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Understanding magnetic properties of arrays of small FePt dots with perpendicular anisotropy
Journal of Magnetism and Magnetic Materials 324 (2012) 3737–3740
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Understanding magnetic properties of arrays of small FePt dots with perpendicular anisotropy Z.J. Yan a,b,n, S. Takahashi a, T. Hasegawa a, S. Ishio a, Y. Kondo c, J. Ariake c, D.S. Xue b a
VBL of Akita University, Gakuen Machi 1-1, Tegata, Akita 010-8502, Japan Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, People’s Republic of China c Akita Research Institute of Advanced Technology (AIT), 4-21 Sanuki, Araya, Akita 010-1623, Japan b
a r t i c l e i n f o
abstract
Article history: Received 22 December 2011 Received in revised form 9 April 2012 Available online 15 June 2012
FePt dot arrays with dot size down to 15 nm are fabricated by film annealing and patterning. The array coercivity shows an increase with dot size decreasing from 100 to 30 nm, and a slight reduction for the 15 nm dot sample. Annealing these dot arrays at higher temperatures results in large enhancements in the coercivities, except the 15 nm dot array where the coercivity increases a little. Micromagnetic models of a 15 nm FePt dot with uniform and nonuniform edges of soft magnetic defects and with inside defects are calculated to reveal the microstructure origins of the dot magnetic properties. It is found that the volume fraction of the L10-phase FePt with perpendicular c-axis orientation is about 50% in the dot and the switching field distribution of the dot array can be influenced significantly by the defect arrangement in the dots. & 2012 Elsevier B.V. All rights reserved.
Keywords: Dot array FePt L10-phase Magnetic switching field distribution Micromagnetic Perpendicular magnetic anisotropy
1. Introduction Bit patterned media (BPM) have attracted increasing attention in recent years because they are possible approaches to overcome the onset of superparamagnetism and consequently increase the recording density of current hard disk drives [1–4]. In BPM, each information bit is stored in one fabricated magnetic nanoscale entity. And hence, reductions of the size and interspace of these nanomagnets are required for higher recording densities. Usually, ‘‘top–down’’ strategy, such as film patterning and lift-off, is utilized as a convenient and precisely controllable production method to synthesize BPM [5,6]. For such techniques, fluctuations in the magnet size, shape, spacing and microstructure which might be induced in the preparation process can result in significant degradations in the recording performance of the BPM. One representative and commonly encountered problem is the widening of the switching field distribution (SFD) [7], which is closely related to the write errors. The above situation becomes more serious in the arrays of smaller magnets. FePt alloy in L10-phase has a large magnetocrystalline anisotropy (Ku 7.0 107 erg/cm3), which makes it an ideal candidate for the ultra-high-density magnetic recording systems and spin electronic devices. Although many reports have been presented on the FePtbased BPM [8–11], a more detailed comprehension of the magnetic
behavior of the arrays composed of smaller dots is still needed. In our previous work, arrays of circular FePt dots smaller than 100 nm were fabricated and investigated experimentally. High perpendicular magnetic anisotropy in these samples were accomplished resulting from the perpendicular orientation of the c-axis of the highly L10ordered FePt grains [12]. In addition, the dependences of the magnetic properties of the dot arrays on the preparation parameters were demonstrated, including the annealing temperature, the sequence of film patterning and annealing, and the patterning parameters. It is found that the procedure of film annealing followed by patterning can increase the array magnetic properties and the reversal of the dot is affected by the structure defects [13]. However, the discrepancy between the experimental results and theoretical predictions, origins of the SFD of the dot arrays, and the reversal mechanism of the dots remains unclear. In the present study, improved preparation process of the FePt dot arrays is adopted in order to increase the array properties. The results show that FePt crystallites of higher quality are developed in most samples, except the array of the smallest dots. On the other hand, micromagnetic simulations are performed to reveal the effect of the underlying microstructure on the magnetic properties of these smallest dots.
2. Methods n
Corresponding author at: Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, People’s Republic of China. Tel.: þ 81 18 889 2405; fax: þ 81 18 837 0403. E-mail address: [email protected] (Z.J. Yan). 0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.06.006
In the experimental procedure, the FePt dot arrays with dot size down to 15 nm were fabricated by film patterning. At first, an FePt continuous film with a thickness of around 6 nm was prepared by
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repeating the alternative sputtering of Fe, Pt and SiO2. Here, the addition of SiO2 promotes a high L10-ordering and a nearly perfect perpendicular orientation of the [001] crystalline axis of the FePt grains during the subsequent rapid annealing process [14]. After the annealing treatment at a relatively low temperature of 550 1C, the degree of order as high as 0.94 of the L10-phase FePt in the continuous film was obtained. The array structure of the FePt dots was then made by electron beam lithography followed by ion etching, as described in detail in [13]. To improve the magnetic properties, the annealing process was carried out again on the arrays at 700 1C. The magnetic properties of the dot arrays were investigated by magnetic force microscopy (MFM) observation, magnetooptic Kerr effect magnetometer (MOKE) and x-ray magnetic circular dichroism (XMCD) measurements. Micromagnetic simulations on a single FePt dot were carried out using a commercial package of LLG Micromagnetics SimulatorTM. The dot was modeled as a plate of 6 nm high and 15 nm in diameter with a discretization cell of 1 nm 1 nm 1 nm. Three different arrangements of the structure defects with soft magnetic properties in the dot were calculated respectively. The material parameters used for the L10-phase FePt were saturation magnetization Ms ¼ 1140 emu/cm3 and exchange constant A¼1 10 6 erg/cm taken from the FePt bulk, and anisotropy constant Ku ¼6 107 erg/ cm3 estimated from the experiments. For the defects, only the anisotropy constant Kud ¼0Ku was varied. The easy magnetization direction was set to be along the dot axis in each cell, i.e., perpendicular anisotropy. In simulation, the applied field was aligned 0.11 off the dot axis to break the symmetry problem.
