- Email: [email protected]
Grain growth and L10 ordering in FePt–SiO2 granular films
Journal of Magnetism and Magnetic Materials 239 (2002) 310–312
Grain growth and L10 ordering in FePt–SiO2 granular films Takeshi Saito, Osamu Kitakami, Yutaka Shimada* Institute for Multidisciplinary Research for Advanced Material, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan
Abstract Ordering process of FePt fine grains in SiO2 matrix is investigated. Study of the grain growth process by annealing FePt–SiO2 granular films with various packing densities monitoring the ordering parameter reveals that ordering is triggered and accelerated by coalescence of grains. r 2002 Published by Elsevier Science B.V. Keywords: L10 ordering; Granular film; Coalescence
FePt alloy with the ordered L10 structure is of interest because of its very high uniaxial anisotropy. Investigation of the thin film of L10 has been performed intensively from the standpoint of applying it to ultrahigh density magnetic recording media. However, formation of L10 films by evaporation or sputtering is not easy. FePt films formed by rapid condensation from vapor phase are disordered and it requires very high temperature annealing to accelerate ordering. This may be due to a high potential barrier between the ordered and disordered state. A basic investigation to determine the activation energy of transformation for FePt films has not been made as far as we know, while a number of reports have been published on the ordering process or increase of coercive force in a variety of fabrication conditions such as annealing temperature [1], additives [2,3], underlayers [4–6] and multilayer structures [7,8]. In this paper, process of ordering is studied for the granular structure of FePt–SiO2. The granular structure is one of the most interesting materials because ferromagnetic metal particles with relatively small distribution of particle size are imbedded in oxide matrix satisfying one of the requirements of high density recording media. In the as-deposited state, FePt particles are disordered and the ordered phase is available after annealing at temperatures as high as 650–7001C. Size *Corresponding author. Fax: +81-22-217-5356. E-mail address: [email protected] (Y. Shimada).
increase of FePt crystals occurs appreciably during this annealing experiment and that makes the process of ordering ambiguous. For this reason, we focus our study on the process of grain growth and ordering during annealing to find out any clue to lower the ordering temperature in the granular structure. The samples were produced by sputtering of FePt chips on a SiO2 target. The thickness of the samples is about 40–50 nm. The packing density (P), which is a ratio of FePt to the whole volume, was determined by measurement of Ms of granular samples after determination of Ms for a disordered single layer FePt film. The packing density was varied widely from 0.89 to 0.25. This is to see different grain growth processes for different P: They were annealed in vacuum at temperatures up to 6501C. The ordering parameter S was determined as the following: ðI1 0 0 Þ=fðI2 0 0 Þ þ ðI0 0 2 Þg 2 ; S ¼ ð1Þ ðI1 0 0 Þc=fðI2 0 0 Þc þ ðI0 0 2 Þcg where ðI1 0 0 Þc is the theoretically calculated diffraction intensity from (1 0 0). The same notation holds for (0 0 2) and (2 0 0). The crystal size ðDÞ or coherent length was estimated by X-ray profile of (1 1 1) by Scherrer’s equation. Transmission electron microscopy was used to observe growth of the ordered phase in the granular structure. Fig. 1 shows the annealing temperature ðTa Þ dependence of the crystal size ðDÞ for various P: The crystal
0304-8853/02/$ - see front matter r 2002 Published by Elsevier Science B.V. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 5 9 5 - 9
T. Saito et al. / Journal of Magnetism and Magnetic Materials 239 (2002) 310–312
sizes are obviously dependent on P: It is also obvious that there are critical annealing temperatures above which the sizes show a very rapid increase. Fig. 2 shows variation of S parameter for D with different values of P: The parameter must not exceed 1.0
18 p= 0.25 p= 0.36 p= 0.42 p= 0.47 p= 0.89
16
D (nm)
14 12 10 8 6 4 2 0
100
200
300
400
500
600
700
Ta (˚C)
Fig. 1. Increase of the crystal size by annealing at Ta with P as a parameter.
1.0
S
0.8 0.6
p=0.25 p=0.36 p=0.42 p=0.47 p=0.89
0.4 0.2
0 2
4
6
8
10
12
14
16
18
D (nm)
Fig. 2. Relation between S and D with P as a parameter.
