Soft magnetic properties of evaporated Fe-Al-Ti alloy films

Journal of Magnetism and Magnetic Materials 102 (1991) 139-143 North-Holland

Soft magnetic properties

of evaporated

Xing-Bo

Yang and T. Miyazaki

Department

of Applied Physics, Faculty of Engineering,

Fe-Al-Ti

alloy films

Tohoku lJniversi@, Sendai 980, Japan

Received 8 April 1991; in revised form 7 June 1991

Fe-Al-Ti alloy films were prepared at 673 K by an electron-beam evaporation method. Excellent soft magnetic properties (a permeability of about 2000 at 5 MHz and a magnetization of about 1.9 T) were obtained. It is suggested that it is possible to develop soft magnetic film materials with high saturation magnetization, high permeability and small magnetostriction in the Fe-Al alloy system.

1. Introduction Soft magnetic film materials with high saturation magnetization, high permeability and small magnetostriction are required for recently-developed high-density magnetic recording technology [ll. A great attention has been paid to Fe-based crystalline alloy films because of the high magnetization of iron [2]. In general, the soft magnetic properties of polycrystalline materials are strongly determined by their grain size and crystallographic orientation. Recent results indicate that the bee iron and Fe-rich alloys whose structures are composed of nanocrystallites show excellent magnetic softness. The refinement of crystalline grains can be achieved by means of multilayering of films [3], homogeneous crystallization of amorphous precursors [4,5] or nitrogenation of films [61. It has also been reported that the soft magnetic properties can be improved by controlling the preferred orientation of crystallites [7,8]. In the above-mentioned investigations, however, the magnetic softness is realized in a nonequilibrium state, and most of them suffer from thermally induced variations. We devoted our effort to searching for new alloy film materials with a good thermal stability and excellent soft magnetic properties. We have extended our study interest to the Fe-Al-Ti alloy system. It is well known that the magnetostric0304-8853/91/$03.50

tion of binary Fe-Al bulk alloys shows a value close to zero at about 10 at% [9]. According to early data, addition of 0.5-1.0 wt% titanium to Fe-Al alloys reduced the hysteresis loss and shghtly raised the permability, while larger additions had the opposite effect [lo]. These two points as well as the high affinity between Al and Ti atoms seem to suggest a possibility of developing a new soft magnetic material in the Fe-Al alloy system by addition of a small amount of Ti (< 1 at%>. This article descirbes the soft magnetic properties of evaporated Fe-Al alloy films containing a small amount of Ti.

2. Experimental procedures Fe-Al-Ti alloy buttons (0 I Al I 20 at%, 0 I Ti I 15 at%> which have been made by arc-melting of 99.5 wt% iron, 99.999 wt% aluminum and 99.9 wt% titanium, were used as the master alloys for film deposition. In order to avoid a large deviation of the actual Ti content in films from the expected values due to the much smaller vapor pressure of Ti, the Ti content in the starting alloys was increased up to 15 at%. The FeAl-Ti alloy films were deposited by an electronbeam evaporation method onto slide glass sub-

0 1991 - Elsevier Science Publishers B.V. All rights reserved

140

Xing-Bo Yang, T. Miyazaki / Soft magnetic properties of Fe-Al-Ti

strate (Matsunami Micro Slide Glass S-1126) in a vacuumsystem maintained at about 1 x lo-’ Torr during the deposition. The system was pumped below 2 X lop6 Torr prior to film deposition and the substrate temperature was kept at 673 K during the deposition. The evaporation rate was about 1.8-2.0 rim/s.. In order to induce anisotropy in the film, a field of 30 Oe was applied along the plane of the substrate and kept until the substrate was cooled down to room temperature. The deposited film had a disk shape with a diameter of 13 mm and thickness of 100-400 nm. The composition of the films was analyzed by an electron probe microanalyzer (EPMA). It was found that the actual concentration of Ti in the films is below 0.5 at% Ti, which significantly deviates from the Ti content of the master alloys. Saturation magnetization, anisotropy field and coercive force were measured by a vibrating sample magnetometer (VSM) in fields up to 200 Oe. The ac hysteresis loop was measured by a homemade ac hysteresis loop tracer at 50 Hz with a maximum ac field of 20 Oe. Effective permeability at high frequencies was measured by an inductive method using a ferrite core. Magnetostriction was determined by an optical cantilever method in fields up to 1 kOe. All samples were analyzed by X-ray diffraction using a diffractometer with CuKa radiation. Additional structural information was obtained by means of scanning electron microscopy (SEM).

jilms

B (kG) I

I

I F%.lA'll

.BTiO.11

Prepared at 673 K

I-

I

:

I

1 I 0 I ; I

I I I I I

_qc

-0

;

I I ; I 1 ;

__ i I

-:

----I lb

H.A. : E. A.

