Magnetic properties of CoPt–SiNx/Ag nanocomposite films

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Magnetic properties of CoPt–SiN

x

/Ag nanocomposite ﬁlms

Y.H. Fang

a

, P.C. Kuo

a,

, P.L. Lin

a

, S.C. Chen

b

, C.T. Kuo

a

, G.P. Lin

a

Institute of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan b

Department of Materials Engineering, MingChi University of Technology, Taipei 243, Taiwan

a r t i c l e

i n f o

Available online 8 August 2008 PACS: 75.50.Kj 75.70.i 75.70.Cn Keywords: Out-of-plane squareness In-plane squareness Out-of-plane coercivity CoPt-SiNx/Ag ﬁlm

a b s t r a c t

When the thickness of Ag under layer is 25 nm, the CoPt/Ag ﬁlm has maximum out-of-plane squareness (S?), minimum in-plane squareness (SJ), and the largest out-of-plane coercivity (Hc?), they are 0.95,

0.35, and 15 kOe, respectively. Different volume percent of SiNxceramic materials were co-sputtered

with Co50Pt50ﬁlms on the Ag under layer to reduce the grain size of the CoPt ﬁlm. Comparing the X-ray

diffraction pattern of CoPt-SiNx/Ag ﬁlms without annealing with that of the ﬁlms which annealed

at 600 and 700 1C, it is found that the intensities of CoPt (0 0 1) and CoPt (0 0 2) superlattice lines were reduced after annealing. As the SiNxcontent is raised to 50 vol%, the particle size of CoPt is reduced

to be about 9 nm.

1. Introduction

Recently, CoPt alloy has been investigated for ultra-high magnetic media application due to its high magnetocrystalline anisotropy and high thermal stability[1–4]. The as-deposited CoPt films possess face-centered-cubic (FCC) phase, which could be transferred to face-centered-tetragonal (FCT) phase by introdu-cing the proper under layer beneath the CoPt films [5–7]. Xu et al.[8]had shown that high perpendicular anisotropy CoPt film could be obtained by adding the Ag layer beneath the CoPt film. On the other hand, the growth rate of recording area density has been increased and expected to exceed 1 Tbit/in2

within few years[9–11]. For high-density recording medium, it was required that a bit dimension is only hundreds of atomic diameters. Therefore, the grain size should be decreased to be smaller than 10 nm as the bit sizes decreased. Moreover, the exchange-coupling effect should be minimized in order to lower the transition noise. Several researchers had controlled the grain size of the magnetic ﬁlm by adding nanomagnetic materials (SiO2[12], Al2O3[13], etc.). In our previous study[14],

we had added the SiNx into the FePt ﬁlms and examined the

magnetic properties of (FePt)1y–(SiNx)yﬁlms. It was found that

the average grain sizes and the in-plane coercivity (H_cJ) of (FePt)70–(SiNx)30 ﬁlm which annealed at 750 1C for 30 min are

40 nm and 8 kOe, respectively. However, there is less paper to

discuss on the SiNxceramic material inserted to the CoPt ﬁlm,

which has many good properties such as oxidation resistance, corrosion resistance, and wear resistance. In this work, we introduced different thicknesses of Ag under layers beneath the CoPt films to achieve a transformation of CoPt films from FCC to FCT structure and to obtain high perpendicular anisotropy of the CoPt film first. Then we added the SiNx ceramic material with

different volume percents to the CoPt ﬁlms and postannealing in an attempt to reduce the grain sizes of CoPt ﬁlms to be smaller than 10 nm.

2. Experiments

First, the Ag under layers were deposited on naturally oxidized Si (1 0 0) wafer and glass substrates with different thicknesses at room temperature. Then different volume percent of SiNx ceramic material and 20 nm Co50Pt50 ﬁlms

were co-sputtered on the Ag ﬁlms at room temperature. Finally, the ﬁlms CoPt-SiNx/Ag were annealed at different

temperatures for 30 min. The composition and thickness of the film were determined by an energy-dispersive X-ray spectrometer and an atomic force microscope, respectively. Magnetic properties of the films were measured by using a vibrating sample magnetometer with a maximum applied field of 15 kOe and a superconducting quantum interference device. The film structure was examined by an X-ray diffractometer and a field emission gun high-resolution transmission electron microscope (FEG-TEM).

