ELSEVIER Journal of Magnetism and Magnetic Materials 176 (1997) 159 163

Journal of magnetism

~14 and magnetic

, ~ i materials

Epitaxial "-(Fe,Co)16N 2 films grown on NaCI(0 0 1) by facing targets sputtering

H.Y. Wang*, E.Y. Jiang, Y. Wang, P. Wu, H.L. B a i , S i . M i n g

Department of Applied Physies, Tian/in UniversiO,, 92 Weijin Road, Nankai District, Tianjin, 300072, China

Received 27 December 1996; received in revised form 11 June 1997

Abstract

Epitaxial ~"-(Fe,Co)16N 2 films have been grown on NaCl(0 0 1) single-crystal substrates by facing targets sputtering (FTS). Both X-ray diffractometer and transmission electron microscope are employed to characterize the crystal structure of the films. These films possess well-defined crystal orientation and exhibit a higher saturation magnetization (4riMs) than that of a-Fe. The perfect electron-diffraction patterns of ~"-(Fe,Co)l 6N2 single crystal in [-0 0 1], [0 1 1] and [1 1 3] directions can be distinctly observed by double tilting. These patterns confirm that the crystal structure of the ~"-(Fe,Co)16N2 films corresponds to a body-centred tetragonal (BCT) lattice. The X-ray diffraction patterns and the selected area-diffraction patterns show that ~"-(Fe,Co)16Nz epitaxially grows on the NaCI(0 0 1) substrate with orienta- tion relationships ~"-(Fe,Co)16N2(0 0 1)IINaCI(00 1) and ~"-(Fe,Co)16N2[0 0 1]I]NaCI[00 1], The saturation magne- tization of the ~"-(Fe,Co)16N2 films is around 2.6-2.7T, which confirms the high-saturation magnetization of ~"-(Fe,Co)16N z. @~ 1997 Elsevier Science B.V, All rights reserved.

PACS: 81.15Cd; 61.16.Bg; 75

Keywords." Sputtering film; Structure; Magnetic property

1. Introduction

Since the discovery of an abnormal ly high mag- netization (2.83T) in ~"-Fex6Nz by K i m and Takahash i [1], a lot of intensive work has been done to clarify the physical propert ies of ~"- FeI6N2. The repor ted values of the sa tura t ion mag-

* Corresponding author. Present address: Department of Physics, Tsinghua University, Beijing, 100084, People's Repub- lic of China. E-mail: wlx@mail.tsinghua.edu.cn.

netization (4nM~) range from about 3 T [2, 3] or 2.5-2.8 T [4 7] to a lmost the same as that of ~-Fe [8]. Moreover , several theoretical studies [9 12] try to expound the high-satura t ion magnet izat ion, but no calculation based on band structure can successfully explain the giant magnet iza t ion and its origin is still myster ious in theory.

We consider that such a contradic t ion was main- ly caused by ambigui ty in phase identification of ~"-Fe16N2 c o m p o u n d and the unreliable me thod for fixing the volume fraction of ~"-Fe16N2 phase in the whole film.

0304-8853/97/$17.00 :,~;: 1997 Elsevier Science B.V. All rights reserved PH $0304-88 53 (97 )00464-2

160 H.E Wang et al. /Journal of'Magnetism and Magnetic Materials 176 (1997) 159 163

In our previous study, we described the prepara- tion of Fe-N films containing the ~"-Fet6N2 phase [13] and epitaxial growth of ~"-Fe16N2 single-crys- tal films by facing targets sputtering [14]. In the present study, in order to see the relation between the structure and magnetic property of ~"-phase, the Fe N system is expanded to Fe-Co N system. From the physical point of view, it is interesting to know whether the high-saturation magnetization for ~x"-Fe16N2 is reduced or not for cx"-(Fe,Co)16N2 films. We have been able to obtain single-crystal ~x"-(Fe,Co)t 6N2 films on NaCI(0 0 1) substrates. We also distinctly observed a series of electron diffrac- tion patterns with perfect symmetry in the direc- tions of [0 0 1], [0 1 1] and [ i 1 3]. The vibrating sample magnetometer (VSM) measurement shows that the FeCoN film with 10 at% Co has saturation magnetization as large as 2.7 T, which confirms the high-saturation magnetization of ~'-(Fe,Co)16N2.

