- Email: [email protected]
Thickness dependence of structural and magnetic properties of FePt films
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 271 (2004) 431–436
Thickness dependence of structural and magnetic properties of FePt films B.C. Lim, J.S. Chen*, J.P. Wang1 Media and Materials, Data Storage Institute, DSI Building, 5 Engineering Drive 1, Singapore 117608, Singapore Received 14 July 2003; received in revised form 18 September 2003
Abstract L10 ordered FePt films of (0 0 1) preferred orientation with different thickness have been prepared by DC magnetron sputtering on CrRu (2 0 0) underlayer. The effect of thin film thickness on the structural and magnetic properties was investigated using X-ray diffractometer (XRD) and vibrating sample magnetometer (VSM). Increasing the FePt magnetic layer thickness from 5 to 20 nm resulted in improvement of the superlattice FCT-FePt (0 0 1) preferred orientation. However, when the FePt thickness is 40 nm the FCT-FePt (0 0 1) preferred orientation began to deteriorate. The degree of long range ordering increases linearly with the FePt layer thickness. Out-of-plane coercivity shows linear relation with thickness only to 20 nm, where a maximum value of 2610 Oe is achieved. The initial growth layer and the intrinsic magnetocrystalline anisotropy are calculated. Initial growth layer thickness was found to be within the range of 2.9–3.5 nm in our films and the intrinsic magnetocrystalline anisotropy ranges from 1.45 107 to 1.91 107 erg/cm3, depending on the demagnetisation factor. r 2003 Published by Elsevier B.V. PACS: 75.50.Ss Keywords: Initial layer; FePt; High anisotropy
1. Introduction Due to the increasing demand of huge capacity storage devices and with the longitudinal recording media nearing an end due to the superparamagnetic effect arising from thermal *Corresponding author. Tel.: 65-6874-8625; fax: 65-67772406. E-mail addresses: chen [email protected] (J.S. Chen), [email protected] (J.P. Wang). 1 Current address: Department of Electrical and Computer Engineering, University of Minnesota, MN, USA. 0304-8853/$ - see front matter r 2003 Published by Elsevier B.V. doi:10.1016/j.jmmm.2003.09.052
instability of recorded bits, perpendicular magnetic media with high magnetic anisotropy is very promising to solve such issues. L10 ordered FePt with face-centered tetragonal structure attracted much attention due to its high magnetocrystalline anisotropy constant, Ku ¼ 7 107 erg/cm3 [1] However, high temperature of 600 C and above is required for phase transformation from soft magnetic face centered cubic (FCC) to hard magnetic face centered tetragonal (FCT) phase either during deposition or post-deposition annealing [2]. The other challenge is to control the easy
ARTICLE IN PRESS 432
B.C. Lim et al. / Journal of Magnetism and Magnetic Materials 271 (2004) 431–436
axis of magnetisation in the out-of-plane direction. Methods to obtain L10 ordered FePt (0 0 1) films include molecular beam epitaxial (MBE) growth on MgO single crystal substrate [3], annealing the sputtered FePt–B2O3 films [4], sputtering the FePt film onto MgO (0 0 1) single crystal substrate [5], Ag (1 0 0)/Si (1 0 0) [6] and MgO/FeSi (1 0 0)/Cr (1 0 0)/MgO/glass [7]. Our previous work reported that the addition of Ru into Cr underlayer aids in lowering the substrate temperature with good c-axis orientation and also base pressure and atmospheric constituents played an important role in determining the control of crystal orientation [8,9]. In order to increase recording resolution, the magnetic recording layer is required to be as thin as possible. Usually, an initial layer exists between magnetic layer and underlayer. Hence, it is important to understand how film thickness affects the magnetic and structural properties of FePt alloy films, which is the purpose of the present work
2. Experiments Films with the structure of glass/CrRu/Pt/FePt were produced by DC sputtering in a home designed UHV sputtering system. Typical chamber base pressure is better than 1.8 107 Torr. Ar gas with 99.999% purity was used as the working gas. The CrRu underlayer and Pt layer thickness were fixed at 30 and 4 nm, respectively. The FePt layer thickness varied from 5 to 40 nm. The substrates were pre-heated to a set-point temperature of 400 C for 10 min prior to sputtering. The magnetic properties were analysed using the vibrating sample magnetometer (VSM) and the structural properties by the X-ray diffractometer (XRD) using Cu-Ka radiation.
