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APFIM study of the compositional inhomogeneity of sputtered CoCr magnetic thin film
al:Oied surface science ELSEVIER
Applied Surface Science 87/88 (1995) 264-270
APFIM study of the compositional inhomogeneity of sputtered Co-Cr magnetic thin film A s t r i d P u n d t a,* C a r s t e n M i c h a e l s e n b a Institut far Metallphysik, Hospitalstrasse 3-7, D-37073 GOttingen, Germany b Institut far Werkstofforschung, GKSS-Forschungszentrum, D-21502 Geesthacht, Germany Received 10 July 1994, accepted for publication 2 October 1994
Abstract An atom probe field ion microscope investigation has been performed to study the decomposition in Co-20at%Cr magnetic recording material. The measurements demonstrate the existence of a decomposition on a one nanometer scale. The concentration of these small dusters varies between 11 at% Cr and 42 at% Cr within at room temperature sputtered films. In films sputtered at 200°C the concentration varies from about 10 at% Cr to 30 at% Cr. Additionally, Cr-enriched precipitates at grain boundaries and concentration variations on a 8 nm scale were found. Annealing the samples at 750°C for 24 h leads to an almost homogeneous alloy. The measured concentrations in the sputtered samples depend on substrate temperature, respectively on annealing temperature. They are interpreted by a calculated miscibility gap in the hcp phase solid solution which is induced by magnetism at low temperatures.
1. Introduction Iwasaki and Ouchi [1] introduced sputtered C o - C r thin films as perpendicular magnetic recording material in 1978. Fibre textured growth of the hexagonal C o - C r grains leads to a magnetic easy axis perpendicular to the surface, as was demanded for this type of magnetic recording films. Magnetic measurements done by Fisher et al. [2] in 1984 on such C o - C r films show higher saturation magnetisation than expected for a homogeneous alloy with concentrations between 12 and 24 at% Cr. A reason for this behaviour could be a phase separation.
* Corresponding author. E-mail: [email protected]; Fax: +49 551 39 5012.
Since phase separation within grains might lead to an additional increase in the magnetic storage density, a lot of research concerning phase separation in C o - C r magnetic material has been done. However, investigations of phase separation by means of transmission electron microscopy or X-ray diffraction are limited because both the atomic size difference and the atomic scattering contrast is small [3]. Therefore Maeda et al. [4,5] etched the films to remove Co in order to investigate the microstructure of Cr. Chrysanthemum like patterns were found in the etched transmission electron microscopy samples suggesting a compositional inhomogeneity on a 10 nm scale. Compositional studies were performed by Parker et al. [6] using M/Sssbauer spectroscopy and Yoshida et al. [7] using nuclear magnetic resonance. Recently, Hono et al. [8-10] analysed as-sputtered
0169-4332/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSDI 0 1 6 9 - 4 3 3 2 ( 9 4 ) 0 0 4 8 0 - 3
A. Pundt, C. Michaelsen /Applied Surface Science 8 7 / 8 8 (1995) 264-270
Co-22at%Cr films deposited at 200°C substrate temperature with the atom probe field ion microscope. Concentration variations were found which were attributed to the chrysanthemum pattern found by Maeda et al. In this paper we present our investigation on microstructure, composition and its dependence on substrate and annealing temperature, respectively, in sputtered C o - C r films on a one nanometer scale using the G/Stfingen field ion microscope including time-of-flight mass spectrometry [11,12].
2. Experimental Sputtered C o - C r magnetic layers of 50 nm and 3 /zm thickness were investigated. From the two types of layers FIM tips were obtained by different preparation methods: in the case of the thinner films, tungsten wires were first electropolished to a tip
265
radius of about 20 nm [13]. Subsequently, a 50 nm Co-20at% Cr layer was sputtered onto these tips at an Ar pressure of 3.7 × 10 -1 Pa. The tip temperature is expected to increase to about 100°C during deposition. In the same sputtering procedure C o - C r films (100 nm) were deposited on planar A1203 substrates to characterise the hcp structure by X-ray diffraction. In the second preparation method tips were prepared from 3 / z m sputtered Co-22at%Cr films. These films were deposited onto flat Cu-coated Si- substrates from a Co-22at%Cr alloy target at Ar pressure of 1.1 Pa. During deposition, the substrate temperature was 200°C. Ribbons with square crosssection (0001 in plane) were produced by lithography technique [14]. Finally, these ribbons were electropolished from both sides to prepare FIM tips. Within these samples only measurements in the planar direction of the film, i.e. perpendicular to the
a.
<0001>
b.
!
!