311
theoretically but there is disagreement between the theoretical model of Eq. (1) that assumes randomly oriented polycrystals and the samples that tend to have a preferred direction of crystal growth. As the samples become thinner, they tend to have (0 0 1) parallel to the film plane. This causes an error in the estimation of S parameter and the values of S thus determined is not fully exact but we can extract an important information on the beginning of ordering. In the figure, rapid increase of S is observed and the critical D for which S begins to rise is dependent on P: Fig. 3 is the TEM observation of a sample with P ¼ 0:23: Variation of the granular structure in the figure is consistent with the results shown in Figs. 1 and 2. After annealing at 5001C and 5251C, the grains grow slowly and after annealing at 5501C and 5751C, they start to grow rapidly. After annealing at 6001C, a wide distribution of grain size is observed. This rapid growth is due to coalescence and S parameter jumps from zero to 0.4 after annealing at 5751C. This suggests that coalescence between grains triggers and accelerates the ordering process. Fig. 4 is a relation between the critical crystal size Ds for which S jumps up from zero and Dx for which the crystal size determined from X-ray diffraction starts to increase rapidly. They are about the same values and indicate again that coalescence accompanies the ordering process. As mentioned previously, L10 phase is an equilibrium state but it seems that there is a high energy barrier between the ordered and disordered phase and very high temperature annealing is needed to give enough thermal fluctuation to accelerate exchange of lattice sites between Fe and Pt atoms. The coalescence process accompanied by intensive atomic
Fig. 3. TEM images of a sample with thickness 40 nm and P ¼ 0:23:
312
T. Saito et al. / Journal of Magnetism and Magnetic Materials 239 (2002) 310–312
Ds (nm)
8.0
p= 0.25 p= 0.36 p= 0.42 p= 0.47 p= 0.89
7.0
6.0
5.0
5.0
6.0
7.0
8.0
D x (nm)
Fig. 4. Relation between the critical size Ds for which S parameters start to increase from zero and Dx that indicates the beginning of coalescence.
TEM observation was made for a sample halfway of ordering. Fig. 5 shows an image of a sample with P ¼ 0:23 after annealing at 6001C. The ordering parameter is estimated to be 0.5. It is seen that grains are composites of ordered and disordered crystals. It is impossible to differentiate FCC(1 1 0) and FCT(1 1 0), but some of these (1 1 0) lattice images should be attributed to FCC because of relatively low S: In this figure, it is clear that the crystals are transformed into an ordered structure through the coalescence process. In conclusion, coalescence of grains in FePt–SiO2 granular films gives rise and accelerates L10 ordering in FePt. From industrial point of view, lowering of ordering temperature is vital for practical use of this material. In further study search for oxides, additives and granular structures to accelerate coalescence are needed to make it possible.
References [1] M. Watanabe, T. Masumoto, D.H. Ping, K. Hono, Appl. Phys. Lett. 76 (2000) 3971. [2] C. Chen, O. Kitakami, S. Okamoto, Y. Shimada, Appl. Phys. Lett. 76 (2000) 3218. [3] O. Kitakami, Y. Shimada, K. Oikawa, H. Daimon, K. Futamichi, Appl. Phys. Lett. 78 (2001) 1104. [4] T. Suzuki, N. Honda, K. Ouchi, J. Magn. Soc. Jpn. 21 (1997) 177. [5] Bo. Bian, K. Sato, Y. Hirotu, A. Makino, J. Appl. Phys. 84 (1998) 4403. [6] Y.N. Hsu, S. Jeong, D. Laughlin, D. Lambeth, Eighth Joint MMM-Intermag Conference DP-05, 2001. [7] C.P. Luo, S.H. Liou, L. Gao, Y. Liu, D.J. Sellmyer, Appl. Phys. Lett. 77 (2000) 2225. [8] Y. Endo, N. Kikuchi, O. Kitakami, Y. Shimada, Eighth Joint MMM-Intermag Conference DP-04, 2001. Fig. 5. TEM image of a sample with thickness 40 nm and P ¼ 0:23 after annealing at 6001C. The ordering parameter is estimated to be about 0.5.
diffusion and high density lattice defects at the interface may decrease the energy barrier and give rise to rearrangement of Fe and Pt atoms into the equilibrium state of L10.