3. Results and discussion Fig. 1 shows the y22y XRD scans of the FePt films with the thickness varied at 5, 10, 20 and 40 nm. The sample where only 5 nm of FePt is deposited, very weak and broad FCT-FePt (0 0 1) peak is observed indicating very few and small
Fig. 1. XRD y22y scans of FePt films with magnetic FePt thickness varied at 5, 10, 20 and 40 nm.
FCT-FePt grains. The increase of the magnetic layer thickness up to 20 nm leads to the sharp increase in the peak intensity and decrease in the peak width of the FCT-FePt (0 0 1) indicating an improvement in the preferred orientation and larger (0 0 1) oriented FePt grains. Further increase to 40 nm causes the FCT-FePt (0 0 1) peak intensity to reduce. The asymmetric y22y peak between 45 and 50 shifts to higher angle with the increase of the FePt layer thickness. Because the c-axis of fct-FePt phase is slightly smaller than a-axis, the shift of the peak between 45 and 50 suggests that the film becomes more ordered. In addition, it is known that the three diffraction peaks of FCT-FePt (0 0 2), (2 0 0) and FCC-FePt (2 0 0) are located at 48.97 , 47.35 and 47.66 , respectively. So the peak between 45 and 50 may be considered to be the overlapping of the FCTFePt (0 0 2), (2 0 0) and FCC-FePt (2 0 0) peaks and thus the lowering of the intensity and shift of the peak between 45 and 50 may imply the reduction of the FCC-FePt phase with the increase of the FePt layer thickness. Therefore, when the magnetic layer is only 5 nm thick, FCC-FePt (2 0 0) is the dominant phase. As the FePt layer thickness increases, the asymmetric peak shifts from FCCFePt (2 0 0) to the FCT-FePt (0 0 2) dominant peak. In addition, when the FePt layer is 40 nm, thick, the FCT-FePt (1 1 1) peak appears. The quality of FCT-FePt (0 0 1) preferred orientation
ARTICLE IN PRESS B.C. Lim et al. / Journal of Magnetism and Magnetic Materials 271 (2004) 431–436
433
The calculation is made using the following formula: " #1=2 I001 Ff 2 ðL A DÞf S¼ I002 Fs ðL A DÞs 1=2 I001 D 0:85 : ð1Þ I002
Fig. 2. Rocking curves of FCT-FePt (0 0 1) peak at FePt thickness of 10, 20 and 40 nm.
Fig. 3. The ordering parameter, S; as a function of the FePt thickness.
with various FePt thickness was examined by the rocking curve of the FCT-FePt (0 0 1) peak as shown in Fig. 2. The full-width at half-maximum (FWHM) decreases from 9 at 40 nm to 6 at 20 nm. The FWHM increases to 10 again as the FePt thickness decreases to 10 nm. This further confirmed that the FCT-FePt (0 0 1) preferred orientation is thickness dependent and the best texture exists at 20 nm FePt thickness. The degree of chemical ordering of the FePt layer can be quantified by the long-range ordering parameter, S: Fig. 3 shows the ordering parameter, S; plotted as a function of FePt layer thickness.
