Preparation and the influence of Co, Pt and Cr additions on the saturation magnetization of α″-Fe16N2 thin films

Journal of Alloys and Compounds 479 (2009) 755–758

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Preparation and the influence of Co, Pt and Cr additions on the saturation magnetization of ␣ -Fe16 N2 thin films Shahid Atiq a,∗ , Hyen-Seok Ko b , Saadat Anwar Siddiqi a , Sung-Chul Shin b a b

Center of Excellence in Solid State Physics, University of the Punjab, Lahore-54590, Pakistan Department of Physics and Center for Nanospinics of Spintronic Materials, Korea Advanced Institute of Science and Technology (KAIST), Daejeon-305-701, South Korea

a r t i c l e

i n f o

Article history: Received 14 October 2008 Received in revised form 9 January 2009 Accepted 15 January 2009 Available online 23 February 2009 Keywords: Epitaxial ␣ -Fe16 N2 Nitrogen partial pressure Alloying elements additions

a b s t r a c t In this paper, the preparation and magnetic characterization of single-phase epitaxial ␣ -Fe16 N2 thin films along with the effect of addition of some alloying elements on the saturation magnetization (Ms ) of the films deposited by dc magnetron sputtering on single crystal Si(1 0 0) substrates have been described. Apart from the other deposition conditions, the epitaxy of ␣ -Fe16 N2 thin films is strongly dependent on nitrogen partial pressure as its variation causes to develop other iron nitride phases like ␥ -Fe4 N and Fe3 N. Single-phase epitaxial ␣ -Fe16 N2 film is found to have maximum Ms value of 1800 ± 20 emu/cc. The effect of Co, Pt, and Cr additions on the Ms value of ␣ -Fe16 N2 thin films has also been investigated. © 2009 Elsevier B.V. All rights reserved.

1. Introduction In spite of the confirmation of giant magnetic moment in ␣ Fe16 N2 thin films [1], it is still a controversial material, attracting considerable attention of the researchers worldwide. The criticality started in 1972, when Kim and Takahashi [2] reported a giant value of saturation magnetization in ␣ -Fe16 N2 films prepared by molecular beam epitaxy (MBE), after about two decades when this material was first structurally discovered [3]. Since then, different thin film processing techniques have been employed to synthesize this compound but mostly led to variable results. For instance, Sugita et al. [4,5] claimed reconfirming the giant magnetization of this phase but the work of Shoji et al. [6,7] seemed to dispute this claim. To resolve this long debated issue, many theoretical attempts [8–10] have also been made but no calculation based on band structure could successfully explain the mystery behind the giant magnetic moment in this phase of iron nitride. The contradiction might be due to the fact that ␣ -Fe16 N2 exists only as a meta-stable phase and thermally less stable [3] as compared to the other strong ferromagnetic phase of iron nitride [11–13]. Jack [14,15] suggested to improve the thermal stability employing both interstitial and substitutional alloying elements to develop ␣ (Fe,X)16 N2 (X = metal) phases, with the aim of incorporating them into thin film heads for high-density disk drives. Therefore, the study of magnetic properties of ␣ -Fe16 N2 phase and the effect

∗ Corresponding author. E-mail address: [email protected] (S. Atiq). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.01.041

of alloying addition on its magnetic properties is very important for both fundamental physical research as well as technological applications. In this study, we have investigated the nitrogen partial pressure dependent preparation of iron nitride and the subsequent effect of Co, Pt, and Cr additions on the saturation magnetization of single-phase epitaxial ␣ -Fe16 N2 films. The reasons for Co additions are twofold: first, Fe maintains a body-centre cubic (bcc) structure even after alloying with the third element, and secondly, the third element does not undergo preferential nitridation [16]. Co meets these two requirements. Pt addition as an alloying element could induce relatively large in-plane coercivity required for the potential applications in recording media [17]. Addition of Cr in iron nitride might be suitable as well, as Cr has less than 1% lattice mismatch with Fe and both the elements have bcc structure [18]. 2. Experimental To optimize the conditions for single-phase growth, several thin films of iron nitride were deposited on single crystal Si(1 0 0) substrates under distinct deposition conditions. The substrates were cleaned ultrasonically with acetone and ethanol, and pre-heated in vacuum for 30 min prior to deposition. The distance between the substrate and highly pure (99.95%) ␣-iron target (diameter: 50 mm) was 10 cm. The base pressure of the magnetron sputtering chamber was better than 2 × 10−6 Torr. Analytically pure Ar (99.95%) was used as a working gas and injected into the chamber at a pressure of 5 mTorr utilizing dc sputtering power of 60 W. Partial pressure of nitrogen (99.95% pure) was varied from 0.2 to 1.0 mTorr. Films of thickness 400 Å were deposited with a deposition rate of 0.88 Å/s at the substrate temperature of 200 ◦ C and in situ annealed for 1 h. The investigation of crystal structure was carried out by Rigaku D/MAX-RC MPA X-ray diffractometer (XRD) utilizing Cu K␣ radiation. Film thickness calibrations being critical in this study for accurate volume estimation of the thin films were performed by a PSIA, XE-100 atomic force microscope (AFM) and the cross-sectional images obtained by a Hitachi S-4800 scanning electron microscope (SEM). The estimation of Co at% was achieved