3. Results and discussion 3.1. Magnetization measurements The magnetic properties of the FePt dot arrays were studied using MFM at first. Two states after magnetic saturation and thermal demagnetization of a sample of 15 nm dots are illustrated in Fig. 1, respectively. The saturation remanent state shows that the circular dots are well arranged in a square lattice with a pitch of 100 nm and all the dots are in single domains with the same contrast, as seen in Fig. 1(a). Please note that there is an enlargement of the objective size in MFM observations and the precise value of the dot size is evaluated by scanning electron microscopy. After thermally demagnetizing the sample, Fig. 1(b) displays that about half of the dots are in the bright contrast with the other half in the dark contrast, implying the reversal of the dot magnetization. No multi-domain states are evidenced in the dots which indicates the perpendicular magnetic anisotropy. By applying perpendicular magnetic fields to the samples in the direction opposite to the original saturation field, remanent states evolving with the field were collected and analyzed in
Fig. 1. MFM images of a 15 nm FePt dot array in (a) saturation remanent state and (b) thermal demagnetization state.
detail. Because of the large magnetic anisotropy of the FePt dots, a homemade electromagnet device generating pulse magnetic fields was utilized to saturate the samples, while the opposite fields were applied by MOKE equipment [13]. Consequently, the remanent hysteresis loops could be constructed and the remanent coercivities Hcr could be determined. Hcr of the dot arrays with dot size ranging from 15 to 100 nm are presented by the closed circles in Fig. 2. When the dot diameter decreases, Hcr firstly increases monotonically from 15.7 kOe for the 100 nm dots, reaches a maximum of 21 kOe at 30 nm, and then descends slightly to 20.8 kOe for the 15 nm dots. The increasing tendency of the coercivity with the decreasing size of the nanofabricated dots is generally observed and is mainly due to a reversal process of defect-related nucleation followed by fast domain wall propagation [7]. Therefore, it is unexpected that the 15 nm dot array has a smaller coercivity. It is noticed that Skomski and co-workers reported a maximum coercivity of around 30 kOe in the ultrasmall FePt particles (3–15 nm in diameter) embedded in a salt matrix [15]. It is well known that during the nanofabrication process, homogeneity of the tiny elements becomes difficult to be maintained. In our samples, as the dot size reduces, the dot is more easily damaged by the ion bombardments in the film patterning, which might lead to the degradation of the chemical order and the increase in structure defects in the L10-phase FePt grains [16]. In order to recover the dots from these imperfections, the arrays were annealed again at 700 1C at different durations. Then the magnetic properties were investigated by XMCD measurements, where the spot size of the x-ray was 50 mm and the maximum applied field amplitude was 100 kOe. The array coercivities with the dot size of 15, 30 and 100 nm are shown in Fig. 2 as open circles, respectively. It is clear that after the thermal treatment, the coercivity increases largely as expected from 15.7 to 25.2 kOe for the 100 nm dots and from 21 to 27.8 kOe for the 30 nm dots, but the coercivity of the 15 nm dot array changes to 21.8 kOe by an addition of only 1 kOe. Here the coercivities are directly compared with the remanent ones, because the irreversible mechanism dominates the magnetization reversal of the highly anisotropic FePt dots. The enhancement of the coercivity is believed to originate from the increase in the L10-phase FePt volume and the development of the L10-crystallite quality (such as the chemical order degree and the c-axis orientation). The magnetic properties, and what is more important, the underlying microstructure of the 15 nm dots seem to be influenced a little by the second annealing. So the question comes to us, what are the structure features of these small dots?
3.2. Micromagnetic simulation To understand the magnetic properties of the small FePt dots, micromagnetic simulations were performed based on three
Fig. 2. Dot diameter dependence of the coercivity of the FePt dot arrays prepared with (open circles) and without (closed circles) the second annealing process. The coercivity of a continuous FePt film is also presented (triangle).