where Ihkl is the integrated intensity; F is the structure factor; L is the Lorentz polarization factor; A is the absorption factor; D is the temperature factor; subscripts f and s refer to fundamental peak and superlattice peak, respectively [10]. Trend shows that S increases with FePt thickness. Fig. 4 shows the in-plane and out-of-plane hysteresis loops of the samples with various FePt thickness. As can be observed, the slope of the outof-plane hysteresis loops, characterised by 4pðdM=dHÞ at H ¼ Hc ; becomes less steep as the thickness increases, indicating that the grains are less exchange coupled. The in-plane, out-of-plane coercivities and out-of-plane magnetisation squareness as a function of the FePt thickness are summarized in Fig. 5. The out-of-plane coercivity increases with FePt thickness, reaching a maximum of 2610 Oe at 20 nm and decreases subsequently. Usually, the coercivity is strongly linearly dependent on the chemical ordering [11,12] due to the strong dependence of the magnetic anisotropy on the chemical ordering. However, the coercivity also strongly depends on other properties such as grain size, domain structure and magnetic reversal mechanism, etc. The in-plane coercivity increases linearly with the FePt thickness. This may be attributed to the increase of the FCT-FePt (2 0 0) component and the emergence of FCT-FePt (1 1 1) texture. The magnetisation squareness, Mr> =Ms> ; of the FePt layer increases with the FePt thickness, reaching a maximum value of 0.93. Further increase of the FePt thickness shows an inverse relationship. The anisotropy field, Hk ; also increases with a thicker FePt layer as show in Table 1. The magnetocrystalcalculated line anisotropy constant, Ku> is calculated by calculated the expression, Ku> ¼ Hk Ms> =2 and the anisotropy field, Hk was estimated by extrapolating the magnetisation curves. Ideally, the in-plane M2H loops should give linear hysteresis behaviour, but since practically it is difficult to achieve, in-plane
ARTICLE IN PRESS 434
B.C. Lim et al. / Journal of Magnetism and Magnetic Materials 271 (2004) 431–436
Fig. 4. The out-of-plane and in-plane hysteresis loops of samples with FePt thickness varied at (a) 5 nm; (b) 10 nm; (c) 20 nm and (d) 40 nm.
Fig. 5. The out-of-plane, in-plane coercivities and the magnetisation squareness, Mr> =Ms> ; plotted as a function of FePt thickness.
loops will contribute to an error bar to the calculation of Kucalculated : The error bar can be calculated by obtaining the area under the in-plane hysteresis loops in the first quadrant of the M–H loops. The magnetocrystalline anisotropy constant shows a linear increase with the FePt thickness, which is consistent with the chemical ordering dependence on the FePt layer thickness. Few studies have been attempted to evaluate the thickness of the initial growth layer and intrinsic magnetocrystalline constant of the FePt films. A methodology has been proposed to compute these attributes [13]. effective calculated Ku> ¼ Ku> þ 12Nd Ms2 ;
ð2Þ
ARTICLE IN PRESS B.C. Lim et al. / Journal of Magnetism and Magnetic Materials 271 (2004) 431–436 effective where Ku> can be considered as the averaged perpendicular magnetic anisotropy over the hard magnetic layer and its initial soft layer, Nd and Ms refers to the demagnetisation factor and saturation magnetisation respectively. Saito et al., had aptly derived an expression to compute the intrinsic magnetocrystalline anisotropy, Kugrain ; of only the hard magnetic FePt layer and initial growth layer thickness, tinitial [13]. effective Ku> tFePt ¼ Kugrain ðtFePt tinitial Þ:
ð3Þ
In this method, columnar growth and the negligible uniaxial anisotropy energy of the initial layer were assumed. The columnar growth of the FePt films on CrRu underlayer was also observed effective [14]. By constructing a plot of Ku> tFePt vs. tFePt as depicted in Fig. 6, the Kugrain value is obtained by determining the gradient and tinitial obtained by extrapolating the line to the tFePt axis.
