nb₁₀Zr₃ and Co₆₄Nb₁₆Zr₂₀ films start to crystallize at temperature higher than 475 ℃. The broad amorphous peak at   2θ=44.2? becomes more narrow and the crystalline peak of Co (0002) is more evident with the increase of annealing time or temperature. Assisted with Jade software it was estimated that the average grain size in

Fig.6 XRD patterns of CoNbZr films isothermally annealed at different temperatures for 15 and 30 min: (a) Co₈₇Nb₁₀Zr₃;         (b) Co₇₆Nb₁₉Zr₅; (c) Co₆₄Nb₂₆Zr₁₀; (d) Co₆₄Nb₁₆Zr₂₀

films after crystallization was 2-19 nm. The soft magnetic property of the nanocrystalline CoNbZr films is still remained to further investigate.

4 Conclusions

(1) The practical composition of CoNbZr amorphous films deviates little from the designed value for there is interaction among individual target during co-sputtering. It is appropriate to select processing parameters for adjusting amorphous film composition in a large range by multi-target co-sputtering.

(2) Soft magnetic property of CoNbZr amorphous films is dominantly determined by Co content. It is advisable to decrease the Co content reasonably to improve the soft magnetic performance. Co₇₆Nb₁₉Zr₅ film with coercivity of about 240 A/m is better than Co₈₇Nb₁₀Zr₃ film as soft magnetic material.

(3) Nb content has a significant effect on the thermal stability of CoNbZr amorphous films. Co₆₄Nb₂₆Zr₁₀ film with practical Nb content of 25.71% (mole fraction) is not crystallized until 550 ℃ under vacuum annealing. Conversely, annealing at temperature of 475 ℃ for Co₈₇Nb₁₀Zr₃ and Co₆₄Nb₁₆Zr₂₀ films induces amorphous films to crystallize with the mean grain size of 2-19 nm.

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(Edited by YANG You-ping)

Foundation item: Project(2007CB607602) supported by the National Basic Research Program of China

Received date: 2008-12-26; Accepted date: 2009-04-21

Corresponding author: MA Xin-xin, Professor, PhD; Tel: +86-451-86402860; E-mail: maxin@hit.edu.cn