Investigations of compositional separation in Co-Cr thin film recording media

information Storage: Basic and Applied

ELSEVIER

Journal

of Magnetism

and Magnetic

Materials

130 (1994) 433-441

Investigations of compositional separation in Co-Cr thin film recording media D.J. Rogersa *, Y. Maeda a, K. Takei a, J.N. Chapman b, J.P.C. Bernards ‘, C.P.G. Schrauwen ’ ’ Nl’TBasic Research Laboratories, Tokai, Ibaraki 319-l 1, Japan b Universiry of Glasgow, Glasgow, G12 8QQ, Scotland, UK ’ Philips Research Laboratories, Prof Holstlaan, 5656 JA Eindhoven, The Netherlands (Received

10 August

1993; in revised

form 7 September 1993)

Abstract We investigated the effect of a Ge underlayer and substrate temperature during film deposition (T,) on the compositional distribution in Co,,Cr,, films using spin echo nuclear magnetic resonance and preferential chemical etching. For films deposited at elevated T, we observed drastic compositional separation (CS) leading to a Co enriched phase with approximately 5 at% Cr on both Ge and polyester. Chemical etching revealed chrysanthemum pattern (CP) type microstructures. For lower T, films we observed less marked CS with a distinct etched microstructure in the film deposited on Ge and no clear etched microstructure in the film deposited on polyester. Results from NMR and chemical etching studies agreed very well with those from X-ray microanalysis,

1. Introduction Co-Cr based alloy thin films are currently under investigation for use as magnetic recording media [l-3]. The compositional distribution in these films has been the subject of much investigation because it is believed that compositional inhomogeneities play an important role in determining the magnetic and recording properties [4-lo]. In particular, grain boundary segregation of Cr has often been proposed as a mechanism whereby intergranular exchange coupling is reduced, giving rise to small, magnetically isolated grains [ll-131. Detailed investigations of the

* Corresponding

author.

0304-8853/94/$07.00 0 1994 Elsevier SSDZ 0304-8853(93)E0558-T

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compositional distribution in Co-Cr based films, however, suggest that compositional separation (CS) may occur in these films, leading to the existence of distinct Co enriched and Cr enriched compositional phases [6]. Using preferential chemical etching of Co, Maeda and co-workers at NTT have been able to find evidence to suggest that CS produces a range of complex in-grain compositional distributions, including the chrysanthemum pattern (CP> type structure [14]. Although recent atom probe field ion microscopy (APFIM) studies [lo] confirm the existence of compositional inhomogeneities consistent with a CP-type structure, the validity of chemical etching studies has come under question because the origin and mechanism of the etching is difficult to prove definitively 115,161.

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In this paper we investigate the effect of the substrate temperature during film deposition (7’.‘.> and the introduction of a Ge underlayer, on the compositional distribution in a series of Co-Cr thin films using spin-echo nuclear magnetic resonance (NMR) and preferential chemical etching. We then compare the results with those of previous studies on the same series of films [12,17-181 using energy dispersive X-ray microanalysis (EDX) in a scanning transmission electron microscope (STEM), and relate them to the magnetic and recording properties.

Table 1). The role of the Ge was to reduce the extent of the initial disordered layer in the Co-Cr film and thus promote homogeneity in the structural and magnetic properties. With Ge underlayers we obtained a large positive effective perpendicular anisotropy (K,,) and a reduced spread in crystallographic c-axis orientation (AIM,,) [19], resulting in experimental recording densities of 100 Mbit/ cm2 [l]. In spin-echo 59Co NMR study of bulk powder and evaporated thin film samples of Co-Cr, Yoshida et al. [20] showed that a spectrum of echo amplitude against input pulse frequency can be used to detect the occurrence of CS, providing a measure of the absolute composition of a Co enriched component. To illustrate this, NMR spectra for a series of compositionally homogeneous samples of vacuum-annealed bulk Co-Cr (700°C for 72 h) are shown in Fig. 1 [81. The spectrum for the Co&r,, bulk alloy (Fig. l(d)) is broad with a peak maximum at N 80 MHz. As Cr content decreases, the spectra show sharper and taller peaks combined with a general shift in the spectrum to higher frequency. The spectrum for bulk Co,,Cr, (Fig. l(a)> is very similar to that for pure Co, with several relatively sharp satellite peaks 6) and a main peak (M) at a frequency around 220MHz. In this study, spin-echo 59Co NMR was conducted at 4.2 K in zero external magnetic field. Pulse amplitude was adjusted to the echo maxi-

