Article ID: 1003-6326(2005)02-0353-05

Induced effects of Cu underlayer on

(111) orientation of Fe₅₀Mn₅₀ thin films

WANG Lei(Íõ ÀÙ), WANG Feng-ping(Íõ·ïƽ), LIU Huan-ping(Áõ»¹Æ½),

WU Ping(Îâ Æ½), QIU Hong(Çñ ºê), PAN Li-qing(ÅËÀñÇì)

(Department of Physics, University of Science and Technology Beijing, Beijing 100083, China)

Abstract: Effects of Cu underlayer on the structure of Fe₅₀Mn₅₀ films were studied. Samples with a structure of Fe₅₀Mn₅₀(200nm)/ Cu(t_Cu) were prepared by magnetron sputtering on thermally oxidized silicon substrates at room temperature. The thickness of Cu underlayer varied from 0 to 60nm in the intervals of 10nm. High-vacuum annealing treatments, at different temperatures of 200, 300 and 400¡æ for 1 h, respectively, on the Fe₅₀Mn₅₀(200nm)/ Cu(20nm) thin films were performed. The surface morphologies and textures of the samples were measured by field emission scan electronic microscope (FE-SEM) and X-ray diffraction(XRD). Energy dispersive X-ray spectroscopy (EDX) and Auger electron spectroscopy(AES) were used to analyze the compositional distribution. It is found that Cu underlayer has an obvious induce effect on (111) orientation of Fe₅₀Mn₅₀ thin films. The induce effects of Cu on (111) orientation of Fe₅₀Mn₅₀ changed with the increase of Cu layer thickness and the best effect was obtained at the Cu layer thickness of 20nm. High-vacuum annealing treatments cause the migration of Mn atoms towards surface of the film and interface between Cu layer and substrate. With the increasing annealing temperature, migration of Mn atoms is more obvious, which leads to a Fe-riched Fe-Mn alloy film.

Key words: Fe₅₀Mn₅₀ films; Cu underlayer; migration CLC number: O484.1; O484.5; O482.54

Document code: A

1 INTRODUCTION

Antiferromagnetic(AFM) materials are widely used in a spin-valve structure in combination with magnetoresistive materials for application in magnetic sensors and magnetic data storage, after a bilayer composed by a ferromagnetic(FM) and an AFM material has been used in an exchange biased spin valve^{[1, 2]}. FexMn_1-x alloys are prototypes for antiferromagnetic materials with a fcc structure in a concentration range between x=45% and x=75% at room temperature^{[3, 4]}. The N¨¦el temperature(T_N) varies with x and reaches the maximum of about 500K at x¡Ö50%. This relatively high T_N of the alloy at equiatomic composition had made Fe₅₀-Mn₅₀ one of the most used materials in spin valve structure^[5-7]. Even if in many applications Fe₅₀Mn₅₀ has been replaced by antiferromagnetic materials with higher corrosion resistance and higher blocking temperature, such as IrMn^[8] and MnPt^[9], it still takes the character of a model system for the investigation of the magnetic coupling at the FM/AFM interfaces.

In addition to the FM and AFM layer thickness, the exchange coupling is also found to depend on the underlayer materials in FM/Fe₅₀Mn₅₀ system^[10-12]. In order to preserve the fcc phase in thin Fe₅₀Mn₅₀ films, a Cu underlayer has been used. As we know that the lattice constant of Cu is very near the one of the Fe₅₀Mn₅₀, the misfit between Fe₅₀Mn₅₀ and Cu is small and it amounts to ¦Ç=(¦Á_Cu-¦Áª­Fe₅₀Mn₅₀)/¦Áª­Fe₅₀Mn₅₀=-0.4%^{[13, 14]}.

Dependence of exchange coupling in permalloy/Cu/Fe₅₀Mn₅₀ on the Cu underlayer thickness has been investigated^[15]. It is found that exchange field H_E and coercivity H_C are strongly related to the thickness of Cu underlayer. No exchange coupling without Cu underlayer, because the metastable phase ¦Ã-Fe₅₀Mn₅₀ cannot exist without transition metal underlayer. The exchange field H_E at room temperature increases approximately linearly with the Cu underlayer thickness up to 30nm. The blocking temperature and the room temperature coercivity (H_C) increase at small Cu underlayer thickness and saturate with further increasing Cu underlayer thickness. These phenomena are related to the changes in the microstructure of the Fe₅₀Mn₅₀ layer, due to the variation of Cu layer thickness. However, the effect of the Cu underlayer on the structure and composition of Fe₅₀Mn₅₀ remains unclear. Fully understanding the detail change of the structure and composition of Fe₅₀Mn₅₀ on the Cu underlayer will be helpful to technological application of spin valve magneto-resistance devices.

