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Notes on Modern Technology

These notes cover some innovations that haven’t found their way into the first edition of the text.

Helium Filled Hard Drives

The hard disk drive that stores data along concentric tracks on the surface of rotating platters is an anachronism. It should do the decent thing and lie down and die. It’s an anachronism because it is an electromechanical device with moving parts. If it weren’t for hard disks (and optical drives), we could probably stick the whole computer on a piece of silicon.

We don’t like disk drives. They are heavy, large, slow, and unreliable. They have nothing going for them except one tiny feature. They are cheap. Very cheap. Solid state memory storage mechanisms are catching up, but hard disk drives still retain a price per bit advantage that is an order of magnitude better than flash-based memory.

One of the reasons that magnetic disk technology has survived is the way in which it has been Moore’s Lawed into the future. Increasingly ingenious tricks have been used to increase bit density per square inch (i.e., areal density) with the result that drives have grown from 5 MB capacities to 4 TB capacities in about six decades).

The access time of hard disk drives has hardly improved over the same period, because little can be done to improve rotational latency. Ultimately, the long access times of magnetic media will ensure their obsolescence. Probably.

A hard drive uses a read/write head mounted on a mechanical arm to read data from and write data to the magnetic coating of a platter. The amount of data that can be stored depends on the size of the magnetized regions on the surface – the smaller the bit, the greater the capacity. The head does not touch the platter’s surface. It flies in the boundary layer of air above the surface of the disk.  Reducing the flying height reduces the region of magnetization and the increases the bit density.

Another way of increasing disk capacity is to increase the number of platters per spindle. The maximum number of platters is limited by the physical size of the disk enclosure, the energy consumption and heat dissipation of the mechanism. One way of reducing power consumption and increasing the number of platters is to fill disk drives with helium rather than air.

Since the introduction of the hard drive, the air above the disk has been either the atmosphere (after removing particulate matter) or dry nitrogen. In 2013 HGST announced the use of helium instead of air. Helium is the second lightest element after hydrogen and is entirely inert (i.e., it does not react chemically with other elements). The advantage of helium is that its density (weight per unit volume) is only 0.15% that of air.

Using a low-density gas means that the aerodynamic drag experienced by the rotating platter, arm and read/write head is much reduced. It is estimated that a helium-filled drive will reduce power consumption by over 20%, increase capacity by 40%, and reduce operating temperatures by about 4° C (remember that the failure rate of electrical systems doubles for approximately every 7° C rise in ambient temperature).

Using helium could increase the number of platters from 5 to 7 and make the leap from 4 TB drives to 5.6 TB drives without increasing the areal density.

The notion of helium-filled drives is not new and has been around for over a decade. However, helium has a very low viscosity and can gradually leak through seals that are impermeable to air (hence the reason that helium filled toy-balloons soon deflate). The technological breakthrough has been in the design of effective seals.

New Magnetoelectric Materials

Electricity and magnetism are intimately related; for example, magnetism produces electrical effects and electricity produces magnetic effects. In general, computer technology is largely concerned with electrical effects; for example, the flow of electrons in semiconductors.

The interaction between electricity and magnetism appears in some highly specialized applications such as GMR read heads in disk drives where the magnetic field from recorded data directly changes the electrical resistance of a material.

In early 2013, scientists at the Argonne National Laboratory in the USA announced the development of a class of electromagnetic material based on europium-titanium oxide, EuTiO3. Europium is a metallic element and a member of a group of elements called lanthanides. Europium has some interesting electromagnetic properties; for example, it becomes superconducting (i.e., no electrical resistance) at low temperatures and high pressures. Europium has a few specialized applications, most of which are connected with phosphorescence (e.g., in CRT-based televisions and florescent lamps). The other metal in this compound is titanium that is widely used to create strong lightweight alloys in the aerospace world.

The structure of a EuTiO3 molecule is rather like a cage with a titanium atom in the center and europium and oxygen atoms forming the perimeter at the following figure from the Argonne lab illustrates.

The europium and titanium atoms control the electrical and magnetic properties of the material, respectively. The ability to control the position of the europium and titanium atoms (e.g., by compression) means the both the electrical and magnetic properties of the molecule can be modified.

The researchers believe that this (or similar) materials could be used as the basis of future memory devices and magnetic field sensors. Electromagnetic materials are potentially advantages because of their relative stability and robustness. Moreover, it is believed that such devices could be used to implement multi-level logic systems; that is, each element could be used to store more than one bit.