2. Experiment Four films with Co-Cr layers 400 nm thick, were produced by radio frequency sputter deposialloy target onto flexible tion from a Co&r,, substrates. Ar pressure during deposition was 1.3 Pa and background pressure was better than 0.1 mPa. T, was measured using a fluoro-optic monitor in thermal contact with the substrate. The first pair of films was deposited directly onto polyester (PET) substrates and the second pair of films was deposited onto 400 nm thick amorphous Ge underlayers which had been sputterdeposited onto polyimide (PI) substrates. On both Ge/PI and PET one film was deposited at higher T, to give improved magnetic properties (see

Table 1 A summary

of previously

reported

magnetic

and recording

results

for all four Co-Cr films

Samples Low HJPET

High H,/ PET

Low H,/ Ge/ PI

High H,/ Ge/ PI

mean = 80°C (30°C + 110°C)

mean = 130°C (90°C + 140°C)

mean = 110°C (60°C + 130°C)

mean = 220°C (200°C + 230°C)

14 kA/m

74 kA/m

20 kA/m

70 kA/m

460 kA/m

500 kA/m

420 kA/m

420 kA/m

Effective perpendicular anisotropy

- 14 kJ/m’

- 58 kJ/m3

20 kJ/m3

23 kJ/m3

Recording performance (SNR = signal to noise ratio)

low SNR and stripe domains

high SNR and small domains

low SNR and stripe domains

high SNR and small domains

Substrate

temperature

a

Coercivity Saturation

magnetisation

a It should be noted that although initial, observed to control the film properties.

final and mean

substrate

temperature

are quoted,

initial

substrate

temperature

was

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D.J. Rogers et al. /Journal

(a)

of Magnetism and Magnetic Materials 130 (1994) 433-441

435

(b) Co-l 1.2at%Cr

Co-5at%Cr N

Y

w’

.E. ::* Sl l: ..

-

L-z!J s2

so @I

2 t i

z

200 (MHz)

sb

250

(d) Y,

c i

9

Fig. 1. Spin-echo

NMR spectra

i. .

:

i

,L

lb0

150

260 (MHz)

250

Co-22at%Cr

N

W’

150 ii0 FREQUENCY

:

FREQUENCY

Sl

i

0

/,

Co-13.4at%Cr

k

M

s3

.

150 lb0 FREQUENCY

N

s2:

Sl

260 (MHz)

for compositionally

260

4 0 5WO

21

50

loo

150

FREQUENCY

homogeneous

mum while pulse width and pulse separation were fixed at 1 l.~s and 15 KS, respectively. The frequency dependence of the echo intensity was corrected for enhancement and Boltzmann factors through division of the echo amplitude by the square of the frequency (F*>. Preferential chemical etching was conducted by preparing planar TEM specimens [21] and immersing them in diluted aqua regia for periods of up to several hours. If X-ray microanalysis revealed a reduced Co content after etching, preferential etching was taken to have occurred. In this case, areas which were formerly Co enriched became thinner, and therefore bright in TEM micrographs. The change in appearance of the microstructure was then assumed to indicate the original distribution of Co enriched regions in the films. Previous investigations of compositional inhomogeneities in the same films were conducted using high resolution EDX microanalysis in a Vacuum Generators HB5 STEM [12,17]. In this technique, a focused electron probe is used to generate characteristic X-rays from an excited

powdered

zoo

250

(MHz)

bulk annealed

samples

of Co-Cr

[S].

