- Email: [email protected]
Thickness dependence of crystallographic and magnetic properties for L10-CoPt thin films
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 303 (2006) e243–e246 www.elsevier.com/locate/jmmm
Thickness dependence of crystallographic and magnetic properties for L10-CoPt thin films W.M. Liaoa, S.K. Chena,, F.T. Yuana, C.W. Hsua, H.Y. Leeb a
Department of Materials Science and Engineering, Feng Chia University, Taichung, Taiwan, ROC b National Synchrotron Radiation Research Center, Hsinchu, Taiwan, ROC Available online 13 February 2006
Abstract Thickness dependence of crystallographic and magnetic properties is investigated from the analyses of the order parameter S, chemically ordered fraction f0, and internal stress of the L10 Co49Pt51 film. Coercivity Hc was increased from 5.1 kOe to a maximum value of 13.3 kOe as the thickness of the film (d) was raised from 10 nm to 50 nm.This is due to the increase of S from 0.30 to 0.64 and the increase of f0 from 0.52 to 0.75. For thicker samples (d^50 nm), a dramatic drop-off in Hc was observed at d ¼ 80 nm. The quantity of ordered phase, measured by X-ray diffractometry, is closely related to the Hc value of the Co49Pt51 thin film for do50 nm. However, the existence of ‘‘domain wall-like magnetization structure’’ in thicker Co49Pt51 samples is harmful for Hc. The decrease in Hc can also be partially attributed to the thermal-stress-induced (0 0 1) texture. r 2006 Elsevier B.V. All rights reserved. PACS: 75.75.Vv; 75.70.i; 75.70.Ak Keywords: Thermal stress; Ordering transformation; Cobalt platinum; Texture; Crystallography
Introduction Ordered L10-alloys such as CoPt and FePt with high magnetic anisotropy (Ku107 erg/cm3) were well known to retain a ferromagnetic state even as their grain sizes are small (o5 nm) [1]. Thermodynamically, below 825 1C equicomposition Co50Pt50 forms a chemically ordered L10 structure from a disorder FCC phase, which is composed of alternating atomic planes of Co and Pt along the c-axis. The FCC to FCT transformation therefore causes a distortion of the disordered FCC unit cell, accompanying with an increased lattice parameter a and a decreased lattice parameter c. High uniaxial anisotropy (Ku), which is essential for high coercivity, is mostly dependent on the degree of long-range order in L10-alloys [2]. Besides, Hc is also determined by some extrinsic properties, such as the quantities, distributions of ordering phase and internal stress, etc. In the FePt L10 thin films, some researchers have observed that the Corresponding author. Tel: 886 4 24512298; fax: 886 4 24510014.
E-mail address: [email protected] (S.K. Chen). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.01.049
diffraction peaks split during ordering transformation [3]. Toney et al. suggested that ultra-thin films (%7 nm) might contain fewer defects, grain boundary or stacking faults, which are usually nucleation sites for ordering transformation [4], thus rise the ordering temperature. However, the phase transformation behaviour is changed as the thickness of the film is increased. Some researchers reported that ‘‘domain wall-like magnetization structure’’ existing between two magnetic particles might decrease the magnetization switching filed (Hs) down to a fraction of anisotropy field (0.07–0.26 Hk) [5]. The reduced Hs can be attributed to the increased number of magnetic particles [5]. This can also satisfactorily explain the decrease of Hc as the thickness of the film is increased. Furthermore, preferred orientation also plays an important role in determining the Hc values of the magnetic films [6]. Oriented films can be produced by using textured substrate or by depositing the magnetic film on specific underlayers [7]. Stress is also considered to be effective to induce texture in the L10 film [8]. However, the mechanism for stress-induced texture is not yet clear. In this study, we try to explore how the ordering parameter S, chemically ordered fraction f0 and
ARTICLE IN PRESS e244
W.M. Liao et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e243–e246
internal stress influence the crystal structure and the magnetic property of the CoPt thin film as the thickness of the film is changed.
