Tikalon Blog by Dev Gualtieri

Writing Magnetic Media

April 26, 2021

Rubbing a plastic drinking straw with most fabric materials will create an electric charge that will attract small pieces of paper to it. The Coulomb's_law force of this electrical attraction is dwarfed by the magnetic force between a refrigerator magnet and a steel paper clip. Such strong magnetic forces enable the electric motors used in industry and many household appliances. Today's mighty motors, such as those used to raise and lower automobile windows, use magnetic alloys of the rare earth elements.

A decade ago, I wrote two articles about a major problem in the supply chain for such elements (Rare Earth Shortage, June 21, 2010 , and Energy Elements, May 11, 2011). The problem is that China is the principal producer of these elements, and it has limited their export. The US was a producer of these elements, but the extraction of rare earths from ore and their purification are not that environmentally friendly, and this led to too high a production cost compared with that of China at that time. At this writing, the price of neodymium metal, an important rare earth used in the most powerful rare earth magnets, is $65/kg.

Principal rare earth elements used in magnetic alloys

Lanthanum (La)	Cerium (Ce)	Praseodymium (Pr)
Neodymium (Nd)	Samarium (Sm)	Europium (Eu)
Gadolinium (Gd)	Terbium (Tb)	Dysprosium (Dy)
Ytterbium (Yb)	Lutetium (Lu)	Scandium (Sc)
Yttrium (Y)

While electric charge exists as separate positive and negative polarities, magnets are all dipoles with two joined poles, named north and south after their similarity to Earth's North Magnetic Pole and South Magnetic Pole. This idea of the dipole ("two-pole") magnet was established by the research of the English natural philosopher, William Gilbert, who devised the first physical model of the magnetic Earth by creating a sphere of lodestone, which he called a terrella. He showed that the magnetic needle of a compass moving on its surface had the same deflection as a compass on the Earth. The compass was known long before Gilbert's time, but not the idea that its deflection was caused by Earth's magnetic field.

Portrait of William Gilbert and a woodcut from Gilbert's De Magnete

William Gilbert (1544-1603) with a woodcut from his De Magnete. The gilbert, a unit of magnetomotive force, was named in honor of Gilbert. The woodcut illustrates how hot iron can be magnetized by being worked on an anvil with its axis aligned north-south. The Latin word for North is septentrio, and the Latin word for South is avster/auster). (Gilbert portrait by Charles Henry Granger (1812-1893), via Wikimedia Commons. De Magnete Illustration no. 125, via Wikimedia Commons.)

Gilbert discovered that breaking magnets produced smaller magnets, still with a north and south pole. Gilbert's book, De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure (On the Magnet and Magnetic Bodies, and on That Great Magnet the Earth), was the first treatise on magnetism.[1] Until that time, magnetism was thought to be like electricity, having two "fluids" which are attracted to each other.

Electric and magnetic monopoles and dipoles.

From left to right, positive and negative electric monopoles, an electric dipole, an analogous pair of magnetic monopoles, and an actual magnetic dipole. Magnetic monopoles, which would be isolated north or south magnetic poles, have not been observed.

In the the late twentieth century there were many experiments designed to detect magnetic monopoles, since they were theorized by Dirac. There were also many failed experiments at the same time to show the existence of tachyons, which are faster than light particles. (Source images, monopoles and dipoles by Maschen, via Wikimedia Commons. Click for larger image.)

Humans were using the natural magnet, lodestone, that Gilbert used for his terrella long before they created their own magnetic ceramics and alloys. Lodestones are magnetized specimens of the mineral, magnetite (Fe₃O₄), and these stones are attracted to themselves, and to iron. The Greek philosopher, Thales, wrote about the attraction of lodestones in the sixth century BC.

Magnetic materials were used for data storage beginning in the mid-20th century, first in audio recording on magnetic metal wire, and then in audio recording and digital data recording on iron oxide powder affixed to flexible tape. Magnetic tape recording was pioneered in the United States by Jack Mullin, who discovered Magnetophon recorders in Germany while serving in the US Army Signal Corps during World War II. Mullin demonstrated tape recording at a May 16, 1946, meeting of the Institute of Radio Engineers.

The magnetization of most materials is non-linear, and this is demonstrated in the material's B-H loop, its induction B as a function of applied magnetic field H. This is a disadvantage for analog recording, so audio tape recorders imposed a high frequency AC biasing on the magnetic particles during recording to reduced the distortion caused by the magnetic hysteresis. However, digital recording is best done on a material with extreme hysteresis and a square-loop response.

B-H loops for a hard and soft magnetic material.

The remanence B_r is the magnetization that remains in the material when the applied magnetic field is removed. The coercivity H_c is a measure of the difficulty of magnetic reversal.

A large H_c is desirable for a magnetic storage medium.

(Created using Inkscape. Click for larger image.)

Reading magnetic media is far easier than its writing, since writing for high areal data density requires application of a strong magnetic field in a small area. The magnetization of materials depends on temperature, making it easier to change its magnetic polarity, so assistive writing techniques such as heat-assisted magnetic recording have been developed.

There's always an inducement for innovation in writing magnetic media, since magnetic media, mostly in the form of magnetic tape, is the principal storage technology for the world's more than 500 exabytes of data (an exabyte is 10¹⁸ bytes).[2] Recently, a team of Japanese scientists from the Toshiba Corporation Corporate Research and Development Center (Kawasaki, Japan) and the Toshiba Electronic Devices and Storage Corporation (Yokohama, Japan) have published an open access paper in the Journal of Applied Physics that describes their particular implementation of microwave-assisted magnetic recording (MAMR).[3-4]

Modern magnetic storage media contain very small magnetic particles that allow their high storage capacity, but such small particles could be easily demagnetized through self-demagnetization and interaction with adjacent areas, unless the magnetic coercivity is high. That results in the difficulty of recording data that microwave-assisted magnetic recording solves.[4] The particular technology for achieving this is a spin-torque oscillator, a device that emits a microwave field that causes the magnetically aligned electrons in the recording medium to wobble like a spinning top. This makes the spins easier to flip over when under the influence of the write head.[4]

This spin-torque oscillator is positioned in the gap of the write head (see figure).[4] In this type of microwave assist, the magnetization of the gap field is completely reversed by an effect known as the flux control effect, and this enhances the amplitude of the magnetic gradient of the recording field.[3-4]

Comparison between a conventional write head and a flux control write head

Comparison between a conventional write head (left) and a flux control write head (right). The magnetization of the flux control device is reversed against the gap field in the write head. This decreases the magnetic flux inside the write head gap and increases the flux outside the write gap to assist writing. (Illustration by Hirofumi Suto, modified for clarity. Click for larger image.)

This improved writing can be used for any magnetic media and not those designed for microwave-assist.[4] A bias current controls the flux control effect, and research showed that higher bias currents caused a faster reversal of magnetization.[4] The flux control device operates at about 3 gigabitsper second.[3-4] Toshiba intends to introduce this writing technology into hard disk drives with 16-18 terabyte capacity.[4]

Writing Magnetic Media

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