Z.J. Yan et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3737–3740
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models of a 15 nm dot with different defect arrangements. As it is pointed out by several groups that the edges of the patterned elements tend to get damaged by the ion milling [16–18], the first model was the dot with defects of lower anisotropy in its circumference which was called the damaged edge. In simulations, the damaged edge was uniform, but its thickness was varied between 0 and 4 nm. The edge anisotropy constant Kud was from 0 to Ku (6 107 erg/cm3) and the temperature was 0 K. Fig. 3 shows the dot coercivity depending on Kud and the thickness of the damaged edge, with the dash line indicating the experimental result. As Kud decreases Hc descends monotonically and the slope of this relationship becomes steeper when the edge thickness increases, because the effective magnetic energy barrier in the dot is reduced. By taking into account the thermal fluctuation, the experimental result implies the edge thickness of around 3 nm when Kud ¼0, i.e., the defects in pure soft phase with a volume fraction larger than 50%. The reversal mechanism of the dot was investigated in detail. For example, different magnetization processes were found with Kud ¼0, depending on the edge thickness. Whereas coherent rotation is evidenced when the damaged edge is thinner than 1.5 nm, edge reverses at first and the hard core rotates quickly in the cases with thicker edge as shown in Fig. 4. In all calculations, rectangle-like hysteresis loops were observed which meant the irreversible feature of the dot. In experiments, the damaged edge of the dot perhaps are not uniform in thickness. To investigate the effect of the edge thickness fluctuation on the dot properties, the second model was built where the hard magnetic core was circular and moved from the center to the circumference of the dot, as illustrated in the inset of Fig. 5. The dependence of dot Hc on the edge volume fraction with Kud ¼0 is shown in Fig. 5, with the dash line denoting the experimental datum. The results based on the first model are also presented for comparison. The temperature was 300 K in these simulations for both the models. It can be seen that the Hc decreases with the hard core moving towards the dot edge. This is caused by the lowering of the nucleation field in the thicker side of the damaged edge. In addition, after depinning of the domain wall from the interface, the core is rotated immediately. The hysteresis curves calculated from the second model are
shown in Fig. 6, in which two different reversal processes can be identified obviously. Moreover, a maximum difference between the Hc values of the two models is about 22 kOe found in Fig. 5, which implies that a tremendous SFD could be induced in the array by the thickness fluctuation of the damaged edge of the dots even though their hard cores are identical. The experimental datum indicates a volume fraction of 46–55% of the damaged phase in the dot. It is interesting to note that when the edge volume fraction increases to be larger than 60%, narrow defectrelated SFD of the dot array can be obtained and meanwhile the coercivity of the dots becomes comparable with the current writing head field. This may be an indication of the possible application for these arrays which behave like the patterned exchange coupling composite media [19], but more detailed research is needed. As the above two models indicate that about half the volume of the dot is in the low anisotropy phase, it is the quantity not the distribution of the defects that seems to be more predominant in the dot magnetic properties. In addition, the exchange coupling between the hard phase and the soft defects likely plays important roles in reducing the coercivity of the small dot. In order to confirm these diagnoses, the FePt dot with magnetic soft phase inside was simulated, as the third model. These calculations were
Fig. 3. Coercivity of a 15 nm FePt dot depending on the anisotropy constant and thickness of the damaged edge. The dash line denotes the experimental value.
Fig. 6. Hysteresis curves of a 15 nm FePt dot with the hard core of different sizes. The core is near the brim of the dot.
Fig. 5. Coercivity of a 15 nm FePt dot depending on the volume fraction of the damaged edge with neglected anisotropy constant. The hard core is at the center and near the brim of the dot, respectively. The dash line denotes the experimental value.
Fig. 4. Snapshots (a)–(d) taken in the reversal process of a 15 nm FePt dot with a damaged edge of 3.5 nm in thickness and zero in anisotropy constant. The red and blue colors represent the out-of-plane up and down directions of the cell moments, respectively. For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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small dots, micromagnetic simulations are performed based on the models of a dot with damaged edges of uniform and nonuniform thickness and with defects inside. It is found that the volume fraction of the L10-phase FePt grains with perpendicular c-axis orientation is only around 50% in the dot, and the defects of the magnetic soft phase are mainly located in the dot circumference after the postannealing. These defects dominate the dot properties by reducing the effective anisotropy of the dot and can increase the SFD of the array. Higher temperatures with large heating rate is supposed to be valid in the post-annealing. On the other hand, direct inspections of the dot structure with high resolution are necessary in the future. Fig. 7. Coercivity of a 15 nm FePt dot depending on the volume fraction of the defects in the damaged edge and inside the dot, respectively. The dash line denotes the experimental value.