Experimental Co49Pt51 thin films were deposited on quartz substrates by RF magnetron sputtering at a substrate temperature 800 1C, followed by a post-annealing at the same temperature for 10 min. After the heat treatment, the samples were naturally cooled in the sputtering chamber at a cooling rate of approximately 2 1C/s. The CoPt target was made by pasting Pt foils on a cobalt target with 2 in in diameter. The background vacuum was better than 2 107 Torr and working pressure of argon was fixed at 1.0 102 Torr. The thickness of the film was varied from 10 to 100 nm. Crystal structure and residual stress analysis were studied by 2-axis X-ray diffractometry using Cu Ka radiation. The stress in the film is evaluated from the slope data obtained by sin2 C method. A slow scanning speed of 0.2 degree/min was used to differentiate the three peaks; L10 (2 0 0), L10 (0 0 2) and FCC (2 0 0). Magnetic properties were measured at 300 K with a SQUID magnetometer under a maximum field 5 T along in-plane direction. Chemical compositions of the films were obtained by inductively coupled plasma (ICP) spectroscopy.
Results and discussion Thickness dependence of the coercivity (Hc) and magnetization data measured at an applied field of 5 T (M5T) for the Co49Pt51 films annealed at 800 1C is indicated in Fig. 1. We found that the variation of Hc and M5T with d is quite different for the samples with thickness either below 30 nm or above 50 nm. As the film thickness was raised from 10 to 50 nm, Hc was increased from 5.1 to 13.3 kOe. Further increase in thickness leads to decreased Hc.
Fig. 1. Thickness dependence of coercivity and magnetization of the Co49Pt51 films annealed at 800 1C.
Fig. 2 shows the thickness dependence of ordering parameter S, fraction of ordered phase f0 (definition reported in Ref. [9]); and volume average of ordered phase Save (Save ¼ S f0). Crystallographic data of S, f0 and Save are increased with the increasing film thickness in the range d%30 nm. Thus, the lower Hc can be due to the lack of nucleation sites [4]. Although the S, f0 and Save values can satisfactorily explain the change in coercivity for the thin films (d%30 nm), the Hc values in the thick samples (d^50 nm) are found to decrease with the thickness of the film, in spite of the rise of S, f0 and Save. The coercivity of the thick sample is dominated by the magnetic reversal behaviour, which will be discussed later. Crystallite sizes of the thick samples (d ¼ 502100 nm) were measured to be 30–35 nm by using Scherrer formula. The obtained data were used to study the size effect of the Co49Pt51 films. Furthermore we found that the number of hard magnetic (L10 phase) particles is increased with the increasing d. It is manifest that more L10 particles indeed increase the inter-particle contact area in the thick samples. The ‘‘domain wall-like magnetization structure’’ between two hard magnetic particles, as reported by Zhao et al. might reduce the magnetization switch field to 0.07–0.26 times of the anisotropy field Hk, thus decreases the Hc values in polycrystalline films [5]. On the contrary, our previous data indicate that Hc might be significantly enhanced in granular structured L10-films due to the elimination of the domain wall-like structure [10–12]. X-ray diffraction patterns for the Co49Pt51 samples are changed with the thicknesses of the film, as indicated in Fig. 3. The (1 1 1) texture was observed in XRD patterns because (1 1 1) is the closest packed plane in the CoPt L10 unit cell [13]. We also found that the intensity of (0 0 1) peak is increased with the thickness of the film. Therefore we take a parameter R, the ratio between the integrated intensities of (0 0 1) and (1 1 0), as an index to estimate the degree of stress induced texture.
Fig. 2. Thickness dependence of the crystallographic data S, f0, and Save for the Co49Pt51 films annealed at 800 1C.
ARTICLE IN PRESS e245
Ο fcc ∆(311) Ο(311) ∆(113)
∆(220) Ο(220) ∆(022)
∆(112)
∆ fct
∆(201)
∆(200) ∆(002) Ο(200)
Intensity (a.u.)
∆(110)
∆(001)
Ο(111)+∆(111)
W.M. Liao et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e243–e246
100nm
80nm 60nm 50nm 30nm
20
30
40
50
60
70
80
90
2θ Fig. 3. X-ray diffraction patterns of the Co49Pt51 films with various thicknesses.