volume of specimen. With the high brightness field emission gun in this microscope, we were able to obtain a resolution of between 1.5 and 2 nm with negligible specimen drift during spectrum acquisition. For count times of 20 s we estimated < 5% error in the Co Ko peak to Cr Ka peak count ratio 141. We investigated the compositional distribution in ion-milled planar sections of the films. Analyses were taken from a number of locations on grain boundaries and at positions within grains. In this way we could quantify the local composition for a particular feature of microstructure. A more detailed description of the technique is given in our previous paper [ 121. Other characterisation work conducted previously on this series of films included TEM, X-ray diffraction, VSM and torque magnetometry studies. We also investigated the recording performance of the films (using a ring head/drum configuration) by applying a sinusoidal write current optimised at short wavelength apd measuring the output at the fundamental frequency of the read signal [l&22,23].

tnformation

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of Magnetism and Magnetic Materials 130 (1994) 433-441

3. Results and discussion

Figure 2 shows the NMR spectra for all four films and Fig. 3 shows corresponding micrographs for typical areas in the chemically etched specimens. The results of EDX studies on these films are represented in the histograms of Co/Cr Ka count ratio for grains and grain boundaries shown in Figs. 4 and 5 and the scans shown in Fig. 6. The Co/Cr Ka count ratio is plotted, rather than the absolute composition, in order to reduce the possibility of a systematic error being incorporated in the conversion [12]. Table 1 gives a brief overview of the results obtained in a previous characterisation of the films. Film coercivity (H,) was observed to be a strong function of T, on both Ge/PI and PET. For each underlayer we obtained a high H, ( > 70 kA/m) at elevated T, and low H, (< 20 kA/m) at lower T,. In recording experiments the high H, films showed higher output signal level and lower (a)

Low Ts, Low HclPET

noise signal level than the low H, films inferring the existence of a smaller domain size in the high H, films. 3.1. Results for the low T,, low H,/PETjilm For the low HJPET film, although the NMR spectrum (Fig. 2(a)) has a similar shape to that for the 22 at% Cr bulk spectrum (Fig. l(d)) the peak is shifted to higher frequency by about 30 MHz. This suggests that the film has slight CS. The etched microstructure for this film (Fig. 3(a)), however, shows no evidence of any compositional inhomogeneity. This is probably because the Co enriched phase revealed by NMR has a composition near the mean for the film as a whole and will therefore exhibit very similar etching characteristics. In our previous work on this film, using EDX microanalysis, we observed less than 1 at% Cr variation between the values for mean grain (b)

High Ts, High HclPET

50

FREQUENCY

(c)

(MHz)

Low Ts, Low HclGelPl

lcm

150

FREQUENCY

(d)

2m

(MHz)

High Ts, High HclGelPI

. : :. I

1

I

102

150

200

FREQUENCY

(MHz)

Fig. 2. Spin-echo

NMR spectra

FREQUENCY

for all four Co-Cr

films.

(MHz)

:

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of Magnetism and Magnetic Materials 130 (1994) 433-441

boundary and mean grain composition (Fig. 4). In the histogram of EDX results in Fig. 4, however, any small compositional inhomogeneity would be concealed by the spread in the Co/Cr Ka count ratio caused by statistical variations in the number of counts. Thus the slight CS suggested by the NMR spectrum is consistent with both the etched microstructure and the spread in the Co/Cr Ka count ratio from EDX microanalysis.

431

3.2. Results for the low T,, low H, / Ge /PI film From the NMR spectrum for the low H,/Ge/ PI film (Fig. 2(c)) we can see, by comparison with the bulk NMR spectra (Fig. 11, that strong CS has occurred. From the main peak frequency of 202 MHz we can estimate that the resultant Co enriched phase contains approximately 13 at% Cr. Fig. 3(c) shows the chemically etched microstruc-

(a) Low Ts, Low He/PET

(b) High Ts, High He/PET

(c) Low Ts, Low Hc/Ge/PI

(d) High Ts, High Hc/Ge/PI

Fig. 3. TEM images of the chemically

etched

microstructures

in all four Co-0

films (all micrographs

same scale).

