Microstructure and magnetic properties of the FeTaCN nanocrystalline
thin films
C. Y. Choua)and P. C. Kuo
Institute of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan Y. D. Yao
Institute of Physics, Academia Sinica, Taipei 115, Taiwan S. C. Chen
Department of Mechanical Engineering, De Lin Institute of Technology, Taipei 236, Taiwan A. C. Sun and C. T. Lie
Institute of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan 共Presented on 13 November 2002兲
FeTaCN films were deposited on quartz substrates by cosputtering of Fe and TaC targets at room temperature with different N2 flow rate ratios in the sputtering gas. The as-deposited films were postannealed in vacuum for 30 min at various temperatures. The effects of annealing temperature on the N2 flow rate ratio and film thickness on the magnetic properties and microstructure of the film
were investigated. X-ray diffraction and transmission electron microscopy analyses show that the as-deposited FeTaCN film has a nanocrystalline structure or mixing phases of nanocrystalline and amorphous. Nanocrystalline as-deposited film with good soft magnetic properties 共in-plane coercivity Hc储⫽1⬃2 Oe and 4M s⫽12– 14 kG) can be obtained by controlling the N2 flow rate
ratio and film thickness. The soft magnetic properties can be improved by postannealing the as-deposited film at 200–300 °C as the N2 flow rate ratio is higher than 5 vol %. For the
Fe71.03Ta6.1C7.2N15.67 film, the Hc储 value decreases as the film thickness is increased when the annealing temperature is lower than 400 °C. After annealing at 300 °C, its Hc储 is about 3.57 Oe as the film thickness is 50 nm and Hc储 will decrease to 0.18 Oe as the film thickness is increased to 1000 nm. © 2003 American Institute of Physics. 关DOI: 10.1063/1.1555904兴
Pure Fe film has a large magnetocrystalline anisotropy and large magnetostriction, which are undesirable from the point of view of soft magnetic properties. It has been known that the addition of some transition metals to Fe film will improve its soft magnetic properties.1Recently, the enhance-ment of the soft magnetic properties of these doped films with nitrogen or carbon incorporation has been extensively investigated.2,3Generally, the soft magnetic properties of Fe-based nanocrystalline films originate from the fine grain size and strong intergranular ferromagnetic exchange coupling, which will reduce the magnetocrystalline anisotropy.4 How-ever, good soft magnetic properties in these Fe-based alloy films are usually obtained by either substrate heating at el-evated temperatures during deposition, or postannealing at temperatures around 400– 600 °C共Ref. 5兲 to nanocrystallize the deposited films. This will restrict the application of them to the magnetic devices which require low temperature fab-rication processes.
In this work, we investigated the magnetic properties and microstructure of the FeTaCN film and make an effort to obtain the as-deposited film with good soft magnetic proper-ties and thermal stability by optimizing the sputtering param-eters.
The FeTaCN film was fabricated on quartz substrate by dc-magnetron reactive cosputtering of Fe and TaC targets at room temperature. The TaC target was made by a Ta disk
overlaid with C chips which covering about 18% of the disk surface area. The sputtering power density was fixed at 3.49 W/cm2for the Fe target and 1.97 W/cm2for the TaC target. The base pressure was below 4⫻10⫺7 Torr. The N2 flow rate ratio, defined as R(N2)⫽F(N2)/关F(Ar)⫹F(N2)兴 ⫻100%, where F(N2) and F(Ar) are the N2 and Ar flow
rates in the sputtering gas, respectively. R(N2) in the
sput-tering gas was varied from 1% to 15%. The film thickness was varied from 50 to 1000 nm. A SiNxcap layer of about 20 nm was deposited on the FeTaCN film by rf magnetron sput-tering of the Si3N4 target to prevent oxidation of the
mag-netic film. After deposition, the films were annealed in vacuum below 1⫻10⫺5Torr for 30 min at a temperature between 200 and 500 °C, then quenched in ice water. The composition of the film was analyzed by x-ray photoelectron spectroscopy共XPS兲. The microstructure and crystal structure of the film were investigated by a Philips Tecnai F30 field emission gun 共FEG兲 transmission electron microscopy
共TEM兲 and a thin-film X-ray diffractometer 共XRD兲 with Cu
K␣ radiation. The magnetic properties of the films were measured by a vibrating sample magnetometer共VSM兲.