Fig. 4 shows the variation of R, Hc, and the slope of sin2 C method (indicated by P) with the thickness of the Co49Pt51 film [14]. We found that for the samples with film thickness d of 50–100 nm, the variation of Hc with d is exactly reverse to the variation of R with d. The results can satisfactorily explain the abnormal drop-off in Hc for the thick sample (d ¼ 80 nm) which possesses the largest R value of 3.96. The (0 0 1) orientation in the Co49Pt51 films is worth studying. It has been reported that for L10-films, the ordering transformation under tensile stress might induce the out-plane (0 0 1) texture [6]. We consider that the stress in our samples was induced by thermal gradient in the cooling step of the annealing treatments. A tensile stress in the Co49Pt51 film was estimated according to a simple equation reported by Rasmussen et al. [6]. The material constants we use in the estimation are listed here: coefficients of thermal expansion are 0.5 106 K1 for quartz substrate and 10.9 106 K1 for CoPt; Young’s modulus and Poisson’s ratio of CoPt bulk are 188 GPa and 0.33, respectively. Since the Young’s modulus of a thin film is 10–20% smaller than that of a bulk, the tensile stress in the film was thus estimated to be 1.8–2.3 GPa. In the analysis using sin2 C method, a positive value of slope indicates the existence of a tensile stress which in turn results in the increase of lattice parameter as the angle of C is increased from 01to 901, where C means the angle between the out-plane direction of the sample and the normal direction of the diffracted plane. The slope of the sin2 C method, P, was obtained from the X-ray diffraction data of the samples, as demonstrated in Fig. 4. The relations between P, internal stress (s) and Young’s modulus (E) can be expressed as PE cot y , s¼ 2ð1 þ nÞ
(1)
Fig. 4. Coercivity, R, and P versus the thickness of the Co49Pt51 films, where R means the ratio between the integrated intensities of (0 0 1) and (1 1 0) diffraction peaks, P the slope obtained from sin2 C method.
where y is the angle of the studied diffraction peak and n is Poisson ratio [14]. From Eq. (1), the internal stress (s) is proportional to E P, which means that a large P is accompanied with a large s. Therefore, the sample with d ¼ 30 nm should have the largest tensile stress and R value. In fact, the elastic modulus of a thin-film is smaller than that of a thick sample due to the effect of ‘‘elastic softening’’ [15]. For this reason, the tensile stress retaining in a thin-film sample (e.g. d ¼ 30 nm) should be smaller than that of a thick one even both samples have the same P value. Therefore the (0 0 1) texture is not obvious in the sample with d ¼ 30 nm. Summary As the thickness of the Co49Pt51 thin films was raised from 10 to 30 nm, Hc was increased from 5.1 to 11.1 kOe due to the increase of S from 0.30 to 0.61 and the increase of f0 from 0.52 to 0.75. The lower Hc might result from the smaller amount of ordered phases in thin-film samples because of the shortage of nucleation sites. A maximum Hc of 13.8 kOe was obtained in the sample with 50 nm in thickness. Coercivity was decreased for thicker films. We suggest that a ‘‘domain wall-like magnetization structure’’ should reduce the magnetization switch field to 0.07–0.26 Hk, thus decreases the Hc values. From the analysis of internal stress, we found that tensile stress tends to induce Co49Pt51 L10 (0 0 1) texture. The (0 0 1) orientation, which to some extent influences the Hc values of the films, can partially explain the irregular change in Hc for a thick sample (d ¼ 80 nm). An effect of ‘‘elastic softening’’ becomes obvious as the thickness of the film is reduced. Acknowledgments This work was supported by the National Science Council of Republic of China. under the Grant no. NSC 94-2216-E-035-009 and NSC 94-2218-E-035-006.