10 n m
100 n m Fig. 1. (a) Sample geometry of a thin C o - C r film deposited on a tungsten-substrate tip. (b) Superimposed SEM micrographs of the FIM tip before and after deposition. The inner bright part is the substrate tungsten tip with radius of curvature of about 20 nm. The thickness of the C o - C r layer is about 50 nm. (c) FIM-micrograph of a 50 nm C o - C r film. The 0001 pole can be seen indicating a grain growth perpendicular to the substrate surface even at the tip apex.
266
A. Pundt, C. Michaelsen /Applied Surface Science 8 7 / 8 8 (1995) 264-270
0001 direction, are possible. Annealing of the samples has been done under HV of 1 × 10 -5 Pa before the electropolishing procedure to avoid oxidation inside the FIM tips, A P analyses were performed under U H V conditions with a tip temperature of about 63 K. A pulse ratio of 16% was taken in all measurements. FIM micrographs were mainly obtained using a gas mixture of 5 × 10 - 4 Pa Ne and 1 × 10 - 4 Pa He gas.
by means of small rises on the tungsten substrate which have to reappear on the surface of the layer (arrows). Atom probe analyses of 50 nm C o - 2 0 a t % C r f i l l s (Ts < 100°C) show regions having solute concentration that corresponds to the nominal composition, as well as regions of compositions suggesting phase separation. In Fig. 2a a typical analysis is plotted as a ladder diagram, i.e. the number of Cr atoms is plotted versus the total number of analysed atoms in which the slope corresponds to the concentration of the analysed region. In such a diagram, concentration variations can be checked in detail: in about 1 nm distances the concentration changes from more than 42 at% Cr to less than 11 at% Cr. Since the analysis cylinder is of the same size as these 1 nm regions the real concentration difference could be larger. A corresponding FIM micrograph is shown in Fig. 2b. Bright regions correspond to low Cr concentrations as measured by a selected area analyses. They are of the same size as determined by A P analysis. Since these concentration variations are on such a fine scale, composition profiles with fixed blocks of ions are not the appropriate method to analyse the compositional inhomogeneities. Every combination of the data into fixed blocks destroys some information of the raw data. Therefore, we choose the ladder
3. R e s u l t s On flat substrates a grain growth in hcp structure with a 0001 fibre texture perpendicular to the surface is required for a perpendicular magnetic recording material. Our FIM measurements show that even with this small radius of curvature existing at the W tip apex, grains grow preferentially with the 0001 direction perpendicular to the tip surface (Fig. la), as can be seen in the FIM micrograph in Fig. lc. In these samples an analysis in 0001 direction is possible. In Fig. l b two scanning electron microscopy (SEM) micrographs of the same tungsten tip before and after film deposition are superimposed, The thickness of the C o - C r layer is determined to be 50 nm. The quality of the coating process can be checked
a.
- 150- l~L~m
23~~
42(_+12)at~
.o 100
0
42(+I1)at% ~e~A~...', _ \ A'-'I I(+7)at%
0
~ 50
0
. . . . . . . . . . . . .
0
300 600 number of Co and Cr ions [1] I
I
10nm Fig. 2. (a) Ladder diagram of the AP analysis of a 50 nm Co-20at%Cr layer. Concentrationvariations on a nanometer scale can be seen. (b) FIM micrograph of a 50 nm Co-20at%Cr film. Bright areas are correlated to Cr-poor regions.
A. Pundt, C. Michaelsen /Applied Surface Science 8 7 / 8 8 (1995) 264-270
Concentrations less than 8 at% Cr and more than 30 at% Cr were observed, indicating a smaller concentration difference within Co-Cr layers deposited at higher substrate temperature. A corresponding FIM micrograph is shown in Fig. 3b, also indicating the bright Cr-depleted regions on a 1 - 2 nm length scale. Because the radius of curvature of the FIM tip differs in two directions, the corresponding magnifications in the micrograph differ, too. In the 3 /xm layers a second type of concentration variation can be found on a scale of about 8 nm, as shown in Fig. 4. The concentration varies between 10 at% Cr and 30 at% Cr. The length scale of this second type corresponds to the scale of the chrysanthemum-like structure. The ladder diagram of a gain boundary analysis is shown in Fig. 5. The angle between the probing direction and the grain boundary plane was about 60 °. It indicates the occurrence of an about 5 nm precipitate with 40 at% Cr that was located at the grain boundary. Cr-depleted regions of about 8 at%
diagram as a more sensitive method. To check for concentration variations in the ladder diagram, the null hypothesis that the mean values of each two regions are equal was tested with a significance level of 1%, providing the distribution of the mean values in each region is a normal distribution [15,16]. Furthermore, we have determined the standard deviation +_2o- = +_2~/c(c- l ) / ( n 1) of each region (_+ 2o- contains the analysed concentration with a probability of 96%), with the mean concentration c and the number of ions n in the concentration region. This is a less sensitive method than the test of the null hypothesis. Only regions are indicated in the ladder diagrams, where the null hypothesis could be rejected and the _+2o- values (indicated in brackets in the diagrams) do not overlap. The mean values of these regions are significantly different and indicate a non-random Cr distribution. Analyses in 3 /xm Co-Cr films (T~ = 200°C) indicate similar compositional variations on a 1 nm scale. In Fig. 3a, the ladder diagram of a typical part of the analyses is shown to demonstrate the details.
b.
a.
