Effect of Co layer thickness on the magnetic properties of Tb30Co70/SiNx/Co thin films

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Effect of Co layer thickness on the magnetic properties of

Tb

30

Co

70

/SiN

x

/Co thin ﬁlms

G.P. Lin

a

, S.C. Chen

b

, P.C. Kuo

a,

, P.L. Lin

a

, K.T. Huang

a

, Y.H. Fang

a

Institute of Materials Science and Engineering, National Taiwan University, Taipei 10617, 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: TbCo Co Exchange effect Perpendicular recording

a b s t r a c t

The SiNx (20 nm)/Tb30Co70 (90 nm)/SiNx (5 nm)/Co (3–37 nm)/SiNx (10 nm)/Si multilayer ﬁlms are

deposited on naturally oxidized Si wafer by magnetron sputtering. The saturation magnetization (Ms) of

the multilayer ﬁlms is increased with the thickness of high Ms ferromagnetic Co layer. The

perpendicular coercivity (Hc?) value is increased with Co layer thickness as the thickness of the Co layer is lower than 15 nm and then decreases drastically when the thickness of the Co layer further increased. The increase of the Hc? value is owing to the interlayer exchange effect [Li Zhang, Physica B 390 (2007) 373] between TbCo and Co layers. Co under-layer with in-plane magnetic anisotropy would pin the magnetic moment of the TbCo layer near by the Co layer and cause the value of Hc?to increase. However, as the Co layer is thicker than a critical thickness, the Hc?value of the multilayer ﬁlm would decrease. Therefore, the Co layer with in-plane magnetic anisotropy and soft magnetic properties is expected to dominate the magnetic properties of the multilayer ﬁlms.

1. Introduction

Heat-assisted magnetic recording (HAMR) method had been proposed for future ultra-high density magnetic recording[1–3]. Very small recording bit size is needed for an ultra-high density magnetic recording. However, the decrease of recording bit size is limited by the thermal stability of recording bit.

Amorphous TbCo alloy ﬁlm is a promising candidate for HAMR medium due to its excellent perpendicular anisotropy and large Hc?. Moreover, the amorphous TbCo without grain boundaries is

expected to have low noise and large signal-to-noise ratio. However, the saturation magnetization (Ms) of the ferrimagnetic

TbCo ﬁlm is too small (150 emu/cm3_{) for GMR head to readout.}

In this paper, we investigate the magnetic properties of sputtered TbCo/SiNx/Co multilayer ﬁlms and discuss its potential

for HAMR medium application. A ferromagnetic Co layer with large Msis introduced under the TbCo alloy ﬁlm in an attempt to

enhance the Msof multilayer ﬁlms. The exchange coupling effect

at the interface of the hard and soft magnetic layers is revealed in the M–H loop.

2. Methods

The SiNx(20 nm)/Tb30Co70(90 nm)/SiNx(5 nm)/Co (3–37 nm)/

SiNx (10 nm)/Si multilayer ﬁlms were deposited on

nature-oxidized Si (1 0 0) wafer at room temperature. In which the SiNx

is used to prevent TbCo and Co from oxidization and diffusion between the TbCo layer and Co layer during the HMAR writing process [3]. The TbCo ﬁlm was fabricated by dc magnetron co-sputtering (base pressure was 5 107_{Torr) of pure Tb (99.95%)}

and Co (99.95%). The power of magnetron gun of Co and Tb was kept at 2.32 and 0.99 W/cm2_{, respectively. The Co ﬁlm was also}

fabricated by dc magnetron. The power of the magnetron gun of the Co target was kept at 2.96 W/cm2_{. The SiN}

xﬁlm was prepared

by rf magnetron sputtering of the Si3N4target. The power of the

magnetron gun of the Si3N4target was kept at 4.93 W/cm2. The Ar

pressure PArwas kept at 3 mTorr for TbCo and Co ﬁlms deposition.

The thickness of the film was measured by an atomic force microscope. The composition of the film was estimated by an energy dispersive spectrometer. The cross-sectional image of films was observed with a transmission electron microscope. Magnetic properties of the films were measured by using a vibrating sample magnetometer with a maximum applied field of 15 kOe.

3. Results and discussion

The HRTEM cross-sectional image of the SiNx/Tb30Co70/SiNx/Co

(37 nm)/SiNx/Si multilayer ﬁlm shows that the TbCo ﬁlm is still

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kept at amorphous state after introducing the Co under layer, as shown in Fig. 1. The roughness between SiNxand TbCo causes

increase in the coercivity of multilayer ﬁlms due to creation of pinning sites[4].

Fig. 2shows the M–H loops of the SiNx/Tb30Co70(90 nm)/SiNx/Si

multilayer film. The TbCo film has excellent perpendicular anisotropy and large Hc?. However, the M_sof the TbCo film is too

small.