ARTICLE IN PRESS e246
W.M. Liao et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e243–e246
References [1] D. Weller, A. Moser, L. Folks, M.E. Best, W. Lee, M.F. Toney, M. Schwickert, J.-U. Thiele, M.F. Doerner, IEEE Trans. Magn. 36 (2000) 10. [2] H. Kanazawa, G. Lauhoff, T. Suzuki, J. Appl. Phys. 87 (2000) 6143. [3] Q.F. Xiao, E. Bruck, Z.D. Zhang, F.R. de Boer, K.H.J. Buschow, J. Alloys Compounds 364 (2004) 64. [4] M.F. Toney, W.Y. Lee, J.A. Hedstrom, A. Kellock, J. Appl. Phys. 93 (2003) 9902. [5] G.P. Zhao, H.S. Lim, Y.P. Feng, C.K. Ong, G.R. Liu, J. Appl. Phys. 91 (2002) 2186. [6] P. Rasmussen, X. Rui, J.E. Shield, Appl. Phys. Lett. 86 (2005) 191915. [7] T. Shima, K. Takanashi, Y.K. Takahashi, K. Hono, Appl. Phys. Lett. 85 (2004) 2571.
[8] C.H. Lai, C.H. Yang, C.C. Chiang, T. Balaji, T.K. Tseng, Appl. Phys. Lett. 85 (2004) 4430. [9] A. Cebollada, R.F.C. Farrow, M.F. Toney, Magnetic Nanostructures, in: H.S. Nalwa (Ed.), American Scientific, Stevenson Ranch, CA, 2002, p. 96. [10] F.T. Yuan, S.K. Chen, W.C. Chang, L. Horng, Appl. Phys. Lett. 85 (2004) 3163. [11] S.K. Chen, F.T. Yuan, T.S. Chin, IEEE Trans. Magn. 41 (2005) 921. [12] S.K. Chen, F.T. Yuan, S.N. Shiao, J. Appl. Phys. 97 (2005) 073902. [13] R. Abbaschian, Physical Metallurgy Principles, third ed, PWS Publishing Company, Boston, 1994 (Chapter 1). [14] H.P. Klug, L.E. Alexande, X-ray Diffraction Procedures second ed., Wiley, New York, 1974 (Chapter 11). [15] M.C. Salvadori, I.G. Brown, A.R. Vaz, L.L. Melo, M. Cattani, Phys. Rev. B 67 (2003) 153404.
Journal of Magnetism and Magnetic Materials 303 (2006) e243–e246 www.elsevier.com/locate/jmmm
Thickness dependence of crystallographic and magnetic properties for L10-CoPt thin films W.M. Liaoa, S.K. Chena,, F.T. Yuana, C.W. Hsua, H.Y. Leeb a
Department of Materials Science and Engineering, Feng Chia University, Taichung, Taiwan, ROC b National Synchrotron Radiation Research Center, Hsinchu, Taiwan, ROC Available online 13 February 2006
Abstract Thickness dependence of crystallographic and magnetic properties is investigated from the analyses of the order parameter S, chemically ordered fraction f0, and internal stress of the L10 Co49Pt51 film. Coercivity Hc was increased from 5.1 kOe to a maximum value of 13.3 kOe as the thickness of the film (d) was raised from 10 nm to 50 nm.This is due to the increase of S from 0.30 to 0.64 and the increase of f0 from 0.52 to 0.75. For thicker samples (d^50 nm), a dramatic drop-off in Hc was observed at d ¼ 80 nm. The quantity of ordered phase, measured by X-ray diffractometry, is closely related to the Hc value of the Co49Pt51 thin film for do50 nm. However, the existence of ‘‘domain wall-like magnetization structure’’ in thicker Co49Pt51 samples is harmful for Hc. The decrease in Hc can also be partially attributed to the thermal-stress-induced (0 0 1) texture. r 2006 Elsevier B.V. All rights reserved. PACS: 75.75.Vv; 75.70.i; 75.70.Ak Keywords: Thermal stress; Ordering transformation; Cobalt platinum; Texture; Crystallography
Introduction Ordered L10-alloys such as CoPt and FePt with high magnetic anisotropy (Ku107 erg/cm3) were well known to retain a ferromagnetic state even as their grain sizes are small (o5 nm) [1]. Thermodynamically, below 825 1C equicomposition Co50Pt50 forms a chemically ordered L10 structure from a disorder FCC phase, which is composed of alternating atomic planes of Co and Pt along the c-axis. The FCC to FCT transformation therefore causes a distortion of the disordered FCC unit cell, accompanying with an increased lattice parameter a and a decreased lattice parameter c. High uniaxial anisotropy (Ku), which is essential for high coercivity, is mostly dependent on the degree of long-range order in L10-alloys [2]. Besides, Hc is also determined by some extrinsic properties, such as the quantities, distributions of ordering phase and internal stress, etc. In the FePt L10 thin films, some researchers have observed that the Corresponding author. Tel: 886 4 24512298; fax: 886 4 24510014.