435
For the thin film of the independent grains mean field theory predicts a demagnetisation factor Nd ¼ 4p: Experimentally, however, this value often causes S-shaped remanence curve after demagnetisation correction. The traditional ‘‘solution’’ for the artifact is to introduce an arbitrary ‘‘effective’’ Nd o4p: However, it is difficult to get a correct and exact value of Nd : The derivation of Nd is still widely studied. In this work, the intrinsic anisotropy and initial growth thickness were estimated by assuming minimum and maximum values of the Nd ¼ 0 and 4p, respectively. Kugrain ¼ 1:45 107 erg=cm3 and tinitial ¼ 3:5 nm when Nd ¼ 0 and Kugrain ¼ 1:91 107 lerg=cm3 and tinitial ¼ 2:9 nm when Nd ¼ 4p: So the correct value for Kugrain and tinitial should vary from 1.45 107 to 1.91 107 erg/cm3 and from 2.9 to 3.5 nm, respectively.
4. Conclusions Table 1 The anisotropy field, Hk and magnetocrystalline anisotropy calculated constant, Ku> ; with respect to the FePt thickness tFePt ðnmÞ
Hk (kOe)
calculated Ku> 107 ðerg=cm3 Þ
5 10 20 40
15 23.5 27 31.5
0.60570.091 0.97270.076 1.10270.024 1.33970.049
In summary, the effect of varying the FePt thickness on magnetic and structural properties with perpendicular anisotropy has been investigated. The out-of-plane coercivity is observed to increase with thickness but reduces drastically when FePt layer is thicker than 20 nm. The degree of long-range ordering increases linearly with the FePt thickness. The best FCT-FePt (0 0 1) texture with the FWHM of the rocking curve of 6 was obtained at the FePt thickness of 20 nm. The initial growth layer and the intrinsic magnetocrystalline anisotropy of FePt are calculated by assuming end-limit values of demagnetisation factor, Nd ¼ 0 or 4p.
Acknowledgements The authors to express gratitude to Dr. Zhou Tiejun for his invaluable discussions.
References effective Fig. 6. Plot of Ku> tFePt vs. tFePt to compute initial layer thickness, tinitial and intrinsic magnetocrystalline anisotropy, Kugrain :
[1] M. Watanabe, M. Homma, Jpn. J. Appl. Phys. 10A (1996) L1264.
ARTICLE IN PRESS 436
B.C. Lim et al. / Journal of Magnetism and Magnetic Materials 271 (2004) 431–436
[2] M.R. Visokay, R. Sinclair, Appl. Phys. Lett. 66 (1995) 1692. [3] R.F.C. Farrow, D. Weller, R.F. Marks, M.F. Toney, A. Cebollada, G.R. Harp, J. Appl. Phys. 79 (1996) 5967. [4] C.P. Luo, S.H. Liou, L. Gao, Y. Liu, D.J. Sellmyer, Appl. Phys. Lett. 77 (2000) 2225. [5] M.R. Visokay, R. Sinclair, Appl. Phys. Lett. 66 (1995) 1692. [6] Y.-N. Hsu, S. Jeong, D.N. Lambeth, D. Laughlin, IEEE Trans. Magn. 36 (2000) 2945. [7] T. Suzuki, T. Kiya, N. Honda, K. Ouchi, J. Magn. Magn. Mater. 235 (2001) 312. [8] Y.F. Xu, J.S. Chen, J.P. Wang, Appl. Phys. Lett. 80 (2002) 3325.
[9] J.S. Chen, B.C. Lim, J.P. Wang, Appl. Phys. Lett. 81 (2002) 1848. [10] J.A. Christodoulides, P. Farber, M. Daniil, H. Okumura, G.C. Hadjipanayis, V. Skumryev, A. Simopoulos, D. Weller, IEEE Trans. Magn. 37 (2001) 1292. [11] J.C. Shih, H.H. Hsiao, J.L. Tsai, T.S. Chin, IEEE Trans. Magn. 37 (2001) 1280. [12] R.A. Ristau, K. Barmak, L.H. Lewis, K.R. Coffey, J.K. Howard, J. Appl. Phys. 86 (1999) 4527. [13] S. Saito, D. Hasegawa, F. Hoshi, D.D. Djayaprawira, M. Takahashi, Appl. Phys. Lett. 80 (2002) 811. [14] Y.F. Xu, J.S. Chen, D.Y. Dai, J.P. Wang, IEEE Trans. Magn. 38 (2002) 2042.
