Exchange Coupled Composite Media for Perpendicular Magnetic Recording

Transcription

1 BB-01 1 Exchange Coupled Composite Media for Perpendicular Magnetic Recording R. H. Victora, Fellow, IEEE, X. Shen Abstract Exchange coupled composite (ECC) media has been shown to possess several major advantages relative to conventional perpendicular media, including a reduction in the switching field of approximately a factor two for the same thermal stability and greater insensitivity to easy axis distribution. In this paper, full magnetostatic interactions are included: this allows comparison between the behavior of multigrain thin films and that of isolated grains as presented earlier. Significant results include hysteresis loops for thin films under various conditions including inadequate and excessive intra granular exchange between the hard and soft materials. An important distinction is made between the coercivity and remnant coercivity as a function of angle between applied field and easy axis. A perpendicular magnetic recording head is used to compare the shape of effective fields for ECC and conventional perpendicular media. Written transitions in the ECC media appear to be similar to those written in perpendicular media at comparable densities. Index Terms Exchange coupled composite media, Perpendicular magnetic recording, Thermal fluctuations. P I. INTRODUCTION ERPENDICULAR magnetic recording has multiple advantages [1] for high-density magnetic recording and was proposed more than 20 years ago as the successor to longitudinal magnetic recording for the next generation of hard disk [2]. Recently, perpendicular recording products with densities over 130 Gbits/in 2 have been announced [3]. However, to greatly increase this recording density is difficult, mostly owing to limitations of the perpendicular recording media itself. One important factor that limits the recording density of perpendicular media is the low writability that reflects the paradox between its thermal stability and switching capability. To increase recording density, grain volumes are reduced and the anisotropy of the media must be increased to maintain thermal stability. This increases the anisotropy field. Thus, extremely high density leads to a very high switching field that cannot be reached by current perpendicular writing heads. To address this problem, we must either increase the head field or decrease the switching field of Manuscript received February 7, This work was supported in part by the National Science Foundation under Grant ECS and in part by the INSIC EHDR program. The authors are with the Center of Micromagnetics and Information Technology (MINT), Electrical and Computer Engineering Department, University of Minnesota, Minneapolis, MN 55455, USA ( victora@ece.umn.edu). the media. Since head field increases are limited by the highest saturation magnetization of head material that can be realized, the other way, decreasing switching field while maintaining the thermal stability of the media seems more applicable. There is a ratio between the thermal barrier of the media and its switching field, which can be defined as! = 2 " E /( H # M # V ) [4]. Here ΔE is the thermal barrier and s s H s, M s, V are switching field, saturation magnetization and volume, respectively. Increasing this ratio can effectively solve the paradox between thermal stability and writability. For example, perpendicular media has a ratio of one and 45 tilted media [5][6] has a ratio of two, which means that at the same thermal stability, 45 tilted media will have half the switching field of perpendicular media. Previously, we have introduced [4] the idea of exchange coupled composite (ECC) media. This media consists of magnetically isolated grains each consisting of two parts: a magnetically hard part with perpendicular anisotropy and a magnetically soft part where the anisotropy can point in any direction provided it is small. For this ECC media, this ratio ξ can also approach two. Taking into account that the fabrication of ECC media is much easier than tilted media [7], it is quite promising for ultra high-density recording. In this paper, ECC media is studied by means of simulating its hysteresis loop and writing behavior under a special perpendicular magnetic recording head [1]. Effects of inter granular magnetostatic interaction and intra- and inter granular exchange coupling are illustrated by hysteresis loops. Angular dependence of coercivity and remnant coercivity are shown. Transition shapes are studied by applying a perpendicular magnetic recording head with a 50 nm thick soft under layer (SUL). II. SIMULATION MODEL We generate a piece of media with 500 composite grains. Voroni cells are used to represent grains within our micromagnetic model. This introduces difficulty in doing surface integrals for calculating the magnetostatic interaction energy, but may give results closer to reality than widely used rectangular grain models. The calculation tricks for Voroni cells arranged in a single layer can be found in [8][9]. For the present calculation, the grain model contains two parts referring to magnetically hard and soft regions of ECC media. The top part sits exactly on the bottom one with the same Voroni shape, but different thickness. Although there is an