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FREQUENCY High l-k/PET

20

n q

(MEAN I GRAIN BOUNDARY (MEAN

3.34

*

0.08)

2.50

5 0.06)

(MEAN = 3.41 f (MEAN = 3.33

0.03) + 0.02)

q

I

n q

30

GRAIN BOUNDARY

Low HclPET

Fig. 4. Histograms of Co/Cr Krx peak count ratios for EDX spectra acquired within grains and at grain boundaries in the low H, /PET and high H, /PET samples [12].

ture for this film. In this micrograph we can see a distribution of large dark spots, approximately 20-30 nm in diameter, spaced about 30-50 nm apart. Around the dark spots there is a fine distribution of bright stripes, indicative of local Co enrichment. This etched microstructure suggests that there may have been grains with Cr enriched cores surrounded by Co enriched stripes prior to chemical etching. The histogram of EDX results for this low H,/Ge/PI film is shown in Fig. 5. This histogram shows a larger spread in Co/ Cr Ko count

FREQUENCY

High HclGelPI

10 *I

n q

I

GRAIN (MEAN = 3.26 + 0.09) BOUNDARY (MEAN E 3.35 + 0.11)

T

n [9

GRAIN (MEAN I 3.35 f 0.04) BOUNDARY (MEAN = 3.40 + 0.03)

Low Hc/Ge/PI

Fig. 5. Histograms of Co/Cr Ka peak count ratios for EDX spectra acquired within grains and at grain boundaries in the low H, /Ge/PI and high H, /Ge/PI samples.

ratio for spectra acquired within grains than for the low HJPET film (Fig. 41, while the spread in the Co/Cr Kol count ratio for spectra recorded at the grain boundaries appears very similar. This suggests that there was more compositional inhomogeneity within grains than in the low H, film deposited directly onto PET. Spectra were also collected for scans across individual grains. In order to do this, the probe was sequentially positioned at spacings of about 5 nm along orthogonal paths across the grains from diametrically opposite points on the grain boundaries. Fig. 6 shows plots of Co/Cr Ka count ratio against probe position for orthogonal scans across two typical grains. From these it can be seen that the grains appear to have a Cr enriched centre and a Co enriched ring between the grain boundary and grain centre. The histogram in Fig. 7 shows the distribution of Co/Cr Kol count ratios for spectra recorded at grain centres and at other positions within grains. From this histogram we can see that the spectra recorded at grain centres had lower mean Co/Cr Ka count ratio than those for other positions within grains, corresponding to a mean enhancement of 3.1 f 0.6 at% Cr relative to the mean for the film as a whole. Thus NMR, chemical etching and EDX results agreed on the occurrence of strong CS in this film, leading to significant Cr enrichment at the grain cores and a fine distribution of Co enriched regions. No such regular pattern of Cr enrichment at the grain centre was observed in EDX studies of the other three samples. 3.3. Results for the high T,, high H, / PET film For the high HJ PET film the NMR spectrum (Fig. 2(b)) shows a main peak at approximately 215 MHz. This is indicative of the occurrence of drastic CS producing a Co enriched phase with approximately 5 to 6 at% Cr [81. The etched microstructure for this film is shown in Fig. 3(b). In this micrograph we can see a CP type structure with bright stripes, approximately 5 nm wide, extending radially from the grain centre towards the grain boundary region. This suggests that these regions were Co enriched prior to etching. The grain boundary regions appear in this micro-

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of Magnetism and Magnetic Materials 130 (1994) 433-441

(4

439

Y

(b)

X

I

I

1 Onm

1 Onm

X POSITION

24

14

X POSITION

24

24

Y POSITION Fig. 6. Schematic diagrams representing orthogonal plots of Co/Cr Ku peak count ratio versus probe

22

12

Y POSITION scans across position.

Low HclGelPl

CdCr PEAK COUNT RATIO

Fig. 7. Histogram of Co/Cr Ke peak count ratios for EDX spectra acquired at the approximate grain centres and at positions away from the grain centre in the low H,/Ge/PI film.

two typical grains

in the low HJGe/PI

film, with corresponding

graph as fine dark bands, suggesting that extensive grain boundary segregation of Cr occurred in this film. A few dark ‘cores’ are also visible, in which the stripes do not extend all the way to the grain centre. This indicates the existence of some Cr enriched grain centres similar to those observed in the low H,/Ge/PI film. From the histogram of EDX results in Fig. 4 we can see that, relative to the low HJPET film, there is a generally large spread in Co/Cr KIZ count ratio for spectra acquired within grains, and that the distribution for the spectra acquired at grain boundaries is shifted to lower Co/ Cr Ka count ratio. This shift corresponds to a 5.3 f 0.7 at% enhancement in the mean concentration of

