IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 7, NO. 4, JULY 2008 409 Structural and Magnetic Properties of Amorphous and Nanocrystalline CoFeSiB Thin Films Jungbum Yoon, Seung-Young Park, Younghun Jo, Myung-Hwa Jung, Chun-Yeol You, Taewan Kim, Jae Youn Hwang, Hae In Yim, Member, IEEE, Jang Roh Rhee, Byong Sun Chun, You Song Kim, and Young Keun Kim Abstract—This study examined the structural, magnetic, and transport properties of CoFeSiB films with various Co com- positions. The main focus was on two samples, amorphous Co 74 Fe 4 Si 14 B 8 and nanocrystalline Co 78 Fe 2 Si 12 B 8 thin films. The results show that the amorphous film is a typical soft magnetic material, while the nanocrystalline film has a large saturation field. It is believed that in a nanocrystalline thin film, a large saturation field is caused by antiferromagnetic exchange at the boundary be- tween the amorphous and nanocrystalline phases. Index Terms—Amorphous, antiferromagnetic exchange cou- pling, nanocrystalline CoFeSiB thin film. I. INTRODUCTION T HE TUNNELING MAGNETORESISTANCE (TMR) ef- fect in magnetic tunnel junctions (MTJs) has attracted considerable research attention over the last decade due to their potential applications in magnetic random access mem- ory (MRAM). Studies on amorphous soft magnetic layers are important for a better free layer performance in MTJs [1], [2]. Soft magnetic free layers are required to ensure a low switching field. Based on the Stoner–Wohlfarth’s single-domain model, the switching field depends on the saturation magnetization (M s ) and magnetic anisotropy energy (K s ) [3]. For a soft mag- netic device, the contribution from shape anisotropy is domi- nant, where K s =2πM 2 s . M s needs to be reduced in order to obtain a low switching field. Among the many candidate amor- phous soft magnetic materials, CoFeSiB has a lower M s value compared with that of the traditionally used CoFe [4], [5]. The amorphous thin film showed soft magnetic behavior due to the lack of crystalline anisotropy. In an effort to reduce the M s ,a series of CoFeSiB thin films with various Co compositions was Manuscript received May 14, 2007; revised October 21, 2007 and March 4, 2008. This work was supported by the Sogang University Re- search Grant of 200810017.01. The review of this paper was arranged by Associate Editor D. Litvinov. J. Yoon is with the Quantum Material Research Team, Korea Basic Science Institute, Daejeon 305-333, Korea, and also with the Department of Physics, University of Inha, Incheon 402-751, Korea. S.-Y. Park and Y. Jo are with the Quantum Material Research Team, Korea Basic Science Institute, Daejeon 305-333, Korea. M.-H. Jung is with the Department of Physics, Sogang University, Seoul 121-742, Korea (e-mail: mhjung@sogang.ac.kr). C.-Y. You is with the Department of Physics, University of Inha, Incheon 402-751, Korea. T. Kim is with the Department of Advanced Materials Engineering, Univer- sity of Sejong, Seoul 143-747, Korea. J. Y. Hwang, H. I. Yim, and J. R. Rhee are with the Department of Physics, University of Sookmyung Women’s, Seoul 140-742, Korea. B. S. Chun, Y. S. Kim, and Y. K. Kim are with the Department of Materials Science and Engineering, University of Korea, Seoul 136-713, Korea. Digital Object Identifier 10.1109/TNANO.2008.926334 prepared, and the structural, magnetic, and transport properties were investigated. With increasing Co composition, the amor- phous CoFeSiB thin film transformed into a nanocrystalline film where the nanocrystalline Co phase was embedded in the amorphous matrix. II. EXPERIMENT The CoFeSiB films were prepared using a six-target dc magnetron sputtering system under a typical base pressure <5 × 10 −8 torr. A Co 70. 5 Fe 4. 5 Si 15 B 10 target on which small 5 mm × 5 mm × 2 mm Co chips were added was used to con- trol the Co composition of the CoFeSiB films. The diameter of the Co 70. 5 Fe 4. 5 Si 15 B 10 target was 50.8 mm. The film thickness is approximately 430 nm. Annealing was carried out in situ at 200 ◦ C in a 5 × 10 −4 torr vacuum under an applied magnetic field of 300 Oe for 2 h. The composition of the CoFeSiB thin films was confirmed by energy dispersive X-ray spectroscopy (EDX) and inductively coupled plasma–atomic emission spec- trometry (ICP-AES). The microstructure was characterized by high-resolution transmission electron microscopy (TEM) and electron diffraction (ED). The magnetic properties at 5 K were characterized using a superconducting quantum interference device (SQUID). The magnetoresistance was measured on a dumbbell-like patterned sample using a two-probe dc technique with a physical property measurement system (PPMS). III. RESULTS AND DISCUSSION The compositions of the prepared samples were changed by the number of Co chips from 0 to 16. The transition of the microstructure from a fully amorphous to nanocrystalline phase occurred at more than eight Co chips. The main focus of this study was on only two samples, Co 74 Fe 4 Si 14 B 8 (Co chip: 6) and Co 78 Fe 2 Si 12 B 8 (Co chip: 10), which have differ- ent microstructure and magnetism. The TEM images and ED patterns showed that the Co 74 Fe 4 Si 14 B 8 film was amorphous while the Co 78 Fe 2 Si 12 B 8 film was partially nanocrystalline, as shown in Fig. 1(a) and (b), respectively. The Co 74 Fe 4 Si 14 B 8 and Co 78 Fe 2 Si 12 B 8 films are denoted as samples-A (amor- phous) and -NC (nanocrystalline), respectively. The microstruc- ture taken from TEM images implied that the average size of nanocrystalline clusters is enlarged with respect to the Co com- position. Fig. 2 shows the magnetization curves of samples-A and -NC. The measurements were performed for an in-plane applied magnetic field at 5 K. Sample-A shows a soft magnetic be- havior due to the lack of crystalline anisotropy, which is usually 1536-125X/$25.00 © 2008 IEEE