Journal of the Korean Physical Society, Vol. 52, No. 5, May 2008, pp. 14351438

Magnetic Properties of (FePt)100 x Cux Thin Films N. T. T.

Van, N. H. Hai,

N. H.

Luong, V. V. Hiep and N. Chau

Center for Materials Science, Hanoi University of Science, 334 Nguyen Trai Road, Hanoi, Vietnam

(Received 16 July 2007) A series of (FePt)100 x Cux (x = 0, 5, 8 and 11) thin lms have been prepared by RF sputtering. The as-deposited lms are nanocrystalline with ne particles. Upon annealing, the lms transfer to the ordered fct FePt phase, which is hard magnetic. The in uences of the Cu content, as well as the heat treatment conditions, on the magnetic properties of the lms have been studied. Addition of 5 { 8 at.% Cu lowers the optimum annealing temperature and improves the magnetic squareness and convexity of the hysteresis loop. Addition of a higher Cu content weakens the hard magnetic properties of the lms. The dependence of the hard phase formation on the annealing conditions is investigated.

PACS numbers: 75.50.Bb, 75.70.Ak, 75.50.+a, 75.50.-i Keywords: L10 structure, Coercivity, FePt I. INTRODUCTION

The stoichiometric FePt phase can exist in a disordered state in which the statistical distribution of the Fe and the Pt atoms is substitutionally random or in a partially or completely ordered state in which the Fe (Pt) atoms occupy 000 ( 12 0 12 ) and 12 12 0 (0 12 12 ) sites, respectively. In the ordered state, Fe and Pt form alternate layers along the c axis, resulting in a distorsion in the face-centered tetragonal (fct) (the so-called L10 structure with c=a = 0.96) with respect to the disordered face-centered cubic (fcc) phase. Because of that, the FePt alloy has a very high anisotropy constant (K = 8 MJ/m3 ) with an easy c axis [1]. In addition, the structure of FePt is formed easily and the materials show a corrosion resistance and stability under ambient conditions [2]. The ordered fct phase can be formed from (i) the disordered phase via a order-disorder transition, (ii) amorphous state through the diusion of Fe and Pt atoms, (iii) Fe and Pt metal via diusion of atoms across the interface. The ordered FePt thin lms have attracted significant attention due to their potential applications in high-density recording media and permanent magnets for micro-electro-mechanical systems (MEMS) applications. In both applications, especially in permanent magnets, nding a way to increase the coercive eld [3] and to reduce the ordering temperature, at which the ordered phase is formed, is important [4, 5]. Adding a small amount of Cu to the FePt thin lm has been reported to reduce the ordering temperature [6]. However, to our knowledge, the eect of Cu addition on the other prop E-mail: [email protected]; Fax: +84-4-858-9496

erties of FePt lms has not been examined yet. In this paper, we study the eect of Cu addition on the ordering temperature, the structure formation and the magnetic properties of the FePt thin lms. We also study the eect of heat treatment on the properties of the FePt lms.

II. EXPERIMENTS

(FePt)100 x Cux (x = 0, 5, 8, 11) lms are deposited on thermally oxidized silicon substrates, which are kept at room temperature, by using an RF sputtering method. We use a target consisting of Pt pieces and Cu pieces placed on a Fe disk. Thus, we can change the composition by using the number of Cu pieces or Pt pieces. After a high-purity Ar gas is introduced, the sputter pressure is xed at 10 mTorr. The lm's deposition is carried out at a 100 W RF power after 10 minutes for cleaning the target. The lms are annealed under a high vacuum at a temperature of 570  C { 700  C for 1 h. The structure and the chemical composition were studied by a D5005 X-ray diractometer and an energy dispersion spectrometer (EDS). The magnetic properties of the lms are measured by using a DMS 880 vibrating sample magnetometer with the applied magnetic eld parallel to the lm's surface.

III. RESULTS AND DISCUSSION

The EDS results for all as-deposited samples show that the samples have a nearly equiatomic composition Fe50 Pt50 . The detailed compositions of the series of

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Fig. 1. XRD pro les of FePt and FePtCu5 lms (a) before and (b) after annealing at 650  C.

