DSpace at VNU: Thickness dependence of the phase transformation in FePt alloy thin films

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 277 (2004) 187–191 Thickness dependence of the phase transformation in FePt alloy thin films P.T.L Minha, N.P Thuya,b,*, N.T.N Chanc a International Training Institute for Materials Science (ITIMS), Dai hoc bach khoa, Dai Co Viet, Hanoi, Viet Nam b Faculty of Technology, Vietnam National University, Hanoi, Viet Nam c Institute of Engineering Physics, Hanoi University of Technology, Hanoi, Viet Nam Received July 2003; received in revised form 17 October 2003 Abstract FePt alloy thin films of different thickness have been prepared and studied Both X-ray analysis and magnetization measurements have been used to detect the FCC (A1)–FCT (L10) phase transformation due to annealing in these films It was found that the ordering process in the thick samples takes place at much lower temperature in comparison to the thinner ones The observed phenomenon can be understood taking into account the kinetics of the FCC–FCT phase transformation and grain growth The obtained experimental results suggest the existence of an optimal annealing temperature for each defined sample thickness r 2003 Elsevier B.V All rights reserved Keywords: Ordering kinetics; Grain growth; Magnetization process; Fe–Pt thin film Introduction Recently, Fe–Pt alloy thin films have attracted much attention for their potential application as high-density magnetic recording materials One of the reasons for that is the possibility to develop high coercivity in the ordered face-centered-tetragonal (FCT) L10 FePt phase To get this phase a post-annealing heat treatment at relatively high temperature (usually larger than 600 C) is required which results in large grains and thus highly exchange coupling [1,2] Several attempts have been made to tailor the film microstructure and *Corresponding author International Training Institute for Materials Science (ITIMS), Dai hoc bach khoa, Dai Co Viet, Hanoi, Viet Nam Tel.: +84-4-8692518; fax: +84-4-8692963 E-mail address: thuy@itims.edu.vn (N.P Thuy) magnetic properties Non-magnetic materials such as BN, B2O3, SiO2, Ag, C have been added to reduce the magnetic coupling and improve the orientation of the magnetic grains in the films [3–6] One of the recent trends is to reduce the FCC–FCT phase transformation temperature of the given alloy thus to avoid the difficulties caused by the high temperature of post-annealing There are some approaches to this problem such as use of some additives like Cu, Zr [5,7–9], preparation of the samples with the substrates at elevated temperature or in type of multilayers [10] However, less attention has been paid to the question how this annealing regime depends on the film thickness In this work X-ray analysis and magnetization studies have been carried out in a series of nearly equiatomic FePt films of different thickness 0304-8853/$ - see front matter r 2003 Elsevier B.V All rights reserved doi:10.1016/j.jmmm.2003.10.032 ARTICLE IN PRESS 188 P.T.L Minh et al / Journal of Magnetism and Magnetic Materials 277 (2004) 187–191 (ranging from 20 to 100 nm) We show that the ordering process clearly depends on sample’s thickness It is a phenomenon, which could provide better understanding of their ordering kinetics t = 40 nm As-deposited t = 20 nm Annealed The nearly equiatomic FePt thin films have been prepared by RF-sputtering on the Si[1 0] substrate The base pressure was  10À6 Torr The films were deposited with a fixed argon pressure of  10À3 Torr A composite target was made putting some Fe (99.95% purity) pieces on the Pt target (99.99% purity) The film thickness was in the range from 20 to 100 nm The samples were annealed under various conditions in Ar atmosphere with the subsequent quenching in the ice water The energy dispersion spectrum (EDS) and X-ray diffractometer (XRD) have been used to study the film composition and microstructure The film thickness was determined by low-angle Xray diffraction Magnetization measurements have been carried out using a vibrating sample magnetometer (VSM) with magnetic field up to 13 kOe Results and discussion In Fig 1, the X-ray patterns of the as-deposited and annealed FePt films of different thickness are demonstrated The films were annealed at 500 C for 15 in Ar atmosphere followed by quenching in ice water It is clearly seen that the asdeposited sample has a face-centered-cubic (FCC) crystalline structure with the characteristic (1 1) peak After annealing for the thinnest film