Static and Dynamic Magnetic Domains of Epitaxial γ -Fe 4 N Thin Films

Transcription

1 Journal of the Korean Physical Society, Vol. 55, No. 3, September 2009, pp Static and Dynamic Magnetic Domains of Epitaxial γ -Fe 4 N Thin Films Shahid Atiq and Saadat Anwar Siddiqi Center of Excellence in Solid State Physics, University of the Punjab, Lahore-54590, Pakistan Hyen-Seok Ko and Sung-Chul Shin Department of Physics and Center for nanospinics of Spintronic Materials, Korea Advanced Institute of Science and Technology, Daejeon (Received 26 August 2008) Highly-ordered single-phase thin films of γ -Fe 4N were deposited by dc magnetron sputtering directly on single-crystal MgO(100) substrates at different temperatures. The surface morphology showed that a very regular nano-sized grain structure gradually turned into a smooth layered growth with increasing temperature up to 450 C. The minimum root-mean-square value of the surface roughness was achieved after 30 minutes in-situ annealing at 450 C. The epitaxial texture of the films was revealed by X-ray diffractometry Bi-axial in-plane magnetic anisotropy, characteristic of strong ferromagnetism, was observed from the quite squarish M-H loops obtained with the help of a vibrating sample magnetometer Bow-tieshaped static domain patterns and a very dense domain structure were direct evidence of strong exchange coupling among magnetic domains. Time-resolved dynamic magnetic domain patterns were observed in γ -Fe 4N thin films by using a magneto-optic microscope magnetometer. PACS numbers: Cr, Ss, Ch, Ej Keywords: Epitaxial growth, Static domain patterns, Dynamic domain patterns I. INTRODUCTION Among iron nitrides, the γ -Fe 4 N phase has been studied extensively due to its excellent ferromagnetic characteristics [1]. In the last few decades, single-phase γ -Fe 4 N thin films were deposited on (100)oriented Si and NaCl substrates [2], and epitaxial γ -Fe 4 N films were reported on Cu(100) [3] and MgO(001) [4, 5] substrates. Quite recently, single-phase epitaxial films have been reported on MgO(100), SrTiO 3 (100) and LaAlO 3 (100) substrates [6]. This material has a face-centered cubic structure with a nitrogen atom at the body-centered site. As compared with the other strong ferromagnetic phase of iron nitride (α -Fe 16 N 2 ) [7], γ -Fe 4 N shows a considerable thermal stability which makes it suitable for applications where high saturation magnetization (M s ), low coercivity, and better chemical stability are required. For instance, the preparation of an all-epitaxial all-nitride magnetic tunnel junction (MTJs) using Cu 3 N as an insulating layer between ferromagnetic γ -Fe 4 N electrodes [8] has triggered potential applications of this material in spintronic device fabrication. The iron nitride system is suitable for examining the effect of a light element on the spin-polarized transport characteristics of shahidatiqpasrur@yahoo.com; Tel: ; Fax: MTJs through the lattice distortion effect. Additionally, the development of MgObased MTJs with γ -Fe 4 N electrodes having relatively large inverse tunneling magnetoresistance (TMR) could find applications in magnetic logic circuits to compose complementary switching elements with conventional normal TMR elements [9]. In addition, high-density magnetic storage media and current perpendicular to plane devices (CCP) could also greatly benefit from this material [10]. II. EXPERIMENTAL PROCEDURES The optimized conditions for single-phase epitaxial growth were achieved by depositing several iron-nitride thin films on MgO(100) substrates by using dc magnetron sputtering under various deposition conditions. A high-purity target (99.95%) of α-fe was placed at a distance of 10 cm from the substrate holder. The substrates were cleaned ultrasonically with acetone and ethanol and were then pre-heated in vacuum for 30 minutes at the deposition temperature. When the chamber base pressure was Torr, a mixture of analytically pure Ar and N 2 was injected into the chamber. The partial pressures of Ar and N 2 were 5 and 0.5 mtorr, respectively. Films of 50 nm in thickness were deposited at substrate temperatures ranging from 200 to 500 C at a dc sputtering -925-