I ’ 20 H (Oe)

I

Fig. 1. Hysteresis curves measured along the easy and hard directions of magnetization for Fe8s~,Al11~sTi,,,I film.

from dc measurement are shown in fig. 2 for Fe 99.WXAlXTi0.11 alloy films. Both E-r, and H, take a minimum around 10 at% Al, showing a similar composition dependence. As was stated in the introduction, we are interested in the large magnetization and highfrequency permeability of the film. Figure 3 shows the composition dependence of the magnetiza-

II4

3. Results and discussions Figure 1 shows the hysteresis loops of Feas.lAL*TicUi film with a thickness of about 100 nm prepared at 673 K. Two curves measured along two directions normal to each other (the easy axis and hard axis> can be observed from this figure. An H, of about 2 Oe along hard axis, 5.5 Oe along the easy axis direction, an anisotropy field (H,) of about 7 Oe and a saturation magnetization of 1.9 T can be obtained. The slight uniaxial in-plane anisotropy was induced by the applied magnetic field present during deposition. The coercive force measured along the hard axis in an ac field and anisotropy field derived

Fe99.89-xA1xT10.11

I

Fig. 2. Coercive force measured along the hard axis and anisotropy field for Fe,,s,_,Al,Ti,,,, alloy films.

Xing-Bo Yang, T. Miyazaki / Soft magnetic properties of Fe-AI-Ti

241

,

,

,

,

,

,

,

,

I

,

,

141

films

2-200 3

,

3_ 1800

%~.v”I~.~“o.II Preuared at 673 K

1600

a? 12

-

- :

Bozorth

1 0

I 4

I

1400

I 8

I

I 12

I

I 16

I

I 20

I 24

=L

800 t

-0

'=99.67-xR'x"0.33

2

t

fp

b \

I 107

the Fe-Al alloy films with different Ti contents. For comparison, the static permeability of binary Fe-Al alloys in bulk state [12] is also shown in fig. 4a by a broken line. The ac and dc permeabilities exhibit a maximum around 9-10 at% Al, being independent of the Ti content of the film. This indicates that at least the magnetostriction and/or anisotropy is very small at about 10 at% Al. As an example, the frequency characteristic

tion measured in a field of 200 Oe for Fe 99.89~X~XTi0.11films. The solid line shown in this figure is the magnetization of Fe-Al alloy in bulk state [ll]. We can see that the Z3, values of both film and bulk alloys are in agreement. The effective permeability measured at 5 MHz along the hard-axis direction is shown in fig. 4 for

1

1

I 106

f (Hz 1 Fig. 5. Frequency dependence of permeability measured along its hard axis for Fe,,,,Al,, sTi,,,, film.

[ill.

t

I IO5

X Fig. 3. Composition dependence of saturation magnetization alloy films. The solid line shows the of Fe sa.sa-&xTio.tt magnetization of Fe-AI alloy in bulk state given by Bozorth

2000

-

I

-4

““““I 0

4

8

12

16

20

,X

“/,__--__.

4

8

12

16

20

24

X

Fig. 4. Composition dependence of effective permeability measured at 5 MHz along the hard-axis direction for the Fe-AI films with different Ti contents. The broken line shows the static permeability of binary Fe-Al alloys in bulk state [12].

