Effects of Tb content on the microstructure and magnetic properties of Co85?XTbXDy15 films

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Journal of Magnetism and Magnetic Materials 272–276 (2004) e485–e486

Effects of Tb content on the microstructure and magnetic

properties of Co

85X

Tb

X

Dy

15

ﬁlms

P.C. Kuo

a,

*, C.T. Lie

a

, S.C. Chen

b

, C.Y. Chou

a

, T.H. Wu

a

, Y.C. Chen

a

Institute of Materials Science and Engineering, and Center for Nanostorage Research, National Taiwan University, Taipei 106, Taiwan b

Department of Mechanical Engineering, De Lin Institute of Technology, Taipei 236, Taiwan

Abstract

The Co85XTbXDy15 ﬁlms ðx ¼ 0224 at%Þ were prepared at room temperature by DC magnetron sputtering.

Transmission electron microscopy analysis indicated that all the ﬁlms were amorphous. The magnetization

measurement revealed that increasing Tb content would reduce the saturation magnetization Ms of the ﬁlm but the

out-plane coercivity Hc> was increased. The Ms and Hc> values of the Co85Dy15 ﬁlm ðx ¼ 0 at%Þ were about

640 emu=cm3and 50 Oe; respectively. However, the Msand Hc> values of the Co61Tb24Dy15 ﬁlm ðx ¼ 24 at%Þ were

PACS: 75.50.Kj; 75.70.i

Keywords: Co85XTbXDy15thin ﬁlms; Saturation magnetization; Out-plane coercivity

Amorphous rare-earth-transition metal (RE-TM) alloy ﬁlms have special perpendicular anisotropy and

have been widely studied by many investigators [1–3].

We have reported an as-deposited CoTb amorphous film with large perpendicular coercivity, and a nearly magnetically isotropic amorphous CoTb film could be obtained by annealing this film in vacuum at 250_{C for}

60 min [4]. In this work, we studied the effects of Tb content on the microstructure and magnetic properties of the Co85XTbXDy15 ﬁlm.

The Co85XTbXDy15 ﬁlms with x ¼ 0224 at% were

deposited on natural-oxidized silicon wafer substrates by DC magnetron sputtering at room temperature. The target was a Co disk overlaid with Dy and Tb pieces, to yield the desired film composition. The film thickness was 100 nm and the film was sandwiched between two

SiNx protective layers that were prepared by rf

magnetron sputtering of Si3N4 target. The thickness of

the SiNx protective layer was 15 nm: The structures of

the films were investigated by transmission electron microscopy (TEM). The composition of the film was determined by energy-dispersive spectroscopy (EDS). The thickness of the film was measured by atomic force microscope (AFM). The magnetic properties of the film were measured by a vibrating sample magnetometer (VSM) at room temperature with a maximum applied field of 13 kOe:

The Co85XTbXDy15ﬁlms were examined by TEM to

understand the effect of Tb content on the structure of the CoDy ﬁlm. It was found that all the Co85XTbXDy15

ﬁlms ðx ¼ 0224 at%Þ were amorphous.Fig. 1(a) shows

a TEM bright ﬁeld image of the Co74Tb11Dy15ﬁlm and

Fig. 1(b) shows, the electron selected area diffraction (SAD) pattern ofFig. 1(a). There are no crystal grains and the shape of the SAD pattern is a broad halo, implying that the ﬁlm is amorphous.

Fig. 2shows the variations of saturation magnetization Ms; out-plane coercivity Hc>and in-plane coercivity Hcjj

with Tb content of the Co85XTbXDy15 ﬁlm. The Ms

value decreases with increasing Tb content. Ms value of

the Co85Dy15 ﬁlm ðx ¼ 0Þ is about 640 emu=cm

3

: It decreases to about 20 emu=cm3 as x is increased to 24. CoDy alloy is sperimagnetic[5], and the magnetization of

ARTICLE IN PRESS

*Corresponding author. Tel.: 2-2364-8881; fax: +886-2-2363-4562.