2 -926- Journal of the Korean Physical Society, Vol. 55, No. 3, September 2009 power of 30 W. All the samples were in-situ annealed for 30 minutes. The crystal structure was investigated by using a Rigaku D/MAX-RC MPA X-ray-diffarctometer (XRD) with Cu K α radiation. Film-thickness calibrations and characterizations of the surface morphology were performed using a PSIA, XE-100 atomic force microscope (AFM) and a Hitachi S-4800 scanning electron microscope (SEM). Micro-structural investigations were performed by using a Tecnai G2 F30 S-Twin transmission electron microscope (TEM). Magnetic hysteresis loops were measured by using a VT-800 (Riken Denshi Co Ltd) vibrating sample magnetometer (VSM) with an applied field up to ±15 koe parallel to the film s plane. The static magnetic domain structure was investigated by using a magnetic force microscope (MFM) which was a non-contact force microscope (PSIA, XE-100) equipped with a magnetic tip (nanosensors). To observe the dynamic magnetic domain evolution patterns, we used a magneto-optic microscope magnetometer (MOMM) [11]. III. RESULTS AND DISCUSSION Figure 1 shows the X-ray diffraction patterns (XRD) of γ -Fe 4 N thin films deposited on single-crystal MgO(100) substrates at different temperatures ranging from 200 C to 500 C and in-situ annealed for 30 minutes. The XRD patterns show the unique (100) and (200) peaks of γ -Fe 4 N at 2θ of and respectively. All other peaks are from the MgO(100) substrate as can be seen in the XRD pattern of the substrate shown in Fig. 1(a). The intensities of the (100) and the (200) peaks of γ - Fe 4 N increase as the deposition temperature is increased from 200 C and become maximum at 450 C, revealing that the degree of epitaxy increases as the temperature is increased from 200 C to 450 C. When the deposition temperature was increased to 500 C, a decrease in intensity was observed for both the peaks along with the appearance of an additional peak (200) of α-fe at 2θ of 65.06, which is obviously due to an excessive escape of nitrogen from some of the interstitial sites of γ -Fe 4 N at a higher temperature converting the deposit partially to pure iron. This illustrates that 450 C is the most appropriate temperature for the growth of γ -Fe 4 N thin films with an epitaxial texture A regular nano-sized grain structure was imaged with the help of a scanning electron microscope for the sample deposited at 200 C. When the temperature was increased, that grain structure turned into tunneling which eventually appeared as a regular smooth layered structure at 450 C. The root-mean-square (RMS) value of the surface roughness for this sample, as measured using an atomic force microscope was 0.17 nm which again illustrates a high degree of smoothness for the epitaxially grown film. Figure 2 shows the magnetic hysteresis loops obtained Fig. 1. X-ray diffraction patterns of (a) MgO(100) substrate and of γ -Fe 4N thin films deposited at (b) 200 C, (c) 250 C, (d) 300 C, (e) 350 C, (f) 400 C, (g) 450 C, and (h) 500 C. using a vibrating sample magnetometer. As can be seen in the figure, quite squarish M-H loops, with a low coercivity in the range of 100 to 200 Oe, were obtained which is a characteristic of a strong ferromagnetic behavior. The maximum value of saturation magnetization was achieved for the sample deposited at 450 C and insitu annealed for 30 minutes while below and above this temperature, the M s value decreased. Figure 3(a) shows a micrograph of γ -Fe 4 N films prepared at 450 C obtained with help of a transmission electron microscope. As compared with the micrograph obtained for a pure iron film deposited on MgO(100) under the same conditions as shown in Fig. 3(b), a very dense and uniform microstructure was observed for γ - Fe 4 N films, which suggests a strong exchange interaction among the magnetic domains resulting in an enhanced magnetic moment in γ -Fe 4 N thin films. The strong ferromagnetic behavior was also verified with the help of a magnetic force microscope for the sam-