Fig. 2. Dependence of lattice parameter on annealing temperature of FePt thin lm.

thin lms are Fe57 Pt43 (hereafter FePt), Fe51 Pt44 Cu5 (FePtCu5 ), Fe52 Pt40 Cu8 (FePtCu8 ) and Fe43 Pt46 Cu11 (FePtCu11 ). The Cu-doped lms are denoted as FePtCu. The X-ray diraction patterns of the as-deposited FePt and FePtCu5 lms (Figure 1(a)) show two broad peaks, one strong peak located at about 41 which is present for the fundamental (111) re ection (present for the disordered fcc phase) and one weak peak located at 47 { 49 , which is present for another fundamental (200) re ection. This indicates that the as-deposited lms are nanocrystalline with ne particles. The nanocrystalline phase is disordered fcc phase, which diers from that of other reports on the as-deposited samples [7]. Upon annealing, in addition to the fact that the (111) peak is narrower, other peaks appear. The (200) peak is split into (200) and (002) peaks due to the contraction of the c axis (Figure 1(b)). The superlattice peaks

Journal of the Korean Physical Society, Vol. 52, No. 5, May 2008

Fig. 3. Hysteresis loops of FePt lm after annealing at dierent tempertures.

(presented for the ordered fct phase) (001) and (110) appear immediately with the other fundamental peaks. This suggests that the formation of the fct ordered phase starts with annealling at relevant temperatures through the order-disorder transition. The lattice parameters a and c are determined from the X-ray diraction (XRD) patterns of the FePt sample (Figure 2). As the annealing temperature (Ta ) was increased from 600  C to 685  C, the value of a expanded from 3.75  A to 3.88  A while the value of c decreased from 3.71  A to 3.67  A. The ratio c=a is deduced to be 0.99, 0.97 and 0.96 for Ta of 600  C, 650  C and 685  C, respectively. The ratio c=a of 0.96 is close to that of the perfectly ordered FePt, which indicates that the fct formation expands when Ta increases and that the formation of the fct phase is almost complete when annealed at 685  C for 1 h. This is also con rmed by the fact that the intensity of all the FePt peaks increases with annealing temperature. Figure 1(b) shows the XRD results for FePt and FePtCu5 after annealing at 650  C. The ratio c=a for FePt is 0.97 while that value for FePtCu5 is 0.96. The relative intensity of the superlattice (001) and fundamental (111) peaks is also an indication of the presence of the ordered fct phase. The value of I(001) =I(111) for FePt is 0.24 and is higher, 0.33, for the FePtCu5 lm. Similar results are obtained for the other FePtCu samples. This fact indicates that the presence of Cu in the FePtCu lms allows the lms to more easily form the fct phase. This is in agreement with other studies on reducing the ordering temperature by adding small amounts of Cu [6]. The as-deposited FePt and FePtCu lms are slightly coercive with coercive elds of few hundred Oe. Mahalingam et al. [7] also reported the same behavior. After annealing, the lms have a high coercivity, Hc . The hysteresis loops of the FePt lm annealed in the temperature range from 600  C to 700  C measured at room temperature are presented in Figure 3. When Ta is 600

Magnetic Properties of (FePt)100 x Cux Thin Films { N. T. T. Van et

al.

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Fig. 4. Hysteresis loops of FePtCu8 lm after annealing at dierent tempertures.

Fig. 5. Dependence of Hc on Cu content in (FePt)100 Cux thin lms at dierent annealing temperatures.

 C, the loop does not show any signi cant change compared to that of the as-deposited lm. When Ta is 650  C, the loop shows a two-phase behavior with a low-

real materials, the value is between 0 and 1. Even though the highest coercivity is obtained for the FePt lm annealed at 685  C, the magnetization curve of this sample does not show high magnetic squareness and convexity. The values of S and C for the optimally annealed FePt lm are 0.6 and 0.3, respectively. These values are far from good values for an ideal hard magnet. The low value of convexity implies that the dipolar interaction, which comes from the microstructure, supports the magnetic reversal [8]. With the presence of Cu, the magnetic properties of FePtCu lms are changed. Figure 4 presents the hysteresis loops of FePtCu8 as an example for other FePtCu samples. Unlike the FePt lm, FePtCu8 shows a single hard phase hysteresis loop when annealed at a lower temperature of 600  C. The optimum heat treatment is 650  C at which the coercivity is 4.5 kOe. The magnetic squareness and convexity for this annealing condition are 0.9 and 0.7, respectively, which are much higher than those for optimally annealed FePt. The high values of S and C suggest a preferred direction in the plane of the lm. Again, we observe a reduction in the ordering temperature with the presence of Cu, eventhough the value of this change is less spectacular than those reported [6]. Thus, the eects of Cu on the properties of the lms is a slight lowering of the ordering temperature and an improved magnetic squareness and convexity. If annealed at temperature higher than 650  C, the magnetic hardness of the lm is reduced. The explanation is similar to that for the FePt case in which the particle size increases to form multidomain structure. The dependence of Hc of the thin lms on the content of Cu at dierent annealing temperatures is presented in Figure 5. The highest Hc is obtained for FePtCu5 and FePtCu8 . The addition of Cu to FePt lms causes an increase in the driving force of the L10 phase. Therefore, the FePtCu alloy is formed easily with low-temperature heat treatments. Figure 5 shows a