E-mail address: [email protected], pckuo@ntu. edu.tw (P.C. Kuo).

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Co subnetwork is antiparallel to that of Dy subnetwork. Net magnetization of Co85Dy15alloy is parallel to that of

Co subnetwork. The decrease of Ms of the

Co85XTbXDy15 ﬁlm with increasing Tb content is due

to the fact that the direction of net moment of the Tb atoms is opposite to that of Co atoms. The variation of Hcjjwith Tb content is small but the Hc>value is increased

rapidly with increasing Tb content as x > 11: The Hc>

value increases from about 1:5 kOe for x ¼ 11 to about 10 kOe for x ¼ 24: Magnetic easy direction of the ﬁlm is changed from isotropy to perpendicular gradually as the Tb content is increased, because Tb atoms have large out-plane magnetic anisotropy constant Ku>[5].

Fig. 3shows the variations of out-plane squareness S> and in-plane squareness Sjj with Tb content of the

Co85XTbXDy15 ﬁlm, where S>¼ Mr>=Ms and Sjj¼

Sjj=Ms Mr> and Mrjj are the out-plane and in-plane

remanent magnetization, respectively. The S>of

CoTb-Dy ﬁlm is increased with increasing Tb content. S>

increases from 0.07 to about 1 as x increases from 0 to 24. This is due to the fact that Tb has large Ku>: The S>was larger than 0.85 as x > 14 at%. The curve of Sjj

versus Tb content is like the peak behavior. The peak of Sjjoccurs at xB11 that is about 0.45. When Tb content

is more than 18 at%; Sjjis increased with increasing Tb

content. As x > 11 at%; S>of the Co85XTbXDy15 ﬁlm

is larger than Sjj:Sjjof the ﬁlm is always lower than 0.5

when 0%x%24; as shown inFig. 3.

We have investigated the effects of Tb content on the

magnetic properties and microstructure of the

Co85XTbXDy15 ﬁlms ðx ¼ 0224 at%Þ: TEM analysis

indicated that all the ﬁlms were amorphous. The magnetization measurement revealed that the addition of Tb would reduce the Msvalue of the Co85Dy15 ﬁlm

but Hc> was increased. S> was about 1 and

Hc>B10 kOe as the Tb content increased to 24 at%:

This work was supported by the National Science Council and Ministry of Economic Affairs of R.O.C. through Grant No. NSC 90-2216-E 002-036 and 91-EC-17-A-08-S1-0006, respectively.

References

[1] H. Katayama, K. Watanabe, K. Takayama, J. Sato, S. Miyanishi, K. Kojima, K. Ohta, Appl. Phys. Lett. 81 (2002) 4994.

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

[3] J. Schmidt, G. Skidmote, S. Foss, E.D. Dahlberg, C. Merton, J. Magn. Magn. Mater. 190 (1998) 81.

[4] P.C. Kuo, C.M. Kuo, J. Appl. Phys. 84 (1998) 3317. [5] P. Hansen, Handbook of Magnetic Materials, in: K.H.J.

Bus-chow (Ed.), Vol. 6, Elsevier, New York, 1991, p. 310 (Chapter 4).

ARTICLE IN PRESS

Fig. 1. (a) TEM bright ﬁeld image and (b) electron diffraction pattern of the Co74Tb11Dy15ﬁlm. 0 5 10 15 20 25 0 200 400 600 800 1000 Ms (e mu /c m 3 ) Tb (at.%) Ms 0 2 4 6 8 10 12 Hc (k Oe ) Hc_⊥ Hc//

Fig. 2. Variations of Ms; Hc>and Hcjjwith Tb content of the

Co85XTbXDy15ﬁlm. 0 5 10 15 20 25 0.0 0.2 0.4 0.6 0.8 1.0 Tb content ( at.% ) Squareness ( Mr/Ms ) S⊥ S_//

Fig. 3. Variations of S> and Sjj with Tb content of the

Co85XTbXDy15ﬁlm.

P.C. Kuo et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e485–e486 e486