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using a Perkin Elmer-4300 (USA) Auger electron spectroscope (AES). Magnetic characterizations were carried using a VT-800 (Riken Denshi Co Ltd.) vibrating sample magnetometer (VSM) with an applied field of up to ±15 kOe. Magnetic hysteresis loops were measured with an applied field parallel to the film plane.

3. Results and discussion Fig. 1 shows the XRD patterns of iron nitride thin films deposited with varying nitrogen partial pressure (PN2 ) from 0.2 to 1.0 mTorr. All the films were deposited at the substrate temperature of 200 ◦ C and in situ annealed for 1 h. The sample deposited at PN2 = 0.2 mTorr showed relatively small X-ray intensity peaks. The pattern revealed bi-phase iron nitride structure as the peaks corresponding to the two different phases, Fe3 N, and ␣ -Fe16 N2 were observed as indicated in Fig. 1(b). All the remaining peaks were from Si substrate. For comparison, the XRD pattern of Si(1 0 0) substrate used in this study is shown in Fig. 1(a). The increase of PN2 to 0.4 mTorr resulted in the enhancement of peak intensity and two relatively clear peaks of ␣ -Fe16 N2 (2 2 0) at 2 = 44.72◦ and that of ␥ -Fe4 N(2 0 0) at 2 = 47.91◦ were observed as shown in Fig. 1(c). Nearly the same bi-phase texture was observed when the PN2 was increased to 0.6 mTorr, with the only difference that an additional (1 1 0) peak of ␣ -Fe16 N2 was also seen at 2 = 21.95◦ (Fig. 1d). Single-phase epitaxial texture of ␣ -Fe16 N2 phase was observed for nitrogen partial pressure of 0.8 mTorr as the only (2 2 0) peak of the phase was observed at 2 = 44.72◦ , as shown in Fig. 1(e). This illustrates that 0.8 mTorr is optimum partial pressure of nitrogen

Fig. 2. Effect of nitrogen partial pressure plotted against saturation magnetization (Ms ).

for the single-phase epitaxial growth of ␣ -Fe16 N2 thin films as the increase in PN2 to a value of 1.0 mTorr resulted again in the appearance of multi-phase iron nitride structure as can be seen in Fig. 1(f). Fig. 2 shows the saturation magnetization (Ms ) of iron nitride thin films plotted as a function of PN2 . Minimum Ms value was observed for the sample prepared using PN2 of 0.2 mTorr. This might be attributed to the presence of Fe3 N phase (Fig. 1b) of iron nitride which exhibits least ferromagnetic behavior as compared to other ferromagnetic iron nitride phases [19]. When the PN2 was increased to 0.4 mTorr, the XRD pattern showed the presence of ␥ -Fe4 N along with ␣ -Fe16 N2 , the two strong ferromagnetic phases of Fe–N system, so the Ms increased. Further increase in PN2 to 0.6 mTorr, resulted in the increase in ␣ -Fe16 N2 concentration, as indicated by the XRD pattern (Fig. 1d), so the corresponding Ms also increased. When the PN2 was increased gradually up to a value of 0.8 mTorr, the Ms reached to a maximum value of 1800 ± 20 emu/cc. XRD pattern (Fig. 1e) of this sample illustrated single-phase epitaxial growth of ␣ -Fe16 N2 films, so a high Ms could justifiably be expected from this sample [4,11]. However, this Ms value of 1800 ± 20 emu/cc is about 14% lesser than the giant value of saturation magnetization reported by Kim and Takahashi for this material [2]. The sample, prepared at PN2 = 1.0 mTorr Torr showed a decrease in Ms which was again due to the mixed phase iron nitride film as revealed by the XRD pattern of the sample (Fig. 1f). 3.1. The effect of Co addition Utilizing the optimized deposition conditions of PN2 = 0.8 mTorr, 200 ◦ C substrate temperature and in situ annealing of 1 h for single-phase growth of ␣ -Fe16 N2 , the FeCoN films were prepared. For this purpose, Co chips (size: 4 mm × 4 mm) were placed on the iron target having diameter of 50 mm. The Co atomic concentration percent (at%) in FeN films was controlled by varying the number of Co chips from 1 to 4 during each deposition. The quantitative estimation of Co at% was evaluated from Survey Scan Data obtained using AES, as shown in Fig. 3. Atomic concentration Cx of an element ‘x’ may be estimated by Cx =

Fig. 1. X-ray diffraction patterns of (a) pure Si(1 0 0) substrate, iron nitride films at nitrogen partial pressure of (b) 0.2 mTorr, (c) 0.4 mTorr, (d) 0.6 mTorr, (e) 0.8 mTorr, and (f) 1.0 mTorr.