E-mail address: [email protected] (S.K. Chen). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.01.049
diffraction peaks split during ordering transformation [3]. Toney et al. suggested that ultra-thin films (%7 nm) might contain fewer defects, grain boundary or stacking faults, which are usually nucleation sites for ordering transformation [4], thus rise the ordering temperature. However, the phase transformation behaviour is changed as the thickness of the film is increased. Some researchers reported that ‘‘domain wall-like magnetization structure’’ existing between two magnetic particles might decrease the magnetization switching filed (Hs) down to a fraction of anisotropy field (0.07–0.26 Hk) [5]. The reduced Hs can be attributed to the increased number of magnetic particles [5]. This can also satisfactorily explain the decrease of Hc as the thickness of the film is increased. Furthermore, preferred orientation also plays an important role in determining the Hc values of the magnetic films [6]. Oriented films can be produced by using textured substrate or by depositing the magnetic film on specific underlayers [7]. Stress is also considered to be effective to induce texture in the L10 film [8]. However, the mechanism for stress-induced texture is not yet clear. In this study, we try to explore how the ordering parameter S, chemically ordered fraction f0 and
ARTICLE IN PRESS e244
W.M. Liao et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e243–e246
internal stress influence the crystal structure and the magnetic property of the CoPt thin film as the thickness of the film is changed.
Experimental Co49Pt51 thin films were deposited on quartz substrates by RF magnetron sputtering at a substrate temperature 800 1C, followed by a post-annealing at the same temperature for 10 min. After the heat treatment, the samples were naturally cooled in the sputtering chamber at a cooling rate of approximately 2 1C/s. The CoPt target was made by pasting Pt foils on a cobalt target with 2 in in diameter. The background vacuum was better than 2 107 Torr and working pressure of argon was fixed at 1.0 102 Torr. The thickness of the film was varied from 10 to 100 nm. Crystal structure and residual stress analysis were studied by 2-axis X-ray diffractometry using Cu Ka radiation. The stress in the film is evaluated from the slope data obtained by sin2 C method. A slow scanning speed of 0.2 degree/min was used to differentiate the three peaks; L10 (2 0 0), L10 (0 0 2) and FCC (2 0 0). Magnetic properties were measured at 300 K with a SQUID magnetometer under a maximum field 5 T along in-plane direction. Chemical compositions of the films were obtained by inductively coupled plasma (ICP) spectroscopy.
Results and discussion Thickness dependence of the coercivity (Hc) and magnetization data measured at an applied field of 5 T (M5T) for the Co49Pt51 films annealed at 800 1C is indicated in Fig. 1. We found that the variation of Hc and M5T with d is quite different for the samples with thickness either below 30 nm or above 50 nm. As the film thickness was raised from 10 to 50 nm, Hc was increased from 5.1 to 13.3 kOe. Further increase in thickness leads to decreased Hc.
Fig. 1. Thickness dependence of coercivity and magnetization of the Co49Pt51 films annealed at 800 1C.