also based on the consideration that after annealing the FePt continuous films, some grains inevitably exist in the residual facecentered cubic phase or the L10-phase with misaligned c-axis, depending on the annealing conditions [20]. When the defects of Kud ¼0 are in cylinder shape at the dot center, the relationship between the dot coercivity and the cylinder volume is shown in Fig. 7, together with the data from the first model. The thermal fluctuation is excluded in these simulations for the sake of computing time. With the expansion of the soft magnetic phase, the two models result in similar tendencies as expected. Although the micromagnetic simulations now cannot determine the microstructure of the small FePt dots definitely, it is convincible that the L10-phase FePt grains with perpendicular caxis are in a low proportion in these 15 nm dots. Except the SiO2 additive which tends to diffuse to the surface of the film and the boundaries between the FePt grains in the annealing, the transition from A1- to L10-phase of FePt can proceed far above 50% (and sometimes near 1) in the continuous films. The large in-plane tensile stresses between the two-dimensional growing grains facilitate this phase transition and the perpendicular c-axis orientation [14]. However, for the small FePt dots the stresses between the growing grains are not sufficient to complete the phase transition even at a higher temperature, because there are fewer grains included in the dot, especially near the dot edge. Therefore, while the inside defects tend to disappear, the damaged edge and the magnetic properties of the dots almost remain.
4. Conclusion FePt dot arrays with dot size down to 15 nm are fabricated by film sputtering, annealing and patterning. The array coercivity shows an increase with dot size decreasing, except the 15 nm dot sample. To recover the dots from the defects induced in the film patterning process, a second annealing is conducted which results in large enhancements in the coercivity of the arrays, once again, except the 15 nm dot array. To reveal the underlying microstructure of these
Acknowledgments One of the authors, Z.J. Yan, would like to thank the NSFC for partial support under Grant no. 51101078.
References [1] H.J. Richter, A.Y. Dobin, R.T. Lynch, D. Weller, R.M. Brockie, O. Heinonen, K.Z. Gao, J. Xue, R.J.M.v.d. Veerdonk, P. Asselin, M.F. Erden, Applied Physics Letters 88 (2006) 222512. [2] O. Hellwig, A. Berger, T. Thomson, E. Dobisz, Z.Z. Bandic, H. Yang, D.S. Kercher, E.E. Fullerton, Applied Physics Letters 90 (2007) 162516. [3] S. Greaves, Y. Kanai, H. Muraoka, IEEE Transactions on Magnetics 44 (2008) 3430. [4] Y. Shiroishi, K. Fukuda, I. Tagawa, H. Iwasaki, S. Takenoiri, H. Tanaka, H. Mutoh, N. Yoshikawa, IEEE Transactions on Magnetics 45 (2009) 3816. [5] T. Seki, T. Shima, K. Yakushiji, K. Takanashi, G.Q. Li, S. Ishio, Journal of Applied Physics 100 (2006) 043915. [6] B.D. Belle, F. Schedin, N. Pilet, T.V. Ashworth, E.W. Hill, P.W. Nutter, H.J. Hug, J.J. Miles, Journal of Applied Physics 101 (2007) 09F517. [7] T. Thomson, G. Hu, B.D. Terris, Physical Review Letters 96 (2006) 257204. [8] T. Suzuki, H. Muraoka, Y. Nakamura, K. Ouchi, IEEE Transactions on Magnetics 39 (2003) 691. [9] A. Martins, M.C.A. Fantini, A.D. Santos, Journal of Magnetism and Magnetic Materials 265 (2003) 13. [10] S. Nakagawa, T. Kamiki, Journal of Magnetism and Magnetic Materials 287 (2005) 204. [11] L. Zhang, Y.K. Takahashi, A. Perumal, K. Hono, Journal of Magnetism and Magnetic Materials 322 (2010) 2658. [12] S. Ishio, T. Narisawa, S. Takahashi, Y. Kamata, S. Shibata, T. Hasegawa, Z. Yan, X. Liu, H. Yamane, Y. Kondo, J. Ariake, Journal of Magnetism and Magnetic Materials 324 (2012) 295. [13] Z.J. Yan, S. Takahashi, Y. Kondo, J. Ariake, T. Sakon, D.S. Xue, S. Ishio, Journal of Physics D: Applied Physics 44 (2011) 185002. [14] T. Narisawa, T. Hasegawa, S. Ishio, H. Yamane, Journal of Applied Physics 109 (2011) 033918. [15] R. Skomski, J.P. Liu, C.B. Rong, D.J. Sellmyer, Journal of Applied Physics 103 (2008) 07E139. [16] T. Seki, T. Shima, K. Yakushiji, K. Takanashi, G.Q. Li, S. Ishio, IEEE Transactions on Magnetics 41 (2005) 3604. [17] J.M. Shaw, S.E. Russek, T. Thomson, M.J. Donahue, B.D. Terris, O. Hellwig, E. Dobisz, M.L. Schneider, Physical Review B 78 (2008) 024414. [18] Y. Kondo, T. Chiba, J. Ariake, K. Taguchi, M. Suzuki, M. Takagaki, N. Kawamura, B.M. Zulfakri, S. Hosaka, N. Honda, Journal of Magnetism and Magnetic Materials 320 (2008) 3157. [19] P. Krone, D. Makarov, T. Schrefl, M. Albrecht, Applied Physics Letters 97 (2010) 082501. [20] H. Yamane, T. Narisawa, T. Hasegawa, S. Ishio, Journal of Applied Physics 108 (2010) 113923.