dispersive X-ray (EDX) analysis. The saturation magnetization was measured at room temperature by VSM with a resolution of 2 × 10 -6 emu in an external magnetic field of 5 kOe which was applied parallel to the sample plane. The hysteresis loop of the NaC1 substrate and the holder was measured and the magnetic moment was about 1-2 orders of magnitude lower than that of samples. The hyster- esis loops of the samples were then measured and the magnetization of the holder and substrates was subtracted automatically by a computer during the measurement. The total error involved in the measurement of the magnetization was safely esti- mated to be no more than 6%. The thickness of the film was measured with a profilometer and the multi-beam interference technique. The composi- tion was evaluated using XPS and electron-probe analysis.

2. Experiment

The FeCoN films were prepared by FTS onto NaCl(0 0 1) single crystal with freshly cloven sur- face. The lattice constant of NaC1 is 0.563 nm, which is smaller by only 1.6% than that of a-axis of ~"-Fe16N2 (0.572 nm). This provides the possibility of epitaxial growth. The sputtering targets are com- posite materials consisting of 30 x 15 mm Co chips placed on ~ 100 x 50 mm Fe(99.99%) targets. The sputtering gas and reactive gas were Ar(99.99%) and N2(99.99%), respectively. After the chamber was evacuated to a base pressure of 6 x 10 .5 Pa, argon gas was introduced. During sputtering, the argon and the nitrogen gas pressures were kept at 0.3 Pa and 0.04-0.06 Pa, respectively. The composi- tion of the FeCoN films was adjusted by varying the number of Co chips mounted on iron targets. The deposition rate was about 0.2 nm/s. The sub- strate temperature was held at 50C. Annealing of the films was sequentially carried out at 150°C for 1 h in vacuum (at least 10 .4 Pa) to obtain ~"-phase. The thickness of the film was 30 70 nm.

The crystal structures of the FeCoN films were examined with an X-ray diffractometer (XRD) us- ing Cu Kcx radiation and a JEM-200CX transmis- sion electron microscope (TEM) capable of energy

3. Results and discussion

Fig. 1 shows the XRD patterns of the FeCoN film with 10 at% Co content. There is an intense cx"(0 0 4) peak and a weak ~x"(0 0 2) peak in the pattern. This reveals that ~"-phase has the same crystallographic orientation as the NaC1 substrate.

For reliable phase identification in FeCoN films, TEM was used to characterize the crystal structure of the films. Fig. 2 shows the selected-area electron diffraction (SAED) patterns. A series of SAED pat- terns are obtained by means of double tilting. Fig. 2a is the SAED pattern with [0 0 1] incidence

I I

20 3o .'o 6o 20( o )

I

70 80

Fig. 1. X-ray diffraction pattern of the ¢('-(Fe,Co)16N2 film with 10 at% Co content on NaCI(0 0 1) substrate.

H. E Wang et al. /Journal of Magnetism and Magnetic Materials 176 (1997) 159 163 161

t

• •

m • $

o m

(b) Table 1 The interplanar spacing of the representative (h k l) in Fig. 2 130 220 310

• • ~ • Q (hk/) doxp(h) ,l .... (~,)

"ilO OOO 150 1 1 0 4.06 4.04 t lh 2 20 2.04 2.02 O O 0 O

3 l 0 1.80 1.80 4 0 0 1.45 1.43 1 2 1 2.5 2.37 0 3 1 1.85 1.82 0 1 1 4.25 4.23 2 00 2.72 2.86

(d)

® ® ® ® ®

211 200 21i ® Q o . ®

0 h ooo oi l # e Q , ,

® o o O m

(0 ~ u ~21

Q o e O 110 000 110

• o 0 • •

® o o O

Fig. 2. The selected area electron diffraction (SAED) patterns of ~"-(Fe,Coh6N2 (50 rim) films with 10 at% Co content: (a) with the incidence of [00 11; (b) schematic illustrations of SAED patterns corresponding to (a); (c) with the incidence of [0 1 1] by means of double tilting; (d) schematic illustrations of SAED patterns corresponding to (c); (e) with the incidence of [1 1 31 by means of double tilting; (f) schematic illustrations of SAED patterns corresponding to (e).