I /Sx

x

I /S␣ ␣ ␣

where Ix is the peak to peak height of the Auger electron signal for the element ‘x’ divided by the proper scale sensitivity and Sx is the sensitivity factor. The AES spectra were obtained at a base pressure of 8.0 × 10−10 Torr and a working gas (Ar: for pre-sputtering) pres-

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Fig. 5. Pt at% in ␣ -Fe16 N2 plotted against Ms (solid line) and number of Pt chips (dashed line).

ing the number of Co chips from 1 to 3. This value of Ms is a little higher than observed by Takahashi et al. [20] for ␣ -(FeCo)16 N2 phase. At 16 at% of Co, corresponding to 4 Co chips, the Ms decreased. 3.2. The effect of Pt addition

sure of 9.0 × 10−8 Torr. All the samples were pre-sputtered for 24 s to remove any contamination and oxide layers up to few nanometers from the sample surface. The Co at% was increased almost linearly by increasing the number of Co chips. Fig. 4 shows the plot of Co at% in iron nitride vs. Ms and the number of Co chips. Ms increased and reached a maximum value of 1660 ± 20 emu/cc as the Co concentration reached to a value of 12.86 at% by chang-

Owing to a high deposition rate of Pt, as the non-ferromagnetic metals are relatively easy to sputter using magnetron sputtering, the size of the Pt chips used was 2 mm × 4 mm. Even then high concentration of Pt was observed in the FePtN films as evaluated by the Survey Scan Data obtained by AES spectra. Fig. 5 shows the plot of Pt at% in iron nitride vs. Ms and the number of Pt chips. The at% of Pt varied from 16.48% to 35.47% as the number of Pt chips were changed from 1 to 4. The maximum Ms of 1510 ± 20 emu/cc was observed at 27.46 at% of Pt. This sample also exhibited maximum value of coercivity (≈1.2 kOe) in the FePtN series of samples. Almost no change in Ms was seen at 35.47 at% of Pt.

Fig. 4. Co at% in ␣ -Fe16 N2 plotted against Ms (solid line) and number of Co chips (dashed line).

Fig. 6. Cr at% in ␣ -Fe16 N2 plotted against Ms (solid line) and number of Cr chips (dashed line).

Fig. 3. AES scan in differential mode using Co chips.

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3.3. The effect of Cr addition

Acknowledgements

The at% of Cr also changed almost linearly with increasing the number of Cr chips (size: 4 mm × 4 mm). Fig. 6 shows the plot of Cr at% in iron nitride vs. Ms and the number of Cr chips. The maximum Ms value observed in this series was 1350 ± 20 emu/cc at 7 at% of Cr, corresponding to 1 Cr chip. The Ms decreased by the increase in Cr at% concentration as the number of Cr chips was further increased. This trend might be attributed to the fact that ferromagnetic–antiferromagnetic (FM–AFM) coupling between Fe and Cr atoms increased as the Cr contents increased [21]. Therefore, Cr could not favor its addition at all, in iron nitride to affect the Ms positively.

The authors would like to thank Huen-Sung Lee for his kind help during some of the experimental measurements. This research work was supported by IRSIP, Higher Education Commission (HEC) of Pakistan, and KOSEF through the National Research Laboratory Project.

4. Conclusions In conclusion, the preparation of single-phase iron nitride is strongly dependent on nitrogen partial pressure in addition to other deposition conditions. A high value of saturation magnetization was observed in epitaxially grown ␣ -Fe16 N2 film at nitrogen partial pressure of 0.8 mTorr. From the alloy addition of three selected elements, it is inferred that, I. The alloying addition of Co in ␣ -Fe16 N2 phase could be favorable as far as stability is concerned, with some compromise on saturation magnetization. II. More systematic investigations are needed to explore FePtN alloy phases to make this system compatible for applications that require high Ms with relatively high coercivity. III. Ferromagnetic–antiferromagnetic coupling between Cr and Fe might be the cause for decreasing Ms with increase in Cr contents in FeCrN system.

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