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3. Results and discussions

3.1. CoPt (20 nm)/Ag (x nm) ﬁlms

According to the past researches[8,15,16], it has been reported that the Ag under layer has 5.7% misﬁt larger than CoPt and result in expanding the CoPt [0 1 0] and [1 0 0] axes. This in-plane expansion cause the shrinkage of the CoPt [0 0 1] axis, which is perpendicular to the ﬁlm plane. It is implied that the distortion of the CoPt unit cell may enhance the CoPt L10ordering and aiding

the formation of the L10 CoPt (0 0 1) variant with the easy-axis

perpendicular to the film plane. Therefore, we introduce the Ag films as the under layer first, and expect that the perpendicular anisotropy CoPt film can be obtained.

From analyzing the I001 and I111 ratio of CoPt/Ag ﬁlms with

different annealing temperatures (where I001 and I111 are the

related integrated intensity of the (0 0 1) and (111) peaks), it is found that the maximum I001/I111ratio occurs at about 25 nm Ag

under layer when the annealing temperatures is 700 1C.

Fig. 1 shows the X-ray diffraction patterns of the CoPt/Ag (x nm) films annealed at 700 1C for 30 min. One regularity feature ofFig. 1is the increase in the superlattice (0 0 1) intensity with the thinner Ag under layer; this indicates that the ordering process is efficiently promoted by decreasing the Ag-layer thickness. The Ag (0 0 2) orientation may suppress the Ag (111) preferred orienta-tion and broke up the epitaxy of CoPt (0 0 1) on the Ag (111) plane. In future work, the high-resolution cross-section FEG-TEM images of the CoPt/Ag (x nm) films annealed at 700 1C for 30 min will be examined to clarify this epitaxial mechanism. We can calculate the lattice parameter of Ag under layers and CoPt films, and investigate the epitaxy between the Ag and CoPt films. Moreover, the ordering parameter (Sorder) of CoPt/Ag (x nm) films were calculated, it is found that the Sorder value of the CoPt/Ag films with different Ag thicknesses are all higher than 0.9 and indicate that the CoPt/Ag films have been ordered.

Fig. 2is the M–H loop of the CoPt (20 nm)/Ag (25 nm) ﬁlms annealed at 700 1C for 30 min. It shows that the magnetic CoPt (0 0 1) ﬁlm exhibits perpendicular magnetic anisotropy with an out-of-plane squareness (S?) and in-plane squareness (SJ) of about

0.9 and 0.55, respectively. The saturated magnetization (Ms) and

out-of-plane coercivity (Hc?) values are about 420 emu/cm3and

14 kOe, respectively. Therefore, the 25 nm Ag ﬁlm is chosen to be the under layer due to the fact that it will enhance large perpendicular magnetic anisotropy of the CoPt/Ag ﬁlms.

3.2. (CoPt)1y–(SiNx)y/Ag ﬁlms

There are no CoPt (0 0 1) and (0 0 2) superlattice peaks in the X-ray diffraction pattern of the as-deposited (CoPt)1y–(SiNx)y/Ag

ﬁlms. After the (CoPt)1y–(SiNx)y/Ag ﬁlms are annealed at 600 1C

for 30 min, it is found that the ordering phase of (0 0 1) superlattice peak only appears in the CoP/Ag ﬁlm without adding SiNx, as shown in Fig. 3. It implies that the SiNxwill break up

the epitaxy between CoPt (0 0 1) and Ag (111) and lower the perpendicular magnetic anisotropy of the ﬁlm. After the (CoPt)1y–(SiNx)y/Ag ﬁlms are annealed at 700 1C for 30 min,

the CoPt (0 0 1) and (0 0 2) superlattice peaks of the (CoPt)1y–(SiNx)y/Ag ﬁlms appear when the SiNx contents are

low, as shown inFig. 4. This is because partial

g

-CoPt disordered phases transformed to

g

1-CoPt ordered phase at this annealing

temperature. This indicated that the addition of SiNx would

impede the transformation of

g

-CoPt to

g

1-CoPt phase.

