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NMR STUDIES OF Co-BASED NITRIDE

AMORPHOUS FILMS

K. Le Dang, P. Veillet, H. Sakakima, R. Krishnan

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, Suppl6ment a u no 12, Tome 49, d6cembre 1988

NMR STUDIES OF CO-BASED NITRIDE AMORPHOUS FILMS

K. Le Dang (I), P. Veillet (I), H. Sakakima (2) and R. Krishnan (3)

(I) Institut d7Electronique Fondamentale, CNRS, UA 22, Baitiment 220, 91405 Orsay, France

(2) Central Research Laboratory, Matswhita Electric Industrial Co, LTD., Osaka 570, Japan (3) Laboratoire de Magnitisme, CNRS, 92195 Meudon, France

Abstract.

-

We have carried out at 4 K spin-echo measurements in both single and multilayer (nitride and nonnitride layers) amorphous films of (Co83NbllTa2Zr4)1-x Nl0oX. The average Co hyperfine field and the local in-plane anisotropy

(K) increase with N (x) content in agreement with magnetization measurements. Annealing leads to a slight increase in

K for single layer but a strong decrease for multilayer which remains amorphous contrary t o the non-nitride films.

Introduction

The Co-based amorphous films with composition (C083NbllTa2Zr4)~_, Nl0oI are soft ferromagnetic materials and their magnetic properties were shown to depend strongly on the N content. The multilayered films are composed of non-nitride and nitride layers alternately deposited on a glass substrate. After an- nealing a t 525 OC for 2 h these films remain amor- phous with an increase in saturation magnetization and permeability [I], in contrast with the rather low crystallization temperature for the non-nitride film. In this paper we report the NMR investigations of these Co-based sputtered films by means of the spin-echo method t o get complementary information of a micro- scopic nature.

Experimental procedure

The Co-based amorphous films were prepared by a sputtering technique in Ar atmosphere for the non- nitride films and in Nz mixed Ar gas for the nitride ones [I]. The N content of the film which increases with the N2 partial pressure was estimated from AES data. The magnetic measurements have shown that the magnetization lies in the film plane with small lo- cal magnetic anisotropy [I]. The NMR measurements were carried out at liquid helium temperature, using a frequency variable spin-echo apparatus. The rf field was parallel t o the film plane within which a dc mag- netic field can be applied. The film surface was about

20 x 1.3 mm2.

Results and discussion

The characteristics of the amorphous films studied in this work are given in table I. The multilayered film is composed of non-nitride and nitride layers having an individual thickness of 200

a.

The Co nuclear sig- nal was observed at 4.2 K, in zero external field. The resonance spectrum is gradually shifted to the high frequency with the N content of the film, revealing

Table I. - Characteristics of amorphous (Cos3Nb11.

Ta2Zr4)1-,N100x films used i n this work. The multi- layered film no 5 was composed of sublayers of 200

A

thickness with x equals alternately 0 and 15.4 %

.

an increase of the Co moment (Fig. 1). The average resonance frequency is indeed roughly proportional to the saturation magnetization. For the multilayered film the non-nitride sublayers contribute to the low- frequency side of the spectrum and the nitride ones to the high-frequency side corresponding to z = 15.4 %.

After annealing this film at 525 O C for 2 h we ob-

served a spectacular narrowing of the spectrum around

220 MHz, revealing that the multilayer structure is de- stroyed with an increase of the Co moment. For the non-nitride films the thermal annealing leads to the crystallization of the sample [l] revealed by sharp res- onance lines arising from hcp Co [2] or both fcc and hcp Co for

T,

= 585 and 710 OC respectively.

In order to obtain more information on the magnetic properties of these films we evaluated the amount of ferromagnetic alloy in these samples represented by the integrated Co signal intensity. For this purpose the spin-echo amplitude was measured in a dc magnetic field of 3 kOe parallel t o the film plane so that the

enhancement factor [3] of the nuclear signal at a given frequency is expected to be the same for all the sam- ples. For the as-prepared samples the integrated signal intensity is practically .independent of the N content. After annealing the signal intensity remains practically unchanged for the nitride films but is reduced by about a factor 2.5 for the non-nitride films. This loss in inten- sity arises chiefly from the formation of CosNb which can retain up to 40 % of cobalt atoms present in the

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C8

-

1714 JOURNAL DE PHYSIQUE C .- ... a Y) multilayer ...' ...' 100 150 200 F C M H z l

Fig. 1. - Co spin-echo spectra at 4.2 K for amorphous (C083NbllTa2Zr4)~-, Nlooz films. The dotted lines are those obtained after thermal annealing. All the spectra are normalized at the same height.

sample, in agreement with the detection of this phase by X-ray diffraction.

In zero external field the enhancement factor is lim- ited only by the local magnetic anisotropy which is low in these soft ferromagnetic films. We can there- fore estimate the local anisotropy field by measuring the decrease of the spin-echo amplitude in a dc mag- netic field. In zero dc field a random distribution of the magnetization direction in the plane is expected, resulting in a reduction of the echo amplitude by about a factor 2. The local anisotropy field HA was then de-

duced from the echo amplitude, taking into account this correction. For single layers HA increases with

the N content (Fig. 2), in agreement with the in-plane magnetization measurements [I].

The anisotropy may arise from columnar structure generated by nitriding. For the multilayered film the N content is taken as the average value of the sublayers. Now, the important point concerns the effects of the thermal annealing on the local anisotropy field showing three distinct behaviours:

(i) for single-layer nitride films HA is slightly in-

creased;

(ii) for multilayered film HA is strongly decreased

and

(iii) for non-nitride films HA is drastically rein-

forced. Thus the HA values for the non-nitride films

Fig. 2. - Local anisotropy field a s a function of N content of the films. The closed symbols represent the values after annealing at 525 OC for 2 h. For non-nitride films see text.

annealed at 585 and 710 OC are estimated to be 400 and 600 Oe respectively which are two orders of magni- tude greater than the initial value. It may be pointed out that the anisotropy field observed in a fcc cobalt film was reported to be 600 Oe [4]. On the other hand the in-plane anisotropy field for a thin hcp cobalt poly- crystalline film [5] has also nearly the same value. The very low anisotropy observed for the multilayered film is an indication of its homogeneous amorphous state.

Conclusion

In conclusion, the NMR studies of these Co-based amorphous films have revealed an important increase of the Co moment by nitriding the samples during the sputtering. The multilayered film is composed of ni- tride and non-nitride sublayers having different magne- tizations. For the single-layer films the local magnetic anisotropy estimated from the spin-echo intensity is shown to increase monotonically with the N content. The as-prepared multilayered film has relatively low magnetic anisotropy. After annealing at a tempera- ture as high as 525 OC its anisotropy is still strongly decreased. This behaviour is contrary to that of non- nitride films which are crystallized after thermal an- nealing resulting in a drastic increase of anisotropy and a loss in ferromagnetic cobalt present in the samples.

[I] Sakakima, H., Osano, K. and Omata, Y., IEEE

Trans. Magn. 23 (1987) 3707.

[2] Kawakami, M., Hihara, T., Koi, Y. and Wakiyama, J. Phys. Soc. Jpn 33 (1972) 1591. [3] Portis, A. M. and Gossard, A. C., J. Appl. Phys.

SuppE. 31 (1960) 205 S.

[4] Rodbell, D. S., J. Phys. Soc. Jpn 17 (1962) 313.

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