Figure 1 shows the x-ray diffraction patterns of the as-deposited and various annealed Fe63.68Ta6.06C4.95N25.31films,
which annealed at different annealing temperatures. The film thickness is 200 nm and the R(N2) during deposition is 15
vol %. No sharp x-ray diffraction peak is observed for the deposited film. We conjecture that the structure of the as-deposited film may be nanocrystalline or mixed phases of
a兲Electronic mail: a3150@ms3.hinet.net
JOURNAL OF APPLIED PHYSICS VOLUME 93, NUMBER 10 15 MAY 2003
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nanocrystalline and amorphous. A broad peak is observed as the film is annealed at 300 °C. The broad peak is still ob-served when the annealing temperature is increased to 400 °C. This may be caused by the poor crystallinity of the film. Besides, no evidence is found for the existence of ni-tride or carbide phases. This may be due to the too small volume fractions of the nitride and carbide phases in the film that were difficult to detect by XRD. It can be also seen that the ␣-Fe共110兲 peak shifts to its typical peak position as the annealing temperature is increased. This means that the lat-tice spacing of the共110兲 planes is decreased as the annealing temperature is increased. It can be related to the stress relief, which is caused by diffusing out of C and N atoms from the ␣-Fe grain to reduce the Gibbs free energy. Some broad TaC共N兲 and -Fe2N diffraction peaks and a more clear ␣-Fe共110兲 peak were observed as the annealing temperature is increased to 500 °C. Precipitation of the-Fe2N phase, as
shown in Fig. 1, will result in deterioration of the soft mag-netic properties of the film.
Figure 2 shows the TEM bright field image and selected
area diffraction 共SAD兲 pattern of the as-deposited
Fe66.87Ta7.09C6.77N19.27 film. The film thickness is 200 nm.
The R(N2) during deposition is 10 vol % for this film. We
can see that this film has a nanocrystalline structure. This film consists of small␣-Fe grains and more smaller Ta共C, N兲 precipitates. The average grain size of ␣-Fe is about 6 nm, which is smaller than that of the Fe71.03Ta6.1C7.2N15.67 film 关R(N2) is 5 vol %兴. The average grain size of␣-Fe observed
by TEM is about 8 nm for the Fe71.03Ta6.1C7.2N15.67 film.
Since the R(N2) of the Fe66.87Ta7.09C6.77N19.27film is larger
than that of the Fe71.03Ta6.1C7.2N15.67film, it is believed that
the number of TaN precipitates increases with R(N2) during
film deposition owing to their large formation enthalpy. As a result, more␣-Fe grains will nucleate from the TaN surface as R(N2) is increased. Therefore, the smaller grain size of
the Fe66.87Ta7.09C6.77N19.27film is formed.
Figures 3共a兲 and 3共b兲 show variations of saturation mag-netization 4M s and in-plane coercivity Hc储 with annealing temperature, respectively, of various FeTaCN films deposited at different R(N2). The film thickness is 200 nm. From Fig.
3共a兲 we can see that the 4M s value is more sensitive to annealing temperature when the N2 flow rate ratio is lower, especially 1 vol % N2. The 4M s value increased drasti-cally from 5.9 to 15.6 kG as the annealing temperature in-creased from 400 to 500 °C for the film with 1 vol % R(N2). The TEM observation shows that rapid increase of 4M s value when annealed at 500 °C is due to the change of mi-crostructure of this film. This film is changed from low 4M s of mixing nanocrystalline and amorphous phases to the high 4M s crystalline␣-Fe phase at this temperature.6
At an R(N2) below 10 vol %, the increase of 4M s
with R(N2) is also due to the increase of crystallinity of the
FIG. 1. X-ray diffraction patterns of the as-deposited and annealed Fe63.68Ta6.06C4.95N25.31films.
FIG. 2. TEM bright field image and electron diffraction pattern of the as-deposited Fe66.87Ta7.09C6.77N19.27film.
FIG. 3. Variations of 共a兲 the saturation magnetization 4M s and共b兲 the
in-plane coercivity Hc储with annealing temperature for the FeTaCN films
with different N2flow rate ratios.
