Mikromagnetische Simulationen

Transcription

1 Mikromagnetische Simulationen Institut: E138, Institut für Festkörperphysik, Advanced Magnetics Group, Kontaktpersonen: Hardware : Software : Univ.Prof. DI Dr. Josef Fidler Privatdozent DI Dr. Dieter Suess PhD Jehyun Lee DI Markus Fuger josef.fidler@tuwien.ac.at dieter.suess@tuwien.ac.at Sun Cluster, IBM Cluster selbstentwickelte Programme Weiterführende WWW-Links : Home-Page der Arbeitsgruppe:

2 Modeling of the write and read back performances of hexagonal Ba-ferrite particulate media for high density tape recording* Jehyun Lee, Markus Fuger, Josef Fidler, Dieter Suess, Thomas Schrefl Advanced Magnetics Group, Institute of Solid State Physics, TU Vienna Wiedner Hauptstraße 8-10, 1040 Vienna, Austria *published in Journal of Magnetism and Magnetic Materials 322 (2010) I. Introduction The areal recording density for magnetic tape recording based on the particulate media has been increased enormously, 42% annually in the past few years. Hexagonal BaFe (h-bafe) particulate media has introduced for advanced tape recording media. In Gb/in 2 was reported [1], and now 29.5 Gb/in 2 was achieved recently [2]. One of the causes for obtaining high areal densities was the application of densely packed hexagonal barium ferrite (h-bafe) platelets, which have many advantages over metal particles (MP): chemical stability, high coercivity, and small particle size. The small aspect ratio of the h-bafe particles enables themselves to be easily aligned by applying of an external field during the coating procedure on the polymer tape. The 6.7 Gbit/in 2 writing and read-back processes were performed using randomly oriented h-bafe media. However, as h-bafe particles have been originally suggested for perpendicular media, recent results reveal a higher signal-to-noise ratio (SNR) when the media particles are aligned to a certain direction. Basically, SNR depends on the quality of the writing and the reading process, as well as on the media quality itself. Since the tape recording media are mostly used as backup media and should be easily removed from the head block, the head to media distances are not as stationary as in the case of hard disk media. The mechanical fluctuation of the media to head distances has a possibility to affect on the read and write processes. In this study we have investigated the SNR performance as a function of the write and read head to media distance, for differently oriented h-bafe particulate media with a constant packing density of 40 %. The magnetization dynamics of the complete finite element model has numerically been calculated by solving the Landau-Lifshitz-Gilbert (LLG) equation using a software package, which has been developed at the Advanced Magnetics Group at the Institute of Solid State Physics of the Vienna University of Technology. II. Results and Discussion Since h-bafe media is developed as a tape recording media, experiments and simulations has been performed by conventional longitudinal head even though the h-bafe particles are mostly aligned perpendicular to the plain even in the randomly oriented media [3-4]. In this study we have introduced single pole tip (SPT) head to generate more efficient perpendicular head field component, and compared the results with Lindholm head and SPT with soft underlayer (SUL). The SPT head was assumed as a saturated magnetic pole with dimension of 200 nm long by 300 nm wide, and the SUL was supposed to behave as a perfect mirror of

3 the SPT. Head field from SPT was calculated following the permanent magnet approximation [5-6]. Fig. 1. Head field profiles of the Lindholm head, SPT head without SUL and SPT head with SUL. The dashed line indicates the position of h-bafe media is supposed to be placed. The numbers attached on the isolines show the strength of the Stoner-Wohlfarth field in Tesla. Figure 1 shows the head field profiles of the Lindholm head, SPT head without SUL and SPT head with SUL. The colors and isolines correspond to the Stoner-Wohlfarth field strength, 2/3 2/3 H ( ) 3/2 SW = Hx + Hz. Lindholm head generates the head field components of upward and downward resulted from the ring shape, whereas the SPT heads generates only downward component. In order to evaluate the head fields, the bit transition parameters (so called a parameter) were obtained. The field profiles w ere applied on the DC-erased perpendicularly ordered h-bafe media (#P) and randomly oriented one (#R), with a velocity of 6 m/s. An average value of 5 different measurements was chosen in a media and a head field. Figure 2 shows the bit transitions as a function of the deviation of Ku of the media. In both #P and #R media, the least bit transition parameter obtained by step-function field was only 2~3 nm. As the Ku has larger deviation, bit transition parameter is increasing in all cases. In comparison between the Lindholm head and SPT head, Lindholm shows the better performance when σ Ku is less than 15%, whereas the SPT without SUL shows better for other cases. The bit transition parameter becomes much less when SUL is introduced, especially in #R. SPT head with SUL shows the least bit transition parameters in all cases.