2 BB-01 2 inter-layer (for example PdSi) between the two parts that controls the exchange coupling strength, in our micromagnetic simulation model, the two parts are adjacent to each other: this slightly exaggerates the magnetostatic interaction. The dimension of the media is 125 nm 100 nm. The mean diameter of the media is set to 5 nm to meet the requirement for 1 Tbits/in 2 recording density. The distribution of the diameter is approximately log normal with a 20% standard deviation. The thickness of the top region (magnetically hard region), t hard is 5 nm and that of the bottom region (magnetically soft region), t soft is 10 nm. The saturation magnetization of the top region, M hard is 150 emu/cm 3 and that of the soft region, M soft is 750 emu/cm 3. Thus, the volume and magnetization ratio of the two regions are ½ and 1/5, respectively, which may be the optimal choice [4]. The anisotropy of the hard region, Ku hard equals erg/cm 3 and for soft region, Ku soft equals 100 erg/cm 3. The easy axes are both in the perpendicular direction. The variation of anisotropy field is not included in our simulation. The exchange coupling constant between the hard and soft region Jex/V is erg/cm 3 [4]. The strength of inter granular exchange coupling is defined in (1) and varied from 0 to erg/cm 3. E exch V!! lij = " Jex( M i# M j ) (1) l norm Here l ij is the length of the neighboring edge of grain i and j and l norm is the average length. III. EFFECT OF MAGNETOSTATIC INTERACTION The effect of the magnetostatic interaction of ECC media can be studied by comparing the hysteresis loops of a single grain and the 500 grain media. The magnetostatic fields are calculated using the definition of H=-Interaction Energy/(Element Volume Saturation Magnetization). In the calculation, the magnetically hard and soft regions of one grain can be considered as two different grains to give the magnetostatic field between the hard and soft layers. To calculate the interaction energy between grains, integration of surface charges is implemented as shown in (2). Here, E(I,J) denotes the interaction energy between grain I and grain J. The various surfaces of the two grains are denoted m and n. The integration is implemented over all the surfaces that are then summed to give the total interaction energy. The charge densities of surface i and j are σ i and σ j. The integration variables u, u, v, v are local coordinates on surface i and j. While calculating the distance between magnetic charges, these coordinates must be transformed to global coordinates, x, x, y, y and z, z. E( I, J ) = n m ' '! i! jdudu dvdv ##$$$$ (2) ' 2 ' 2 ' 2 i= 1 j= 1 i j ( x " x ) + ( y " y ) + ( z " z ) The difficulty here is that the top and bottom surfaces of grains have voroni shapes, as shown in Figure 2. How to integrate these irregular shapes becomes the greatest problem that needs to be addressed in the calculation. The solution we use is first dividing these irregular shapes into several quadrilaterals or triangles and then mapping the quadrilaterals and triangles to squares [8][9]. Fig. 1. Schematic view of the perpendicular magnetic recording head The perpendicular recording head contains a tip, 150 nm in length, 50 nm in width and 250 nm in height and a collar surrounding the top part of the tip. Figure 1 gives a schematic view of the head. Detailed parameters of the head and head field distribution can be found in [1]. The SUL consists of two layers of 25 nm cubic soft cell arrays with saturation magnetization the same as that of the head. The dimension of the SUL is 500 nm 500 nm. When simulating hysteresis loops, no SUL is used. Fig. 2. Illustration of the surface integration between two voroni shaped grains A trick is used when calculating the magnetostatic interaction energy between the top and bottom regions of the same grain. Since these two regions are adjacent to each other, a singularity problem may occur. In other words, since the distance between the bottom surface of the top region and the top surface of the bottom region vanishes, the magnetostatic interaction energy relative to this pair of surfaces, according to (2), must be infinity.