of ~"-single-crystal. Fig. 2b is the illustration of SAED pattern with [ 0 0 1] incidence of ~"- (Fe,Co)16N2 single crystal corresponding to Fig. 2a. The representative indexing results are listed in Table 1. Fig. 2c shows the SAED pattern in the [0 1 1] direction at an angle of 42 ° to the [0 0 1] direction, while Fig. 2e is the SAED pattern

at an angle of 23 "~ to the [0 0 1] which is the inci- dence direction of [1 1 3]. The representative in- dexing results are also listed in Table 1. Fig. 2d and Fig. 2f are the illustrations of SAED patterns with [0 1 1] and [ i 1 3] incidence of cF-single-crystal corresponding to Fig. 2c and Fig. 2e, respectively. One can see that the three patterns display a perfect symmetry. The superlattice reflections from (1 1 0), (01 1 ) , ( 2 0 0 ) , ( 2 0 2 ) , ( 1 1 2 ) , ( 2 1 1 ) , ( 3 1 0 ) , ( 0 3 1 ) and (0 0 4) of the ~"-phase are clearly observed. By using these superlattice reflections, determined values of lattice constants a = 0.574 _+ 0.002 nm and c = 0.630 + 0.004 nm of the ~"-phase are very close to the results obtained by Jack [15] (a = b = 0.572 nm, c = 0.629 nm). This indicates that Co containing ~"-alloys phase has the same structure a s o ( ' - F e l 6 N 2 phase. EDX analysis was made of the same area of the SAD to determine the composi t ion of the SAD area in the film. The com- position of the area was determined to be 79 a t% Fe, 10 a t% Co and 11 a t% N. There are no diffrac- tion spots or rings of Fe Co or C o - N compounds in the SAD patterns. Therefore, the phase corre- sponding to the single-crystal spots in the SAD was determined to be a Co-containing 1 6 : 2 nitride, which can be denoted by ~"-(Fe,Co)t6N 2.

F rom the point of view of X-ray diffractometer and SAED patterns, we deduced that ~"- (Fe,Co)16N2 grows epitaxially on the NaC1 sub- strates with the following orientat ion relationships:

~"-(Fe,Co)16N2(0 0 1)bINaCl(0 0 1),

~"-(Fe,Co)16N2[0 0 13 IINaCI[0 0 1],

162 H.E Wang et al. /Journal of Magnetism and Magnetic Materials 176 (1997) 159 163

i.e., the growth plane is (0 0 1) with the c-axis per- pendicular to the film plane, which is quite different from the case of(l 1 0) growth on InGaAs substrate [16]. The interplanar spacing of e('(200) is 0.286 nm, while that of NaCI(2 0 0) is 0.2815 nm. The unit cell of ~"-(Fe,Co)~,N2 grows on one single unit cell of NaC1, the mismatch between ~"- (Fe,Coh6N2 and NaC1 is only 1.6%. This guaran- tees the epitaxial growth with the above-mentioned orientation relationships.

Previously, the Hitachi group gave the SAED pattern of Fel (,N2 single crystal in the [0 0 11 direc- tion by a cross sectional TEM [161. However, the spatial distribution of the diffraction spots did not exhibit perfect symmetry. This paper gives the per- fect electron diffraction patterns of ct"-(Fe,Coh 6N2 single crystal in the three directions of [00 1], [0 1 11 and [1 1 3] simultaneously for the first time.

It is worth mentioning that the temperature con- ditions during the deposition and annealing were critical to obtain the cz"-phase. Because the stable Fe4N has a similar structure with exactly twice as many octahedral interstices filled as the ~"-phase, the thermal in situ post deposition treatments to order the nitrogen have to be very gentle to prevent the stable-compound forming. We found that a good annealing temperature was 150-180C.