4 Fig. 2. Bit transition parameters of #P and #R under various head fields, as a function of the standard deviation of crystal anisotropy Ku. Fig. 3. Snapshots of the writing processes using (a) Lindholm head, (b) SPT head without SUL and (c) SPT head with SUL.

5 Fig. 4. A schematic of read back process using reciprocity theorem. In order to estimate the Signal to noise ratio (SNR), 20 alternating bits were written in a media model. The writing process and reading process are schematically shown in Fig. 3 and Fig. 4, respectively. In Fig.3, the cross sections of the bit structure are shown clearly. The SPT with SUL results in the bit structures of which interfaces are more vertically aligned than others. For the read back process, the reciprocity theorem was applied. The field box generated by the free layer of the read head moves overlapped on the written bits, then calculates the interaction energy between the field box and the media. SNR was obtained for the case of σ Ku is 0, as shown in Fig 5. The SNRs were calculated by Fast Fourier Transformation (FFT) of artificial 18 µm media which was prepared by adding on synchronized read back signals. The written bit patterns for read back simulation have 60 nm bit length, corresponds to 423 kfci. The writing head velocity was fixed as 6 m/s and 20 m/s. In most cases SPT head without SUL shows the worst results and SPT head with SUL shows the best one, up to 47 db in #P media with 6m/s writing speed. The results agree very well with the bit parameter estimation in Fig. 2. Fig. 5 SNRs of #P and #R media, obtained by the bits written with Lindholm head, SPT without SUL and SPT with SUL.

6 III. Conclusion The signal-to-noise ratio (SNR) performances of longitudinally, randomly, and perpendicularly oriented particles, based on hexagonal barium ferrite (h-bafe) platelets with an average volume of 2400 nm 3 have been studied as a function of the recording head to media distance by numerical micromagnetic simulations. The distances from the write head to media and from the read head to media were varied independently. For a fixed read distance and varied writing distances, the SNR was decreasing in larger write distance. An optimum write distance of 40 nm and 50 nm was found for the longitudinally oriented media and the perpendicularly oriented media, respectively. The optimum write distance for longitudinally oriented media, 40 nm, resulted in the local minimum SNR for the perpendicularly oriented media. In most write distances the perpendicularly oriented media show the outstanding best performance, but near the write distance of 40 nm the longitudinally oriented media works as good as the perpendicularly oriented media. In a fixed write distance with various read distances, the SNR was almost constant in each media whereas the average signal amplitude was exponentially decayed in larger read head to media distance. The best SNR was found in the perpendicularly oriented media at write head to media distance d write = 20 nm and read head to media distance d read = 40 nm. The best SNR value is 11.9 db and 24.4 db in time domain and frequency domain, respectively. References [1] D. Berman, R. Biskeborn, N. Bui, E. Childers, R.D. Cideciyan, W. Dyer, E. Eleftheriou, D. Hellman, R. Hutchins, W. Imaino, G. Jaquette, J. Jelitto, P.O. Jubert, C. Lo, G. McClelland, S. Narayan, S. Oelcer, T. Topuria, T. Harasawa, A. Hashimoto, T. Nagata, H. Ohtsu, S. Saito, 6.7 Gb/in2 recording areal density on barium ferrite tape, IEEE Trans. Magn., 43 (2007) [2] G. Cherubini, R.D. Cideciyan, L. Delimann, E. Eleftheriou, W. Haeberle, J. Jelitto, V. Kartik, M.A. Lantz, S. Oelcer, A. Pantazi, D. Berman, W. Imaino, P.-O. Jubert, G. Mcclelland, P. Koeppe, K. Tsuruta, T. Harasawa, Y. Murata, A. Musha, H. Noguchi, H. Ohtsu, O. Shimizu, S. Ryota, 29.5 Gb/in2 Recording Areal Density on Barium Ferrite tape, in: TMRC 2010, Jolla California, 2010, pp. C3. [3] T. Harasawa, R. Suzuki, O. Shimizu, S. Ölçer, E. Eleftheriou, Barium-Ferrite particulate media for high-recording-density tape storage systems, IEEE Trans. Magn., 46 (2010) [4] P.O. Jubert, B. Biskeborn, D. Qiu, H. Nuguchi, O. Shimizu, Noise properties of barium ferrite particulate media studied by micromagnetic modeling, IEEE Trans. Magn., (2010) in press. [5] S. Khizroev, D. Litvinov, Perpendicular magnetic recording: Writing process, J Appl Phys, 95 (2004) [6] J. Norpoth, et al., Straightforward field calculations for uniaxial hardmagnetic prisms: stray field distributions and dipolar coupling in regular arrays, Journal of Physics D: Applied Physics, 41 (2008)