The M–H loops of the SiNx/Tb30Co70/SiNx/Co/SiNxmultilayer

film with 9 nm of ferromagnetic Co layers are shown in Fig. 3. Because the two layers reverse independently in the in-plane direction, there are two different switching fields and a large step was observed in the in-plane M–H loop. The magnetic moment of the Co layer reversed at small field (less than 500 Oe). But, the magnetic moment reversion of TbCo occurred at larger switching field. However, when the field is in the out-plane direction, the

two layers reversed almost simultaneously due to the strong coupling effect [5], and only a small step is observed at 500–1000 Oe ﬁeld.

When the thickness of the Co under-layer in TbCo/SiNx/Co

multilayer ﬁlms exceeded the critical thickness, the Hc? value

decreased as shown inFig. 4. The Co layer thickness ofFig. 4is 37 nm. Large loss of perpendicular anisotropy in the TbCo/SiNx/Co

multilayer ﬁlm occurs. Because the two layers reverse indepen-dently, a small step is observed at about 1000 Oe ﬁeld.

Because the TbCo and Co layer have magnetic anisotropies perpendicular and parallel to the ﬁlm, respectively, the coupling effect make the magnetic moments dispersive around the TbCo/ SiNx/Co interface, as shown inFig. 5 [6]. The out-plane squareness

of the multilayer ﬁlm is reduced as Co under-layer is being introduced.

It was found that the Hc?value of the TbCo/SiN_x/Co multilayer

ﬁlms is increased with increase in the thickness of the Co layer, as shown inFig. 6. The exchange coupling effect exists between TbCo and Co layers[7]. The Co layer has in-plane magnetic anisotropy, it would affect part of the TbCo layer which is near by the Co layer varying from perpendicular to in-plane anisotropy and form a pinned TbCo layer, as shown inFig. 5. The exchange coupling effect will increase the Hc? value of multilayer ﬁlms. However,

when the Co layer is thicker than a critical thickness, the Co layer

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Fig. 1. The cross-sectional HRTEM image of the SiNx/Tb30Co70(90 nm)/SiNx(5 nm)/

Co (37 nm)/SiNx(10 nm)/Si multilayer.

Fig. 2. M–H loops of the SiNx/Tb30Co70(90 nm)/SiNx/Si multilayer ﬁlm.

Fig. 3. M–H loops of the SiNx/Tb30Co70(90 nm)/SiNx/Co (9 nm)/SiNx/Si multilayer

ﬁlm.

Fig. 4. M–H loops of the SiNx/Tb30Co70(90 nm)/SiNx/Co (37 nm)/SiNx/Si multilayer

ﬁlm.

G.P. Lin et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 3117–3119 3118

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with in-plane magnetic anisotropy and soft property would dominate the magnetic properties of the multilayer ﬁlms and the Hc? value would decrease seriously. Therefore, the interlayer

exchange effect would increase with the thickness of the Co under-layer. The multilayer ﬁlm with critical thickness of the Co layer would have maximum Hc?value. But, when the Co layer is

thicker than a critical thickness, the magnetic properties of ferromagnetic Co would overcome the exchange effect.

The saturation magnetization of the TbCo/SiNx/Co multilayer

ﬁlms is increased with increase in the thickness of the Co layer, as shown inFig. 6. The enhancement of Msis due to the increase of

the volume fraction of the ferromagnetic Co layer, which has much larger value of Msthan that of the ferrimagnetic TbCo layer.

4. Conclusions

Introducing a ferromagnetic Co under-layer with thickness lower than a critical thickness under the TbCo alloy ﬁlm could

enhance its Ms and Hc? values. The enhancement of M_s is

due to the increase of the volume fraction of the ferromagnetic Co layer. An exchange coupling effect was found between a TbCo hard magnetic layer and a soft Co layer. The increase of Hc?value

is owing to the exchange coupling effect. But, the TbCo and Co layers have magnetic anisotropy in different directions; the coupling effect makes the magnetic moment become dispersive around the TbCo/SiNx/Co interface and the out-plane squareness

decrease.

Acknowledgements

This research was support by the National Science Council and Ministry of Economic Affairs of Taiwan through NSC Grants 95-2221-E-002-119-MY3 and 97-EC-17-A-08-S1-006, respectively.

References

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

[3] Robert E. Rottmayer, et al., IEEE Trans. Magn. 42 (2006) 2417.

[4] J. Swerts, K. Temst, N. Vandamme, C. Van Haesendonck, Y. Bruynseraede, J. Magn. Magn. Mater. 240 (2002) 380.

[5] K.C. Schuermann, J.D. Dutson, J. Appl. Phys. 99 (2006) 08Q904. [6] O. Redon, P.P. Freitas, J. Appl. Phys. 83 (1998) 2851.

[7] Neil Smith, William C. Cain, J. Appl. Phys. 69 (1991) 2471.

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TbCo

Co SiNx

Fig. 5. Exchange effect between TbCo and Co layers.

Fig. 6. Effects of Co layer thickness on the Msand Hc?value of the SiNx/Tb30Co70/

SiNx/Co/SiNx/Si multilayer ﬁlms.