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Since the 1970s, commercial mass data storage has been available with continuously diminishing costs and increasing storage capacity. The old eight-inch floppy disk displayed a storage capacity of 250 kB, which today might seem like an incredibly low capacity, however large-scale production was a technological prowess back then.
From the first floppies to the currently available terabyte hard disk drives (HDDs), the principle of data storage continues to be the same: the active component is a magnetic layer that stores information in the form of 0s and 1s by being magnetized locally in one direction or its opposite. The layer is placed on a spinning disk that can be written and read by a mobile head.
One remarkable progress in magnetic media recording was accomplished with the discovery of the Giant Magnetoresistance effect (GMR) by Albert Fert and Peter Grünberg, which got them the Nobel Prize in 2007. This effect facilitated moving from in-plane magnetic media recording to perpendicular media recording (PMR), thus expanding the storage capacities by over three orders of magnitude! Technologists today are discovering new tricks to improve density and pack more terabytes in hard disks measuring just a couple of inches wide.
This article examines the fascinating race for ever growing storage capacity through the “eye” of the Magnetic Force Microscope (MFM). MFM is a type of Scanning Probe Microscopy where magnetic material is coated on the sensing tip so as to be sensitive to the vertical component of the magnetic stray field originating from the sample surface.
In our measurements, we used phase-locked loop technology to track the resonance of a cantilever as it interacts with the surface. The cantilever is excited at its first longitudinal resonance (typically 50 kHz) while magnetic interaction is detected as a shift of the resonance, typically only of a few Hertz.
Brighter contrast means positive frequency shift which in turn depicts repulsive interaction or a magnetic stray field of the surface anti-parallel to the magnetization of the tip. Darker contrast translates as negative frequency shift, or attractive interaction and a magnetic stray field of the surface parallel to the tip magnetization.
When conducted under suitable conditions – ideally in high-vacuum – a magnetic lateral resolution of just 10 nm can be accomplished, which is the length of today's bits in HDDs. MFM, along with the similar near-field measurement mode of spin-polarized STM, remains the only tool to image bits in real space without the burden of a large instrument such as a synchrotron.
A NanoScan microscope has been used to record all MFM images.
1987: 3.5-Inch Floppy Disk
In the 1970s, magnetic storage made its appearance and a decade later became a widespread consumer good. The bits of an 8 inch floppy disk were macroscopic so that they can hardly be imaged with an MFM! From 1987 on, the next-generation floppies, the 3.5-inch disks, already had bits that had shrunk to a microscopic 2x100 µm2, as shown in the MFM image below:
Figure 1. 75x200 µm2 MFM image of a floppy disk. Magnetization is in-plane, contrast arises from the magnetic stray field at the domain boundaries. The orange square illustrates the dimensions of Figure 3. (25x25 µm2 MFM image of a zip drive)
Magnetization in this case is oriented in the plane of the layer and hence the MFM detects the vertical magnetic stray field originating at domain boundaries, as shown in the figure below:
Figure 2. Principle of Magnetic Force Microscopy over a sample surface with in-plane magnetization. The magnetically-coated tip detects the vertical component of the stray field (grey arrows) emanating from the surface. Hence, the MFM signal (in red) exhibits peaks at the domain boundaries.
The floppy disk shown here is a “High Density” disk with 1.44 MBytes of storage, or 11.52 Mbits (1 Byte = 8 bits) dispatched on a 3.5 inch platter. Taking into consideration the one-inch rotor, that makes an area of 8.84 in2 of active layer. However, not the entire area is available for data storage: for example the rims of the disk cannot be written and formatting also occupies space.
Counting the bits in the 75x200 µm2 MFM image (approximately 37 bits), a density of 1.6 Mbits/in2 can be established. Thus, 7.2 in2 or 81% of the whole surface is requested to store 11.52 Mbits of information.
1.6 Mbits/in2, to put the number to scale with the following Hard Disk Drives that is a density of 0.0016 Gbits/in2.
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1994: Zip Drive
After the arrival of the floppy disk, the zip drive was introduced in 1994. The size of the medium was still a 3.5 inch platter, and magnetic information was still stored in-plane, but due to its decreased bit size, the storage capacity could be expanded to 100 MB or 800 Mbits.
On the following MFM image of 25x25 µm2, the bit dimension is assessed to be 1x10 µm2: that is 20 times smaller than in the floppy disk.
Figure 3. 25x25 µm2 MFM image of a zip drive. Magnetization is again in-plane. The red square illustrates the scale of Fig.5 to 8. (500x500 nm2 MFM image of hard disk drives)
On the MFM data, 54 bits are counted, amounting to a density of 56 Mbits/in2, which makes 403 Mbits on the available 7.2 in2 (assuming the same active area as on a floppy disk) or approximately 800 Mbits when considering both faces of the zip. This figure is in seamless agreement with the Manufacturer’s specifications.
But again: 56 Mbits/in2, that is just 0.0056 Gbits/in2.
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2007: Fujitsu HDD
After the discovery of the GMR effect, the bits could be written vertically in the medium and much more information could be packed into the same area. From the early 2000s onwards, Perpendicular Magnetic Recording (PMR) (see schematic representation below) became the norm and Hard Disk Drives were launched.
Figure 4. Principle of Magnetic Force Microscopy over a sample with Perpendicular Magnetic Recording (PMR). There is always a vertical component of the magnetic stray field; hence the MFM signal always exhibits contrast.
