Magnetic properties and microstructure of Ag 2 Se/FePt particulate films Jai-Lin Tsai, a) Hsueh-Wei Tai, and Hsin-Te Tzeng Department of Materials Science and Engineering, National Chung Hsing University, Taichung, Taiwan (Presented 16 November 2010; received 14 September 2010; accepted 18 November 2010; published online 24 March 2011) Multilayer Ag/[Ag 2 Se(t)/FePt(1nm)] 10 (thickness t ¼ 0.1–0.4 nm) were alternately deposited on a glass substrate and subsequently annealed by rapid thermal process (RTP) at 800 C for 3 min. After RTP, the interface between FePt and Ag 2 Se was intermixed to form particulate films. The grains size of the L1 0 FePt decreased from 9.8 to 7.7 nm when the total thickness of Ag 2 Se intermediate layer increases to 1 nm. The Ag/(Ag 2 Se/FePt) 10 particulate film showed perpendicular magnetization and a slight increase in out-of-plane coercivity over that of the original thickness of Ag 2 Se. The Ag 2 Se atoms were immiscible with FePt phase but the FePt grains were refined and separated well from the Ag 2 Se phase. Compared with Ag/FePt bilayer, the grains of the FePt were refined and uniformly separated in the Ag/[Ag 2 Se(t)/FePt] 10 multilayer with t ¼ 0.1 nm. V C 2011 American Institute of Physics. [doi:10.1063/1.3553943] I. INTRODUCTION The granular magnetic recording media was used con- ventionally. Smaller grain size in magnetic media is required to maintain the signal to noise ratio as the densities involved in recording increase. The (001) textured L1 0 FePt thin film has been much discussed because of its high intrinsic magne- tocrystalline anisotropy (K u ) which is required for hard mag- netic materials and high-density magnetic recording media. Thermal stability has been achieved even with grain size as small as 5 nm, because of its high uniaxial anisotropy. 1–4 To attain the smaller grain size, films are annealed at specific temperatures to embed the FePt grains into the nonmagnetic matrix or by preparing the Fe/Pt multilayer with intermediate nonmagnetic layers. Such processes also bestow the charac- teristic to isolate FePt grains and to form a particulate struc- ture. The L1 0 FePt films with perpendicular magnetic anisotropy have been prepared through many processes such as homogeneous or heterogeneous epitaxy, sputtering, sput- tering Fe/Pt multilayer on a single-crystal substrate with appropriate underlayers, 5–7 or by forming the c-axis texture on an amorphous substrate. 8–10 FePt films with an Ag top layer, underlayer, FePt/Ag multilayer, and FePtAg-C granular films have been discussed extensively due to the immiscibility between FePt and Ag. 11,12 A (001)-textured FePt-(TiO 2 , SiO 2 ) nanocomposite films composed of isolated grains of 5 nm FePt have also been reported. 13,14 In our pre- vious work, the Ag/FePt bilayer and Ag/FePt/Ag trilayer annealed by rapid thermal process (RTP) at 800 C had per- pendicular magnetization. 15 Ag has high thermal diffusivity that reduces the ordering temperature of FePt but enhances grain growth during the RTP. In this study, we compared the magnetic properties and microstructure of multilayer Ag/ (Ag 2 Se/FePt) 10 and Ag/FePt bilayer. Compound Ag 2 Se has a lower melting point (897 C) than that of Ag (962 C) and is characterized by a cubic structure with high ionic conductivity above the transition temperature. Ag 2 Se is known for its polymorphism and high ionic conductivity and has traditionally been considered a potential thermoelectric material and super-ionic conductor. 16 The recent discovery of high magnetoresistance (MR) has greatly enhanced inter- est in silver selenide. 17 II. EXPERIMENTAL Multilayer Ag(1 nm)/[Ag 2 Se(t)/FePt(1 nm)] 10 (thickness t ¼ 0–0.4 nm) were prepared by DC (Direct Current) magnetron sputtering. The base pressure of the sputtering system was 5 10 8 Telsa with a working pressure of 1.5 10 3 Telsa under high purity argon gas. FePt, Ag 2 Se alloy target, and Ag element target were used to deposit the films on the glass sub- strate. The Ag with thickness of 1nm was covered on multilay- ered (Ag 2 Se/FePt) films. After deposition, the films were annealed by using a rapid thermal annealing (RTA) system at 800 C for 3 min. The crystal structure of the samples was iden- tified by grazing incident x-ray diffractometry (XRD) with Cu K a radiation. The microstructure of the films was observed by high-resolution transmission electron microscopy (HRTEM). Magnetic hysteresis loops were measured at room temperature using a vibration sample magnetometer (VSM) with the maxi- mum magnetic field of 2 Telsa. III. RESULTS AND DISCUSSIONS Figure 1 shows XRD patterns of Ag/FePt bilayer and Ag/ [Ag 2 Se(t)/FePt(1 nm)] 10 multilayer with thickness (t ¼ 0.1–0.4 nm) annealed at 800 C for 3 min. For the Ag/FePt bilayer indicated in Fig. 1(a), the film preferred an orientation in the 001 direction. The relative intensity of fundamental peaks (111) was low. The XRD patterns of multilayer Ag/[Ag 2 Se(t)/ FePt(1 nm)] 10 (t ¼ 0.1, 0.2, 0.3, 0.4 nm) are plotted in Figs. 1(b)–1(e), respectively. A super-lattice diffraction peak (001) dominates and a low intensity of (111) peak is indexed. The ordering degree S was not easy to estimate by lattice constant c (c-axis spacing) over a (a-axis spacing), c/a ratio, from (001) a) Author to whom correspondence should be addressed. Electronic mail: [email protected]. Tel.: 886-4-22875741. Fax: 886-4-22857017. 