Media technology is one of the cornerstones of a tape system. In order to continue to scale the areal recording density of future magnetic tape systems, the media must also be continuously improved. We work together with tape media manufactures to test and evaluate prototype media in order to help enable the development of media that will meet the requirements of future tape systems. Much of our current work focuses on the use of tape based on barium ferrite (BaFe) particles which can be produced using low cost particulate coating technologies. Currently, we are investigating three complementary strategies for improving the SNR of particulate tape media:
- reduced particle volume and improved size control,
- perpendicular particle orientation, and
- reduced media roughness to reduce magnetic spacing.
Particle volume
Noise in the read-back signal of a magnetic tape recording channel typically consists of two components, electronic noise due to noise in the read sensor and analogue front end and media noise which arises from the finite size and from variations in size of the magnetic particle used in the recording layer. Media noise can be reduced by reducing the volume of the magnetic particles and the variation in the particle volume.
Recent LTO tape drives (LTO Gen 1-5) utilize iron-cobalt-based metal particles (MP) media, whereas the latest enterprise tape drives use tape based on non-oriented BaFe particles. The origin of the coercivity of MP media is shape anisotropy which makes it difficult to maintain a high coercivity as the particle volume is reduced. In contrast, the coercivity of BaFe particles arises from crystalline magnetic anisotropy, rendering the scaling of BaFe particles more favorable.
Another issue with traditional MP media results from the oxidation of the material which leads to degraded performance. To prevent this, the particles are covered with a protective, nonmagnetic "shell", which further hinders scaling to finer particle sizes. BaFe, in contrast, is already an oxide and therefore does not need a protective shell. Hence the full particle contributes to the magnetic signal and the particles can be scaled to much smaller particle volumes. Figure 1 shows TEM images of the 1600 nm3 BaFe particles used in a recent areal density demo [1] and the metal particles used in LTO-5 media with a volume of 4650 nm3. The reduced particle volume of the BaFe particles results in reduced media noise and to a significant improvement in SNR relative to LTO-5 media.
In general, as the size of a magnetic particle is reduced, the thermal stability of the particles is also reduced. This can be quantified by the thermal stability given by: KuV/kB T, where Ku is the anisotropy energy density, V the particle volume, kB the Boltzmann constant and T the absolute temperature. For BaFe particles, the anisotropy can be adjusted by the substitution of Fe with other elements [1]. The BaFe particles shown in Figure 1 have a thermal stability factor of 75, which provides sufficient stability for long-term archival storage applications.
Perpendicular orientation
Another advantage of BaFe particles is their platelet shape and easy axis which is orthogonal to the surface of the platelets. This makes it easier to orient the easy axis of BaFe media in the perpendicular direction of the tape surface by applying a magnetic field during the coating process. Perpendicular orientation is desirable as it results in an increase in signal amplitude and a reduction in the demagnetizing field at high linear densities. Figure 2 compares cross sectional TEM images of a perpendicularly oriented BaFe tape and a non-oriented tape: the improved orientation of the perpendicular sample is clearly visible.
Figure 3 shows M-H loops of a perpendicular BaFe tape (Tape A), a non oriented BeFe tape sample (Tape B) and an LTO-5 tape sample. The M-H loops where taken in the perpendicular direction for Tape A and B and in the longitudinal direction for the LTO-5 tape. The squareness ratio of the perpendicularly oriented BaFe tape is 0.86, which is comparable to the squareness of the LTO-5 tape in the longitudinal direction, indicating a similar degree of orientation between to the two tape types but in orthogonal directions.
- Figure 1. TEM images of BaFe particles with a volume of 1600 nm3 on the left, and the metal particles used in LTO5 media with a volume of 4650 nm3.
- Figure 2. Cross-sectional TEM micrograph of (a) perpendicularly oriented BaFe media and (b) non-oriented BaFe media. The scale bars are 20 nm.
- Figure 3. M-H loops taken in the perpendicular direction of a perpendicularly oriented BaFe tape (Tape A) and a non-oriented BaFe (Tape B). The dashed red line shows an M-H loop taken in the longitudinal direction of an LTO-5 tape.
Media roughness
The SNR performance of tape media can be improved by making the media smoother which reduces the magnetic spacing, i.e. the distance between the read/write transducers and the magnetic coating. However, increasing the tape smoothness tends to increase tape-head friction which in turn reduces the "runability" of the media and leads to high frequency velocity variations that can degrade the performance of the data detection process. This dilemma can be resolved through careful engineering of the tape surface roughness in which the long-range roughness or "waviness" is minimized while maintaining a moderate roughness measured over a shorter length scale. Reducing the waviness results in reduced magnetic spacing while the moderate short range roughness acts to minimize and control the tape head contact area and hence to control friction. This combination of low surface waviness and moderate short-range roughness results in an increased signal from the medium while excellent durability and runability is maintained.
References
[1] 29.5 Gb/in2 Recording Areal Density on Barium Ferrite Tape,
G. Cherubini, R.D. Cideciyan, L. Dellmann, E. Eleftheriou, W. Haeberle, J. Jelitto, V. Kartik, M. Lantz, S. Oelcer, A. Pantazi, H. Rothuizen, 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, and R. Suzuki,
IEEE Transactions on Magnetics 47(1) (January 2011).