Journal of Magnetism and Magnetic Materials 271 (2004) 431–436
Thickness dependence of structural and magnetic properties of FePt films B.C. Lim, J.S. Chen*, J.P. Wang1 Media and Materials, Data Storage Institute, DSI Building, 5 Engineering Drive 1, Singapore 117608, Singapore Received 14 July 2003; received in revised form 18 September 2003
Abstract L10 ordered FePt films of (0 0 1) preferred orientation with different thickness have been prepared by DC magnetron sputtering on CrRu (2 0 0) underlayer. The effect of thin film thickness on the structural and magnetic properties was investigated using X-ray diffractometer (XRD) and vibrating sample magnetometer (VSM). Increasing the FePt magnetic layer thickness from 5 to 20 nm resulted in improvement of the superlattice FCT-FePt (0 0 1) preferred orientation. However, when the FePt thickness is 40 nm the FCT-FePt (0 0 1) preferred orientation began to deteriorate. The degree of long range ordering increases linearly with the FePt layer thickness. Out-of-plane coercivity shows linear relation with thickness only to 20 nm, where a maximum value of 2610 Oe is achieved. The initial growth layer and the intrinsic magnetocrystalline anisotropy are calculated. Initial growth layer thickness was found to be within the range of 2.9–3.5 nm in our films and the intrinsic magnetocrystalline anisotropy ranges from 1.45 107 to 1.91 107 erg/cm3, depending on the demagnetisation factor. r 2003 Published by Elsevier B.V. PACS: 75.50.Ss Keywords: Initial layer; FePt; High anisotropy
1. Introduction Due to the increasing demand of huge capacity storage devices and with the longitudinal recording media nearing an end due to the superparamagnetic effect arising from thermal *Corresponding author. Tel.: 65-6874-8625; fax: 65-67772406. E-mail addresses: chen [email protected] (J.S. Chen), [email protected] (J.P. Wang). 1 Current address: Department of Electrical and Computer Engineering, University of Minnesota, MN, USA. 0304-8853/$ - see front matter r 2003 Published by Elsevier B.V. doi:10.1016/j.jmmm.2003.09.052
instability of recorded bits, perpendicular magnetic media with high magnetic anisotropy is very promising to solve such issues. L10 ordered FePt with face-centered tetragonal structure attracted much attention due to its high magnetocrystalline anisotropy constant, Ku ¼ 7 107 erg/cm3 [1] However, high temperature of 600 C and above is required for phase transformation from soft magnetic face centered cubic (FCC) to hard magnetic face centered tetragonal (FCT) phase either during deposition or post-deposition annealing [2]. The other challenge is to control the easy
ARTICLE IN PRESS 432
B.C. Lim et al. / Journal of Magnetism and Magnetic Materials 271 (2004) 431–436
axis of magnetisation in the out-of-plane direction. Methods to obtain L10 ordered FePt (0 0 1) films include molecular beam epitaxial (MBE) growth on MgO single crystal substrate [3], annealing the sputtered FePt–B2O3 films [4], sputtering the FePt film onto MgO (0 0 1) single crystal substrate [5], Ag (1 0 0)/Si (1 0 0) [6] and MgO/FeSi (1 0 0)/Cr (1 0 0)/MgO/glass [7]. Our previous work reported that the addition of Ru into Cr underlayer aids in lowering the substrate temperature with good c-axis orientation and also base pressure and atmospheric constituents played an important role in determining the control of crystal orientation [8,9]. In order to increase recording resolution, the magnetic recording layer is required to be as thin as possible. Usually, an initial layer exists between magnetic layer and underlayer. Hence, it is important to understand how film thickness affects the magnetic and structural properties of FePt alloy films, which is the purpose of the present work
2. Experiments Films with the structure of glass/CrRu/Pt/FePt were produced by DC sputtering in a home designed UHV sputtering system. Typical chamber base pressure is better than 1.8 107 Torr. Ar gas with 99.999% purity was used as the working gas. The CrRu underlayer and Pt layer thickness were fixed at 30 and 4 nm, respectively. The FePt layer thickness varied from 5 to 40 nm. The substrates were pre-heated to a set-point temperature of 400 C for 10 min prior to sputtering. The magnetic properties were analysed using the vibrating sample magnetometer (VSM) and the structural properties by the X-ray diffractometer (XRD) using Cu-Ka radiation.