~150!
tl nm I
9(+_.5)a~/~ 10(+_4)at%
"2 100: O
267
/
I IL 2~(-+81at% 8(+4)at% f~__..I._ I [I
50i 0
0
500 I000 number of Co and Cr ions [1]
10 nm
(0001 )
Fig. 3. (a) Ladder diagram of the AP analysis of a 3 p~m Co-22at%Cr film. Concentration variations on a nanometer scale are evident. (b) FIM micrograph of a 3 p,m Co-22at%Cr film. Bright regions correspond to Cr-poor concentrations. Arrows indicate grain boundaries.
A. Pundt, C. Michaelsen /Applied Surface Science 87/88 (1995) 264,270
268
lOO
~
10 nm I
i
300 l
1~nm
17(+4)at%~ :j j
o ..~
5o 10
o L)
0
100 200 number of blocks
0
300 [1]
Fig. 4. Concentration variations in a 3 / x m C o - 2 2 a t % C r film on a scale of about 8 nm. Each block contains 69 ions.
Cr were often observed in the vicinity of these precipitates. Note, that the analyses of the grain boundaries have only been performed in the 3 /xm films. To homogenise the decomposed samples we annealed them just below the temperature of phase transformation. Annealing of the samples had to be performed under HV conditions to reduce oxide formation at the sample surface. Phase transformation from hcp to fcc starts at 800°C, as can be seen from an in situ X-ray diffraction heating measurement shown in Fig. 6. Above 800°C the (200) reflex occurs, indicating the fcc structure. No compositional change within the samples is expected for low temperatures and short annealing times, as was determined from magnetic measurements: no change in the saturation magnetisation was indicated in measurements of the 100 nm thin films annealed at temperatures between 300 and 500°C for 1 h. Because of the sluggish diffusion we annealed our samples at 7500C for 24 h. A FIM micrograph of an annealed sample can be seen in Fig. 7a, the corresponding concentration profile is shown in Fig. 7b. The long-range concentration variation inside the grains has almost disappeared.
500 10oo number of Co and Cr ions
[1]
Fig. 5. Ladder diagram of the grain boundary analysis of a 3 /xm Co-22at%Cr film. The aperature size was about 1.2 nm, the angle between probing direction and grain boundary plane was about 60°. A 40at%Cr-enriched precipitate of about 5 nm can be found. by a fit to the experimental data by the C A L P H A D method (calculation of phase diagrams, for details, see Ref. [17]). Since sputtered and annealed films exhibited an hcp structure, we plotted the hcp free energy curves for several low temperatures in Fig. 8. At temperatures lower than 800°C, the occurrence of magnetism induces a miscibility gap: the ferromagnetic Cr-depleted solid solution is thermodynamically stabilised with respect to the paramagnetic phase. Note, that the measured data used for these calculations were obtained above 800°C, i.e. the
hcp
(0002)
Co - Cr
7800C
>, o~ m e,
_=
790°C
800"C
810°c
fcc
,
-
,..--J
(111)
........
(200)
4. Discussion ...J
820°C
The results can be discussed with respect to the calculated C o - C r free energy curves of Hasebe et al. [17]. Hasebe has given analytical expressions for the free energy curves of each phase as a function of temperature and composition, which were obtained
~.
, 1
35
40
45 2e(°)
50
55
Fig. 6. In situ X-ray diffraction heating measurement of a C o 22at%Cr thin film (heating rate: 10°C/20 min). The phase transformation hcp --->fcc starts at 800°C.
A. Pundt, C. Michaelsen /Applied Surface Science 8 7 / 8 8 (1995) 264-270
269
10000
influence of magnetism has only been determined mathematically yet. A phase separation into Cr-depleted (ferromagnetic) and Cr-rich (paramagnetic) hcp phases can in fact be seen in the AP measurements of the as-sputtered layers. Additionally, the AP studies show an increase of the concentration difference with decreasing substrate temperature, in good qualitative agreement with the calculated free energy curves. The calculations indicate an upper end of the miscibility gap at about 40-45 at% for the 100°C free energy curve and about 35-40 at% for the 200°C free energy curve, in good agreement with our APFIM results. Thus, we conclude that the measurements support the existence of the calculated miscibility gap in the hcp phase. As the magnetic measurements indicate, bulk diffusion, which is known to be very sluggish [18], can be neglected at low temperatures. Therefore, the decomposition must have taken place during deposition at the growing surface of the films. The measurements suggest that small concentration variations on a nanometer scale are formed within the grains. Due to their open structure the surface mobility is apparently most pronounced at grain boundaries,
O
5000 0
-5000 -10000
~
~
*
C
-15000 Co
20
40
60
O 200"C " • 400°C • 600°C " [] 700*C :_ 80
Cr
[at%] Fig. 8. Free energy curves of the hcp phase, obtained by plotting the expression of Hasebe et al. [17] for low temperatures. They show that magnetism induces a miscibility gap at temperatures lower than about 700°C. The measurements support the existence of this calculated low-temperature miscibility gap.