142

Xing-Bo Yang, T. Miyazaki / Soft magnetic properties of Fe-Al-Ti

of the permeability of Fe,,,Al,,,sTiO,,, film along its hard axis is shown in fig. 5. A nearly frequency-independent permeability of about 2000 is obtained up to frequencies of 10 MHz. In order to clarify the reasons for the high permeability of Fe-Al-Ti films, measurements of magnetostriction and X-ray diffraction and SEM observation were carried out. We find that the magnetostriction of the film around the composition corresponding to the maximum of permeability is as small as 2 x 10m6. Therefore, the maximum of permeability around 10 at% Al can be attributed to the small magnetostriction. The X-ray diffractograms of the films with a large permeability and saturation magnetization are shown in fig. 6, indicating the films are composed of a single bee phase. The SEM in-plane image of Fe,,,,Al,,,sTi,,ii film is given in fig. 7. The average size of the crystallites obtained from SEM observation and from Scherrer’s equation by using the half width of the (110) X-ray diffraction peak is about 20-40 nm. The small crystallite size can result in a decrease of anisotropy fluctuation and thus a relatively large permeability can be realized. Summarizing the above results, we can say that the large permeability realized in the Fe-Al-Ti alloy films results from the small magnetostriction and the relatively small grain size of crystallites. The addition of the small amount of Ti can

I

30

40

I

I

50

60

I

I

100

Fig. 6. Typical X-ray diffractograms of the Fe-Al-Ti

films.

( degree

80

I

90

213

70

1

films

Fig. 7. SEM in-plane image of Fe,,,AllI,sTio,l,

film.

give rise to the following effects; i> the decrease of grain size; ii) the solution of Ti in Fe-Al alloys which reduces the magnetostriction and/or anisotropy of Fe-Al alloys; iii> the purification of the alloy films during the evaporation process because the presence of a suitable amount of Ti in the master alloys can tie up many impurity elements in the master alloys; and iv) the improvement of the local vacuum between the evaporation source and the substrate. Although the reasons for the high permeability obtained in the Fe-Al-Ti alloy films are not clear yet, the present study at least suggests the possibility to develop excellent soft magnetic film materials with high permeability and large magnetization in the Fe-Al alloy systems. Therefore, a considerable amount of research work in the other Fe-Al alloy film systems is still needed.

4. Summary Fe-Al alloy films with the addition of a small amount of Ti were prepared at 673 K by an

Xing-Bo Yang, T. Miyazaki / Soft magnetic properties of Fe-AI-Ti

electron-beam evaporation method. A high permeability of about 2000 (at 5 MHz) combined with a large magnetization of 1.9 T was obtained. A possibility to develop soft magnetic film materials with high saturation magnetization, high permeability and small magnetostriction is suggested.

Acknowledgements The authors would like to thank Mr. R. Goto, Electron-Devices Department, Moka work, Hitachi Metals Ltd. and Mr. F. Sato, Department of Applied Physics, Tohoku University, for their helpful discussions. This work was supported by The Kurata Research Grant, Japan.

films

143

References [l] J.K. Howard, J. Vat. Sci. Technol. A 4 (1986) 1. [2] K. Hayashi, M. Hayakawa, W. Ishikawa, Y. Ochiai, H. Matsuda, Y. Iwasaki and K. Aso, J. Appl. Phys. 61 (1987) 3514. [3] R. Nakatani, T. Kobayashi, S. Otomo and N. Kumasaka, Gyo Butsuri 59 (1990) 688 (in Japanese). [4] Y. Yoshizawa, S. Oguma and K. Yamauchi, J. Appl. Phys. 64 (1988) 6044. [5] N. Hasegawa and M. Saito, J. Magn. Sot. Jpn. 14 (1990) 313. [6] M. Takahashi and T. Shimatsu, IEEE Trans. Magn. MAG-26 (1990) 1485. [7] F.W.A. Dime, M. Brouha, C.H.M. Witmer and J.L.C. Daams, Mater. Sci. Eng. 99 (1988) 57. [8] A. Hosono and Y. Shimada, J. Appl. Phys. 67 (1990) 6981. [9] H. Honda, H. Masumoto, Y. Shirakawa and T. Kobayashi, Nippon Kinzoku Gakkai-si 12 (1948) 2 (in Japanese). [lo] R.M. Bozorth, Ferromagnetism (Van Nostrand, New York, 1964) p. 256. [ll] R.M. Bozorth, Ferromagnetism (Van Nostrand, New York, 1964) p. 216. [12] M. Sugihara, J. Phys. Sot. Jpn. 15 (1960) 14556.