coercivity phase similar to the as-deposited lm and a high-coercivity phase. The magnetic curve is highly coercive for the sample annealed at 685  C with a coercivity of 4.4 kOe. However, when the annealing temperature is higher, the magnetic hardness of the lm is reduced. The magnetic hardness of a lm, in which the coercivity is an indication, depends on the intrinsic properties of the material (anisotropy constant, saturation magnetization) and the extrinsic properties (microstructure). The ordered fct phase possesses a high crystalline anisotropy (intrinsic) that causes the annealed samples to have a much higher Hc than the as-deposited one. Microstructure details, such as defects, antiphase boundaries, dislocations can form pinning sites that impede the movement of magnetic domain walls, leading to high coercivity [6]. The microstructure changes with annealing temperature and, as a result, magnetic reversal comes easily. When annealed at 685  C, the FePt lm is in an ordered fct phase with a relevant particle size that causes a high coercivity. At higher annealing temperatures, larger particles may appear and the microstructure in the lm may change, which forms a multidomain structure leading to a reduction of the coercivity. Besides the coercivity, other parameters playing important roles in obtaining a high maximum energy product are the magnetic squareness and the magnetic convexity. The magnetic squareness, S , is de ned by the ratio of the remanent magnetization Mr to the saturation magnetization Ms (S = Mr =Ms ). The magnetic convexity C is de ned as the ratio of the area limited by the magnetization curve in the second quadrant of the hysteresis loop (AH ) to the area of a rectangle with two lengths of Hc and Ms ; C = AH =(Hc Ms ). For ideal hard magnetic materials, the values of S and C are unity. For

x

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Journal of the Korean Physical Society, Vol. 52, No. 5, May 2008

maximum value of around 4.5 kOe for the FePtCu8 lm.

406506) for its nancial support.

IV. CONCLUSIONS

REFERENCES

The eects of adding Cu and of the heat treatment on the magnetic properties of FePt thin lms are studied. Before annealing, the FePt and the FePtCu thin lms are nanocrystalline with ne particles. After annealing, the lms are highly coercive materials. The optimal annealing temperature for the FePt lm is 685  C whereas the optimal annealing temperature for the Cu-doped FePt lms is 650  C. In addition, the presence of Cu improves the coercivity, the squareness and the convexity, which enhance the maximum energy product of the lms.

[1] N. H. Hai, N. M. Dempsey and D. Givord, IEEE Trans. Magn. 39, 2914 (2003). [2] N. H. Luong, V. V. Hiep, D. M. Hong, N, Chau, N. D. Linh, M. Kurisu, D. T. K. Anh and G. Nakamoto, J. Magn. Magn. Mater. 290-291, 559 (2005). [3] X.-H. Xu, H.-S. Wu, F. Wang and X. LiLi, Appl. Surf. Sci. 233, 1 (2004). [4] K. Nishimura, K. TaKahashi, H. Uchida and M. Inoue, J. Magn. Magn. Mater 272-276, 2189 (2004). [5] S.-R. Lee, S. Yang, Y. K. Kim and J. G. Na, Appl. Phys. Lett. 78, 4001 (2001). [6] T. Maeda, T. Kai, A. Kikitsu, T. Nagase and J. Akiyama, Appl. Phys. Lett. 80, 2147 (2002). [7] T. Mahalingam, J. P. Chu, J. H. Chen, S. F. Wang and K. Inoue, J. Phys: Condensed. Matter. 15, 2561 (2003). [8] D. Girovd and M. F. Rossignol, Coercivity, in: Rare-Earth Iron Permanent Magnet, edited by J. M. D. Coey (Clarendon Press, Oxford, 1996), p. 218.

ACKNOWLEDGMENTS

We would like to thank the Vietnam National Fundamental Research Program in Natural Science (Project