0021-8979/2011/109(7)/07A713/3/$30.00 V C 2011 American Institute of Physics 109, 07A713-1 JOURNAL OF APPLIED PHYSICS 109, 07A713 (2011) Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
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Magnetic properties and microstructure of Ag2Se/FePt particulate films
Jai-Lin Tsai,a) Hsueh-Wei Tai, and Hsin-Te TzengDepartment of Materials Science and Engineering, National Chung Hsing University, Taichung, Taiwan
(Presented 16 November 2010; received 14 September 2010; accepted 18 November 2010;
published online 24 March 2011)
Multilayer Ag/[Ag2Se(t)/FePt(1nm)]10 (thickness t ¼ 0.1–0.4 nm) were alternately deposited on a
glass substrate and subsequently annealed by rapid thermal process (RTP) at 800 �C for 3 min.
After RTP, the interface between FePt and Ag2Se was intermixed to form particulate films. The
grains size of the L10 FePt decreased from 9.8 to 7.7 nm when the total thickness of Ag2Se
intermediate layer increases to 1 nm. The Ag/(Ag2Se/FePt)10 particulate film showed perpendicular
magnetization and a slight increase in out-of-plane coercivity over that of the original thickness of
Ag2Se. The Ag2Se atoms were immiscible with FePt phase but the FePt grains were refined and
separated well from the Ag2Se phase. Compared with Ag/FePt bilayer, the grains of the FePt were
refined and uniformly separated in the Ag/[Ag2Se(t)/FePt]10 multilayer with t¼ 0.1 nm.VC 2011 American Institute of Physics. [doi:10.1063/1.3553943]
I. INTRODUCTION
The granular magnetic recording media was used con-
ventionally. Smaller grain size in magnetic media is required
to maintain the signal to noise ratio as the densities involved
in recording increase. The (001) textured L10 FePt thin film
has been much discussed because of its high intrinsic magne-
tocrystalline anisotropy (Ku) which is required for hard mag-
netic materials and high-density magnetic recording media.
Thermal stability has been achieved even with grain size as
small as 5 nm, because of its high uniaxial anisotropy.1–4 To
attain the smaller grain size, films are annealed at specific
temperatures to embed the FePt grains into the nonmagnetic
matrix or by preparing the Fe/Pt multilayer with intermediate
nonmagnetic layers. Such processes also bestow the charac-
teristic to isolate FePt grains and to form a particulate struc-
ture. The L10 FePt films with perpendicular magnetic
anisotropy have been prepared through many processes such
as homogeneous or heterogeneous epitaxy, sputtering, sput-
tering Fe/Pt multilayer on a single-crystal substrate with
appropriate underlayers,5–7 or by forming the c-axis texture
on an amorphous substrate.8–10 FePt films with an Ag
top layer, underlayer, FePt/Ag multilayer, and FePtAg-C
granular films have been discussed extensively due to the
immiscibility between FePt and Ag.11,12 A (001)-textured
FePt-(TiO2, SiO2) nanocomposite films composed of isolated
grains of 5 nm FePt have also been reported.13,14 In our pre-
vious work, the Ag/FePt bilayer and Ag/FePt/Ag trilayer
annealed by rapid thermal process (RTP) at 800 �C had per-
pendicular magnetization.15 Ag has high thermal diffusivity
that reduces the ordering temperature of FePt but enhances
grain growth during the RTP. In this study, we compared
the magnetic properties and microstructure of multilayer Ag/
(Ag2Se/FePt)10 and Ag/FePt bilayer. Compound Ag2Se has a
lower melting point (897 �C) than that of Ag (962 �C) and is
characterized by a cubic structure with high ionic
conductivity above the transition temperature. Ag2Se is
known for its polymorphism and high ionic conductivity and
has traditionally been considered a potential thermoelectric
material and super-ionic conductor.16 The recent discovery
of high magnetoresistance (MR) has greatly enhanced inter-
(d) t¼ 0.3 nm, and (e) t¼ 0.4 nm. FIG. 2. (Color online) In-plane and out-of-plane magnetic hysteresis loops
of (a) Ag/FePt bilayer, (b) Ag/[Ag2Se(t)/FePt]10 multilayer, t¼ 0.1 nm,
(c) t¼ 0.2 nm, and (d) t¼ 0.4 nm.
