Microstructure and magnetic properties of CoPt-SiNx/Ag thin films

Microstructure and magnetic properties of CoPt-SiN

/ Ag thin films

1

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

2

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

共Presented on 8 November 2007; received 11 September 2007; accepted 22 October 2007; published online 30 January 2008兲

Different amounts of amorphous SiNx nonmagnetic material and magnetic Co50Pt50 alloy were cosputtered on the different thicknesses of Ag films at room temperature then annealed at different temperatures. When the thickness of Ag underlayer is 25 nm, the CoPt/Ag film has a minimum in-plane squareness 共S储兲 which is about 0.35. The out-of-plane squareness 共S⬜兲, out-of-plane coercivity共Hc⬜兲, and saturated magnetization 共Ms兲 values of the CoPt/Ag 共25 nm兲 film are about

0.95, 15 kOe, and 420 emu/cm3_{, respectively. From the microstructure analysis of CoPt-SiN}

x/Ag

共25 nm兲 films with different volume percent of SiNxcontent, it is found that the average grain size

of CoPt decreases from about 80 to 9 nm when the volume percent of SiNxis increased from 0%

to 50%. The S_⬜, Hc⬜, and Msvalues of the共CoPt兲50-共SiNx兲50/Ag films are about 0.5, 7.5 kOe, and 200 emu/cm3_{, respectively. © 2008 American Institute of Physics.关DOI:10.1063/1.2832342兴}

Both the CoPt and FePt alloy films have high magneto-crystalline anisotropy and high thermal stability as they ap-ply to the ultrahigh density magnetic recording medium.1 These two films possess a face-centered-cubic 共fcc兲 phase which could be transferred to a face-centered-tetragonal共fct兲 phase by introducing the proper underlayer behind the FePt or CoPt films.2–4 It has been reported that the strain energy caused by the misfit between the CoPt layer and Ag layer provides a driving force for the ordering of CoPt film as the Ag underlayer is introduced beneath the CoPt films.5 Accord-ing to Zhang et al.,6in order to decrease the recording noise, the grain size of the magnetic film has to be smaller than 10 nm. Moreover, the exchange coupling effect should be minimized in order to lower the transition noise. Several re-searchers controlled the grain size of the magnetic film by adding nanomagnetic materials关BN,7SiO2,8 Al2O3,9and C 共Ref. 10兲兴. In our previous study,11we added the SiNx into

the FePt films and examined the magnetic properties of 共FePt兲1−y-共SiNx兲y films. In this work, we will discuss the

addition of SiNxceramic material to the CoPt films that have

many good properties such as oxidation resistance, corrosion resistance, and wear resistance. Here, we introduced the dif-ferent thicknesses of Ag underlayers beneath the CoPt films to achieve a transformation of CoPt films from fcc to fct structure, and added SiNxceramic materials to the CoPt films

in an attempt to reduce the grain sizes of CoPt films. Different thicknesses of Ag underlayers were deposited on naturally oxidized Si 共100兲 wafer and glass substrates at room temperature. Different volume percent of SiNxceramic

material and 20 nm Co₅₀Pt₅₀ films were cosputtered on the Ag films with different thicknesses at room temperature. Fi-nally, 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 spec-trometer and an atomic force microscope, respectively.

Mag-netic properties of the films were measured by using a vi-brating sample magnetometer with a maximum applied field of 15 kOe and a superconducting quantum interference de-vice. The film structure was examined by an x-ray diffracto-meter共XRD兲 and a field emission gun high resolution trans-mission electron microscope共FEG-TEM兲.

Figure1shows the ratio of I001/I111as a function of Ag underlayer thickness with different annealing temperatures, where I001and I111are the related integrated intensities of the 共001兲 and 共111兲 peaks. It is found that the I001/I111ratio of CoPt/Ag film is increased from 0 to 13 as Ag thickness increases from 0 to 50 nm, then decreases rapidly with fur-ther increase of Ag thickness when the annealing tempera-ture is 700 ° C. The maximum I001/I111ratio occurs at about 100 nm Ag underlayer when the annealing temperatures are 600 and 500 ° C. It has been found that the orientation of the easy axis of CoPt magnetic layer was strongly influenced by the Ag underlayer, and the Ag underlayer would also pro-mote the formation of the CoPt共001兲 orientation; these re-sulted in enhancing the CoPt L10ordering easily.12,13For the CoPt/Ag film annealed at 700 °C, the I001/I111ratio of the CoPt films decreases rapidly as the Ag underlayer thickness

a兲_{Tel. No.: ⫹886-2-2364881. FAX: ⫹886-2-23634562. Electronic mail:} [email protected].