In this paper, we focus on the effect of Cu underlayer on the structure and composition of Fe₅₀-Mn₅₀ films. The dependence of the orientation of thin Fe₅₀Mn₅₀ films on Cu thickness and the thermal stability of the Fe₅₀Mn₅₀/Cu bilayer during annealing are investigated.

2 EXPERIMENTAL

Samples with a structure of Fe₅₀Mn₅₀(200nm)/ Cu(t_Cu) were deposited by magnetron sputtering on thermal oxidized silicon substrate at room temperature. The thickness of the Cu underlayer varied from 0 to 60nm in the intervals of 10nm. The base pressure was better than 2¡Á10^-4Pa and the Ar pressure was 0.45Pa. The deposition rate was 0.08nm/s for Fe₅₀Mn₅₀ and 0.66nm/s for Cu. The film thickness is determined by the sputtering time. High-vacuum isothermal annealing treatments for the as-deposited Fe₅₀Mn₅₀(200nm)/ Cu(20nm) films at the temperatures of 200, 300 and 400¡æ for 1h, respectively, were performed.

The crystal structure of the films was examined by X-ray diffraction(XRD). The surface morphology and composition of the films were characterized using field emission scan electronic microscope(FE-SEM) and energy dispersive X-ray spectroscopy (EDX). Auger electron spectroscopy(AES) was used to analyze the extent of diffusion of the Fe₅₀Mn₅₀(200nm)/ Cu(20nm) films annealed at 400¡æ.

3 RESULTS AND DISCUSSION

The effects of the Cu thickness on textures of as-deposited Fe₅₀Mn₅₀(200nm)/ Cu(t_Cu) bilayer were investigated. Fig.1 shows the X-ray diffraction patterns for the Cu (t_Cu) films and the Fe₅₀-Mn₅₀(200nm) films with underlayer Cu(t_Cu). From Fig.1(a), it is easy to observe that the crystallization of Cu layers presented preferential (111) orientation with the increase of Cu thickness. The XRD intensity of Cu (111) peak reaches the maximum when the thickness of Cu layer is 40nm and does not increase anymore with the increase of the thickness of Cu layer. From Fig.1(b), one can find that Cu underlayer produces an obvious inducing effect on (111) orientation of Fe₅₀Mn₅₀ thin films. There is no obvious diffraction peak of Fe₅₀-Mn₅₀ (111) appears in the films without Cu underlayer. Comparing Fig.1(a) with Fig.1(b), it can be seen that the crystallization of Fe₅₀Mn₅₀ fcc structure along the (111) orientation presents a fluctuant change with the increasing Cu underlayer thickness. The crystallization of Fe₅₀Mn₅₀ fcc structure along the (111) orientation intensifies with the increasing Cu underlayer thickness from 0 to 20nm and comparatively strong and sharp diffraction peak of Fe₅₀Mn₅₀ (111) appears for the film grown on the underlayer of 20nm Cu. When the thickness of Cu underlayer exceeds 20nm, the XRD intensity of Fe₅₀Mn₅₀ (111) begins to decrease. However, the Fe₅₀Mn₅₀ (111) XRD peak becomes strong again when the thickness of Cu reaches 60nm. It can also be seen that the Fe₅₀-Mn₅₀ (111) diffraction peak position in the Fe₅₀-Mn₅₀(200nm)/ Cu(20nm) film has a shift to the left and is closer to the standard peak position of bulk Fe₅₀Mn₅₀.

Fig.1  XRD patterns with different Cu thickness

   The surface morphologies were characterized using FE-SEM. The film grew with columnar grain structure. The interface between the Cu underlayer and Fe₅₀Mn₅₀ layer was not sharp. This is due to a low misfit between fcc Cu (111) and fcc Fe₅₀Mn₅₀ (111) (less than 1 %) and close average atomic volume^{[16, 17]}. The FE-SEM images for the Fe₅₀Mn₅₀/Cu films are shown in Fig.2. As can be seen, the grains of Fe₅₀Mn₅₀ without Cu underlayer seem to be small, while the grains of Fe₅₀Mn₅₀ deposited on the Cu underlayer begin to get together.With the increasing thickness of Cu underlayer from 10 to 50nm, owing to the collection of the grains on the surface of Fe₅₀Mn₅₀/Cu, the surface roughness of the films is enhanced. When the thickness of the Cu underlayer increases to 60nm, the surface morphology of the Fe₅₀Mn₅₀ film presents notable change. The grains have grown up and presented clubbed.