7206 J. Appl. Phys., Vol. 93, No. 10, Parts 2 & 3, 15 May 2003 Chouet al.
film. As R(N2) further increases to 15 vol %, the decrease of
4M s may be due to the reaction of supersaturated N atoms with Fe and Ta atoms, forming a nonmagnetic TaN phase and weaker ferromagnetic or nonmagnetic FeNx phase. There-fore, the 4M s value of this film is lower than that of 10 and 5 vol % R(N2) films, as shown in Fig. 3共a兲. From Fig. 3共b兲,
we can see that the as-deposited film has higher Hc储 value than that of the annealed film and Hc储 increases with the R(N2) for the as-deposited film. The higher Hc储 value for larger R(N2) film is ascribed to the distortion of crystal
lat-tice that come from the occupation of N atoms in the inter-stitial sites. This resulting in large internal stress in the film, thus, impeding the domain wall motion and degrading the soft magnetic properties. The soft magnetic properties can be improved by postannealing the film at 200 to 300 °C as shown in Fig. 3共b兲. This is due to the stress relief result from the diffusing of C and N atoms out from ␣-Fe grains to reduce the Gibbs free energy, as discuss above. The Hc储 value increases as the annealing temperature is higher than 400 °C. It is related to the large residual stress resulting from quenching the film in ice water after high temperature an-nealing and the grain growth after annealed above 400 °C.
It should be noted that the Hc储 value of the 5 vol % R(N2) film (Fe71.03Ta6.1C7.2N15.67film兲 is lower than that of
10 vol % R(N2) film (Fe66.87Ta7.09C6.77N19.27film兲 after
be-ing annealed at 500 °C. Although the crystal phases of are
identical, the average ␣-Fe grain size of the
Fe71.03Ta6.1C7.2N15.67 film is larger than that of the
Fe66.87Ta7.09C6.77N19.27 film after being annealed at 500 °C.
From the TEM observation, the average ␣-Fe grain size of the Fe71.03Ta6.1C7.2N15.67 film is about 9 nm and the
Fe66.87Ta7.09C6.77N19.27 film is about 6 nm after being
an-nealed at 500 °C. The smaller Hc储 value of the
Fe71.03Ta6.1C7.2N15.67film is attributed to that it has
appropri-ate size and amount of TaC or TaN particles. Owing to the
excessive TaC or TaN precipitates in the
Fe66.87Ta7.09C6.77N19.27 film that reduce the exchange cou-pling force between ferromagnetic grains, the Hc储 value of this film is larger than that of the Fe71.03Ta6.1C7.2N15.67film as shown in Fig. 3共b兲.
Figures 4共a兲 and 4共b兲 are the variations of 4M s and Hc储 with annealing temperature, respectively, of the Fe71.03Ta6.1C7.2N15.67films with various film thickness. It can
be seen that the 4M s value is lower than 12.2 kG as the annealing temperature below 400 °C and the film thickness higher than 400 nm. This low 4M s value is related to more nonmagnetic atoms, such as Ta, C, and N atoms, dissolved into the Fe-based matrix. From Fig. 4共b兲, we can see that the Hc储 value decreases with increasing film thickness when the annealing temperature is lower than 400 °C. The decrease of the coercivity with increasing film thickness is consistence with the Hoffmann’s magnetization ripple theory.7,8After be-ing annealed at 300 °C, the Hc储 value is about 3.57 Oe as the film thickness is 50 nm, the Hc储 value will decrease rapidly to 0.18 Oe as the film thickness is increased to 1000 nm. But, the Hc储 value increases drastically as the film thickness is increased as the annealing temperature further increases to 500 °C, especially as the film thickness is larger than 400
nm. This is due to the large thermal stress forming in the film that is caused by the thermal expansion coefficient difference between the FeTaCN film and the quartz substrate after quenching from high temperature.
We have successfully achieved good soft magnetic prop-erties for the as-deposited film by simultaneous addition of C and N to the FeTa alloy film. Comparing the conventional FeTaC and FeTaN alloy films, the high temperature anneal-ing process is avoided by combination addition of C and N to FeTa alloy film. The magnetic properties of the FeTaCN films are very sensitive to the N2 flow rate ratio. The fine
crystalline ␣-Fe grain together with appropriate size and amount of TaC or TaN precipitates in the film are the essen-tial factors to obtain the good soft magnetic properties.
This work was supported by the National Science Coun-cil of ROC through Grant No. NSC 90-2112-M-001-065. The authors would like to thank Hsueh-Ren Chen for TEM observations.
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FIG. 4. Variations of 共a兲 the saturation magnetization 4M s and共b兲 the
in-plane coercivity Hc储 with annealing temperature for the
Fe71.03Ta6.1C7.2N15.67films with different thicknesses.
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