with t ¼ 20 nm the only (1 1) peak is observed meaning that the crystalline structure in this sample still remains FCC and no transformation has taken place In case of the thicker samples, with t=50 and 100 nm, which were annealed at the same conditions the situation is quite different The appearance of the (1 0) and the split of (2 0) peaks in the X-ray diagrams of these samples have been observed which indicate the occurrence of the phase transformation from the FCC to the FCT Intensity (a.u) Experimental procedure t = 50 nm Annealed t = 100 nm Annealed (200) (001) 20 (111) (110) 30 40 2θ (degree) (002) 50 Fig X-ray patterns of the as-deposited FePt thin films and those annealed at 500 C for 15 The film thickness is indicated in the figure The vertical solid lines are standard peaks for bulk polycrystalline sample of FCT structure crystal structure A clear decrease of the full-width at half-maximum (FWHM) of (1 1) peak with the sample thickness is a proof of the fact that in the thick films the grain growth is occurred at much higher rate Fig 2a,b presents the initial magnetization curves measured in the direction parallel to the film plane for Fe–Pt thin films of different thickness annealed at 425 and 500 C for 15 min, respectively As the film thickness increases the mechanism of the magnetization process exhibit tendency to change from domain nucleation to domain wall pinning type The thin samples with to75 nm get saturation state at very low field whereas in the thicker samples the magnetization slowly increases at low field and only at higher field it augments faster It is interesting to note a close correspondence between the observed tendency in mechanism of the magnetization process and the hysteresis curves as shown in Fig 3: high ARTICLE IN PRESS P.T.L Minh et al / Journal of Magnetism and Magnetic Materials 277 (2004) 187–191 1.0 189 1200 t = 20 nm parallel perpendicular 0.8 M/M max M/M max 0.6 t = 20 nm t = 25 nm t = 40 nm t = 50 nm t = 75 nm t = 100 nm 0.4 0.2 -1200 H (kOe) H (kOe) 1200 t = 25 nm 0.0 M/M max (a) 1.0 -1200 1200 0.8 t = 20 nm t = 25 nm t = 40 nm t = 50 nm t = 75 nm t = 100 nm 0.4 0.2 0 H (kOe) 1200 t = 75 nm 12 H (kOe) Fig Initial magnetization curves for FePt thin films with different thickness t as indicated in the figure All the films were annealed for 15 at temperature of (a) 425 C, (b) 500 C M/M max (b) 13 -1200 0.0 M/M max M/Mmax 0.6 M (emu/cm3) t = 40 nm -1200 H (kOe) 13 H (kOe) 13 1200 t =100 nm M/M max coercivity value corresponds to a pinning mechanism whereas the low coercivity one—a nucleation mechanism As can be seen in this figure, when the sample thickness rises a considerable increase in the coercivity is obtained A thin sample only has the coercive value of hundreds oersteds while a large value of coercivity of about 7.5 kOe has been developed in the thick sample with t ¼ 100 nm although they have been all annealed at the same conditions As mentioned above in the X-ray analysis the phase transformation evolution presents the film thickness dependence: in the studied ranges of thickness the thicker the sample is, the higher the transformation rate and thus the larger -1200 H (kOe) Fig Hysteresis loops for FePt alloy thin films annealed at 500 C for 15 Two curves corresponding to measurements parallel and perpendicular to the film plane are demonstrated The film thickness is indicated in the figure The insets show initial magnetization curves ARTICLE IN PRESS 190 P.T.L Minh et al / Journal of Magnetism and Magnetic Materials 277 (2004) 187–191 the FCT volume fraction This suggests that the appearance of the (1 0) peak and the grain growth observed in X-ray experiments are correlated with the tendency to change to the domain wall pinning type of magnetization process leading to high value of coercivity in the samples as shown in Fig These results are in good agreement with those reported by Ristau et al [11] on the relationship between the high coercivity in the annealed Fe–Pt alloy films and the volume fraction of the FCT phase Thus, our obtained experimental results showed that at the same annealing temperature, there is a critical thickness above it the fct-phase is formed leading to a high value of coercivity Generally, the ordering kinetics can be described by standard Johnson–Mehl–Avrami equation f =1Àexp(Àktn ) where f is the transformed volume fraction, t is time, k is a constant and n is the Avrami exponent Both k and n depend on the nucleation and diffusion rates, which in their turn depend on the activation energy and temperature through Arrhenius equation, possible growth mechanism and spatial dimensionality of the growing region In thinner films transformation process appears to be limited by the sample thickness Thus, the Avrami exponent should be low that leads to longer time transformation or higher temperature required for ordering to be completed [12] In our case the temperature needed for the ordering in the thick sample with t ¼ 100 nm is about 400 C At this temperature the diffusion rate is rather low and in the order of 10À11 cm/h This value cannot explain the observed phenomenon It is well known that in the disorder–order transformation high activation energy is needed to promote nucleation and growth processes From the obtained experimental results we deduce that the driving force of the disorder–order transformation in thick films is larger than that in the thinner films In other words the barrier energy for nucleation of new phase in the case of thicker samples is lower than that of thin samples Therefore, the ordering process in the thick samples occurs more easily at low temperature These results are certainly related to the thickness dependence of the film microstructural character- istics such as film density and film intrinsic stress Study on the kinetics of grain growth also supports the above explanation As reported by Barmak et al [12] the activation energy for phase transformation is much higher than that for grain growth in FePt thin films At low annealing temperatures the grain growth is dominated In the early stage of annealing the major driving force for this process is surface energy reduction For fcc metal and alloy the (1 1) plane represents the lowest surface energy plane Then the randomly oriented grains are consumed until each grain has mostly [1 1] oriented neighbors leading to decrease of the interface energy due to coherence between matrix and product phase Thus, the ordering process is triggered and proceeded as a result of the grain coalescence in the films [4,13] In summary, the magnetization process and coercivity variation in the FePt alloy thin films of different thickness have been analyzed in close relation to the kinetics of the FCC–FCT phase transformation and grain growth The phase transformation observed at low temperature in thick films is attributed to the relatively high value of the driving force or low value of the barrier to nucleation for the ordering, which strongly depend on the film microstructure and the kinetics of grain growth Further study on the phase transformation evolution with the film microstructure and its thermodynamic aspect is needed to fully understand the observed phenomenon Acknowledgements This work was supported by the State Program on Fundamental Research under Grant No 421001 References [1] Y.K Takahashi, M Ohnuma, K Hono, Jpn J Appl Phys 40 (2001) L1367 [2] P.T.L Minh, N.P Thuy, N.D Van, N.T.N Chan, J Magn Magn Mater 239 (2002) 335 ARTICLE IN PRESS P.T.L Minh et al / Journal of Magnetism and Magnetic Materials 277 (2004) 187–191 [3] P.T.L Minh, N.D Van, N.P Thuy, L.T Nguyen, J.C Lodder, P.D Thang, N.T.N Chan, Physica B 327 (2003) 360 [4] S.C Chen, P.C Kuo, A.C Sun, C.T Lie, W.C Hsu, Mater Sci Eng B88 (2002) 91 [5] C.L Platt, K.W Wierman, E.B Svedberg, R van de Veerdonk, J.K Howard, A.G Roy, D.E Laughlin, J Appl Phys 92 (2002) 6104 [6] T Saito, O Kitakami, Y Shimada, J Magn Magn Mater 239 (2002) 310 [7] T Maeda, T Kai, A Kikitsu, T Nagase, J Akiyama, Appl Phys Lett 80 (2002) 2147 191 [8] Y.K Takahashi, M Ohnuma, K Hono, J Magn Magn Mater 246 (2002) 259 [9] S.R Lee, S Yang, Y.K Kim, J.G Na, J Appl Phys 91 (2002) 6857 [10] Y Endo, N Kikouchi, O Kitakami, Y Shimada, J Appl Phys 89 (2001) 7065 [11] R.A Ristau, K Barmak, L.H Lewis, K.R Coffey, J Appl Phys 86 (1999) 4527 [12] K Barmak, J Kim, S Shell, E.B Svedberg, J.K Howard, Appl Phys Lett 80 (2002) 4268 [13] R.A Ristau, K Barmak, K.R Coffey, J.K Howard, J Mater Res 14 (1999) 3263  ... magnetization process and coercivity variation in the FePt alloy thin films of different thickness have been analyzed in close relation to the kinetics of the FCC–FCT phase transformation and grain... transformation is much higher than that for grain growth in FePt thin films At low annealing temperatures the grain growth is dominated In the early stage of annealing the major driving force for this process... than that of thin samples Therefore, the ordering process in the thick samples occurs more easily at low temperature These results are certainly related to the thickness dependence of the film