Fig. 2 shows the thickness dependence of ordering parameter S, fraction of ordered phase f0 (definition reported in Ref. [9]); and volume average of ordered phase Save (Save ¼ S f0). Crystallographic data of S, f0 and Save are increased with the increasing film thickness in the range d%30 nm. Thus, the lower Hc can be due to the lack of nucleation sites [4]. Although the S, f0 and Save values can satisfactorily explain the change in coercivity for the thin films (d%30 nm), the Hc values in the thick samples (d^50 nm) are found to decrease with the thickness of the film, in spite of the rise of S, f0 and Save. The coercivity of the thick sample is dominated by the magnetic reversal behaviour, which will be discussed later. Crystallite sizes of the thick samples (d ¼ 502100 nm) were measured to be 30–35 nm by using Scherrer formula. The obtained data were used to study the size effect of the Co49Pt51 films. Furthermore we found that the number of hard magnetic (L10 phase) particles is increased with the increasing d. It is manifest that more L10 particles indeed increase the inter-particle contact area in the thick samples. The ‘‘domain wall-like magnetization structure’’ between two hard magnetic particles, as reported by Zhao et al. might reduce the magnetization switch field to 0.07–0.26 times of the anisotropy field Hk, thus decreases the Hc values in polycrystalline films [5]. On the contrary, our previous data indicate that Hc might be significantly enhanced in granular structured L10-films due to the elimination of the domain wall-like structure [10–12]. X-ray diffraction patterns for the Co49Pt51 samples are changed with the thicknesses of the film, as indicated in Fig. 3. The (1 1 1) texture was observed in XRD patterns because (1 1 1) is the closest packed plane in the CoPt L10 unit cell [13]. We also found that the intensity of (0 0 1) peak is increased with the thickness of the film. Therefore we take a parameter R, the ratio between the integrated intensities of (0 0 1) and (1 1 0), as an index to estimate the degree of stress induced texture.
Fig. 2. Thickness dependence of the crystallographic data S, f0, and Save for the Co49Pt51 films annealed at 800 1C.
ARTICLE IN PRESS e245
Ο fcc ∆(311) Ο(311) ∆(113)
∆(220) Ο(220) ∆(022)
∆(112)
∆ fct
∆(201)
∆(200) ∆(002) Ο(200)
Intensity (a.u.)
∆(110)
∆(001)
Ο(111)+∆(111)
W.M. Liao et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e243–e246
100nm
80nm 60nm 50nm 30nm
20
30
40
50
60
70
80
90
2θ Fig. 3. X-ray diffraction patterns of the Co49Pt51 films with various thicknesses.
Fig. 4 shows the variation of R, Hc, and the slope of sin2 C method (indicated by P) with the thickness of the Co49Pt51 film [14]. We found that for the samples with film thickness d of 50–100 nm, the variation of Hc with d is exactly reverse to the variation of R with d. The results can satisfactorily explain the abnormal drop-off in Hc for the thick sample (d ¼ 80 nm) which possesses the largest R value of 3.96. The (0 0 1) orientation in the Co49Pt51 films is worth studying. It has been reported that for L10-films, the ordering transformation under tensile stress might induce the out-plane (0 0 1) texture [6]. We consider that the stress in our samples was induced by thermal gradient in the cooling step of the annealing treatments. A tensile stress in the Co49Pt51 film was estimated according to a simple equation reported by Rasmussen et al. [6]. The material constants we use in the estimation are listed here: coefficients of thermal expansion are 0.5 106 K1 for quartz substrate and 10.9 106 K1 for CoPt; Young’s modulus and Poisson’s ratio of CoPt bulk are 188 GPa and 0.33, respectively. Since the Young’s modulus of a thin film is 10–20% smaller than that of a bulk, the tensile stress in the film was thus estimated to be 1.8–2.3 GPa. In the analysis using sin2 C method, a positive value of slope indicates the existence of a tensile stress which in turn results in the increase of lattice parameter as the angle of C is increased from 01to 901, where C means the angle between the out-plane direction of the sample and the normal direction of the diffracted plane. The slope of the sin2 C method, P, was obtained from the X-ray diffraction data of the samples, as demonstrated in Fig. 4. The relations between P, internal stress (s) and Young’s modulus (E) can be expressed as PE cot y , s¼ 2ð1 þ nÞ
(1)
Fig. 4. Coercivity, R, and P versus the thickness of the Co49Pt51 films, where R means the ratio between the integrated intensities of (0 0 1) and (1 1 0) diffraction peaks, P the slope obtained from sin2 C method.
