Thermal Stability of a00-Fe16N2 Thin Films Preparedby Facing-Target Sputtering

Yinghua Xu, Enyong Jiang, Yi Wang, Denglu Hou, Shiwei Ren, and Haili Bai

Department of Applied Physics, Faculty of Science, Tianjin University,Tianjin 300072, P.R. China

(Received October 4, 2000; in revised form January 25, 2000; accepted January 25, 2001)

Subject classification: 68.37.Lp; 68.37.Yz; 68.60.Dv; 75.70.Ak; S1.62

a00-Fe16N2 thin films have been prepared by facing-target sputtering. Their structures, magneticproperties and thermal stability were studied by X-ray diffraction (XRD), transmission electronmicroscope (TEM) and vibrating sample magnetometer (VSM). It was found that the saturationmagnetization (Ms) of a00-Fe16N2 decreases with increasing temperature, and at about 523 K, asudden change was observed, which can be attributed to the decomposition of a00-Fe16N2 into a-Feand g0-Fe4N.

1. Introduction

In 1972, Kim and Takahashi [1] observed a giant saturated magnetic flux density(Bs ¼ 2.58 T) which was 17% stronger than that of pure iron. Believing that the Fe–Nfilm was a polycrystalline mixture of Fe and a00-Fe16N2, they deduced that Bs of a00-Fe16N2

would be 2.83 T from the volume ratio of a00-Fe16N2 in the film. Since the large satu-rated flux density is of importance, both theoretically and from the point of view ofpractical applications, many researchers have made great efforts using a variety ofmethods to synthesize a00-Fe16N2 into bulk materials [2–4] and thin films [5–10].

It is well-know that the a00-phase is metastable, and it may decompose into a and g0

phases at high temperature, but the researchers who investigated the temperature de-pendence of Ms in a00-Fe16N2 films, came to a different conclusion. Kim and Takahashi[1] reported that Ms decreases gradually when the temperature increases to 573 K, thendecreases sharply when the temperature goes to around 673 K. While on cooling, Ms

increases slightly with decreasing temperature, it increases remarkably at about 503 K,and then increases gradually when the temperature goes below 423 K. While the Hita-chi group [10] described a continuous and reversible change up to 673 K in their Bs

measurements versus temperature. They explained that beyond 673 K a chemical reac-tion between InGaAs and Fe16N2, and the formation of Fe4N compound occurred.

In our work, we got different conclusions from the above mentioned. We preparedFe–N thin films by facing-target sputtering, made detail studies about the stability ofthe structure and the magnetic behavior of a00-Fe16N2, and found that Ms of a00-Fe16N2

gradually decreases with increasing temperature. Around 523 K there is a sudden de-crease of Ms, accompanied by a decomposition of a00-Fe16N2 into two phases of a-Feand g0-Fe4N. And Ms of a00-Fe16N2 shows no remarkable increase during cooling.

2. Experimental Procedure

The Fe–N thin films were prepared by FTS on NaCl (001) single crystals with a freshlycleaved surface. The sputtering Fe (99.99%) targets have a diameter of 100 � 50 mm2.

phys. stat. sol. (a) 184, No. 2, 297–302 (2001)

Ar (99.99%) and N2 (99.99%) are the sputtering and reactive gases, respectively. Afterthe chamber has been evacuated to a base pressure of 6 � 10––5 Pa, argon gas wasintroduced. During sputtering, the argon and the nitrogen gas pressures were kept at0.3 and 0.5 Pa, respectively. The deposition rate was about 0.2 nm/s. The substratetemperature was held at 373 K.

The crystal structure of the Fe–N thin films were examined by an X-ray diffract-ometer (XRD) using CuKa radiation and a JEM-200CX transmission electron micro-scope (TEM). The saturation magnetization was measured at room temperature byVSM with a resolution of 2 � 10––6 emu in an external magnetic field of 5 kOe whichwas applied parallel to the sample plane. The hysteresis loop of the NaCl substrate andthe holder was measured and the magnetic moment was about one to two orders ofmagnitude lower than that of the samples. The hysteresis loops of the samples werethen measured and the magnetization of the holder and substrates were subtractedautomatically by a computer during the measurement of the magnetization being esti-mated to be not larger than 6%. The thickness of the film was measured with a profil-ometer and multi-beam interference technique.

3. Results and Discussion

According to the phase diagram of the Fe–N system, the a00-Fe16N2 phase is metastableand will decompose into a and g0 phases at high temperature [11]; then Ms may changeat the decomposition temperature.

Figure 1 shows the XRD patterns of as-deposited Fe–N films whose thickness is 50nm. There appear two intensive peaks of a00(002), a00(004) and some weak peaks ofa00(022), a00(220) and g0(111), g0(221, 300). From the figure, one can see that a00-Fe16N2

is the main phase of the film.Figure 2a shows the SAD pattern and TEM bright field image of the same as-depos-

ited Fe–N films as shown in Fig. 1. The indexing results are listed in Table 1. There area00, g0 peaks picked out from the SAD pattern. From the figure, we can find that theSAD pattern is more precise than the XRD in clarifying the phase structure. Someweak peaks or some information be covered in the intensive NaCl(002) and (004)

peaks, which cannot be found in XRD,emerged in the SAD pattern, such as a00

(222), g0(110), g0(210), and so on. Fromthe bright field image, we also can findthat the size of the crystalline grains ofthe film is about 10–15 nm. After an-nealing, the average size of the crystal-line grains increased, furthermore, fromthe SAD, we know that the structure of