07A713-2 Tsai, Tai, and Tzeng J. Appl. Phys. 109, 07A713 (2011)
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
FePt grains in oxide matrices such as SiO2, TiO2, and MgO.
The intermetallic compound Ag2Se has a much lower melt-
ing point than oxide which may reduce the ordering
temperature of FePt phase.
IV. CONCLUSIONS
Multilayer Ag/(Ag2Se/FePt)10 with perpendicular magnet-
ization were fabricated on a glass substrate with FePt grains
well isolated by Ag2Se phase. The size of FePt grains reduced
from the Ag/FePt bilayer to particulate Ag/[Ag2Se(0.1 nm)/
FePt]10 film. The average FePt grains size was distributed
more uniformly in particulate Ag/[Ag2Se(0.1 nm)/FePt]10 films
than Ag/FePt bilayer.
ACKNOWLEDGMENTS
The authors acknowledge the NSC for financial support
under grant number NSC 99-2221-E-005-071. They also
acknowledge the Center of Nanoscience and Nanotechnology
in NCHU for the TEM investigation.
1D. Weller, A. Moser, L. Folks, M. E. Best, W. Lee, M. F. Toney,
M. Schwickert, J.-U. Thiele, and M. F. Doerner, IEEE Trans. Magn. 36,
10 (2000).
2M. H. Hong, K. Hono, and M. Watanabe, J. Appl. Phys. 84, 4403 (1998).3T. Shima, K. Takanashi, Y. K. Takahashi, and K. Hono, Appl. Phys. Lett.
88, 063117 (2006).4J. P. Liu, C. P. Luo, Y. Liu, and D. J. Sellmyer, Appl. Phys. Lett. 27, 483
(1998).5G. R. Trichy, D. Chakraborti, J. Narayan, and J. T. Prater, Appl. Phys.
Lett. 92, 102504 (2008).6C. Feng, Q. Zhan, B. Li, J. Teng, M. Li, Y. Jiang, and G. Yu, Appl. Phys.
Lett. 93, 152513 (2008).7M. L. Yan, N. Powers, and D. J. Sellmyer, J. Appl. Phys. 93, 8292 (2003).8Y. Xu, J. S. Chen, and J. P. Wang, Appl. Phys. Lett. 80, 3325 (2002).9Y. C. Wu, L. W. Wang, and C. H. Lai, Appl. Phys. Lett. 91, 072502
(2007).10J. S. Kim, Y. M. Koo, B. J. Lee, and S. R. Lee, J. Appl. Phys. 99, 053906
(2006).11L. Zhang, Y. K. Takahashi, A. Perumal, and K. Hono, J. Magn. Magn.
Mater. 322, 2658 (2010).12Z. L. Zhao, J. Ding, J. B. Yi, J. S. Chen, J. H. Zeng, and J. P. Wang,
J. Appl. Phys. 97, 10H502 (2005).13T. J. Zhou, B. C. Lim, and B. Liu, Appl. Phys. Lett. 94, 152505 (2009).14Y. C. Wu, L. W. Wang, and C. H. Lai, Appl. Phys. Lett. 93, 242501
(2008).15J. L. Tsai, H. T. Tzeng, and G. B. Lin, J. Alloys Compd. 487, 18–23
(2009).16B. C. Mohanty and S. Kasiviswanathan, Thin Solid Films 515, 2059
(2006).17R. Xu, A. Husmann, T. F. Rosenbaum, M. L. Saboungi, J. E. Enderby, and
P. B. Littlewood, Nature 390, 57 (1997).18J. S. Kim and Y. M. Koo, Thin Solid Films 516, 1147 (2008).
FIG. 3. (Color online) TEM images, selective area
diffraction (SAD) patterns, and average grains
size of (a) Ag/FePt bilayer, (b) Ag/[Ag2Se(t)/FePt]10 multilayer, t¼ 0.1 nm, (c) t¼ 0.2 nm, and
(d) t¼ 0.4 nm.
07A713-3 Tsai, Tai, and Tzeng J. Appl. Phys. 109, 07A713 (2011)
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp