Microstructure of Fe/Pt film anneled in magnetic field
YU Yong-sheng(于永生), GUO Feng-yun(国凤云), FEI Wei-dong (费维栋), ZHAO Lian-cheng(赵连城)
Department of Materials Physics and Chemistry, Harbin Institute of Technology, Harbin 150001, China
Received 10 April 2006; accepted 25 April 2006
Abstract: An Si(001)/SiO2/Ti/Pt/Fe/Cu multilayer was prepared by direct-current magnetic sputtering system. The phase composition of the film was characterized by X-ray diffractometry(XRD), and the microstructure was observed by scanning electronic microscopy(SEM). Through the film annealed in magnetic field perpendicular to the surface of the film, FCC FePt film with (001) texture was obtained. And the density of the particle in the film annealed without magnetic field is very small compared with that in the film annealed with magnetic field. And the effect of magnetic field annealing on the microstructure of Fe/Pt film and the segregation of FCC FePt phase were also discussed.
Key words: L10-FePt; magnetic recording system; uniaxial magnetic anisotropy; magnetic field annealing; texture
1 Introduction
With the increasing areal density in magnetic recording system, CoCrPtTaB, the currently main magnetic recording media, has got to the superparamagnetism limitation[1]. Because of its very large uniaxial magnetic anisotropy (Ku=7.0×107 erg/cm3)[2] and huge coercivity [3], chemically ordered L10-FePt has been a major interest of magnetic data storage researchers for decades. The superparamagnetic limit of chemically ordered L10-FePt is about only 2.8 nm [4], and its areal density in magnetic recording system can reach 1Tbit/in2[5]. But as-deposited FePt film at room temperature is chemically disordered FCC phase with soft magnetism. Through post-annealing or the deposition at elevated temperature, FePt film can turn to chemically ordered FCT phase with hard magnetism. The annealing temperature is usually above 550 ℃[6], and it makes grain grown and brings magnetic coupling among the grains, which produces more read/write noise and is not beneficial from the application in magnetic recording system. So it is the most important researching direction to reduce process temperature. Recently, several endeavors have been paid for reducing process temperature, such as the introduction of underlayers[7], the addition of third elements[8], ion irradiation[9,10], dynamic stress-induced [11], and multilayering[12].
Because the easy axis of L10-FePt is [001] direction and Fe and Pt are alternatively arranged along [001] direction, it is very important to obtain a highly oriented L10-FePt with (001) texture for the optimization of L10-FePt film properties. Because (001) of L10-FePt is parallel to (001) of disordered FCC Fe-Pt solution during ordering process, the more important technology is to obtain the disordered FCC Fe-Pt solution with high (001) texture. However, the research on the preparation of highly textured disordered FCC Fe-Pt solution is limited. In the present paper, highly textured FCC Fe-Pt particles embedded in Pt-Cu matrix were obtained using magnetic field annealing, and the effect of magnetic field annealing on the microstructure of Fe/Pt film was discussed.
2 Experimental
The film was prepared using magnetic sputtering technique, the film structure is Si(001)/SiO2/Ti/Pt/Fe/Cu. Before the depositions of Fe and Cu, the substrate was annealed for the crystallization of textured Pt. The Ti layer is buffer to relax the residual stress, and Cu layer is top layer to prevent from the oxidation of Fe layer during post-annealing. Copper (the purity of 99.97%) and iron (the purity of 99.7%) targets were used to prepare the film. The base pressure is 4.0×10-3 Pa, the working gas is Ar with the purity of 99.99%, and the working gas pressure and sputtering current are 0.2 Pa and 0.1 A, respectively. After sputtering, the film was annealed at 420 ℃ under the pressure of 10-2 Pa for 50 min in magnetic field perpendicular to the film surface with the field intensity of 560 kA/m. For the comparison, the annealing without magnetic field was also carried out.
The phase composition of the film was characterized using X-ray diffraction (XRD) on Philips X’Pert X-ray diffractometer with the radiation of CuKα, and the microstructure was observed using scanning electronic microscope (SEM) on an S-3000 SEM.
3 Results and discussion
Fig.1 shows the XRD pattern of as-deposited film, the diffractions from Ti, Pt, Fe and Cu can be clearly found, which suggests that the as-deposited film is multi-layer, or the diffusions among layers are slight. PtTi3 compound forms during the crystallization of Pt layer.
Fig. 1 XRD pattern of as-deposited film
Fig.2 shows the XRD patterns of the films subjected different annealing treatment. For the film annealed without magnetic field, the all diffraction peaks in the XRD pattern are similar to those in Fig. 1, no additional diffraction peaks can be found, which suggests that the quantity of compound formed during annealing is very little. However, (002) diffraction of FCC FePt compound can be clearly found at the diffraction angle of 46.9? for the film annealed in magnetic field. Because no other diffractions of FCC FePt can be detected, so we think the FCC FePt is (001)-textured. Because FCC FePt is ferromagnetic, the results indicate that the magnetic field is beneficial to the formation of magnetic phase.
In the secondary electron image(SEI) shown in Fig. 3(a), many particles can be seen with the random distribution in the film annealed in magnetic field. In the corresponding backscattering electron image (BEI) of SEM shown in Fig.3(b), one can find brightness of the particles is relatively dark compared with the matrix, so
Fig. 2 XRD patterns of annealed films
Fig.3 SEM images of film annealed in magnetic field: (a) SEI; (b) BEI
the particle can be considered as Fe-rich phase according to the composition of the film, because the atomic number of Fe is smallest in the film. On the basis of XRD results, we think that the particles are FCC FePt compound.
As illustrated in Fig.4 of the microstructure of the film annealed without magnetic field, although some particles can also be found, the density of the particles is very small compared with that in the film annealed in magnetic field. That the diffraction of FCC FePt particles in the film was not detected by the XRD analysis as shown in Fig.1 may result from the quantity of the particles is small and the orientation of the particle is random.
Fig.4 SEM images of film annealed without magnetic field: (a) SEI; (b) BEI
The formation of FCC FePt phase can be analyzed as follows: 1) The diffusions must take place during annealing treatment; 2) Because of the ferromagnetic properties of FCC FePt phase, when it segregates, the free energy of the system must be decreased by the static magnetic energy. So the quantity of the particle in thefilm annealed in magnetic field is much higher than that in the film annealed without magnetic field. In addition, the absolute Zeemen energy (negative) is large when [001] direction of FePt particle is parallel to the magnetic field, because the easy axis of the phase is [001] direction.
4 Conclusions
1) The atom diffusion takes place in Fe/Pt film during annealing, resulting in the formation of FCC FePt compound.
2) The magnetic field is very beneficial to the formation of FePt with FCC structure.
3) The highly textured FCC FePt particles with (001) texture can be obtained through magnetic field annealing.
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(Edited by LONG Huai-zhong)
Corresponding author: FEI Wei-dong; Tel; +86-451-86418647; E-mail: wdfei@hit.edu.cn