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Understanding magnetic properties of arrays of small FePt dots with perpendicular anisotropy Z.J. Yan a,b,n, S. Takahashi a, T. Hasegawa a, S. Ishio a, Y. Kondo c, J. Ariake c, D.S. Xue b a
VBL of Akita University, Gakuen Machi 1-1, Tegata, Akita 010-8502, Japan Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, People’s Republic of China c Akita Research Institute of Advanced Technology (AIT), 4-21 Sanuki, Araya, Akita 010-1623, Japan b
a r t i c l e i n f o
abstract
Article history: Received 22 December 2011 Received in revised form 9 April 2012 Available online 15 June 2012
FePt dot arrays with dot size down to 15 nm are fabricated by film annealing and patterning. The array coercivity shows an increase with dot size decreasing from 100 to 30 nm, and a slight reduction for the 15 nm dot sample. Annealing these dot arrays at higher temperatures results in large enhancements in the coercivities, except the 15 nm dot array where the coercivity increases a little. Micromagnetic models of a 15 nm FePt dot with uniform and nonuniform edges of soft magnetic defects and with inside defects are calculated to reveal the microstructure origins of the dot magnetic properties. It is found that the volume fraction of the L10-phase FePt with perpendicular c-axis orientation is about 50% in the dot and the switching field distribution of the dot array can be influenced significantly by the defect arrangement in the dots. & 2012 Elsevier B.V. All rights reserved.
Keywords: Dot array FePt L10-phase Magnetic switching field distribution Micromagnetic Perpendicular magnetic anisotropy
1. Introduction Bit patterned media (BPM) have attracted increasing attention in recent years because they are possible approaches to overcome the onset of superparamagnetism and consequently increase the recording density of current hard disk drives [1–4]. In BPM, each information bit is stored in one fabricated magnetic nanoscale entity. And hence, reductions of the size and interspace of these nanomagnets are required for higher recording densities. Usually, ‘‘top–down’’ strategy, such as film patterning and lift-off, is utilized as a convenient and precisely controllable production method to synthesize BPM [5,6]. For such techniques, fluctuations in the magnet size, shape, spacing and microstructure which might be induced in the preparation process can result in significant degradations in the recording performance of the BPM. One representative and commonly encountered problem is the widening of the switching field distribution (SFD) [7], which is closely related to the write errors. The above situation becomes more serious in the arrays of smaller magnets. FePt alloy in L10-phase has a large magnetocrystalline anisotropy (Ku 7.0 107 erg/cm3), which makes it an ideal candidate for the ultra-high-density magnetic recording systems and spin electronic devices. Although many reports have been presented on the FePtbased BPM [8–11], a more detailed comprehension of the magnetic
behavior of the arrays composed of smaller dots is still needed. In our previous work, arrays of circular FePt dots smaller than 100 nm were fabricated and investigated experimentally. High perpendicular magnetic anisotropy in these samples were accomplished resulting from the perpendicular orientation of the c-axis of the highly L10ordered FePt grains [12]. In addition, the dependences of the magnetic properties of the dot arrays on the preparation parameters were demonstrated, including the annealing temperature, the sequence of film patterning and annealing, and the patterning parameters. It is found that the procedure of film annealing followed by patterning can increase the array magnetic properties and the reversal of the dot is affected by the structure defects [13]. However, the discrepancy between the experimental results and theoretical predictions, origins of the SFD of the dot arrays, and the reversal mechanism of the dots remains unclear. In the present study, improved preparation process of the FePt dot arrays is adopted in order to increase the array properties. The results show that FePt crystallites of higher quality are developed in most samples, except the array of the smallest dots. On the other hand, micromagnetic simulations are performed to reveal the effect of the underlying microstructure on the magnetic properties of these smallest dots.
2. Methods n
Corresponding author at: Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, People’s Republic of China. Tel.: þ 81 18 889 2405; fax: þ 81 18 837 0403. E-mail address: [email protected] (Z.J. Yan). 0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.06.006
In the experimental procedure, the FePt dot arrays with dot size down to 15 nm were fabricated by film patterning. At first, an FePt continuous film with a thickness of around 6 nm was prepared by
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Z.J. Yan et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3737–3740
repeating the alternative sputtering of Fe, Pt and SiO2. Here, the addition of SiO2 promotes a high L10-ordering and a nearly perfect perpendicular orientation of the [001] crystalline axis of the FePt grains during the subsequent rapid annealing process [14]. After the annealing treatment at a relatively low temperature of 550 1C, the degree of order as high as 0.94 of the L10-phase FePt in the continuous film was obtained. The array structure of the FePt dots was then made by electron beam lithography followed by ion etching, as described in detail in [13]. To improve the magnetic properties, the annealing process was carried out again on the arrays at 700 1C. The magnetic properties of the dot arrays were investigated by magnetic force microscopy (MFM) observation, magnetooptic Kerr effect magnetometer (MOKE) and x-ray magnetic circular dichroism (XMCD) measurements. Micromagnetic simulations on a single FePt dot were carried out using a commercial package of LLG Micromagnetics SimulatorTM. The dot was modeled as a plate of 6 nm high and 15 nm in diameter with a discretization cell of 1 nm 1 nm 1 nm. Three different arrangements of the structure defects with soft magnetic properties in the dot were calculated respectively. The material parameters used for the L10-phase FePt were saturation magnetization Ms ¼ 1140 emu/cm3 and exchange constant A¼1 10 6 erg/cm taken from the FePt bulk, and anisotropy constant Ku ¼6 107 erg/ cm3 estimated from the experiments. For the defects, only the anisotropy constant Kud ¼0Ku was varied. The easy magnetization direction was set to be along the dot axis in each cell, i.e., perpendicular anisotropy. In simulation, the applied field was aligned 0.11 off the dot axis to break the symmetry problem.