Fig. 5 is the effect of SiNx volume percent on the Hc of

(CoPt)1y–(SiNx)y/Ag ﬁlms annealed at 600 and 700 1C for 30 min.

We can see that the increase of SiNx contents in the ﬁlms

will increase the annealing temperature required for phase

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Intensity (a.u.) 20 25 30 35 40 45 50 55 2θ (°) (001) (111 ) (111 ) (002) ₍₀₀₂₎ (200) FCT CoPt FCC CoPt Ag 200 nm 100 nm 50 nm 25 nm

Fig. 1. X-ray diffraction patterns of the CoPt/Ag ﬁlms annealed at 700 1C for 30 min.

600 400 200 0 -200 -400 -600 -60000 -40000 -20000 0 20000 40000 60000 H (Oe) M (emu/cm 3) in-plane out-plane

Fig. 2. M–H loop of the CoPt (20 nm)/Ag (25 nm) ﬁlm annealed at 700 1C for 30 min. Intensity (a.u.) 20 25 30 35 40 45 50 55 2θ (°) (111 ) (002) (002) (200) (111 ) (001) FCC CoPt FCT CoPt Ag 0 vol% 4.3 vol% 5.7 vol% 13.7 vol% 22.2 vol% 34.4 vol% 50 vol%

Fig. 3. X-ray diffraction patterns of the (CoPt)1y–(SiNx)y/Ag ﬁlms annealed at 600 1C for 30 min.

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transformation. To obtain the coercivity which is larger than 3 kOe, the pure Co50Pt50ﬁlm must be annealed at temperature which is

higher than 550 1C. This means that the phase transformation temperature of pure Co50Pt50is about 550 1C. However, it will raise

the phase transformation temperature to be higher than 700 1C as the SiNxis added to the Co50Pt50ﬁlm. When the amount of the

magnetic phase is ﬁxed, not only the magnetic grain sizes but also the intergranular distances are increased with the increase in annealing temperature. Moreover, as the SiNxcontent is 4.3 vol%, the

CoPt particles are isolated partially and the particle sizes are not uniform. It is suggested that some CoPt particles below the single domain diameter (Ds) cause a decrease of Hc? and HcJ values

of (CoPt)95.7–(SiNx)4.3/Ag ﬁlm. As the SiNxcontent increase from 4.3

to 22.2 vol%, the particle-size distributions of CoPt become broader. From the FEG-TEM images of (CoPt)77.8–(SiNx)22.2/Ag ﬁlm

(not shown here), we found that partial CoPt grains are isolated. Therefore, the magnetization of (CoPt)77.8–(SiNx)22.2/Ag ﬁlm

may be the domain rotation [17] and it will lead to higher Hc?

and H_cJ values of (CoPt)94.3–(SiNx)5.7/Ag ﬁlm than those of the

(CoPt)95.7–(SiNx)4.3/Ag ﬁlm. As the SiNx content is higher than

22.2 vol%, the Hc? and HcJ of (CoPt)1y–(SiNx)y/Ag ﬁlms decrease

rapidly when the SiNx content is increased. From the FEG-TEM

image of the (CoPt)50–(SiNx)50/Ag ﬁlm, it shows that the

particle-size distributions of CoPt seems more uniform than that of the

(CoPt)77.8–(SiNx)22.2/Ag ﬁlm, as shown in Fig. 6(a). From the

enlargement of FEG-TEM image of the (CoPt)50–(SiNx)50/Ag ﬁlm

(as shown inFig. 6(b)), it is found that some particles are smaller than 8 nm (Fig. 6(b)). By calculating the Dp (minimal stable

particle diameter) with anisotropy energy density (Ku ¼ 3.6 106

erg/cm3_{) and anisotropy ﬁeld (H}

k¼30 kOe) of CoPt [18], we

obtained that the Dpis about 8.8 nm. As the particle size is smaller

than 8.8 nm, the thermal-agitation effect will cause the Hc?and HcJ

of (CoPt)1y–(SiNx)y/Ag ﬁlms decrease drastically.