3 BB-01 3 This singularity problem is addressed by defining the magnetostatic interaction energy between the top and soft regions of one grain in the way depicted in (3). Here, I and J denotes the top and bottom regions. E(I,J) is the interaction energy between them. E(I,I) and E(J,J) are self magnetostatic energies of region I and J, respectively. If we consider the grain that contains these two regions, I and J, as a single grain and denote it as grain K, then the difference between E(K,K) and the sum of E(I,I) and E(J,J) is just the interaction energy between I and J. E( I, J ) = E( K, K)! E( I, I)! E( J, J ) (3) After the magnetostatic interaction energy is calculated successfully and divided by the element volume and saturation magnetization, the magnetostatic field is given. We then use standard rotation matrices to transform it from the local (grain) coordinates to the common film axes. in Figure 3. During this step, the magnetizations of hard regions also rotate, but to a small angle compared with that of the soft regions. It is also worthwhile to notice that, during this step, all the soft regions and the hard regions rotate coherently. No grain switches in this step. This coherent rotation can be seen in Figure 4 that shows the standard deviation of normalized magnetizations of all the soft and hard regions of the 500 grains. In the range of 0 to 12 koe, the standard deviation is very small revealing that all the magnetizations are almost in the same direction. The benefit of this step is that, owing to the rotation of the magnetizations of the soft region, the effective field at the hard region of the media, comprising both the applied field and the exchange field generated by the soft region and others such as magnetostatic fields, is at an angle to the magnetization of the hard region, making it much easier to switch compared to a perfectly aligned field. After this coherent rotation step, the applied field continues to increase and grains start to switch, which corresponds to the second step of the hysteresis loop. Fig. 3. Comparison of hysteresis loops of single grain and 500 grain media. The dashed loop is loop for single grain and the solid one is for 500 grain media. The hysteresis loop of the 500 grain media is shown in figure 3 with a comparison to that of a single grain. There is a good agreement between the coercivities of the two hysteresis loops. However, the loop for 500 grains is tilted owing to the effect of magnetostatic interactions. For conventional perpendicular media, the idealized tangent of the tilted loop, α, should be approximately one, indicating that the maximum magnetostatic field is 4π M s. However, for the ECC media, the loop has two regions that are tilted at different angles. However, if we draw a line to connect the nucleation point and the closure point, the tangent of that line can be seen as the effective α of ECC media, which is approximately one. The switching of the grains complies with a two-step process that is described in [4]. This can successfully explain the two different regions of the hysteresis loop. While the applied field is increasing from 0 to about 12 koe in the negative direction, the magnetizations of the soft regions rotate leading to the deduction of the average magnetization from 550 emu/cm 3 to approximately 200 emu/cm 3, as shown Fig. 4. Standard deviation of the granular magnetization evaluated over a 500 grain specimen. The solid and dotted curves are for the soft and hard regions, respectively. IV. EFFECT OF INTRA GRANULAR EXCHANGE COUPLING The switching field of ECC media depends on the exchange coupling strength between the soft and hard regions. With zero exchange coupling, the soft region has no effect on the hard region, and the switching field will be as high as that of the hard material. While increasing the exchange coupling strength, because of the effect of the soft region, the switching field of the media decreases to a minimum. Further increasing the exchange coupling will reduce the benefit of ECC media because the magnetization of the soft region is tightly bound to that of the hard region, thus losing its freedom to rotate before switching. The switching field of the media will increase. Figure 5 shows the hysteresis loops of ECC media when exchange coupling strength between the soft and hard regions, Jex/V, equals 0, and erg/cm 3. For the zero exchange coupling case, the loop shows clearly the separated switching of the hard and soft regions. The