FIG. 1.共Color online兲 I001/I111ratio of the共CoPt兲1−y-共SiNx兲y/Ag films as a

function of Ag underlayer thickness annealed at different temperatures. JOURNAL OF APPLIED PHYSICS 103, 07F514共2008兲

0021-8979/2008/103共7兲/07F514/3/$23.00 103, 07F514-1 © 2008 American Institute of Physics

is more than 50 nm. This is because the Ag共002兲 orientation will be formed and increased gradually as the Ag thickness is increased. However, the Ag 共002兲 orientation will suppress the Ag共111兲 preferred orientation and also break up the ep-itaxy between Ag 共111兲 and CoPt 共001兲. Figures 2共a兲 and 2共b兲are the relationships among squareness共S_⬜兲, squareness 共S储兲, and the thickness of the Ag underlayer of CoPt/Ag films at different annealing temperatures. It is found that as the Ag underlayer thickness increases from 0 to 200 nm, the

S_⬜ value of CoPt/Ag film is kept above 0.9 and the

mini-mum S储value is about 0.35, which occurs at 25 nm Ag, when the CoPt/Ag film is annealed at 700 °C. From the XRD analysis of the CoPt/Ag 共25 nm兲 film annealed at 700 °C, it is observed that the fct共001兲 and fct 共002兲 superlattice lines appeared, whereas the intensity of the 共111兲 peak is sup-pressed. This means that the Ag underlayer is effective in promoting both the growth of the L10phase and the chemical ordering of CoPt, as shown in Fig.3. The S_⬜, S储, Hc⬜, and

Msvalues of the CoPt/Ag 共25 nm兲 film annealed at 700 °C

are about 0.95, 0.35, 15 kOe, and 420 emu/cm3, respec-tively. Therefore, the 25 nm Ag film is chosen to be the underlayer due to its large perpendicular magnetic aniso-tropy.

Figure 3 shows the x-ray diffraction patterns of the 共CoPt兲1−y-共SiNx兲y/Ag films annealed at 700 °C for 30 min.

It is found that the peak intensity of CoPt 共001兲 and CoPt 共002兲 peaks are very high for the CoPt/Ag film 共y=0兲. This indicates that the CoPt/Ag film without adding the SiNx

pos-sesses good perpendicular magnetic anisotropy. As the SiNx

content of共CoPt兲1−y-共SiNx兲y/Ag films increases, the

intensi-ties of CoPt共001兲 and CoPt 共002兲 superlattice lines decrease rapidly. This implies that the SiNxwill break up the epitaxy

between CoPt共001兲 and Ag 共111兲, and lower the perpendicu-lar magnetic anisotropy of the film.

Figures 4共a兲–4共d兲 show the FEG-TEM images of CoPt/Ag, 共CoPt兲_95.7-共SiNx兲4.3/Ag, 共CoPt兲77.8-共SiNx兲22.2/Ag, and共CoPt兲50-共SiNx兲50/Ag films, respectively. The bright re-gions and the dark rere-gions are SiNxmatrix and CoPt grains,

respectively. The average grain sizes of Figs. 4共a兲–4共d兲 are about 80, 50, 30, and 8 nm, respectively. For the 共CoPt兲1−y-共SiNx兲y/Ag films with low SiNxcontents, it

con-tains a nonuniform distribution of CoPt particles connected to each other, as shown in Figs. 4共b兲 and4共c兲. As the SiNx

content achieves 50 vol %, the CoPt particle sizes are de-creased drastically to about 9 nm, as shown in Fig.4共d兲. In this film, the CoPt grains are isolated by the SiNxmatrix and

are more uniform than that of lower SiNx contents

共CoPt兲1−y-共SiNx兲y/Ag films. To observe the effect of SiNx

volume percent on the coercivity 共Hc兲 of

共CoPt兲1−y-共SiNx兲y/Ag films annealed at 700 °C, the changes

of Hcvalues with SiNxcontents are shown in Fig.5. As the

FIG. 2. 共Color online兲 关共a兲 and 共b兲兴 Relationships among the S_⬜, S储and

thickness of Ag underlayers of CoPt/Ag films annealed at different temperatures.