3 Static and Dynamic Magnetic Domains of Epitaxial γ -Fe 4N Thin Films Shahid Atiq et al Fig. 2. Magnetic hystersis loops of γ -Fe 4N films deposited at (a) 350 C, (b) 400 C, (c) 450 C, and (d) 500 C. Fig. 3. Micro-structural image of (a) γ -Fe 4N films and (b) pure iron obtained by using a field emission transmission electron microscope. ple deposited at 450 C. Bow-tie-shaped static magneticdomain structures were observed, as shown in Fig. 4(a). Traditionally this type of domain structure is expected from ferromagnetic materials with perpendicular magnetic anisotropy (PMA). The corresponding topography of the same sample is also shown in Fig. 4(b), which shows a quite smooth layered texture for the grown films. In Figure 5, we present the time-resolved dynamic magnetic-domain evolution patterns of γ -Fe 4 N thin films deposited at 450 C. These patterns are observed successively by using a magneto-optic microscope magnetometer to investigate the same area of the film with a size of µm 2. As can be seen in the figure, the domain evolution patterns in each figure clearly exhibit discrete and sudden jumps in the magnetization reversal process. Since these experiments were carried out repeatedly on the same area of the film, the magnetization reversal proceeds with quite different jumps every time. Hence, these jumps can be related to Barkhausen avalanches which have been observed in ferromagnetic

4 -928- Journal of the Korean Physical Society, Vol. 55, No. 3, September 2009 Fig. 4. (a) Static magnetic domain structure and (b) corresponding surface topography γ -Fe 4N films. Fig. 5. Dynamic magnetic domain evolution patterns obtained by using a magneto-optic microscope magnetometer. thin films [12]. IV. CONCLUSION of Pakistan, and the Korea Science and Engineering Fund (KOSEF) through the National Research Laboratory Project. In conclusion, ferromagnetic γ -Fe 4 N thin films were deposited at different temperatures ranging from 200 C to 500 C. Single-phase epitaxial growth of the films was achieved at 450 C after 30 minutes of post annealing. Smooth-layered growth and strong ferromagnetic behavior were observed. Static and dynamic magnetic domain patterns in the γ -Fe 4 N films were investigated ACKNOWLEDGMENTS The authors would like to thank Sung-Huen Kim and Huen-Sung Lee for their kind help during some of the experimental measurements. This research work was supported by International Research Support Initiative Program (IRSIP), the Higher Education Commission (HEC) REFERENCES [1] J. L. Costa-Kramer, D. M. Borsa, J. M. Gracia-Martin, M. S. Martin-Gonzalez, D. O. Boerma and F. Brinoes, Phys. Rev. B 69, (2004). [2] L. L. Wang, X. Wang, N. Ma, W. T. Zheng, D. H. Jin and Y. Y. Zhao, Surf. Coat. Technol. 201, 786 (2006). [3] D. Ecija, E. Jimenez, J. Camarero, J. M. Gallego, J. Vogel, N. Mikuszeit, N. Sacristan and R. Miranda, J. Magn. Magn. Mater. 316, 321 (2007). [4] J. E. Mattson, C. D. Potter, M. J. Conover, C. H. Sowers and S. D. Bader, Phys. Rev. B 55, 70 (1997). [5] R. Lolee, K. R. Nikolaev and W. P. Pratt, Appl. Phys. Lett. 82, 3281 (2003). [6] S. Atiq, H. S. Ko, S. A. Siddiqi and S. C. Shin, Appl. Phys. Lett. 92, (2008). [7] K. H. Jack, Proc. R. Soc. Lond. Ser. A 208, 216 (1951).

5 Static and Dynamic Magnetic Domains of Epitaxial γ -Fe 4N Thin Films Shahid Atiq et al [8] D. M. Borsa, S. Grachev, C. Presura and D. O. Boerma, Appl. Phys. Lett. 80, 1823 (2002). [9] K. Sunaga, M. Tsunoda, K. Komagaki, Y. Uehara and M. Takahashi, J. Appl. Phys. 102, (2007). [10] K. R. Nikolaev, I. N. Krivorotov, E. D. Dahlberg, V. A. Vas ko, S. Urazhdin, R. Loloee and W. P. Pratt, Jr., Appl. Phys. Lett. 82, 4534 (2003). [11] S. B. Choe, D. H. Kim, Y. C. Cho, H. J. Jang, K. S. Ryu, H. S. Lee and S. C. Shin, Rev. Sci. Instrum. 73, 2910 (2002). [12] D. H. Kim, S. B. Choe and S. C. Shin, J. Appl. Phys. 93, 6564 (2003).