Grain growth and L10 ordering in FePt–SiO2 granular films Takeshi Saito, Osamu Kitakami, Yutaka Shimada* Institute for Multidisciplinary Research for Advanced Material, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan
Abstract Ordering process of FePt fine grains in SiO2 matrix is investigated. Study of the grain growth process by annealing FePt–SiO2 granular films with various packing densities monitoring the ordering parameter reveals that ordering is triggered and accelerated by coalescence of grains. r 2002 Published by Elsevier Science B.V. Keywords: L10 ordering; Granular film; Coalescence
FePt alloy with the ordered L10 structure is of interest because of its very high uniaxial anisotropy. Investigation of the thin film of L10 has been performed intensively from the standpoint of applying it to ultrahigh density magnetic recording media. However, formation of L10 films by evaporation or sputtering is not easy. FePt films formed by rapid condensation from vapor phase are disordered and it requires very high temperature annealing to accelerate ordering. This may be due to a high potential barrier between the ordered and disordered state. A basic investigation to determine the activation energy of transformation for FePt films has not been made as far as we know, while a number of reports have been published on the ordering process or increase of coercive force in a variety of fabrication conditions such as annealing temperature [1], additives [2,3], underlayers [4–6] and multilayer structures [7,8]. In this paper, process of ordering is studied for the granular structure of FePt–SiO2. The granular structure is one of the most interesting materials because ferromagnetic metal particles with relatively small distribution of particle size are imbedded in oxide matrix satisfying one of the requirements of high density recording media. In the as-deposited state, FePt particles are disordered and the ordered phase is available after annealing at temperatures as high as 650–7001C. Size *Corresponding author. Fax: +81-22-217-5356. E-mail address: [email protected] (Y. Shimada).
increase of FePt crystals occurs appreciably during this annealing experiment and that makes the process of ordering ambiguous. For this reason, we focus our study on the process of grain growth and ordering during annealing to find out any clue to lower the ordering temperature in the granular structure. The samples were produced by sputtering of FePt chips on a SiO2 target. The thickness of the samples is about 40–50 nm. The packing density (P), which is a ratio of FePt to the whole volume, was determined by measurement of Ms of granular samples after determination of Ms for a disordered single layer FePt film. The packing density was varied widely from 0.89 to 0.25. This is to see different grain growth processes for different P: They were annealed in vacuum at temperatures up to 6501C. The ordering parameter S was determined as the following: ðI1 0 0 Þ=fðI2 0 0 Þ þ ðI0 0 2 Þg 2 ; S ¼ ð1Þ ðI1 0 0 Þc=fðI2 0 0 Þc þ ðI0 0 2 Þcg where ðI1 0 0 Þc is the theoretically calculated diffraction intensity from (1 0 0). The same notation holds for (0 0 2) and (2 0 0). The crystal size ðDÞ or coherent length was estimated by X-ray profile of (1 1 1) by Scherrer’s equation. Transmission electron microscopy was used to observe growth of the ordered phase in the granular structure. Fig. 1 shows the annealing temperature ðTa Þ dependence of the crystal size ðDÞ for various P: The crystal
0304-8853/02/$ - see front matter r 2002 Published by Elsevier Science B.V. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 5 9 5 - 9
T. Saito et al. / Journal of Magnetism and Magnetic Materials 239 (2002) 310–312
sizes are obviously dependent on P: It is also obvious that there are critical annealing temperatures above which the sizes show a very rapid increase. Fig. 2 shows variation of S parameter for D with different values of P: The parameter must not exceed 1.0
18 p= 0.25 p= 0.36 p= 0.42 p= 0.47 p= 0.89
16
D (nm)
14 12 10 8 6 4 2 0
100
200
300
400
500
600
700
Ta (˚C)
Fig. 1. Increase of the crystal size by annealing at Ta with P as a parameter.
1.0
S
0.8 0.6
p=0.25 p=0.36 p=0.42 p=0.47 p=0.89
0.4 0.2
0 2
4
6
8
10
12
14
16
18
D (nm)
Fig. 2. Relation between S and D with P as a parameter.
