Effect of Co/Sn doping on the thermal stability of magneticproperties of Ba-ferrite thin films

IEEE TRANSACTIONS ON MAGNETICS, VOL. 34, NO. 2, MARCH 1998 381

Effect of

Co/Sn Doping on the Thermal Stability of Magnetic Properties of Ba-Ferrite

Thin Films

Chih-Ming Kuo, P. C. Kuo and Yung-Yuan Wu

Institute of Materials Science and Engineering, National Taiwan University, Tgipei, Taiwan J. W. Chen

Department of Physics, National Taiwan University, Taipei, Taiwan

Abstram-BaFe,,.,,Co,Sn,O,, (where x=O

-

1.03) thin films were made by rf magnetron sputtering in Ar-0, mixed atmosphere. These films were then post-annealed in air between 850°C and 1000

"C

for 10 minutes, in order to get M-type phase. The contents of Co/Sn on the magnetic properties and thermal stability of the magnetic properties of the films were discussed.

I. INTRODUCTION

Recently, there has been interested in using thin film BaM for high density magnetic recording media, because it possess not only excellent magnetic properties but also has good mechanical and chemical stability. Sputtered Ba-ferrite thin films under normal conditions are amorphous structure in nature and they are nonmagnetic [I]. Hence, crystalline BaM thin films have to be obtained by either applying in-situ substrate heating during deposition [2] or through a post- deposition annealing treatment [3].

Many investigators [4-61 indicate that Co/Ti doped BaM particles could reduce grain size of the particles. However, their temperature coefficient of coercivity dHc/dT is about +3-+40ePC [ 7 ] . Kubo et al. [7] have substituted various third elements into the Ba-ferrite particles. They found that Sn is the most effective element in reducing dHc/dT of the particles. Li and Sinclair [SI found that temperature coefficient of coercivity dHc/dT of the Ti/Zn doped BaM thin film is about + O S OePC for perpendicular coercivity and

-

30e/"C for in-plane coercivity.

In this report, we investigate the effect of Co/Sn doping on the thermal stability of the magnetic properties of the BaM films. The effects of Co/Sn contents and annealing temperature on the perpendicular and in-plane magnetic properties of the Co/Sn doped BaM thin films were also examined.

11. EXPERIMENT

BaFe,,,,Co,Sn,O,, (where x=O- 1.03) thin films were made by rf magnetron sputtering system in the Ar

-

0, mixed

Manuscript received June 30, 1997

Chih-Ming Kuo. 886-2-363-023 1-3399, fax 886-2-363-4562, f3508035 This work was supported by the National Science Council of ROC @cc ntu edu.tw, P C Kuo, 886-2-363-0231-3038, pc!ao@ccms ntu edu tw through Grant No NSC 86-2216-E 002-029

atmosphere, then post-annealed in air between 850°C and 1000"Cfor 10 minutes. Co/Sn doped BaM targets were used in this experiment. All of the films were deposited onto a room-temperature fused silica glass substrate. The base pressure in the system was 1 X IO" Torr, and after the high purity Ar-0, mixed gas (Ar : O2 = 9 : I) was introduced, sputtering pressure was 5 X Torr. Thickness of the films was 100

nm.

The structure and phase types of the films were determined by using x-ray diffractometer (XRD). Magnetic properties of the films were measured with vibrating sample magnetometer (VSM) at room temperature with maximum applied field of 12 kOe. Composition and homogeneity of the films were determined by energy disperse x-ray spectrometer (EDS). Thickness of the films was measured by a a-step.

111. RESULTS AND DISCUSSION

Figure 1 shows the coercivity Hc and saturation magnetization Ms of the BaFeI2~,,Co,S~O,, thin films as a function of Co/Sn content x. These films were post-annealed at 950°C for 10 minutes in air. It can be seen that both the out-plane coercivity Hci and in-plane coercivity Hal decrease dramatically with increasing x while the saturation magnetization Ms decreases slowly with increasing

x.

For pure BaM film, its Hci is about 3200 Oe, Hal is about 2200 Oe, and Ms is about 320 emu/cm3. As x is increased to 0.83, they decrease to about 1 100 Oe, 850 Oe, and 240 emu/cc, respectively for x=0.83. The out-plane coercivity always higher than that of in-plane for the same x. The increasing of Co/Sa content

x

will suppress the out-plane coercivity more

than that of the in-plane coercivity. Hence, we can get nearly

magnetic isotropy BaM thin films ( i.e., Hcl 3 Hal ) by

doping proper amount of Co/Sn.