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D.J. Rogers et al. /Journal

of Magnetism and Magnetic Materials 130 (1994) 433-441

Cr at the grain boundary, relative to the mean for the film as a whole. Since a local elemental concentration of Cr greater than approximately 25 at% renders a region non-ferromagnetic in nature [24] this concentration of Cr is sufficient to reduce, or possibly eliminate, exchange coupling between grains. Such a reduction in intergranular exchange coupling is also consistent with the small domain size suggested by recording experiments. Thus NMR and chemical etching studies on this film confirmed the occurrence of both grain boundary segregation of Cr and strong in-grain CS. It is interesting to note that in our EDX study there were some spectra acquired within grains which showed exceptionally high Co content relative to the mean for the film as a whole. These can now be recognised as spectra for which the probe was located on one of the Co enriched stripes revealed by chemical etching. 3.4. Results for the high T,, high H, / Ge /PI film The NMR spectrum for the high H,/ Ge/PI film (Fig. 2(d)) is very similar to that for the high HJ PET film (Fig. 2(b)). Thus this film also shows evidence for the occurrence of drastic CS, leading to a Co enriched phase composed of between 5 and 6 at% Cr. The chemically etched microstructure for this film is shown in Fig. 3(d). In this micrograph we can see evidence of strong in-grain CS leading to a disordered CP type structure. Although it is difficult to distinguish grain boundaries for most grains we can see that in a few grains part of the grain boundary is clearly visible as a thin dark band. This suggests that grain boundary segregation of Cr may have occurred locally in the film. As for the high HJPET film a few grains exhibited dark ‘core’ regions indicative of Cr enrichment. From the histogram in Fig. 5 we see a similar spread in Co/Cr Kcx count ratio to that for the high HJPET film, suggesting a similar extent of CS within grains. For spectra recorded at grain boundaries, however, there is no shift to lower Co/Cr Kol count ratio. Thus, from EDX studies, we did not see any evidence for grain boundary segregation of Cr. This is probably due to the

local nature of the grain boundary segregation revealed by chemical etching, combined with the fact that it was hard to position the probe on a grain boundary because of the disordered nature of the unetched microstructure. Previously, it was suggested that the Cr boundary segregation observed in the high HJPET film could act to reduce magnetic exchange coupling between grains, producing a more particulate-type magnetic microstructure and a rotational-type domain reversal mechanism [12, 13, 17-181. This, however, did not take into account the role of compositional inhomogeneities within grains. In this work, we observed good magnetic and recording properties in both the high HJ PET and high HJGe/PI films. Both these films exhibited strong in-grain CS, but only the high HJPET film exhibited extensive grain boundary Cr segregation. If the magnetic microstructure is indeed strongly influenced by the compositional distribution, this result suggests that the in-grain CS in the high HJGe/PI film may be effective for producing good magnetic and recording properties [93. In-grain CS may become important for the future development of high density recording media because of the potential for small, regular, magnetically isolated, ferromagnetic regions within individual grains [251.

4. Conclusions In this study we made use of spin echo NMR and preferential chemical etching to investigate CS in a series of Co-Cr thin films. The results were then correlated with results of previous studies on the same series of films, which included high resolution EDX microanalysis. Combination of the merits of these techniques enabled us to form a more detailed picture of the compositional distribution in the films, and draw more extensive conclusions on the relation to the film properties. Using spin-echo NMR we detected the occurrence of CS in all films investigated, including the low HJPET film, which was found from previous EDX study [12] to be the most homogeneous.

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From the frequency of the main peak in the NMR spectrum we obtained a measure of the composition of the Co enriched phases. The good correlation between the NMR, chemical etching and the EDX results in this study suggests that chemical etching was a valid technique for revealing the pattern of CS. Using chemical etching we confirmed the existence of Cr enriched grain cores suggested by EDX study of a low H, film deposited on Ge. Drastic CS, producing a CP type compositional distribution, was observed to occur at elevated T, in films deposited on both Ge and PET. The good magnetic and recording properties of these films were attributed to both in-grain CS and grain boundary segregation of Cr.