leading to larger Cr-enriched precipitates in these regions. Furthermore, composition variations on an 8 nm scale have been measured which might correspond to the chrysanthemum-like pattern found previously. We note that surface thermodynamics would be required if decomposition truly occurs at the growing surface. However, since good agreement between bulk and surface thermodynamics is known
b.
a.
100 10 nm
0 o~D
50-
0 a
U 0
50
1O0
150
200
number of blocks [1]
10 n m
(0001)
Fig. 7. (a) Concentration profile of a 3 /xm sample, annealed for 24 h at 750°C. Each block contains 50 ions. The concentration profile is almost homogeneous. (b) FIM micrograph of 3 p.m Co-22at%Cr film, annealed for 24 h at 750°C.
270
A. Pundt, C. Michaelsen /Applied Surface Science 8 7 / 8 8 (1995) 264-270
from semiconductors and the important role of bulk thermodynamics on phase formation during deposition is known from metals [19], we conclude that the description using bulk free energy is appropriate. Homogenisation of the decomposed films is observed upon annealing the samples for 24 h at 750°C. The miscibility gap in the hcp phase is nearly closed at these temperatures, as can be seen from the free energy curves. The results indicate that the bulk thermodynamic interpretation is a reasonable approximation to describe the experimental observation within C o - C r thin films.
Acknowledgements The authors would like to acknowledge the support of their advisor, the late Professor P. Haasen. They also wish to thank Dr. K. Hono for kindly providing the lithography samples and thank all members of the GSttingen FIM group, Dr. R. Busch, Professor R. Kirchheim and Professor R. Wagner for helpful discussions. This work was supported by the Deutsche Forschungsgemeinschaft via Sonderforschungsbereich 345.
References [1] s. Iwasaki and K. Ouchi, IEEE Trans. Magn., MAG 14 (1978) 849.
[2] R.D. Fisher, V.S. Au-Yeung and B.B. Sabo, IEEE Trans. Magn., MAG 20 (1984) 806. [3] K. Kobayashiand G. Ishida, J. Appl. Phys. 52 (1981) 2453. [4] Y. Maeda, S. Hirono and M. Asahi, Jpn. J. Appl. Phys. 24 (1985) L951. [5] Y. Maeda and M. Takahashi, IEEE Trans. Magn., MAG 24 (1988) 3012. [6] F.T. Parker, H. Oesterreicher and E. Fullerton,J. Appl. Phys. 66 (1989) 5988. [7] K. Yoshida, H. Kakibayashiand H. Yasuoka, J. Appl. Phys. 68 (1990) 705. [8] K. Hono, Y. Maeda, J.-L. Li and T. Sakurai, J. Magn. Magn. Mater. 110 (1992) L254. [9] K. Hono, Y. Maeda, J.-L. Li and T. Sakurai, IEEE Trans. Magn., MAG 29 (1993) 3745. [10] tC Hono, S.S. Babu, Y. Maeda, N. Hasegawa and T. Sakurai, Appl. Phys. Lett. 62 (1993) 2504. [11] G.P. Geber, T. A1-Kassab, D. Isheim, R. Busch and P. Haasen, Z. Metallkd. 83 (1992) 449. [12] A. Pundt, R. Busch and C. Michaelsen, MRS Proceedings 1994, to be published. [13] R. Busch and S. Schneider, MRS Proceedings 1994, to be publised. [14] N. Hasegawa, K. Hono, R. Okano, H. Fujimori and T. Sakurai, Appl. Surf. Sci. 67 (1993) 407. [15] E. Kreyszig, Statistische Methoden und ihre Anwendungen (Vandenhoeck& Ruprecht, GSttlngen,1965). [16] M.G. Hetherington,A. Cerezo, J. Hyde, G.D.W. Smith and G.M. Won'all, J. Phys. 11 (1986) C7-495. [17] M. Hasebe, K. Oikawa and T. Nishizawa, J. Jpn. Inst. Met. 46 (1982) 577. [18] A. Green, D.P. Whittle, J. Stringer and N. SwindeUs, Scr. MetaU. 7 (1973) 1079. [19] R. Bormann, Habilitations Thesis, Universit~it GSttingen, 1988.
