Magnetic properties of CoTb/FePt(001) nano-bilayer films

INSTITUTE OFPHYSICSPUBLISHING NANOTECHNOLOGY

Nanotechnology 17 (2006) 2411–2414 doi:10.1088/0957-4484/17/9/057

Magnetic properties of CoTb/FePt(001)

nano-bilayer films

Y H Fang

, P C Kuo

, T H Yang

, A C Sun

, C Y Chou

, S C Chen

and P L Lin

1_{Institute of Materials Science and Engineering, National Taiwan University, Taipei 106,} Taiwan

2_{Department of Materials Engineering, MingChi University of Technology, Taipei 243,} Taiwan

E-mail:[email protected]

Received 21 November 2005, in final form 13 March 2006

Published 19 April 2006

Online at

stacks.iop.org/Nano/17/2411

Abstract

The magnetic properties and microstructure of CoTb/FePt bilayer films are

investigated. The magnetic anisotropy of the Co

Tb

/FePt

(001) film is

perpendicular to the film plane. The saturated magnetization (M

) and

perpendicular coercivity (H

) of the Co

Tb

/FePt

(001) bilayer films are

higher than those of single-layered Co

Tb

films. The H

and M

are

5450 Oe, and 403 emu cm

, respectively. As the temperature is increased to

200

C, there is a rapid decrease in the H

value.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

The heat-assisted magnetic recording (HAMR) method, which is a combination of thermal magnetic writing and magnetic flux detection, has been suggested for increasing the recording density of magnetic disks [1, 2]. The medium used for HAMR must (i) provide satisfactory magneto-optical writing performance, (ii) have a large saturation magnetization to generate sufficient magnetic flux for giant magnetoresistive (GMR) head readout, and (iii) possess a large Hc⊥ to resist thermal agitation of the magnetic moments. CoTb is a rare-earth metal alloy possessing a large Hc⊥value, perpendicular anisotropy (Ku⊥), and amorphous structure [3]. Moreover, the amorphous CoTb alloy has a short-range order without grain boundaries [3]. Therefore, it is a potential medium, which is expected to have low noise and high recording density, in HAMR.

Recently, the surface/interface roughness of magnetic thin films has been found to influence magnetic properties, such as magnetic anisotropy, coercivity, magnetoresistance, and magnetic domain structure [4, 5]. Various works on the relationship between the surface roughness and coercivity of thin films have been carried out [6–8]. In addition to the surface roughness, it is known that the thickness, composition, and crystalline structure of the magnetic film, as well as the 3 _{Author to whom any correspondence should be addressed.}

preparation condition, also determine the magnetic properties of films. In this work, bilayer films of CoTb with an FePt(001) under-layer were fabricated. The magnetic properties of this bilayer system were investigated to assess its potential use as an HAMR medium.

2. Experimental procedure

Fe50Pt50(001)/Pt(001)/Cr(200)films were fabricated on 7059 Corning glass substrates by conventional dc magnetron sputtering in an ultra-high vacuum sputtering chamber. The base pressure was better than 5×10−9Torr before sputtering. The substrate was heated to 350◦C to prepare the Cr(200) under-layer and the Pt(001) buffer layer. The Cr and Pt layer thicknesses were 70 nm and 2 nm, respectively. The Fe50Pt50 (20 nm) magnetic layer was deposited at 420◦C in order to form the desired L10 FePt phase [9], followed by room-temperature deposition of 20–60 nm Co70Tb30 films by dc magnetron sputtering. To avoid the oxidation of the films, a protective layer of Si3N4(20 nm) was deposited.

The film structure was examined by an x-ray diffractome-ter (XRD) and a field emission gun transmission electron mi-croscope (FEG-TEM). Composition and homogeneity of the CoTb film were assessed from the energy dispersive spectrum (EDS). The film thickness was measured by an atomic force microscope (AFM). Magnetic properties of the films were

Y H Fang et al Out-plane 200 100 -100 -200 Co₇₀Tb₃₀ thickness=500 Å -15000 -10000 -5000 0 5000 10000 15000 H(Oe) M( emu/cm 3 )

Figure 1.M–H loop of the Co70Tb30film.

1000 900 800 700 600 500 400 300 200 100

Intensity (arb. unit)

20 30 40 50

2θ (degree) 60 70

L1₀FePt (001)

L1₀FePt (002)

Cr(002)

Figure 2. The x-ray diffraction pattern of FePt/Pt/Cr tri-layer films.

measured by a vibrating sample magnetometer (VSM) with a maximum applied field of 12 kOe.