298 Yinghua Xu et al.: Thermal Stability of a00-Fe16N2 Thin Films

Fig. 1. X-ray diffraction patterns of the as-deposited Fe–N thin film

the film also changed with the annealing temperature. Figure 2b shows the SAD pat-tern and TEM bright image of Fe–N films annealed at 473 K. There are polycrystallinerings of a00(022), a00(222), a00(331), and g0(111), g0(221, 300), as indexed in Table 2. Theannealing helped to bring about the growth of a00-Fe16N2, the intensity of the polycrys-talline rings of a00-phase became sharper. The diffraction spots of a00-Fe16N2 single crystal

phys. stat. sol. (a) 184, No. 2 (2001) 299

Fig. 2. Selected area electron diffraction patterns and bright-field images of Fe–N thin films, a) as-deposited, and annealed at b) 473, c) 573, d) 673 K

300 Yinghua Xu et al.: Thermal Stability of a00-Fe16N2 Thin Films

Ta b l e 1Indexing results for the as-deposited Fe–N film

dexp (�A) phase (hkl) dcalc (�A) intensity

3.1292.6962.1932.1321.7101.6661.5741.256

a00 (002)g0 (110)g0 (111)a00 (022)a00 (222)g0 (210)a00 (004)g0 (221, 300)

3.1452.6842.1912.1161.7011.6971.5731.265

strongmediumweakmediumweakweakstrongweak

Ta b l e 2Indexing results for the Fe–N film annealed at 473 K (except the pattern of the Fe16N2

single crystal)

dexp (�A) phase (hkl) dcalc (�A) intensity

2.1932.1321.7101.4541.3151.256

g0 (111)a00 (022)a00 (222)a00 (114)a00 (331)g0 (221, 300)

2.1912.1161.7011.4661.3181.265

weakstrongstrongmediummediummedium

Ta b l e 3Indexing results for the Fe–N film annealed at 573 K

dexp (�A) phase (hkl) dcalc (�A) intensity

2.6962.4672.1931.7101.545

g0 (110)a00 (112)g0 (111)a00 (222)g0 (211)

2.6842.4832.1911.7011.549

strongweakmediummediumstrong

Ta b l e 4Indexing results for the Fe–N film annealed at 673 K

dexp (�A) phase (hkl) dcalc (�A) intensity

2.1931.4291.250

g0 (111)a (200)g0 (221, 300)

2.1911.4301.265

strongmediumstrong

Fig. 3. Illustration of the electron diffraction pattern correspond-ing to Fig. 2b of a00-Fe16N2 single crystal with [011] incidence

came out, which are marked as shown in Fig. 3. The spots are integrated and display aquasi-sixfold symmetry. The superlattice reflections from (111), (200), (211) and (022)of the a00-phase are clearly observed. By using these values the lattice constants a = b =(5.74 � 0.02) �A and c = (6.30 � 0.04) �A of the a00-phase are very close to the resultsobtained by Jack [11] (a = b = 5.72 �A, c = 6.29 �A), and denotes a tetragonal body-centred lattice. This pattern is a stereographic projection in the [011] direction of single-crystal a-Fe16N2. After annealing at 573 K, as shown in Fig. 2c (see also Table 3), theintensity of the a00-Fe16N2 polycrystalline rings weakened, simultaneously, the polycrys-talline rings of g0-Fe4N increased, this means that the a00-phase has begun to decom-pose, but not completely. While annealing at 673 K, as shown in Fig. 2d (see also Table4), the polycrystalline rings of a00-Fe16N2 disappeared, the SAD is only constituted of a-Fe and g0-Fe4N. This means that a00-Fe16N2 has decomposed completely into two phasesof a-Fe and g0-Fe4N. From the bright field image, we find that the average size of crys-talline grains increased to increased 30–40 nm.

Figure 4 shows the temperature dependence of saturation magnetization in a00-Fe16N2

films. As shown in the figure, Ms gradually decreases with increasing temperature.Around 523 K there is a sudden decrease of Ms. With further increase of temperature,Ms decreases gradually. When the film is cooled from 673 K, Ms of a00-Fe16N2 increasedgradually until room temperature, So we think that at 523 K, the sudden change isattributed to the phase decomposition of a00-Fe16N2 as shown in Fig. 2. But on cooling,around 523 K, there is no sudden change to return to its original route. Ms increasedslowly with the temperature. This means that after decomposition, the a00-phase couldno longer be reformed while decreasing the temperature, which resulted in the heatingcurve of Ms which does not coincide with the cooling curve.

4. Conclusion

Fe–N thin films were prepared by facing-target sputtering. In this work, we made de-tailed studies on the stability of the structure and the magnetic behavior of a00-Fe16N2,and found that Ms of a00-Fe16N2 gradually decreases with increasing temperature.

phys. stat. sol. (a) 184, No. 2 (2001) 301

Fig. 4. Temperature dependence ofsaturation magnetization for the a00-Fe16N2 films

Around 523 K there is a sudden decrease of Ms, attributed to the decomposition of a00-Fe16N2 into two phases of a-Fe and g0-Fe4N, and a00-Fe16N2 is no longer reformed duringcooling.

Acknowledgements This work is supported by the National Natural Science Founda-tion of China (No. 50072015 and 59801006), Key Teacher Supporting Project (G00032),Starting Foundation for Returned Researchers (B29904), Key Project of Science andTechnology Research of National Education Committee (00022), and Tianjin YouthFoundation of 21st Century (983701111). The authors are grateful to the researchers inthe TEM laboratory of Peking University and in the X-ray laboratory of Nankai Uni-versity for their assistance.

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