On the other hand, as the SiNxcontent is higher than 22.2 vol%,

the Hc?and HcJof (CoPt)1y–(SiNx)y/Ag ﬁlms which annealed at

700 1C for 30 min are higher than 7 kOe. According to Xu et al.[8], the Ag may diffuse into the CoPt ﬁlms as the Ag (111) peaks shift slightly to a high angle. This phenomenon could induce vacancies, as well as increase the mobility of Co and Pt atoms. This also results in enhancing the kinetics for transformation and promot-ing the CoPt orderpromot-ing. From the diffraction patterns of our ﬁlm (seeFig. 6(a)), it can be found that the Ag3Pt diffraction rings are

formed. This implies that the Ag has diffused into the CoPt ﬁlms and promoted CoPt ordering.

4. Conclusion

We have investigated the magnetic properties of co-sputtered nano-composited (CoPt)1y–(SiNx)y/Ag ﬁlms over the varieties of

annealing temperatures and SiNxcontents. On adding 25 nm Ag

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Intensity (a.u.) 20 25 30 35 40 45 50 55 2θ (°) (001) (111 ) (111 ) (002) (002) (200) FCC CoPt FCT CoPt Ag 34.4 vol% 50 vol% 22.2 vol% 13.7 vol% 5.7 vol% 4.3 vol% 0 vol%

Fig. 4. X-ray diffraction patterns of the (CoPt)1y–(SiNx)y/Ag ﬁlms annealed at 700 1C for 30 min. 16000 14000 12000 10000 8000 6000 4000 2000 0 10 20 30 40 50 SiNx vol% Hc (Oe) 600°C in-plane 600°C out-of-plane 700°C in-plane 700°C out-of-plane

Fig. 5. Effect of SiNxvolume percent on the Hcvalues of (CoPt)1y–(SiNx)y/Ag ﬁlms annealed at 600 and 700 1C for 30 min.

Ag3Pt (311) Ag3Pt (220) Ag3Pt (110) CoPt (001) CoPt (002) CoPt (200) CoPt SiNx

Fig. 6. FEG-TEM images of (CoPt)50–(SiNx)50/Ag films annealed at 700 1C for 30 min. (a) Low magnification and (b) high magnification.

Y.H. Fang et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 3032–3035 3034

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under layer to the CoPt (20 nm) ﬁlm, a maximum perpendicular magnetic anisotropy CoPt/Ag ﬁlm would be obtained. The S?, Hc?,

and Ms values of the CoPt(20 nm)/Ag (25 nm) ﬁlms which

annealed at 700 1C are about 0.95, 15 kOe, and 420 emu/cm3_,

respectively. Granular structure of CoPt–SiNxﬁlms with average

grain size about 9 nm could be obtained by annealing the (CoPt)50–(SiNx)50/Ag ﬁlms at 700 1C.

However, this annealing temperature is little high, the further important research is to reduce the ordering temperature of CoPt/Ag ﬁlm and (CoPt)1y–(SiNx)y/Ag ﬁlms. Therefore, the experiments about

adding the Cu into the (CoPt)1y–(SiNx)yﬁlm are going on in our

laboratory. We expect that the (0 0 1) texture of (CoPt)1y–(SiNx)y/Ag

films can be maintained and the ordering temperature can be lowered. Moreover, in order to realize perpendicular films with the easy-axis of magnetization of all grains perpendicular to the film plane, the MgO (2 0 0) under-layer can be inserted to the CoPt/Ag films to enhance the CoPt fct (0 0 2) orientation.

Acknowledgement

This work was supported by the National Science Council and Ministry of Economic Affairs of Taiwan through the NSC 95-2221-E-002-119-MY3 and 95-EC-17-A-08-S1-0006 Grants, respectively.

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