The magnetic moment of Ba-ferrite unit cell is coming

from the resultant moment of Fe" ions at spin-up (12k,2b)

and spin-down (4fJ sites [9]. The decrease of Ms value with increasing x is due to that the substitution of these magnetic

Fe3+ ions with nonmagnetic Sn4' ions. On the other hand, the Fe3' ions at 12k and 2b sub-lattices are major contributors to the overall uniaxial anisotropy of BaM ferrite [IO]. The substitution of Fe3' ions with Cozcand Sn4+ ions at 12k and 2b 0018-9464/98$10.00 0 1998 IEEE

3 82

sites will destroy the exchange interaction between adjacent 12k and 2b sites and reduce the coercivity of BaM .

-0- parallel to film plane

3000 -0- saturation magnetization

8

U

I

X

and the saturation magnetization Ms as function of

,,

thin films These films were Co/Sn-doping x for the B

post-annealed at 950°C for 1

ows the variation of Ms with temperature between 20°C and 100°C for various BaFe,,,,Co,Sn,O,, thin films. It is obviously that the value of Ms decreases with increasing temperature. In figure 2, we can see that the slope of Ms vs. T curve is decreased with increasing

x.

This is due to that the variation o temperature is dominated by the interactions of 12k and 12k-12k ions for small x ted by the perturbing effect of

(~=0.62-1.02) [ll]. 350 --I-x=O -0--=0,62

"6

.

3

5

v

r"

- X = O 21 -A- x=O 83 -v-x=l 03 0- 1 0 250 - A-___ -A v---V., 1 v---v----__ 40 60 80 100 *0° 2 0 ' " " ' ' y Temperature ("C)

Fig 2 The Ms value of BaFe,,,,Co,Sn,O,, thin films as function of temperature between 20°C and 100°C

The temperature coefficient of coercivity, defined by dHc/dT, was determined as dHc/dT = [ Hc (100°C) - Hc (20

"C)]

/ 80°C. Where Hc (100°C) and Hc (20°C) are the

coercivities at 1OO"C a dHcidT as a f i c t i o n

and in-plane dHc/dT

Fig 3 The temper for various BaFe,,,,Co,Sn,O,,

grain growth. It reveals t be obtained when the a 950°C. As TA is lower nonmagnetic

a

-Fe >950"C. Hc is als

"C,

but it decreases <950°C, the increase phase as shown in Fi decrcase of Hc is due was calculated

383

r

I

20 30 40 50 60

2 0 (degree)

Fig. 4 X-ray diffraction patterns of the B&eio&Oo62Sb6& thin films which are annealed at various temperatures: (a) 850°C ; (b) 900°C; (c) 950°C ; (d) 1000°C.

-0- parallel to film plane -0- saturation magnetization 2000

8

₀

r

1000 I 5 O 0 l I 500

1

./---o-a

-1

i

-w

O ‘ 850 ’ 900 950 1000 Annealing temperature TA (“C)

Fig 5 Coercivities and the saturation magnetization as function of annealing temperature for the BaFe,,,6Co,62Sn,,,0,, thin films. These films were post- annealed for 10 minutes in air.

IV. CONCLUSION

The CofSn doped barium ferrite (BaM) thin films had been prepared by using rf magnetron sputtering. Ms, Hcl, and Hcli of the film was decreased with increasing Co/Sn concentration. For the BaFe,, ,6C00 6$& ,,0i9 thin film, both

in-plane and out-plane coercivities almost keep constant with

temperature between 20 “C and 100 “C

.

Its out-plane coercivity is about 1400 Oe and in-plane coercivity is about 1100 Oe. The saturation magnetization Ms is about 270 emu/cm3 and the average grain size is about 20 nm. It reveals that this BaFe,, 76C00 6$&, 6 2 0 i 9 thin film has excellent

magnetic thenbal stability.

I

850 ’ 900 950 1000

Annealing temperature TA (“C)

Fig. 6. Average grain size as function of annealing temperature for the Bak,o&O,&~62019 thin films. These films were post-annealed at 850°C

-1000°C for 10 minutes in air.

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1981

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--

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