3. Results and discussion Fig. 1 shows the y22y XRD scans of the FePt films with the thickness varied at 5, 10, 20 and 40 nm. The sample where only 5 nm of FePt is deposited, very weak and broad FCT-FePt (0 0 1) peak is observed indicating very few and small
Fig. 1. XRD y22y scans of FePt films with magnetic FePt thickness varied at 5, 10, 20 and 40 nm.
FCT-FePt grains. The increase of the magnetic layer thickness up to 20 nm leads to the sharp increase in the peak intensity and decrease in the peak width of the FCT-FePt (0 0 1) indicating an improvement in the preferred orientation and larger (0 0 1) oriented FePt grains. Further increase to 40 nm causes the FCT-FePt (0 0 1) peak intensity to reduce. The asymmetric y22y peak between 45 and 50 shifts to higher angle with the increase of the FePt layer thickness. Because the c-axis of fct-FePt phase is slightly smaller than a-axis, the shift of the peak between 45 and 50 suggests that the film becomes more ordered. In addition, it is known that the three diffraction peaks of FCT-FePt (0 0 2), (2 0 0) and FCC-FePt (2 0 0) are located at 48.97 , 47.35 and 47.66 , respectively. So the peak between 45 and 50 may be considered to be the overlapping of the FCTFePt (0 0 2), (2 0 0) and FCC-FePt (2 0 0) peaks and thus the lowering of the intensity and shift of the peak between 45 and 50 may imply the reduction of the FCC-FePt phase with the increase of the FePt layer thickness. Therefore, when the magnetic layer is only 5 nm thick, FCC-FePt (2 0 0) is the dominant phase. As the FePt layer thickness increases, the asymmetric peak shifts from FCCFePt (2 0 0) to the FCT-FePt (0 0 2) dominant peak. In addition, when the FePt layer is 40 nm, thick, the FCT-FePt (1 1 1) peak appears. The quality of FCT-FePt (0 0 1) preferred orientation
ARTICLE IN PRESS B.C. Lim et al. / Journal of Magnetism and Magnetic Materials 271 (2004) 431–436
433
The calculation is made using the following formula: " #1=2 I001 Ff 2 ðL A DÞf S¼ I002 Fs ðL A DÞs 1=2 I001 D 0:85 : ð1Þ I002
Fig. 2. Rocking curves of FCT-FePt (0 0 1) peak at FePt thickness of 10, 20 and 40 nm.
Fig. 3. The ordering parameter, S; as a function of the FePt thickness.
with various FePt thickness was examined by the rocking curve of the FCT-FePt (0 0 1) peak as shown in Fig. 2. The full-width at half-maximum (FWHM) decreases from 9 at 40 nm to 6 at 20 nm. The FWHM increases to 10 again as the FePt thickness decreases to 10 nm. This further confirmed that the FCT-FePt (0 0 1) preferred orientation is thickness dependent and the best texture exists at 20 nm FePt thickness. The degree of chemical ordering of the FePt layer can be quantified by the long-range ordering parameter, S: Fig. 3 shows the ordering parameter, S; plotted as a function of FePt layer thickness.