The volume fraction of ~"-alloys phase was esti- mated from the XRD pattern. The integrated inten- sity in X-ray scattering is given by Ihk~ = AIFh k IIZLP P [171, in which A is a constant, Lp is the combined Lorentz and polarization factor, and P the multiplicity factor. The Lorentz factor, cos 0(1/sin 20) 2, is modified to (1/sin 20) 2 since the cos 0 term is only included for randomly oriented specimens. The structure factors were calculated using formula derived by Nakajima and Okamoto [181, and atomic scattering factors for Fe and N were those reported by Cullity [17]. Calculations were based on an ideal structure in which the Bragg angles (0B) are 14.2 and 29.4 '~' for e('(00 2) and ~"(0 0 4), respectively. The sample area was large compared to the beam size, but the thickness was smaller than the effective X-ray penetration depth. Therefore, the integrated intensity was multiplied by the factor G = 1 - e x p ( - 2 t ~ t / s i n O u ) [17], in which/~ denotes the line absorption coefficient. The ideal integrated intensity ratio, I~., m o 41/I~"1o o 21

was calculated to be 5.8 for a single-crystal ~" film. In addition, to determine the volume fraction of ~'-FesN and ~"-Fe16N2, the ideal I~,lo o 2)/I~,,m o 2~ integrated intensity ratio for a 50/50 wt% mixture of ~' and ~" was calculated to be 9.3 using similar parameters and values. It can be shown that the X-ray intensities of ~' /~" mixtures satisfy the rela- tionship

I~,'m 0 4~ 1 - x I~,~0 0 2) -}- _ _ _ _

I ' ~ " (0 0 21 X I~,,(o 0 2)

= (1:~'(002)+1,z"(004)~ , 1~,,10 0 21 , / e x p t .

in which x is the volume fraction (assume similar densities for ~' and ~") of a". The left-hand side of the equation contains the calculated ratios while the right-hand side contains the measured ratio. Using the above formula and the experimental data, the volume fraction of ~" was estimated to be 80 82 vol%.

In order to clarify the saturation magnetization of ct"-(Fe,Co)16N2 single-crystal films, VSM mea- surement is employed and the results are listed in Table 2. The VSM data were calibrated using :~-Fe thin film grown under identical sputtering condi- tions. The values are distributed around 2.6-2.7 T, which is larger than that of Fe Co alloys (2.2-2.3 T) sputtered under the same deposition conditions. Considering the errors involved in the VSM measurement and the volume estimation, the 4rrM~ of ~"-(Fe,Co)16N2 single crystal is reasonably ex- pected to be 2.8 2.9 T, which is in good agreement with the results of Kim et al. [1] and Sugita et al. [2]. This result demonstrates that ~"- (Fe,Co)16N2 obviously has a very high-saturation magnetization, which is as large as that of ~"- Fe16N2, indicating that proper Co addition does not induce notable changes in the giant magnetic moment of :t"-alloys phase.

Table 2 The saturation magnetization from VSM measurements

Sample No. 1 2 3 4

Thickness (nm) 34 40 50 68 4~zM~ (T) 2.7 2.72 2.68 2.66

H. E Wang et al. / Journal of Magnetism and Magnetic" Materials" 176 (1997) 159 -163 163

4. Conclusions References

~"-(Fe,Co)~6N 2 (0 0 1) epitaxial films were grown on NaCI(00 1) single-crystal substrates by facing targets sputtering. The epitaxial growth relation- ships can be figured out as

~"-(Fe ,Co)x6N2(0 0 1)IINaCI(0 0 1),

a"-(Fe,Coh(,N2[0 0 1] IkNaCl[0 0 1].

The perfect electron diffraction patterns of ~"- (Fe,Co)16N2 single crystal in directions of [0 0 1], [-0 1 1] and [ i 1 3] were distinctly observed. These patterns confirm that the crystal structure of the films corresponds to a BCT lattice. The saturation magnetization of the FeCoN film is measured to be around 2.6 2.7 T. The saturation magnetization of ~"- (Fe ,Co) l 6N 2 is expected to be 2.8-2.9 T, which is as large as that of ~"-FeI6N2 reported by Sugita et al. [16]. This result indicates proper Co addition does not reduce the high saturation magnetization of ~"- (Fe ,Co)16N 2 alloys phase.

Acknowledgements

This work is supported in part by the National Natural Science Foundation of China, Tianjin Youth Science Foundation of 21st Century and Zhongguanchun Union Measurement Foundation. The authors are grateful to Dr. Y.P. Wang and others in the TEM Lab of Peking University.

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