Applied Surface Science 87/88 (1995) 264-270
APFIM study of the compositional inhomogeneity of sputtered Co-Cr magnetic thin film A s t r i d P u n d t a,* C a r s t e n M i c h a e l s e n b a Institut far Metallphysik, Hospitalstrasse 3-7, D-37073 GOttingen, Germany b Institut far Werkstofforschung, GKSS-Forschungszentrum, D-21502 Geesthacht, Germany Received 10 July 1994, accepted for publication 2 October 1994
Abstract An atom probe field ion microscope investigation has been performed to study the decomposition in Co-20at%Cr magnetic recording material. The measurements demonstrate the existence of a decomposition on a one nanometer scale. The concentration of these small dusters varies between 11 at% Cr and 42 at% Cr within at room temperature sputtered films. In films sputtered at 200°C the concentration varies from about 10 at% Cr to 30 at% Cr. Additionally, Cr-enriched precipitates at grain boundaries and concentration variations on a 8 nm scale were found. Annealing the samples at 750°C for 24 h leads to an almost homogeneous alloy. The measured concentrations in the sputtered samples depend on substrate temperature, respectively on annealing temperature. They are interpreted by a calculated miscibility gap in the hcp phase solid solution which is induced by magnetism at low temperatures.
1. Introduction Iwasaki and Ouchi [1] introduced sputtered C o - C r thin films as perpendicular magnetic recording material in 1978. Fibre textured growth of the hexagonal C o - C r grains leads to a magnetic easy axis perpendicular to the surface, as was demanded for this type of magnetic recording films. Magnetic measurements done by Fisher et al. [2] in 1984 on such C o - C r films show higher saturation magnetisation than expected for a homogeneous alloy with concentrations between 12 and 24 at% Cr. A reason for this behaviour could be a phase separation.
* Corresponding author. E-mail: [email protected]; Fax: +49 551 39 5012.
Since phase separation within grains might lead to an additional increase in the magnetic storage density, a lot of research concerning phase separation in C o - C r magnetic material has been done. However, investigations of phase separation by means of transmission electron microscopy or X-ray diffraction are limited because both the atomic size difference and the atomic scattering contrast is small [3]. Therefore Maeda et al. [4,5] etched the films to remove Co in order to investigate the microstructure of Cr. Chrysanthemum like patterns were found in the etched transmission electron microscopy samples suggesting a compositional inhomogeneity on a 10 nm scale. Compositional studies were performed by Parker et al. [6] using M/Sssbauer spectroscopy and Yoshida et al. [7] using nuclear magnetic resonance. Recently, Hono et al. [8-10] analysed as-sputtered
0169-4332/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSDI 0 1 6 9 - 4 3 3 2 ( 9 4 ) 0 0 4 8 0 - 3
A. Pundt, C. Michaelsen /Applied Surface Science 8 7 / 8 8 (1995) 264-270
Co-22at%Cr films deposited at 200°C substrate temperature with the atom probe field ion microscope. Concentration variations were found which were attributed to the chrysanthemum pattern found by Maeda et al. In this paper we present our investigation on microstructure, composition and its dependence on substrate and annealing temperature, respectively, in sputtered C o - C r films on a one nanometer scale using the G/Stfingen field ion microscope including time-of-flight mass spectrometry [11,12].
2. Experimental Sputtered C o - C r magnetic layers of 50 nm and 3 /zm thickness were investigated. From the two types of layers FIM tips were obtained by different preparation methods: in the case of the thinner films, tungsten wires were first electropolished to a tip
265
radius of about 20 nm [13]. Subsequently, a 50 nm Co-20at% Cr layer was sputtered onto these tips at an Ar pressure of 3.7 × 10 -1 Pa. The tip temperature is expected to increase to about 100°C during deposition. In the same sputtering procedure C o - C r films (100 nm) were deposited on planar A1203 substrates to characterise the hcp structure by X-ray diffraction. In the second preparation method tips were prepared from 3 / z m sputtered Co-22at%Cr films. These films were deposited onto flat Cu-coated Si- substrates from a Co-22at%Cr alloy target at Ar pressure of 1.1 Pa. During deposition, the substrate temperature was 200°C. Ribbons with square crosssection (0001 in plane) were produced by lithography technique [14]. Finally, these ribbons were electropolished from both sides to prepare FIM tips. Within these samples only measurements in the planar direction of the film, i.e. perpendicular to the
a.
<0001>
b.
!
!
10 n m
100 n m Fig. 1. (a) Sample geometry of a thin C o - C r film deposited on a tungsten-substrate tip. (b) Superimposed SEM micrographs of the FIM tip before and after deposition. The inner bright part is the substrate tungsten tip with radius of curvature of about 20 nm. The thickness of the C o - C r layer is about 50 nm. (c) FIM-micrograph of a 50 nm C o - C r film. The 0001 pole can be seen indicating a grain growth perpendicular to the substrate surface even at the tip apex.