5. Acknowledgements

We would like to thank all those at Philips Research Laboratories and Glasgow University who contributed to this project and also the Science and Engineering Research Council for their assistance in funding part of this research. We would also like to thank M. Asahi and Y. Ohki for performing scanning electron microscope study which aided our understanding of the CS.

6. References [ll S.B. Luitjens,

R.W. de Bie, V. Zieren, J.P.C. Bernards, C.P.G. Schrauwen and H.A.J. Cramer, IEEE Trans. Magn. 24 (1988) 2338. 121S. Iwasaki, J. Magn. Sot. Jpn. 15, Suppl. S2 (1991) 1. F. Kugiya, M. Suzuki, H. Takano, Y. 131 M. Futamoto, Matsuda, N. Inaba, Y. Miyamura, K. Akagi, T. Nakao, H. Sawaguchi, H. Fukuoka, T. Munemoto and T. Takagaki, IEEE Trans. Magn. 27 (1991) 5280.

141J.N. Chapman,

441

I.R. McFadyen and J.P.C. Bernards, J. Magn. Magn. Mater. 62 (1986) 359. and E. Fullerton, J. Appi. [51 F.T. Parker, H. Oesterreicher Phys. 66 (1989) 5988. J. Appl. Phys. 68 (1990) [61 Y. Maeda and M. Takahashi, 4751. C.P.G. Schrauwen and H.W. van 171 J.P.C. Bernards, Kesteren, IEEE Trans. Magn. 26 (1990) 33. k31K. Takei and Y. Maeda, Jpn. J. Appl. Phys. 30 (1991) L1125. 191Y. Maeda and K. Takei, IEEE Trans. J. Magn. Japn. 7 (1992) 919. [lOI K. Hono, Y. Maeda, S. Babu, J. Li and T. Sakurai, presented at Intermag ‘93 Conference, Stockholm, Sweden (April 1993). 1111S. Iwasaki, K. Ouchi and N. Honda, IEEE Trans. Magn. 16 (1987) 1111. ml D.J. Rogers, J.N. Chapman, J.P.C. Bernards and S.B. Luitjens, IEEE Trans. Magn. 25 (1989) 4180. [131 T. Yogi, C. Tsang, T.A. Nguyen, K. Ju, G.L. Gorman and G. Castillo, IEEE Trans. Magn. 26 (1990) 2271. [14] Y. Maeda, M. Asahi and M. Seki, Jpn. J. Appl. Phys. 25 (19861 L668. 1151 M. Takahashi and Y. Maeda, Jpn. J. Appl. Phys. 29 (19901 1705. [16] A. Fartash and H. Oesterreicher, J. Appl. Phys. 66 (1989) 3282. [17] D.J. Rogers, Ph.D. thesis, University of Glasgow (1990). [18] J.P.C. Bernards and C.P.G. Schrauwen, Ph.D. thesis, University of Twente (1990). [19] C.P.G. Schrauwen, J.P.C. Bernards, R.W. de Bie, G.J.P. van Engelen, H.H. Stel, V. Zieren and S.B. Luitjens, IEEE Trans. Magn. 24 (1988) 1901. [20] K. Yoshida, H. Kakibayashi and H. Yasuoka, J. Appl. Phys. 68 (1990) 705. [21] Y. Maeda and M. Asahi, J. Appl. Phys. 61 (1987) 1972. [22] J.P.C. Bernards, C.P.G. Schrauwen, S.B. Luitjens, V. Zieren and R.W. de Bie, IEEE Trans. Magn. 23 (1987) 125. 1231 J.P.C. Bernards, C.P.G. Schrauwen, S.B. Luitjens, Appl. Phys. A 49 (1989) 491. [24] F. Bolzoni, F. Leccabue, R. Pannizzieri and L. Paretti, J. Magn. Magn. Mater. 31-34 (1983) 845. [251 E.S. Murdock, R.F. Simmons and R. Davidson, IEEE Trans. Magn. 28 (1992) 3078.