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-

est in silver selenide.17

II. EXPERIMENTAL

Multilayer Ag(1 nm)/[Ag2Se(t)/FePt(1 nm)]10 (thickness

t¼ 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, Ag2Se 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 (Ag2Se/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

Ka 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/

[Ag2Se(t)/FePt(1 nm)]10 multilayer with thickness (t¼ 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/[Ag2Se(t)/FePt(1 nm)]10 (t¼ 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:

tsaijl@dragon.nchu.edu.tw. Tel.: 886-4-22875741. Fax: 886-4-22857017.

0021-8979/2011/109(7)/07A713/3/$30.00 VC 2011 American Institute of Physics109, 07A713-1

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and (110) peaks or from the intensity ratio of (001) and (002)

peaks (I(001)/I(002)) due to the weak diffraction peaks of (110)

and (002). However, the (001) peak gradually shifted to higher

angles, and this is evident in Fig. 1(a)–1(c), indicating that the

lattice constant c had decreased. The ordering of multilayered

Ag/[Ag2Se(t)/FePt(1 nm)]10 (t¼ 0.1, 0.2, 0.3 nm) films is supe-

rior to that of Ag/FePt bilayer. The semiquantitative method,

Lotgering orientation factor (LOF) was used to analyze varia-

tions in the (001) preferred orientation. The LOF represents

the degree of specific texture with values from 0 to 1, where

“0” and “1” indicated a random distribution and perfect crystal

orientation, respectively. When the specific orientation is

{001}, the LOF is defined as follows:18

LOF ¼ P� P0

1� P0

where P ¼P

001ð ÞsampleP

hklð Þsample; P0 ¼

P001ð Þpowder

PðhklÞpowder

(1)

To calculate the orientation of the sample, the P value

was estimated from the intensity summation of {00l} orienta-

tion over the intensity summation of hkl (the reflection planes

appeared in XRD diffraction patterns) reflections ranging

from 20 to 80�. For the nonoriented sample (P0 value), the

simulation data were used to replace the free textured FePt

powder sample. The LOF value of Ag/FePt bilayer was 0.65,

and the LOF values of the multilayer Ag/[Ag2Se(t)/FePt(1 nm)]10 (t¼ 0.1, 0.2, 0.3 nm) were 0.64, 0.63, 0.64,

respectively. The orientation had still not deteriorated when

the amorphous Ag2Se with amount of 40 % was inserted into

the FePt layer.

Figure 2 shows magnetic hysteresis loops of Ag/FePt

bilayer and multilayer Ag/[Ag2Se(t)/FePt(1 nm)]10 (t¼ 0.1,

0.2, 0.4 nm). Figure 2(a) shows in-plane and out-of-plane

hysteresis loops of Ag/FePt bilayer annealed at 800 �C and

presenting perpendicular magnetization. In Fig. 2(a), the

out-of-plane Hc and remanence ratio are 12.4 and 0.92 kOe,

respectively. The high value of in-plane Hc was due to c-axis

dispersion, but the remanence ratio (Mr/Ms¼ 0.20) was low.

Figures 2(b)–2(d) show in-plane and out-of-plane hysteresis

loops of multilayer Ag/[Ag2Se(t)/FePt(1 nm)]10 (t¼ 0.1, 0.2,

0.4 nm) annealed at 800 �C. The out-of-plane coercivity and

remanence were much higher than the in-plane properties

for Ag/(Ag2Se/FePt) films. The Ag/(Ag2Se/FePt) films pre-

sented perpendicular magnetization with out-of-plane Hc

(11.6, 11.9, and 12.0 kOe) and remanence ratio (0.89, 0.87,

and 0.80), when the total thickness of Ag2Se reached 1, 2, 4

nm (0.1, 0.2, 0.4 nm per pair).

Figure 3 shows plane view transmission electron micros-

copy (TEM) images, selective area diffraction (SAD) pat-

terns, and average grains size of [Fig. 3(a)] Ag/FePt bilayer

and [Figs. 3(b)–3(d)] multilayer Ag/[Ag2Se(t)/FePt]10

(t¼ 0.1, 0.2, 0.4 nm), respectively. In Fig. 3(a), the image of

the Ag/FePt film annealed at 800 �C for 3 min shows that the

FePt grains were distributed on the glass substrate separately

but not uniformly. The average grain size was 9.8 nm rang-

ing widely between 1 and 21 nm. The ring patterns of (002)

and (111) planes were indexed. Figures 3(b)–3(d) show an

image of multilayer Ag/[Ag2Se(t)/FePt]10 (t¼ 0.1, 0.2, 0.4

nm) annealed at 800 �C for 3 min with the FePt grains

isolated by amorphous Ag2Se present in the particulate struc-

ture. Compared to the image of Ag/FePt bilayer, the

FePt grains size was reduced and was uniform only in Ag/

[Ag2Se(0.1 nm)/FePt]10 film.

In Fig. 3(b), the FePt grains were separated by 1 nm

Ag2Se to form the particulate structure. The FePt grains

were rounded and individually distributed. Twin bands used

to release phase transition strains were observed in a number

of FePt grains with an average grains size of 7.7 nm. Figures

3(b)–3(c) show TEM images of Ag/[Ag2Se(0.2, 0.4 nm)/

FePt]10 film, in which the grains have growth and are not

uniformly separated, with an average grain size (FePt) of 13

and 17 nm, respectively. In summary, the FePt grains were

uniformly refined and isolated by an Ag2Se layer of 1 nm

thickness. When the total thickness of Ag2Se layer was

increased to 2 and 4 nm, FePt grains have growth and are not

uniformly separated. These results are similar to those of the

FIG. 1. XRD patterns of films annealed at 800 �C for 3 min: (a) Ag/FePt

bilayer, (b) Ag/[Ag2Se(t)/FePt]10 multilayer, t¼ 0.1 nm, (c) t¼ 0.2 nm,

(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)

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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.

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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