4 BB-01 4 coercivity of the soft and hard regions are at 100 Oe and Oe, respectively, just being equal to the anisotropy fields of the two regions. When the exchange coupling strength is erg/cm 3 which is the proper coupling case, the coercivity is reduced from Oe to 12.8 koe. Although the proper coupling case and the zero coupling case both have two-step hysteresis loops, the switching processes are totally different in that the switching of the hard and soft regions for the zero coupling case are separated but for the proper coupling case, these two regions switch together. When the exchange coupling strength is increased to erg/cm 3, the coercivity is increased. the inter granular exchange coupling constant equals 0, 0.5, 1.0 and 1.5 times (M 1 M 2 ) where M 1 and M 2 are saturation magnetizations of the soft and hard region, respectively. From the figure, we can see that inter granular exchange coupling of ECC media does increase the steepness of the loop of the second step of switching but has little effect on the first step which is dominated by the rotation of magnetizations of the soft region. VI. ANGULAR DEPENDENCE OF COERCIVITY One of the most serious problems of conventional perpendicular media is its sensitivity to easy axis distribution. According to the Stoner-Wolfarth model, when the applied field is almost aligned with the easy axis of the media, a small variation of the angle between the easy axis and the field will cause a great change in switching field of the media, which contributes greatly to recording noise. However, if the applied field is at about a 45 angle to the easy axis of the media, the switching field is quite independent to variation of that angle. This is one reason why 45 tilted media is considered advantageous to conventional perpendicular media in highdensity recording. Fig. 5. Comparison of hysteresis loops at different intra granular exchange coupling strength. In the top figure, solid line and dashed line represent loops for exchange coupling strength at and erg/cm 3, respectively. The bottom figure gives loop for the zero exchange coupling case Fig. 6. Comparison of hysteresis loops with different inter granular exchange coupling of ECC media V. EFFECT OF INTER GRANULAR EXCHANGE COUPLING Exchange coupling between neighboring grains can affect the nucleation field and the steepness of the hysteresis loop for conventional perpendicular media. The effects of inter granular exchange coupling on ECC media is also studied. Figure 6 shows several hysteresis loops of ECC media when Fig. 7. Hysteresis loops of ECC media when applied field is tilted at 0, 30, 45 and 85. In the case of ECC media, the angular dependence of switching field is also insensitive to the variation of angle between the easy axis of the hard region and the applied field. The switching field versus applied field angle is studied in [4] showing that when the angle between applied field and easy axis is less than 10, the switching field of ECC media is almost constant. This insensitivity can be explained by the mechanism that switches ECC media when it is properly coupled. Although the applied perpendicular field is almost aligned with the easy axis of the hard region of ECC media, the effective field that switches the media is not the applied field, but a combination of the applied field and the exchange field from the soft region. This effective field is tilted at an angle, near 45, to the easy axis. Thus, ECC media has a similar angular dependence to tilted media.

5 BB-01 5 Fig. 8. Comparison of switching fields, coercivity and remnant coercivity of single grain and 500 grains media at tilted field angles. Figure 7 shows hysteresis loops of ECC media when the angle between applied field and easy axis of hard region is 0, 30, 45 and 85. Along with the increase of tilted angle of the field, the coercivity of the media reduces and the remnant coercivity increases. Figure 8 shows a comparison of coercivity and remnant coercivity of ECC media at different tilted field angles. The dashed and dotted curves are switching fields and coercivity of a single grain, respectively. The filled and open circles are remnant coercivity and coercivity, respectively, of 500 grain media at several tilted field angles. The difficulty in switching of ECC media at a large field angle is quite promising for addressing the problem of adjacent track erasure. For conventional perpendicular media, although the head field at the center is larger than that at the edge, grains right under the center of the head usually resist switching and grains at the edge of neighboring tracks are likely to switch unintentionally. This is because the switching of grains not only depends on