311
theoretically but there is disagreement between the theoretical model of Eq. (1) that assumes randomly oriented polycrystals and the samples that tend to have a preferred direction of crystal growth. As the samples become thinner, they tend to have (0 0 1) parallel to the film plane. This causes an error in the estimation of S parameter and the values of S thus determined is not fully exact but we can extract an important information on the beginning of ordering. In the figure, rapid increase of S is observed and the critical D for which S begins to rise is dependent on P: Fig. 3 is the TEM observation of a sample with P ¼ 0:23: Variation of the granular structure in the figure is consistent with the results shown in Figs. 1 and 2. After annealing at 5001C and 5251C, the grains grow slowly and after annealing at 5501C and 5751C, they start to grow rapidly. After annealing at 6001C, a wide distribution of grain size is observed. This rapid growth is due to coalescence and S parameter jumps from zero to 0.4 after annealing at 5751C. This suggests that coalescence between grains triggers and accelerates the ordering process. Fig. 4 is a relation between the critical crystal size Ds for which S jumps up from zero and Dx for which the crystal size determined from X-ray diffraction starts to increase rapidly. They are about the same values and indicate again that coalescence accompanies the ordering process. As mentioned previously, L10 phase is an equilibrium state but it seems that there is a high energy barrier between the ordered and disordered phase and very high temperature annealing is needed to give enough thermal fluctuation to accelerate exchange of lattice sites between Fe and Pt atoms. The coalescence process accompanied by intensive atomic
Fig. 3. TEM images of a sample with thickness 40 nm and P ¼ 0:23:
312
T. Saito et al. / Journal of Magnetism and Magnetic Materials 239 (2002) 310–312
Ds (nm)
8.0
p= 0.25 p= 0.36 p= 0.42 p= 0.47 p= 0.89
7.0
6.0
5.0
5.0
6.0
7.0
8.0
D x (nm)
Fig. 4. Relation between the critical size Ds for which S parameters start to increase from zero and Dx that indicates the beginning of coalescence.
TEM observation was made for a sample halfway of ordering. Fig. 5 shows an image of a sample with P ¼ 0:23 after annealing at 6001C. The ordering parameter is estimated to be 0.5. It is seen that grains are composites of ordered and disordered crystals. It is impossible to differentiate FCC(1 1 0) and FCT(1 1 0), but some of these (1 1 0) lattice images should be attributed to FCC because of relatively low S: In this figure, it is clear that the crystals are transformed into an ordered structure through the coalescence process. In conclusion, coalescence of grains in FePt–SiO2 granular films gives rise and accelerates L10 ordering in FePt. From industrial point of view, lowering of ordering temperature is vital for practical use of this material. In further study search for oxides, additives and granular structures to accelerate coalescence are needed to make it possible.
References [1] M. Watanabe, T. Masumoto, D.H. Ping, K. Hono, Appl. Phys. Lett. 76 (2000) 3971. [2] C. Chen, O. Kitakami, S. Okamoto, Y. Shimada, Appl. Phys. Lett. 76 (2000) 3218. [3] O. Kitakami, Y. Shimada, K. Oikawa, H. Daimon, K. Futamichi, Appl. Phys. Lett. 78 (2001) 1104. [4] T. Suzuki, N. Honda, K. Ouchi, J. Magn. Soc. Jpn. 21 (1997) 177. [5] Bo. Bian, K. Sato, Y. Hirotu, A. Makino, J. Appl. Phys. 84 (1998) 4403. [6] Y.N. Hsu, S. Jeong, D. Laughlin, D. Lambeth, Eighth Joint MMM-Intermag Conference DP-05, 2001. [7] C.P. Luo, S.H. Liou, L. Gao, Y. Liu, D.J. Sellmyer, Appl. Phys. Lett. 77 (2000) 2225. [8] Y. Endo, N. Kikuchi, O. Kitakami, Y. Shimada, Eighth Joint MMM-Intermag Conference DP-04, 2001. Fig. 5. TEM image of a sample with thickness 40 nm and P ¼ 0:23 after annealing at 6001C. The ordering parameter is estimated to be about 0.5.
diffusion and high density lattice defects at the interface may decrease the energy barrier and give rise to rearrangement of Fe and Pt atoms into the equilibrium state of L10.