266
A. Pundt, C. Michaelsen /Applied Surface Science 8 7 / 8 8 (1995) 264-270
0001 direction, are possible. Annealing of the samples has been done under HV of 1 × 10 -5 Pa before the electropolishing procedure to avoid oxidation inside the FIM tips, A P analyses were performed under U H V conditions with a tip temperature of about 63 K. A pulse ratio of 16% was taken in all measurements. FIM micrographs were mainly obtained using a gas mixture of 5 × 10 - 4 Pa Ne and 1 × 10 - 4 Pa He gas.
by means of small rises on the tungsten substrate which have to reappear on the surface of the layer (arrows). Atom probe analyses of 50 nm C o - 2 0 a t % C r f i l l s (Ts < 100°C) show regions having solute concentration that corresponds to the nominal composition, as well as regions of compositions suggesting phase separation. In Fig. 2a a typical analysis is plotted as a ladder diagram, i.e. the number of Cr atoms is plotted versus the total number of analysed atoms in which the slope corresponds to the concentration of the analysed region. In such a diagram, concentration variations can be checked in detail: in about 1 nm distances the concentration changes from more than 42 at% Cr to less than 11 at% Cr. Since the analysis cylinder is of the same size as these 1 nm regions the real concentration difference could be larger. A corresponding FIM micrograph is shown in Fig. 2b. Bright regions correspond to low Cr concentrations as measured by a selected area analyses. They are of the same size as determined by A P analysis. Since these concentration variations are on such a fine scale, composition profiles with fixed blocks of ions are not the appropriate method to analyse the compositional inhomogeneities. Every combination of the data into fixed blocks destroys some information of the raw data. Therefore, we choose the ladder
3. R e s u l t s On flat substrates a grain growth in hcp structure with a 0001 fibre texture perpendicular to the surface is required for a perpendicular magnetic recording material. Our FIM measurements show that even with this small radius of curvature existing at the W tip apex, grains grow preferentially with the 0001 direction perpendicular to the tip surface (Fig. la), as can be seen in the FIM micrograph in Fig. lc. In these samples an analysis in 0001 direction is possible. In Fig. l b two scanning electron microscopy (SEM) micrographs of the same tungsten tip before and after film deposition are superimposed, The thickness of the C o - C r layer is determined to be 50 nm. The quality of the coating process can be checked
a.
- 150- l~L~m
23~~
42(_+12)at~
.o 100
0
42(+I1)at% ~e~A~...', _ \ A'-'I I(+7)at%
0
~ 50
0
. . . . . . . . . . . . .
0
300 600 number of Co and Cr ions [1] I
I
10nm Fig. 2. (a) Ladder diagram of the AP analysis of a 50 nm Co-20at%Cr layer. Concentrationvariations on a nanometer scale can be seen. (b) FIM micrograph of a 50 nm Co-20at%Cr film. Bright areas are correlated to Cr-poor regions.
A. Pundt, C. Michaelsen /Applied Surface Science 8 7 / 8 8 (1995) 264-270
Concentrations less than 8 at% Cr and more than 30 at% Cr were observed, indicating a smaller concentration difference within Co-Cr layers deposited at higher substrate temperature. A corresponding FIM micrograph is shown in Fig. 3b, also indicating the bright Cr-depleted regions on a 1 - 2 nm length scale. Because the radius of curvature of the FIM tip differs in two directions, the corresponding magnifications in the micrograph differ, too. In the 3 /xm layers a second type of concentration variation can be found on a scale of about 8 nm, as shown in Fig. 4. The concentration varies between 10 at% Cr and 30 at% Cr. The length scale of this second type corresponds to the scale of the chrysanthemum-like structure. The ladder diagram of a gain boundary analysis is shown in Fig. 5. The angle between the probing direction and the grain boundary plane was about 60 °. It indicates the occurrence of an about 5 nm precipitate with 40 at% Cr that was located at the grain boundary. Cr-depleted regions of about 8 at%
diagram as a more sensitive method. To check for concentration variations in the ladder diagram, the null hypothesis that the mean values of each two regions are equal was tested with a significance level of 1%, providing the distribution of the mean values in each region is a normal distribution [15,16]. Furthermore, we have determined the standard deviation +_2o- = +_2~/c(c- l ) / ( n 1) of each region (_+ 2o- contains the analysed concentration with a probability of 96%), with the mean concentration c and the number of ions n in the concentration region. This is a less sensitive method than the test of the null hypothesis. Only regions are indicated in the ladder diagrams, where the null hypothesis could be rejected and the _+2o- values (indicated in brackets in the diagrams) do not overlap. The mean values of these regions are significantly different and indicate a non-random Cr distribution. Analyses in 3 /xm Co-Cr films (T~ = 200°C) indicate similar compositional variations on a 1 nm scale. In Fig. 3a, the ladder diagram of a typical part of the analyses is shown to demonstrate the details.
b.
a.