the amplitude of the field, but also the angle at which the field tilts. To consider the effect of field angle, we can define the effective field, which is generated by normalizing those real head fields as if they are perpendicular to the media so that the amplitude of the effective field is the only index to evaluate the switching of the grain. Figure 9 and 10 show a comparison of effective field (normalized to its maximum value) for conventional perpendicular media and ECC media based on the perpendicular recording head described in section two. It can be seen that, at the edge of the adjacent track (40nm in cross track direction), the effective field for conventional perpendicular media is about 0.68 while for ECC media, it is reduced to The field available to write a 40 nm track is 0.88 and 0.89 in the two cases. This means that the effective write field has only declined by 22% (=1-0.68/0.88) for conventional perpendicular recording, but 40% for recording on an ECC media. This should translate into a significant reduction in adjacent track erasure that is known to be a major problem for terabit/inch 2 recording. [10] Fig. 9. Effective field of conventional perpendicular media Fig. 10. Effective field of ECC media Fig. 11. Transition shapes of ECC media VII. TRANSITION Several transitions are written on the 500 grain media using the head mentioned above. For conventional perpendicular media, transitions have curved shapes. The transitions written on ECC media appear to be similar to those written on

6 BB-01 6 conventional perpendicular media. Figure 11 shows an example of 5 transitions written on DC erased media. VIII. CONCLUSION Exchange coupled composite media has been studied using a newly developed simulation code that allows for two layers of granular media based on Voronoi cells. Three important points are made. First, the apparently small nucleation field caused by the magnetostatic interaction between grains (film demagnetization) is immaterial because it only represents the magnetically soft elements of each grain rotating in unison with the soft elements of other grains. Actual switching does not occur until much larger fields comparable to the isolated grain case. Second, it was shown that, as the angle between the applied field and the perpendicular axis increases, there is an increased divergence between the remnant coercivity (indicative of switching) and the coercivity. This is important for experimentally assessing the properties of proposed media. Finally, the write field profile associated with ECC media is narrower in the cross track direction than that associated with conventional perpendicular media. This may have important implications for adjacent track erasure, a problem of great current importance [5][10] ACKNOWLEDGMENT The authors would like to thank the Minnesota Super Computing Institute for computing time. REFERENCES [1] R. H. Victora, J. Xue, and M. Patwari, Areal density limits for perpendicular magnetic recording, IEEE. Trans. Magn., vol. 38, pp , [2] S. Iwasaki, Perpendicular magnetic recording evolution and future, IEEE. Trans. Magn., vol. 20, pp , 1984 [3] Dec. 14, 2004, Toshiba leads industry in bringing perpendicular data recording to HDD sets new record for storage capacity with tow new HDDs, Available: downloads/pressrelease/hdd_pressrelease_v4final.pdf [4] R. H. Victora, X. Shen, Composite media for perpendicular magnetic recording, will be published on IEEE. Trans. Magn., vol. 41, 2005 [5] H. N. Bertram and K. Z. Gao, Magnetic recording configuration for densities beyond 1 Tb/in 2 and data rates beyond 1 Gb/s, IEEE Trans. Magn., vol. 38, no. 5, pp , 2002 [6] J. P. Wang, Y. Y. Zou, C. H. Hee, T. C. Chong, and Y. F. Zheng, Approaches to tilted magnetic recording for extremely high areal density, IEEE Trans. Magn., vol. 39, no. 4, pp , 2003 [7] W. Shen, J. Bai, R. H. Victora, J. H. Judy and J. Wang, Composite media for perpendicular magnetic recording using [Co/PdSi]n hard layer and Fe-Si-O soft layer, accepted by J. Appl. Phys., vol. 97, 2005 [8] J. Xue, Micromagnetic simulation of hysteresis characteristics for thin magnetic films, Ph.D. thesis, Dept. Elect. Eng., University of Minnesota, Minneapolis, MN, 2004 [9] R. H. Victora and Mujahid Khan, Micromagnetic model of perpendicular head and double-layer media for 100 Gb/in 2, IEEE Trans. Magn., vol. 38, pp , 2002 [10] J. A. Bain, M. L. Williams and R. H. Victora, Analysis of transition shape and adjacent track aging for 1 Tb/in 2 write head designs, IEEE. Trans. Magn., vol. 40, pp , 2004 TMAG

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