3. Results and discussion 3.1. Magnetization measurements The magnetic properties of the FePt dot arrays were studied using MFM at first. Two states after magnetic saturation and thermal demagnetization of a sample of 15 nm dots are illustrated in Fig. 1, respectively. The saturation remanent state shows that the circular dots are well arranged in a square lattice with a pitch of 100 nm and all the dots are in single domains with the same contrast, as seen in Fig. 1(a). Please note that there is an enlargement of the objective size in MFM observations and the precise value of the dot size is evaluated by scanning electron microscopy. After thermally demagnetizing the sample, Fig. 1(b) displays that about half of the dots are in the bright contrast with the other half in the dark contrast, implying the reversal of the dot magnetization. No multi-domain states are evidenced in the dots which indicates the perpendicular magnetic anisotropy. By applying perpendicular magnetic fields to the samples in the direction opposite to the original saturation field, remanent states evolving with the field were collected and analyzed in
Fig. 1. MFM images of a 15 nm FePt dot array in (a) saturation remanent state and (b) thermal demagnetization state.
detail. Because of the large magnetic anisotropy of the FePt dots, a homemade electromagnet device generating pulse magnetic fields was utilized to saturate the samples, while the opposite fields were applied by MOKE equipment [13]. Consequently, the remanent hysteresis loops could be constructed and the remanent coercivities Hcr could be determined. Hcr of the dot arrays with dot size ranging from 15 to 100 nm are presented by the closed circles in Fig. 2. When the dot diameter decreases, Hcr firstly increases monotonically from 15.7 kOe for the 100 nm dots, reaches a maximum of 21 kOe at 30 nm, and then descends slightly to 20.8 kOe for the 15 nm dots. The increasing tendency of the coercivity with the decreasing size of the nanofabricated dots is generally observed and is mainly due to a reversal process of defect-related nucleation followed by fast domain wall propagation [7]. Therefore, it is unexpected that the 15 nm dot array has a smaller coercivity. It is noticed that Skomski and co-workers reported a maximum coercivity of around 30 kOe in the ultrasmall FePt particles (3–15 nm in diameter) embedded in a salt matrix [15]. It is well known that during the nanofabrication process, homogeneity of the tiny elements becomes difficult to be maintained. In our samples, as the dot size reduces, the dot is more easily damaged by the ion bombardments in the film patterning, which might lead to the degradation of the chemical order and the increase in structure defects in the L10-phase FePt grains [16]. In order to recover the dots from these imperfections, the arrays were annealed again at 700 1C at different durations. Then the magnetic properties were investigated by XMCD measurements, where the spot size of the x-ray was 50 mm and the maximum applied field amplitude was 100 kOe. The array coercivities with the dot size of 15, 30 and 100 nm are shown in Fig. 2 as open circles, respectively. It is clear that after the thermal treatment, the coercivity increases largely as expected from 15.7 to 25.2 kOe for the 100 nm dots and from 21 to 27.8 kOe for the 30 nm dots, but the coercivity of the 15 nm dot array changes to 21.8 kOe by an addition of only 1 kOe. Here the coercivities are directly compared with the remanent ones, because the irreversible mechanism dominates the magnetization reversal of the highly anisotropic FePt dots. The enhancement of the coercivity is believed to originate from the increase in the L10-phase FePt volume and the development of the L10-crystallite quality (such as the chemical order degree and the c-axis orientation). The magnetic properties, and what is more important, the underlying microstructure of the 15 nm dots seem to be influenced a little by the second annealing. So the question comes to us, what are the structure features of these small dots?
3.2. Micromagnetic simulation To understand the magnetic properties of the small FePt dots, micromagnetic simulations were performed based on three
Fig. 2. Dot diameter dependence of the coercivity of the FePt dot arrays prepared with (open circles) and without (closed circles) the second annealing process. The coercivity of a continuous FePt film is also presented (triangle).