~150!
tl nm I
9(+_.5)a~/~ 10(+_4)at%
"2 100: O
267
/
I IL 2~(-+81at% 8(+4)at% f~__..I._ I [I
50i 0
0
500 I000 number of Co and Cr ions [1]
10 nm
(0001 )
Fig. 3. (a) Ladder diagram of the AP analysis of a 3 p~m Co-22at%Cr film. Concentration variations on a nanometer scale are evident. (b) FIM micrograph of a 3 p,m Co-22at%Cr film. Bright regions correspond to Cr-poor concentrations. Arrows indicate grain boundaries.
A. Pundt, C. Michaelsen /Applied Surface Science 87/88 (1995) 264,270
268
lOO
~
10 nm I
i
300 l
1~nm
17(+4)at%~ :j j
o ..~
5o 10
o L)
0
100 200 number of blocks
0
300 [1]
Fig. 4. Concentration variations in a 3 / x m C o - 2 2 a t % C r film on a scale of about 8 nm. Each block contains 69 ions.
Cr were often observed in the vicinity of these precipitates. Note, that the analyses of the grain boundaries have only been performed in the 3 /xm films. To homogenise the decomposed samples we annealed them just below the temperature of phase transformation. Annealing of the samples had to be performed under HV conditions to reduce oxide formation at the sample surface. Phase transformation from hcp to fcc starts at 800°C, as can be seen from an in situ X-ray diffraction heating measurement shown in Fig. 6. Above 800°C the (200) reflex occurs, indicating the fcc structure. No compositional change within the samples is expected for low temperatures and short annealing times, as was determined from magnetic measurements: no change in the saturation magnetisation was indicated in measurements of the 100 nm thin films annealed at temperatures between 300 and 500°C for 1 h. Because of the sluggish diffusion we annealed our samples at 7500C for 24 h. A FIM micrograph of an annealed sample can be seen in Fig. 7a, the corresponding concentration profile is shown in Fig. 7b. The long-range concentration variation inside the grains has almost disappeared.
500 10oo number of Co and Cr ions
[1]
Fig. 5. Ladder diagram of the grain boundary analysis of a 3 /xm Co-22at%Cr film. The aperature size was about 1.2 nm, the angle between probing direction and grain boundary plane was about 60°. A 40at%Cr-enriched precipitate of about 5 nm can be found. by a fit to the experimental data by the C A L P H A D method (calculation of phase diagrams, for details, see Ref. [17]). Since sputtered and annealed films exhibited an hcp structure, we plotted the hcp free energy curves for several low temperatures in Fig. 8. At temperatures lower than 800°C, the occurrence of magnetism induces a miscibility gap: the ferromagnetic Cr-depleted solid solution is thermodynamically stabilised with respect to the paramagnetic phase. Note, that the measured data used for these calculations were obtained above 800°C, i.e. the
hcp
(0002)
Co - Cr
7800C
>, o~ m e,
_=
790°C
800"C
810°c
fcc
,
-
,..--J
(111)
........
(200)
4. Discussion ...J
820°C
The results can be discussed with respect to the calculated C o - C r free energy curves of Hasebe et al. [17]. Hasebe has given analytical expressions for the free energy curves of each phase as a function of temperature and composition, which were obtained
~.
, 1
35
40
45 2e(°)
50
55
Fig. 6. In situ X-ray diffraction heating measurement of a C o 22at%Cr thin film (heating rate: 10°C/20 min). The phase transformation hcp --->fcc starts at 800°C.
A. Pundt, C. Michaelsen /Applied Surface Science 8 7 / 8 8 (1995) 264-270
269
10000
influence of magnetism has only been determined mathematically yet. A phase separation into Cr-depleted (ferromagnetic) and Cr-rich (paramagnetic) hcp phases can in fact be seen in the AP measurements of the as-sputtered layers. Additionally, the AP studies show an increase of the concentration difference with decreasing substrate temperature, in good qualitative agreement with the calculated free energy curves. The calculations indicate an upper end of the miscibility gap at about 40-45 at% for the 100°C free energy curve and about 35-40 at% for the 200°C free energy curve, in good agreement with our APFIM results. Thus, we conclude that the measurements support the existence of the calculated miscibility gap in the hcp phase. As the magnetic measurements indicate, bulk diffusion, which is known to be very sluggish [18], can be neglected at low temperatures. Therefore, the decomposition must have taken place during deposition at the growing surface of the films. The measurements suggest that small concentration variations on a nanometer scale are formed within the grains. Due to their open structure the surface mobility is apparently most pronounced at grain boundaries,
O
5000 0
-5000 -10000
~
~
*
C
-15000 Co
20
40
60
O 200"C " • 400°C • 600°C " [] 700*C :_ 80
Cr
[at%] Fig. 8. Free energy curves of the hcp phase, obtained by plotting the expression of Hasebe et al. [17] for low temperatures. They show that magnetism induces a miscibility gap at temperatures lower than about 700°C. The measurements support the existence of this calculated low-temperature miscibility gap.