where y is the angle of the studied diffraction peak and n is Poisson ratio [14]. From Eq. (1), the internal stress (s) is proportional to E P, which means that a large P is accompanied with a large s. Therefore, the sample with d ¼ 30 nm should have the largest tensile stress and R value. In fact, the elastic modulus of a thin-film is smaller than that of a thick sample due to the effect of ‘‘elastic softening’’ [15]. For this reason, the tensile stress retaining in a thin-film sample (e.g. d ¼ 30 nm) should be smaller than that of a thick one even both samples have the same P value. Therefore the (0 0 1) texture is not obvious in the sample with d ¼ 30 nm. Summary As the thickness of the Co49Pt51 thin films was raised from 10 to 30 nm, Hc was increased from 5.1 to 11.1 kOe due to the increase of S from 0.30 to 0.61 and the increase of f0 from 0.52 to 0.75. The lower Hc might result from the smaller amount of ordered phases in thin-film samples because of the shortage of nucleation sites. A maximum Hc of 13.8 kOe was obtained in the sample with 50 nm in thickness. Coercivity was decreased for thicker films. We suggest that a ‘‘domain wall-like magnetization structure’’ should reduce the magnetization switch field to 0.07–0.26 Hk, thus decreases the Hc values. From the analysis of internal stress, we found that tensile stress tends to induce Co49Pt51 L10 (0 0 1) texture. The (0 0 1) orientation, which to some extent influences the Hc values of the films, can partially explain the irregular change in Hc for a thick sample (d ¼ 80 nm). An effect of ‘‘elastic softening’’ becomes obvious as the thickness of the film is reduced. Acknowledgments This work was supported by the National Science Council of Republic of China. under the Grant no. NSC 94-2216-E-035-009 and NSC 94-2218-E-035-006.
ARTICLE IN PRESS e246
W.M. Liao et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e243–e246
References [1] D. Weller, A. Moser, L. Folks, M.E. Best, W. Lee, M.F. Toney, M. Schwickert, J.-U. Thiele, M.F. Doerner, IEEE Trans. Magn. 36 (2000) 10. [2] H. Kanazawa, G. Lauhoff, T. Suzuki, J. Appl. Phys. 87 (2000) 6143. [3] Q.F. Xiao, E. Bruck, Z.D. Zhang, F.R. de Boer, K.H.J. Buschow, J. Alloys Compounds 364 (2004) 64. [4] M.F. Toney, W.Y. Lee, J.A. Hedstrom, A. Kellock, J. Appl. Phys. 93 (2003) 9902. [5] G.P. Zhao, H.S. Lim, Y.P. Feng, C.K. Ong, G.R. Liu, J. Appl. Phys. 91 (2002) 2186. [6] P. Rasmussen, X. Rui, J.E. Shield, Appl. Phys. Lett. 86 (2005) 191915. [7] T. Shima, K. Takanashi, Y.K. Takahashi, K. Hono, Appl. Phys. Lett. 85 (2004) 2571.
[8] C.H. Lai, C.H. Yang, C.C. Chiang, T. Balaji, T.K. Tseng, Appl. Phys. Lett. 85 (2004) 4430. [9] A. Cebollada, R.F.C. Farrow, M.F. Toney, Magnetic Nanostructures, in: H.S. Nalwa (Ed.), American Scientific, Stevenson Ranch, CA, 2002, p. 96. [10] F.T. Yuan, S.K. Chen, W.C. Chang, L. Horng, Appl. Phys. Lett. 85 (2004) 3163. [11] S.K. Chen, F.T. Yuan, T.S. Chin, IEEE Trans. Magn. 41 (2005) 921. [12] S.K. Chen, F.T. Yuan, S.N. Shiao, J. Appl. Phys. 97 (2005) 073902. [13] R. Abbaschian, Physical Metallurgy Principles, third ed, PWS Publishing Company, Boston, 1994 (Chapter 1). [14] H.P. Klug, L.E. Alexande, X-ray Diffraction Procedures second ed., Wiley, New York, 1974 (Chapter 11). [15] M.C. Salvadori, I.G. Brown, A.R. Vaz, L.L. Melo, M. Cattani, Phys. Rev. B 67 (2003) 153404.