Z.J. Yan et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3737–3740
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models of a 15 nm dot with different defect arrangements. As it is pointed out by several groups that the edges of the patterned elements tend to get damaged by the ion milling [16–18], the first model was the dot with defects of lower anisotropy in its circumference which was called the damaged edge. In simulations, the damaged edge was uniform, but its thickness was varied between 0 and 4 nm. The edge anisotropy constant Kud was from 0 to Ku (6 107 erg/cm3) and the temperature was 0 K. Fig. 3 shows the dot coercivity depending on Kud and the thickness of the damaged edge, with the dash line indicating the experimental result. As Kud decreases Hc descends monotonically and the slope of this relationship becomes steeper when the edge thickness increases, because the effective magnetic energy barrier in the dot is reduced. By taking into account the thermal fluctuation, the experimental result implies the edge thickness of around 3 nm when Kud ¼0, i.e., the defects in pure soft phase with a volume fraction larger than 50%. The reversal mechanism of the dot was investigated in detail. For example, different magnetization processes were found with Kud ¼0, depending on the edge thickness. Whereas coherent rotation is evidenced when the damaged edge is thinner than 1.5 nm, edge reverses at first and the hard core rotates quickly in the cases with thicker edge as shown in Fig. 4. In all calculations, rectangle-like hysteresis loops were observed which meant the irreversible feature of the dot. In experiments, the damaged edge of the dot perhaps are not uniform in thickness. To investigate the effect of the edge thickness fluctuation on the dot properties, the second model was built where the hard magnetic core was circular and moved from the center to the circumference of the dot, as illustrated in the inset of Fig. 5. The dependence of dot Hc on the edge volume fraction with Kud ¼0 is shown in Fig. 5, with the dash line denoting the experimental datum. The results based on the first model are also presented for comparison. The temperature was 300 K in these simulations for both the models. It can be seen that the Hc decreases with the hard core moving towards the dot edge. This is caused by the lowering of the nucleation field in the thicker side of the damaged edge. In addition, after depinning of the domain wall from the interface, the core is rotated immediately. The hysteresis curves calculated from the second model are
shown in Fig. 6, in which two different reversal processes can be identified obviously. Moreover, a maximum difference between the Hc values of the two models is about 22 kOe found in Fig. 5, which implies that a tremendous SFD could be induced in the array by the thickness fluctuation of the damaged edge of the dots even though their hard cores are identical. The experimental datum indicates a volume fraction of 46–55% of the damaged phase in the dot. It is interesting to note that when the edge volume fraction increases to be larger than 60%, narrow defectrelated SFD of the dot array can be obtained and meanwhile the coercivity of the dots becomes comparable with the current writing head field. This may be an indication of the possible application for these arrays which behave like the patterned exchange coupling composite media [19], but more detailed research is needed. As the above two models indicate that about half the volume of the dot is in the low anisotropy phase, it is the quantity not the distribution of the defects that seems to be more predominant in the dot magnetic properties. In addition, the exchange coupling between the hard phase and the soft defects likely plays important roles in reducing the coercivity of the small dot. In order to confirm these diagnoses, the FePt dot with magnetic soft phase inside was simulated, as the third model. These calculations were
Fig. 3. Coercivity of a 15 nm FePt dot depending on the anisotropy constant and thickness of the damaged edge. The dash line denotes the experimental value.
Fig. 6. Hysteresis curves of a 15 nm FePt dot with the hard core of different sizes. The core is near the brim of the dot.
Fig. 5. Coercivity of a 15 nm FePt dot depending on the volume fraction of the damaged edge with neglected anisotropy constant. The hard core is at the center and near the brim of the dot, respectively. The dash line denotes the experimental value.
Fig. 4. Snapshots (a)–(d) taken in the reversal process of a 15 nm FePt dot with a damaged edge of 3.5 nm in thickness and zero in anisotropy constant. The red and blue colors represent the out-of-plane up and down directions of the cell moments, respectively. For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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small dots, micromagnetic simulations are performed based on the models of a dot with damaged edges of uniform and nonuniform thickness and with defects inside. It is found that the volume fraction of the L10-phase FePt grains with perpendicular c-axis orientation is only around 50% in the dot, and the defects of the magnetic soft phase are mainly located in the dot circumference after the postannealing. These defects dominate the dot properties by reducing the effective anisotropy of the dot and can increase the SFD of the array. Higher temperatures with large heating rate is supposed to be valid in the post-annealing. On the other hand, direct inspections of the dot structure with high resolution are necessary in the future. Fig. 7. Coercivity of a 15 nm FePt dot depending on the volume fraction of the defects in the damaged edge and inside the dot, respectively. The dash line denotes the experimental value.