Fig.2  FE-SEM images of Fe₅₀Mn₅₀(a), Fe₅₀Mn₅₀/ Cu(20nm)(b),

Fe₅₀Mn₅₀/ Cu(50nm)(c) and Fe₅₀Mn₅₀/ Cu(60nm)(d)

From the results, it can be seen that the effects of the Cu underlayer on textures of as-deposited Fe₅₀Mn₅₀/Cu (t_Cu) films show a complex dependence on Cu thickness. Both XRD and FE-SEM indicate that the 20nm Cu underlayer has an obvious effect on (111) orientation of Fe₅₀-Mn₅₀ thin films. A strong and sharp diffraction peak of Fe₅₀Mn₅₀ (111) appears for the film. The surface of sample is smooth and a small quantity of grains combine to form a large one. When the Cu underlayer is thinner than 20nm, due to small and scattered grains of the sample, the Fe₅₀Mn₅₀ shows a weak XRD peak in Fe₅₀Mn₅₀/Cu. However, with the increasing Cu thickness, more and more grains combine to form a large one, which results in the rough surface and cave increase. These caves also lead to a lower XRD intensity. The most interesting result is observed in the Fe₅₀Mn₅₀/ Cu(60nm) film. Both XRD and FE-SEM results show that the film has a notable change in the structure and surface morphology. It is necessary to investigate the mechanism in our future work.

Effects of annealing on the structure of the Fe₅₀Mn₅₀(200nm)/ Cu(20nm) films and the thermal stability of the antiferromagnetic Fe₅₀Mn₅₀ layer were investigated.

The structure of the Fe₅₀Mn₅₀(200nm)/ Cu(20nm) and Cu(20nm) films annealed at 200, 300 and 400¡æ for 1h, respectively, was also characterized by XRD. The results in Fig.3 show that with the increasing annealing temperature, the XRD intensity of Fe₅₀Mn₅₀ (111) decreases and the one of Cu (111) increases. Also some diffraction peaks of Mn-O presented in Fig.3(b) indicate that the oxide of Mn forms on the films.

EDX and AES were used to study the compositional distribution of Fe, Mn, and Cu in the films region. Table 1 shows the qualitative mass and atomic proportion of Fe₅₀Mn₅₀. Along the direction of film thickness, Fe and Mn shows the 1¡Ã1 composition ratio. The Auger depth profile analysis for the Fe₅₀Mn₅₀(200nm)/ Cu(20nm) films annealed at 400¡æ was carried out and the result is shown in Fig.4. As can be seen, a substantial amount of Mn atoms have migrated to the surface and migrated through the Cu layer to the substrate. And a Fe-riched Fe-Mn in the inner of the film was observed.

Table 1 Qualitative mass and atomic proportion of Mn and Fe in Fe₅₀Mn₅₀ film

Fig.3  XRD patterns for Cu films(a) and Fe₅₀Mn₅₀/Cu films(b) at

different annealing temperatures for 1h

Fig.4  AES depth profile for Fe₅₀Mn₅₀ (200nm)/ Cu(20nm) films after annealed at 400¡æ

It has been observed that antiferromagnetic Fe-Mn thin film has shown to spontaneously oxidize to form a surface oxide at room temperature and 0.1Pa^[18]. As the Mn atoms have been oxidized on the surface, the oxidation promotes the Mn migration towards the surface during the anneal^[19]. From the results of XRD (Fig.3) and AES profile (Fig.4), it can be presumed that Mn atoms have been oxidized on the surface and Cu-SiO₂ interface during annealing. At the same time, some Mn atoms have migrated through Cu layer and combined with the oxygen of substrate surface. The migration of Mn atoms leads to Fe-riched Fe-Mn in the inner of the film and then destroyed the structure of Fe₅₀Mn₅₀ layer, which results in the decrease of diffraction peak intensity at 400¡æ as seen in Fig.3(b). The formation of Mn-O compound may have important implications for the magnetic properties of Fe₅₀Mn₅₀/Cu films. The migration of Mn atoms toward the surface of the film and Cu-SiO₂ interface leads to the breakage of Fe₅₀Mn₅₀ layer composition ratio. Therefore the antiferromagnetic property Fe₅₀Mn₅₀ would be destroyed.

4 CONCLUSIONS

Effects of Cu underlayer on the structure of Fe₅₀Mn₅₀ films are studied. Cu underlayer produces an obvious inducing effect on (111) orientation of Fe₅₀Mn₅₀ thin films. A strong diffraction peak (111) of Fe₅₀Mn₅₀ appears for the films grown on the underlayer of 20nm Cu. Annealing treatment brought remarkable effects on the structure and composition of Fe₅₀Mn₅₀ thin film. With the increasing annealing temperature, the XRD intensity of Fe₅₀Mn₅₀ (111) decreases. The results of AES show that Mn atoms have migrated to the surface and Cu-SiO₂ interface. The migration of Mn atoms destroys the structural and compositional properties of Fe₅₀Mn₅₀ layer.

ACKNOWLEDGEMENT

The authors would like to thank C. Fan for his technical support.

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Foundation item: Project(19974005) supported by the National Natural Science Foundation of China

Received date: 2004-12-06; Accepted date: 2005-01-18

Correspondence: WANG Feng-ping; Tel: +86-10-62333786; E-mail: fpwang@sas.ustb.edu.cn

(Edited by LONG Huai-zhong)