where Ihkl is the integrated intensity; F is the structure factor; L is the Lorentz polarization factor; A is the absorption factor; D is the temperature factor; subscripts f and s refer to fundamental peak and superlattice peak, respectively [10]. Trend shows that S increases with FePt thickness. Fig. 4 shows the in-plane and out-of-plane hysteresis loops of the samples with various FePt thickness. As can be observed, the slope of the outof-plane hysteresis loops, characterised by 4pðdM=dHÞ at H ¼ Hc ; becomes less steep as the thickness increases, indicating that the grains are less exchange coupled. The in-plane, out-of-plane coercivities and out-of-plane magnetisation squareness as a function of the FePt thickness are summarized in Fig. 5. The out-of-plane coercivity increases with FePt thickness, reaching a maximum of 2610 Oe at 20 nm and decreases subsequently. Usually, the coercivity is strongly linearly dependent on the chemical ordering [11,12] due to the strong dependence of the magnetic anisotropy on the chemical ordering. However, the coercivity also strongly depends on other properties such as grain size, domain structure and magnetic reversal mechanism, etc. The in-plane coercivity increases linearly with the FePt thickness. This may be attributed to the increase of the FCT-FePt (2 0 0) component and the emergence of FCT-FePt (1 1 1) texture. The magnetisation squareness, Mr> =Ms> ; of the FePt layer increases with the FePt thickness, reaching a maximum value of 0.93. Further increase of the FePt thickness shows an inverse relationship. The anisotropy field, Hk ; also increases with a thicker FePt layer as show in Table 1. The magnetocrystalcalculated line anisotropy constant, Ku> is calculated by calculated the expression, Ku> ¼ Hk Ms> =2 and the anisotropy field, Hk was estimated by extrapolating the magnetisation curves. Ideally, the in-plane M2H loops should give linear hysteresis behaviour, but since practically it is difficult to achieve, in-plane
ARTICLE IN PRESS 434
B.C. Lim et al. / Journal of Magnetism and Magnetic Materials 271 (2004) 431–436
Fig. 4. The out-of-plane and in-plane hysteresis loops of samples with FePt thickness varied at (a) 5 nm; (b) 10 nm; (c) 20 nm and (d) 40 nm.
Fig. 5. The out-of-plane, in-plane coercivities and the magnetisation squareness, Mr> =Ms> ; plotted as a function of FePt thickness.
loops will contribute to an error bar to the calculation of Kucalculated : The error bar can be calculated by obtaining the area under the in-plane hysteresis loops in the first quadrant of the M–H loops. The magnetocrystalline anisotropy constant shows a linear increase with the FePt thickness, which is consistent with the chemical ordering dependence on the FePt layer thickness. Few studies have been attempted to evaluate the thickness of the initial growth layer and intrinsic magnetocrystalline constant of the FePt films. A methodology has been proposed to compute these attributes [13]. effective calculated Ku> ¼ Ku> þ 12Nd Ms2 ;
ð2Þ
ARTICLE IN PRESS B.C. Lim et al. / Journal of Magnetism and Magnetic Materials 271 (2004) 431–436 effective where Ku> can be considered as the averaged perpendicular magnetic anisotropy over the hard magnetic layer and its initial soft layer, Nd and Ms refers to the demagnetisation factor and saturation magnetisation respectively. Saito et al., had aptly derived an expression to compute the intrinsic magnetocrystalline anisotropy, Kugrain ; of only the hard magnetic FePt layer and initial growth layer thickness, tinitial [13]. effective Ku> tFePt ¼ Kugrain ðtFePt tinitial Þ:
ð3Þ
In this method, columnar growth and the negligible uniaxial anisotropy energy of the initial layer were assumed. The columnar growth of the FePt films on CrRu underlayer was also observed effective [14]. By constructing a plot of Ku> tFePt vs. tFePt as depicted in Fig. 6, the Kugrain value is obtained by determining the gradient and tinitial obtained by extrapolating the line to the tFePt axis.
435
For the thin film of the independent grains mean field theory predicts a demagnetisation factor Nd ¼ 4p: Experimentally, however, this value often causes S-shaped remanence curve after demagnetisation correction. The traditional ‘‘solution’’ for the artifact is to introduce an arbitrary ‘‘effective’’ Nd o4p: However, it is difficult to get a correct and exact value of Nd : The derivation of Nd is still widely studied. In this work, the intrinsic anisotropy and initial growth thickness were estimated by assuming minimum and maximum values of the Nd ¼ 0 and 4p, respectively. Kugrain ¼ 1:45 107 erg=cm3 and tinitial ¼ 3:5 nm when Nd ¼ 0 and Kugrain ¼ 1:91 107 lerg=cm3 and tinitial ¼ 2:9 nm when Nd ¼ 4p: So the correct value for Kugrain and tinitial should vary from 1.45 107 to 1.91 107 erg/cm3 and from 2.9 to 3.5 nm, respectively.