FIG. 3. 共Color online兲 X-ray diffraction patterns of the

共CoPt兲1−y-共SiNx兲y/Ag films annealed at 700 °C for 30 min.

FIG. 4. FEG-TEM images of共a兲 CoPt/Ag, 共b兲 共CoPt兲95.7-共SiNx兲4.3/Ag, 共c兲 共CoPt兲77.8-共SiNx兲22.2/Ag, and 共d兲 共CoPt兲50-共SiNx兲50/Ag films.

07F514-2 Fang et al. J. Appl. Phys. 103, 07F514共2008兲

SiNxcontent is 4.3 vol %关Fig.4共b兲兴, the CoPt particles are

isolated partially and the particle sizes are not uniform. It is suggested that a few CoPt particles below the single particle domain diameter共Ds兲 cause a decrease of Hc⬜and Hc储of the 共CoPt兲95.7-共SiNx兲4.3/Ag film. When the SiNx content is

be-tween 4.3 and 22.2 vol %, the particle size distribution of CoPt becomes more uniform. In these films, the SiNxmatrix

will impede the reverse of the spin moments of CoPt. There-fore, the high Hc⬜ and Hc储 values of these films are due to pinning site effect of SiNxmatrix. When the SiNxcontent is

higher than 22.2 vol %, the Hc⬜ and Hc储 of 共CoPt兲1−y-共SiNx兲y/Ag films decrease rapidly when the SiNx

content is increased. The FEG-TEM image of Fig. 4共d兲 showed that the particle size distribution of CoPt is uniform and some particles are smaller than 8 nm. By calculating the

Dp共minimal stable particle diameter兲 with anisotropy energy

density 共Ku= 3.6⫻106erg/cm3兲 and anisotropy field 共Hk

= 30 kOe兲 of CoPt,14

we obtained a Dp of about 8.8 nm.

When the particle size is smaller than 8.8 nm, the thermal agitation effect will cause the Hc⬜ and Hc储 of 共CoPt兲1−y-共SiNx兲y/Ag films to decrease drastically.

A further important research is to reduce the ordering temperature of CoPt/Ag film and 共CoPt兲1−y-共SiNx兲y/Ag

films. To obtain the ordered CoPt phase at low temperatures, some researchers focused on the addition of Ag into the CoPt film to reduce the activation energy and promote the ordering process.15,16 Moreover, in order to lower the ordering tem-perature of FePt films, some researchers increased the atomic diffusivity, thereby enhancing the kinetics of ordering of FePtCu films.17,18 These studies suggest that lowering the ordering temperature of fcc to L1₀FePt phase transformation can be obtained by adding Cu nanoparticles. Based on the above references, experiments involving the addition of Cu into the共CoPt兲_1−y-共SiNx兲y/Ag films are on-going in our

labo-ratory. We expect that the 共001兲 texture of 共CoPt兲1−y-共SiNx兲y/Ag films can be maintained and the

order-ing temperature can be lowered. On the other hand, the ad-dition of Ag onto共CoPt兲1−y-共SiNx兲y/Ag films will be

inves-tigated in the near future.

In summary, a large perpendicular magnetic anisotropy of a CoPt/Ag film can be obtained by adding 25 nm Ag underlayer to the CoPt 共20 nm兲 film. The S_⬜, S储, Hc_⬜, and

Msvalues of the CoPt共20 nm兲/Ag 共25 nm兲 film annealed at

700 ° C are about 0.95, 0.35, 15 kOe, and 420 emu/cm3_, re-spectively. On the other hand, adding amorphous SiNx

non-magnetic material to the CoPt film can isolate CoPt particles and reduce the particle size of CoPt film efficiently, and a uniform particle size distribution granular structure 共CoPt兲50-共SiNx兲50/Ag film with an average particle size of about 9 nm was obtained.

This work was supported by the National Science Coun-cil 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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FIG. 5. 共Color online兲 Effect of SiNx volume percent on the Hc of

共CoPt兲1−y-共SiNx兲y/Ag films annealed at 700 °C.

07F514-3 Fang et al. J. Appl. Phys. 103, 07F514共2008兲