also based on the consideration that after annealing the FePt continuous films, some grains inevitably exist in the residual facecentered cubic phase or the L10-phase with misaligned c-axis, depending on the annealing conditions [20]. When the defects of Kud ¼0 are in cylinder shape at the dot center, the relationship between the dot coercivity and the cylinder volume is shown in Fig. 7, together with the data from the first model. The thermal fluctuation is excluded in these simulations for the sake of computing time. With the expansion of the soft magnetic phase, the two models result in similar tendencies as expected. Although the micromagnetic simulations now cannot determine the microstructure of the small FePt dots definitely, it is convincible that the L10-phase FePt grains with perpendicular caxis are in a low proportion in these 15 nm dots. Except the SiO2 additive which tends to diffuse to the surface of the film and the boundaries between the FePt grains in the annealing, the transition from A1- to L10-phase of FePt can proceed far above 50% (and sometimes near 1) in the continuous films. The large in-plane tensile stresses between the two-dimensional growing grains facilitate this phase transition and the perpendicular c-axis orientation [14]. However, for the small FePt dots the stresses between the growing grains are not sufficient to complete the phase transition even at a higher temperature, because there are fewer grains included in the dot, especially near the dot edge. Therefore, while the inside defects tend to disappear, the damaged edge and the magnetic properties of the dots almost remain.
4. Conclusion FePt dot arrays with dot size down to 15 nm are fabricated by film sputtering, annealing and patterning. The array coercivity shows an increase with dot size decreasing, except the 15 nm dot sample. To recover the dots from the defects induced in the film patterning process, a second annealing is conducted which results in large enhancements in the coercivity of the arrays, once again, except the 15 nm dot array. To reveal the underlying microstructure of these
Acknowledgments One of the authors, Z.J. Yan, would like to thank the NSFC for partial support under Grant no. 51101078.
References [1] H.J. Richter, A.Y. Dobin, R.T. Lynch, D. Weller, R.M. Brockie, O. Heinonen, K.Z. Gao, J. Xue, R.J.M.v.d. Veerdonk, P. Asselin, M.F. Erden, Applied Physics Letters 88 (2006) 222512. [2] O. Hellwig, A. Berger, T. Thomson, E. Dobisz, Z.Z. Bandic, H. Yang, D.S. Kercher, E.E. Fullerton, Applied Physics Letters 90 (2007) 162516. [3] S. Greaves, Y. Kanai, H. Muraoka, IEEE Transactions on Magnetics 44 (2008) 3430. [4] Y. Shiroishi, K. Fukuda, I. Tagawa, H. Iwasaki, S. Takenoiri, H. Tanaka, H. Mutoh, N. Yoshikawa, IEEE Transactions on Magnetics 45 (2009) 3816. [5] T. Seki, T. Shima, K. Yakushiji, K. Takanashi, G.Q. Li, S. Ishio, Journal of Applied Physics 100 (2006) 043915. [6] B.D. Belle, F. Schedin, N. Pilet, T.V. Ashworth, E.W. Hill, P.W. Nutter, H.J. Hug, J.J. Miles, Journal of Applied Physics 101 (2007) 09F517. [7] T. Thomson, G. Hu, B.D. Terris, Physical Review Letters 96 (2006) 257204. [8] T. Suzuki, H. Muraoka, Y. Nakamura, K. Ouchi, IEEE Transactions on Magnetics 39 (2003) 691. [9] A. Martins, M.C.A. Fantini, A.D. Santos, Journal of Magnetism and Magnetic Materials 265 (2003) 13. [10] S. Nakagawa, T. Kamiki, Journal of Magnetism and Magnetic Materials 287 (2005) 204. [11] L. Zhang, Y.K. Takahashi, A. Perumal, K. Hono, Journal of Magnetism and Magnetic Materials 322 (2010) 2658. [12] S. Ishio, T. Narisawa, S. Takahashi, Y. Kamata, S. Shibata, T. Hasegawa, Z. Yan, X. Liu, H. Yamane, Y. Kondo, J. Ariake, Journal of Magnetism and Magnetic Materials 324 (2012) 295. [13] Z.J. Yan, S. Takahashi, Y. Kondo, J. Ariake, T. Sakon, D.S. Xue, S. Ishio, Journal of Physics D: Applied Physics 44 (2011) 185002. [14] T. Narisawa, T. Hasegawa, S. Ishio, H. Yamane, Journal of Applied Physics 109 (2011) 033918. [15] R. Skomski, J.P. Liu, C.B. Rong, D.J. Sellmyer, Journal of Applied Physics 103 (2008) 07E139. [16] T. Seki, T. Shima, K. Yakushiji, K. Takanashi, G.Q. Li, S. Ishio, IEEE Transactions on Magnetics 41 (2005) 3604. [17] J.M. Shaw, S.E. Russek, T. Thomson, M.J. Donahue, B.D. Terris, O. Hellwig, E. Dobisz, M.L. Schneider, Physical Review B 78 (2008) 024414. [18] Y. Kondo, T. Chiba, J. Ariake, K. Taguchi, M. Suzuki, M. Takagaki, N. Kawamura, B.M. Zulfakri, S. Hosaka, N. Honda, Journal of Magnetism and Magnetic Materials 320 (2008) 3157. [19] P. Krone, D. Makarov, T. Schrefl, M. Albrecht, Applied Physics Letters 97 (2010) 082501. [20] H. Yamane, T. Narisawa, T. Hasegawa, S. Ishio, Journal of Applied Physics 108 (2010) 113923.