3. Results and discussion

If the Tb content was greater than 39 at.%, the CoTb films exhibit no magnetic properties because the Curie temperature (Tc) is lower than room temperature [10]. It was previously found that the Tc for a Co70Tb30 film was greater than 500 K [10–13], thus satisfying the criteria required for HAMR media. Figure1shows theM–H loop of the Co70Tb30film at room temperature. The Hc⊥andMsvalues are approximately 3600 Oe and 176 emu cm−3, respectively. This small Ms makes detection by a high-resolution giant magnetoresistive head (GMR) or a tunneling magnetoresistive (TMR) head difficult. Therefore, a FePt(001) film is introduced beneath the Co70Tb30film in an attempt to increase theMsvalue.

From a previous study [14], it was revealed that, in the absence of a Pt intermediate layer, the Cr atoms of the Cr(200) under-layer diffused directly into the FePt magnetic layer and prevented the formation of L10 FePt(001) orientation. However, by depositing Pt(001)/Cr(200) under-layers, the L10 FePt(001) and magnetic anisotropy was perpendicular to the surface [14]. In figure 2, peaks of Cr(002), along with (001) and (002)FePt are observed. This would suggest

In-plane Out-plane FePt/Pt/Cr M (emu/cm3₎ -15000 -10000 -5000 0 5000 10000 15000 800 600 400 200 -200 -500 -400 -800 H(Oe)

Figure 3.M–H loops of FePt/Pt/Cr tri-layer films.

6000 5000 4000 3000 2000 1000 0 600 500 400 300 200 100 0 20 30 40 Co₇₀Tb₃₀ thickness (nm) HC ⊥ (Oe) Ms(emu/cm 3 ) HC_⊥ Ms 50 60

Figure 4. Variations of Hc⊥and Msvalues of

Co70Tb30/FePt(001)/Pt/Cr films with the film thickness of Co70Tb30 in Co70Tb30/FePt(001)/Pt/Cr.

that the FePt/Pt/Cr trilayer films have perpendicular magnetic anisotropy. Indeed, figure3shows that the magnetic FePt(001) film exhibits perpendicular magnetic anisotropy with an out-of-plane squareness (S_⊥) of around 0.8. TheMsandHc⊥values are about 691 emu cm−3and 3100 Oe, respectively.

The variation ofHc⊥andMswith Co70Tb30film thickness at room temperature in the Co70Tb30/FePt(001)/Pt/Cr system is shown in figure 4. It is noticed that, as the thickness of the Co70Tb30film ranges between 20 nm and 60 nm, the Hc⊥ value of the Co70Tb30/FePt(001)/Pt/Cr films increases from about 2100 Oe to 5700 Oe and the Msvalue decreases from 530 emu cm−3to 365 emu cm−3. According to the study of Wan et al [15], the interaction of the domain wall increases with film thickness. This is considered to be the explanation behind the increased Hc⊥ value of the Co70Tb30 films with increasing thickness. As an HAMR medium is required to have large values ofHc⊥andMsat room temperature, the Co70Tb30 film with a thickness of 50 nm is examined further.

After introducing the under-layer of L10 FePt(001) film into the Co70Tb30 (50 nm) film, the M–H loops of the Co70Tb30/FePt(001)/Pt/Cr films are shown in figure5. It can be seen that the Hc⊥, Ms, S⊥ and in-plane squareness (S) are 5450 Oe, 403 emu cm−3, 0.82 and 0.38, respectively. Figure6shows the M–H loops of the Co70Tb30, FePt(001) and Co70Tb30/FePt(001)/Pt/Cr films alone. It is seen that 2412

Magnetic properties of CoTb/FePt(001) nano-bilayer films Out-plane In-plane CoTb/FePt/Pt/Cr M (emu/cm3) -15000 -10000 -5000 0 5000 10000 15000 400 200 -200 -400 H(Oe)

Figure 5.M–H loops of the Co70Tb30/FePt(001)/Pt/Cr films.

CoTb/FePt/Pt/Cr FePt/Pt/Cr CoTb M (emu/cm3) -10000 -5000 0 5000 10000 800 600 400 200 -200 -500 -400 -800 H(Oe) ∆Ms = 227 emu/cm3

Figure 6. Different M–H loop of Co70Tb30/FePt(001)/Pt/Cr, Co70Tb30and FePt(001)/Pt/Cr film.

the Ms value of the Co70Tb30/FePt(001)/Pt/Cr film (Ms = 403 emu cm−3) lies between those of the Co70Tb30 (Ms = 176 emu cm−3) and the FePt (Ms = 691 emu cm−3) single-layered films. This is attributed to the fact that Msis related to the moments of the domain per unit volume of the material. On the other hand, theHc⊥value of Co70Tb30/FePt(001)/Pt/Cr films is larger than that of either single-layered Co70Tb30or FePt(001) films. This is due to the roughness of the interface. The roughness of the interface is inferred from the cross-sectional transmission electron microscope (TEM) image of the Co70Tb30/FePt(001)/Pt/Cr film, shown in figure 7. The surface roughness between the Co70Tb30and FePt(001) films is found to be quite high. The magnetic properties are very sensitive to surface roughness. This roughness could create areas of different domain orientation. Thus the uncompensated surfaces are increased and this causes the total number of spins in one direction to be reduced [16]. Moreover, the rough surface provides more pinning sites and impedes the motion of domain walls at the interface between the Co70Tb30 and FePt(001) films [17–22]. It indicates that the magnetic moment needs a larger magnetic field to reverse, so the Hc⊥ value is increased due to high surface roughness.