leading to larger Cr-enriched precipitates in these regions. Furthermore, composition variations on an 8 nm scale have been measured which might correspond to the chrysanthemum-like pattern found previously. We note that surface thermodynamics would be required if decomposition truly occurs at the growing surface. However, since good agreement between bulk and surface thermodynamics is known
b.
a.
100 10 nm
0 o~D
50-
0 a
U 0
50
1O0
150
200
number of blocks [1]
10 n m
(0001)
Fig. 7. (a) Concentration profile of a 3 /xm sample, annealed for 24 h at 750°C. Each block contains 50 ions. The concentration profile is almost homogeneous. (b) FIM micrograph of 3 p.m Co-22at%Cr film, annealed for 24 h at 750°C.
270
A. Pundt, C. Michaelsen /Applied Surface Science 8 7 / 8 8 (1995) 264-270
from semiconductors and the important role of bulk thermodynamics on phase formation during deposition is known from metals [19], we conclude that the description using bulk free energy is appropriate. Homogenisation of the decomposed films is observed upon annealing the samples for 24 h at 750°C. The miscibility gap in the hcp phase is nearly closed at these temperatures, as can be seen from the free energy curves. The results indicate that the bulk thermodynamic interpretation is a reasonable approximation to describe the experimental observation within C o - C r thin films.
Acknowledgements The authors would like to acknowledge the support of their advisor, the late Professor P. Haasen. They also wish to thank Dr. K. Hono for kindly providing the lithography samples and thank all members of the GSttingen FIM group, Dr. R. Busch, Professor R. Kirchheim and Professor R. Wagner for helpful discussions. This work was supported by the Deutsche Forschungsgemeinschaft via Sonderforschungsbereich 345.
References [1] s. Iwasaki and K. Ouchi, IEEE Trans. Magn., MAG 14 (1978) 849.
[2] R.D. Fisher, V.S. Au-Yeung and B.B. Sabo, IEEE Trans. Magn., MAG 20 (1984) 806. [3] K. Kobayashiand G. Ishida, J. Appl. Phys. 52 (1981) 2453. [4] Y. Maeda, S. Hirono and M. Asahi, Jpn. J. Appl. Phys. 24 (1985) L951. [5] Y. Maeda and M. Takahashi, IEEE Trans. Magn., MAG 24 (1988) 3012. [6] F.T. Parker, H. Oesterreicher and E. Fullerton,J. Appl. Phys. 66 (1989) 5988. [7] K. Yoshida, H. Kakibayashiand H. Yasuoka, J. Appl. Phys. 68 (1990) 705. [8] K. Hono, Y. Maeda, J.-L. Li and T. Sakurai, J. Magn. Magn. Mater. 110 (1992) L254. [9] K. Hono, Y. Maeda, J.-L. Li and T. Sakurai, IEEE Trans. Magn., MAG 29 (1993) 3745. [10] tC Hono, S.S. Babu, Y. Maeda, N. Hasegawa and T. Sakurai, Appl. Phys. Lett. 62 (1993) 2504. [11] G.P. Geber, T. A1-Kassab, D. Isheim, R. Busch and P. Haasen, Z. Metallkd. 83 (1992) 449. [12] A. Pundt, R. Busch and C. Michaelsen, MRS Proceedings 1994, to be published. [13] R. Busch and S. Schneider, MRS Proceedings 1994, to be publised. [14] N. Hasegawa, K. Hono, R. Okano, H. Fujimori and T. Sakurai, Appl. Surf. Sci. 67 (1993) 407. [15] E. Kreyszig, Statistische Methoden und ihre Anwendungen (Vandenhoeck& Ruprecht, GSttlngen,1965). [16] M.G. Hetherington,A. Cerezo, J. Hyde, G.D.W. Smith and G.M. Won'all, J. Phys. 11 (1986) C7-495. [17] M. Hasebe, K. Oikawa and T. Nishizawa, J. Jpn. Inst. Met. 46 (1982) 577. [18] A. Green, D.P. Whittle, J. Stringer and N. SwindeUs, Scr. MetaU. 7 (1973) 1079. [19] R. Bormann, Habilitations Thesis, Universit~it GSttingen, 1988.