4. Conclusions Table 1 The anisotropy field, Hk and magnetocrystalline anisotropy calculated constant, Ku> ; with respect to the FePt thickness tFePt ðnmÞ
Hk (kOe)
calculated Ku> 107 ðerg=cm3 Þ
5 10 20 40
15 23.5 27 31.5
0.60570.091 0.97270.076 1.10270.024 1.33970.049
In summary, the effect of varying the FePt thickness on magnetic and structural properties with perpendicular anisotropy has been investigated. The out-of-plane coercivity is observed to increase with thickness but reduces drastically when FePt layer is thicker than 20 nm. The degree of long-range ordering increases linearly with the FePt thickness. The best FCT-FePt (0 0 1) texture with the FWHM of the rocking curve of 6 was obtained at the FePt thickness of 20 nm. The initial growth layer and the intrinsic magnetocrystalline anisotropy of FePt are calculated by assuming end-limit values of demagnetisation factor, Nd ¼ 0 or 4p.
Acknowledgements The authors to express gratitude to Dr. Zhou Tiejun for his invaluable discussions.
References effective Fig. 6. Plot of Ku> tFePt vs. tFePt to compute initial layer thickness, tinitial and intrinsic magnetocrystalline anisotropy, Kugrain :
[1] M. Watanabe, M. Homma, Jpn. J. Appl. Phys. 10A (1996) L1264.
ARTICLE IN PRESS 436
B.C. Lim et al. / Journal of Magnetism and Magnetic Materials 271 (2004) 431–436
[2] M.R. Visokay, R. Sinclair, Appl. Phys. Lett. 66 (1995) 1692. [3] R.F.C. Farrow, D. Weller, R.F. Marks, M.F. Toney, A. Cebollada, G.R. Harp, J. Appl. Phys. 79 (1996) 5967. [4] C.P. Luo, S.H. Liou, L. Gao, Y. Liu, D.J. Sellmyer, Appl. Phys. Lett. 77 (2000) 2225. [5] M.R. Visokay, R. Sinclair, Appl. Phys. Lett. 66 (1995) 1692. [6] Y.-N. Hsu, S. Jeong, D.N. Lambeth, D. Laughlin, IEEE Trans. Magn. 36 (2000) 2945. [7] T. Suzuki, T. Kiya, N. Honda, K. Ouchi, J. Magn. Magn. Mater. 235 (2001) 312. [8] Y.F. Xu, J.S. Chen, J.P. Wang, Appl. Phys. Lett. 80 (2002) 3325.
[9] J.S. Chen, B.C. Lim, J.P. Wang, Appl. Phys. Lett. 81 (2002) 1848. [10] J.A. Christodoulides, P. Farber, M. Daniil, H. Okumura, G.C. Hadjipanayis, V. Skumryev, A. Simopoulos, D. Weller, IEEE Trans. Magn. 37 (2001) 1292. [11] J.C. Shih, H.H. Hsiao, J.L. Tsai, T.S. Chin, IEEE Trans. Magn. 37 (2001) 1280. [12] R.A. Ristau, K. Barmak, L.H. Lewis, K.R. Coffey, J.K. Howard, J. Appl. Phys. 86 (1999) 4527. [13] S. Saito, D. Hasegawa, F. Hoshi, D.D. Djayaprawira, M. Takahashi, Appl. Phys. Lett. 80 (2002) 811. [14] Y.F. Xu, J.S. Chen, D.Y. Dai, J.P. Wang, IEEE Trans. Magn. 38 (2002) 2042.