TheHc⊥andMsvalues of the Co70Tb30/FePt(001)bilayer films at various temperatures are plotted in figure 8. It can be seen that Ms remains at an almost constant value of around 400 emu cm−3 across the temperature range studied. However, the Hc⊥ value decreases from 5450 Oe to 730 Oe

Figure 7. The cross-sectional TEM image of the

Co70Tb30/FePt(001)/Pt/Cr film. 450 400 350 300 250 200 Ms(emu/cm 3) Hc(Oe) 150 100 50 0 0 50 100 T(°C) 150 200 250 6000 5000 4000 3000 2000 1000 0

Figure 8. The relationship of the Msvalue, Hc⊥value, and the temperature of the Co70Tb30/FePt(001) bilayer films.

as the temperature increases from 25◦C to 200◦C. The rapid decrease in theHc⊥value with temperature satisfies the writing requirements of the HAMR medium.

4. Conclusions

It has been shown that Co70Tb30/FePt(001)films have a high Msvalue at room temperature. On the other hand, due to a high interface roughness between the Co70Tb30and FePt(001) films, the Co70Tb30/FePt(001)bilayers also have a largeHc⊥value at room temperature. The Co70Tb30/FePt(001)/Pt/Cr films thus appear to be a promising material as a HAMR medium.

Acknowledgments

This work was supported by the National Science Council and Ministry of Economic Affairs of Taiwan through the NSC grants 94-2216-E-002-009 and 94-EC-17-A-08-S1-0006, respectively.

References

[1] Saga H, Nemoto H, Sukeda H and Kesteren H W 1999 Japan. J. Appl. Phys. 138 1839

[2] Ruigrok J J M, Coehoorn R, Cumpson S R and Kesteren H W 2000 J. Appl. Phys.87 5398

Y H Fang et al

[3] Kuo P C and Kuo C H 1998 J. Appl. Phys.84 3317 [4] Chang C H and Kryder M H 1994 J. Appl. Phys.75 6864 [5] Bruno P, Bayureuther G, Beauvillain P, Chappert C, Lugert G,

Renard D, Renard J P and Seiden J 1990 J. Appl. Phys. 68 5759

[6] Malyutin V I, Osukhovskii V E, Vorobiev Yu D,

Shishkov A G and Yudin V V 1981 Phys. Status Solidi a 65 45

[7] Vilain S, Ebothe J and Troyon M 1996 J. Magn. Magn. Mater. 157 274

[8] Jiang Q, Yang H N and Wang G C 1997 Surf. Sci.373 181 [9] Sun A C, Kuo P C, Chen S C, Chou C Y, Huang H L and

Hsu J H 2004 J. Appl. Phys.95 7624

[10] Betz J, Mackay K and Givord D 1999 J. Magn. Magn. Mater. 207 180

[11] Hansen P, Raasch D and Mergel D 1994 J. Appl. Phys.75 5267 [12] Hansen P, Clausen C, Much G, Rosenkranz M and

Witter K 1989 J. Appl. Phys.66 756

[13] Hansen P, Klahn S, Clausen C, Much G and Witter K 1991 J. Appl. Phys.69 3194

[14] Sun A-C, Kuo P C, Hsu J-H, Huang H L and Sun J-M 2005 J. Appl. Phys.98 076109

[15] Wan H, Tsoukatos A and Hadjipanayis G C 1993 J. Magn. Magn. Mater.125 157

[16] Nogues J and Schuller I K 1999 J. Magn. Magn. Mater. 192 203

[17] Yang W, Lambeth D N, Tang L and Laughlin D E 1997 J. Appl. Phys.81 4370

[18] Matsumoto K, Ozaki K and Chekanov A 2000 Japan. J. Appl. Phys.39 1161

[19] Katayama H, Watanabe K, Takayama K, Sato J-I, Miyanishi S and Ohta K 2002 Appl. Phys. Lett.81 4994 [20] Li M, Zhao Y P and Wang G C 1998 J. Appl. Phys.83 6287 [21] Ng V, Hu J F, Adeyeye A O and Wang J P 2002 J. Appl. Phys.

91 7206

[22] Chang C-H and Kryder M H 1994 J. Appl. Phys.75 6864