The following 500x500 nm2 MFM image was captured on a Fujitsu HDD from 2007 comprising of a single 2.5 inch platter and stipulating a total storage capacity of 40 GB or 320 Gbits.
Figure 5. 500x500 nm2 MFM image of a Fujitsu HDD from 2007. Magnetization is perpendicular to the surface.
The bit dimension derived from the image is approximately 25x 200 nm2 which makes it 2000 times smaller than in a zip drive. Logically, the density also makes a massive leap: on average 37.4 bits are counted on this MFM data, on an area of only 0.25 µm2: that comprises a density of 100 Gbits/in2!
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2009: Seagate HDD
Since then, the race for ever-shrinking bits was propelled. In the spring of 2009, Seagate commercialized a Hard Disk Drive totaling 500 GB (4000 Gbits or 4 Terabits) split on two 2.5 inch platters.
The 500x500 nm2 MFM image below represents 83.2 bits on average, the bit sizes amounts to ca. 20x 125 nm2. This signifies a gain of over a factor of two in bit size over the Fujitsu HDD. Logically, the bit density increases to 215 Gbits/in2.
Figure 6. 500x500 nm2 MFM image of a Seagate HDD from 2009.
2012: Western Digital HDD
In 2012, Western Digital commercialized a HDD that had 1 TB or 8 Tbits of storage capacity divided over two 2.5 inch platters.
The 500x500 nm2 MFM image below displays 190 bits on average, the bit sizes totals to ca. 17x80 nm2, which is becoming hard for an MFM to image. The bit area represents again a gain of over a factor of two in bit size over the earlier Seagate HDD, the bit density increasing to 490 Gbits/in2.
Figure 7. 500x500 nm2 MFM image of a Western Digital HDD from 2012.
2016: Seagate HDD
After 2015, the battle for bit size reduction became harder as the physical limitations to the stability of the bits were increasingly reached. HDD Manufacturers have examined two tricks as workarounds in the existing Perpendicular Magnetic Recording technology. Both have the objective of reducing the width of the bits:
- Shingled Magnetic Recording (SMR): similar to roof tiles, a track is written overlapping the earlier neighboring track.
- Two-Dimensional Magnetic Recording (TDMR): so as to prevent inter-track interference during readout of very narrow tracks, an array of read-heads is used to read adjacent tracks and correlate data. This technology, combined to SMR would gain another 5 to 10% of storage capacity.
The HDD from Seagate illustrated below is from the BarraCuda series (up to 5 TB of storage on three 2.5 inch platters) and according to the Manufacturer employs the SMR technology. However, from the MFM image, only a weak increase of storage density can be noticed. The 500x500 nm2 MFM image below displays 208 bits on average, the bit size amounts to ca. 16x 77 nm2: compared to the earlier Western Digital HDD there is only a minor improvement of the bit size of a factor of 1.1. The bit density is 540 Gbits/in2.
Both TDMR and SMR technologies are thought to be stop-gap measures where read heads can be made smaller but write heads not. Moreover, SMR highlights the issue of re-writing a track, which has an incidence on the following adjacent tracks.
Figure 8. 500x500 nm2 MFM image of a Seagate HDD from 2016.
Beating the 1 Tbits/in2 limit is the next challenge HDD Manufacturers are currently facing. Bear in mind: a storage density of 1 Tbits/in2, that is a bit size of just 12.7x50 nm2.
The active layer of present day HDDs is made up of magnetic grains (usually CoPtCr alloys) clustered together, with dimensions of just few nanometers, typically 8 nm. The actual bit size is so small that less than 20 grains make up each bit.
Intuitively, to expand capacity it would be enough to reduce additional bit size and hence have fewer grains composing a bit. But this would weaken the Signal-to-Noise ratio and would make readout more complex.
Alternatively, one could think of decreasing the grain size. However physics stalls this: a smaller grain cannot hold its magnetization anymore as thermal activation at room temperature will serve to flip its spin.
However, there are technological solutions, at various states of development. One possible technology enabler is to employ other alloys with higher anisotropy and hence greater thermal stability. The downside to it is that writing becomes a lot more laborious and requires to locally heat the active layer to provisionally lower the magnetization barrier. This so-called Heat-Assisted Magnetic REcodring (HAMR) will hence not only need new materials but also new and intricate read/write heads.
Another solution that can be considered is to pattern the active layer so as to physically define the shape of the storage unit: these are called Bit-Patterned Media and are also under investigation, as shown in Figure 9. Production of the layers would then need additional steps, which is a huge disadvantage in keeping production costs low.
Figure 9. 500x500 nm2 MFM image of a Bit-Patterned Medium (BPM). Sample courtesy of Seagate.
In the Figure above, there are on average 92.5 dots over an area of 500x500 nm2 which makes a density of just 240 Gbits/in2. The pitch between each dot is 50 nm in this sample from 2009. To realize a density of 1 Tbits/in2, the pitch between each dot would have to be halved to 25 nm.
HAMR, BPM, or a combination thereof: in the days to come it will be known which path technologists have chosen to take to solve the challenge of data storage capacity. The Advanced Storage Technology Consortium (ASTC) roadmap, issued by the International Disk Drive Equipment and Materials Association (IDEMA) provides a hint of what is anticipated in the next 10-15 years.
Figure 10. The ASTC Technology Roadmap gives an insight of the evolution of storage density in the coming 5-10 years in relation with the development of new technologies.
This information has been sourced, reviewed and adapted from materials provided by NanoScan AG.
For more information on this source, please visit NanoScan AG.