30 Magnetic Data Storage

Dr. Anchal Srivastava

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Contents of this Unit

 

1. Introduction.

2. Magnetic data storage.

3. The first hard disk.

4. Primary storage device.

5. Memory storage mechanism.

6. Various types of secondary storage devices are as followings. 4.1. Winchester disk.

4.2. Magnetic drum.

4.3. Magnetic tapes.

4.4. Optical disks.

4.5. Magnetic disks.

7. Addresses, seek time, latency time, data transfer time.

8.Analog recording & digital recording.

9. Magnetic tape sound recording.

10. Magneto-optical recording.

11. Domain propagation memory.

12. Solids classified into two main categories.

12.1. Crystalline materials.

12.2. Amorphous materials.

13.   Nanostructured materials and its two components.

13.1. The crystalline component.

13.2. The interfacial component.

14.    Magnetic nanostructures and their properties and morphologies.

15.    Summary.

 

Learning Outcomes

 

After studying this module, you shall be able to

 

  1. Learn a few basic processes which show how the memory storage devices function and magnetic data storage and its growth with the advent of time and advancements in technologies.
  2. Learn about the various types of Secondary Storage Devices which are Winchester Disk, Magnetic Drum, Magnetic Tapes, Optical Disks, Magnetic Disks etc.
  3. Learn about important parameters for data storage are Addresses, Seek Time, Latency Time, Data Transfer Time.
  4. Learn how Nanostructured magnetic materials exhibit interesting and unexpected properties due to their very high surface area to volume ratio. And learn about the magnetic nanostructures can be divided into different groups based on their properties and morphologies.

 

1. INTRODUCTION

 

Magnetic materials are used in various day-to-day applications such as in motors, generators, electromagnets, low and high frequency transformers, magnetic field screening, permanent magnets, loudspeakers, analogue and digital data storage, telecommunications, etc. Some of the widely used permanent magnetic materials are magnetite, magnetic carbon steel, alnico magnets (alloys based on Al, Co, and Ni), cobalt platinum magnets, hard ferrite magnets (SrFe12O19 and BaFe12O19), samarium cobalt, neodymium iron boron magnets, and samarium iron nitride. The oxide ferrimagnetic materials (ferrites, garnets, etc.) are discovered in the early 1940s and they still occupy a key position among the technologically important magnetic materials.

 

In simple words, without magnetic materials, we cannot think about modern technological revolutions. The present trend in magnetic materials research is on nanometer sized materials. The main emphases, at present, are on the synthesis, characterization, studies on the differences in the properties with respect to the corresponding bulk counterparts and on the application of magnetic nanomaterials for the benefit of mankind.

Fig: 1- Comparison of Optical Drives Storage Capacities and Magnetic Drives Storage Capacities.

 

Magnetic nanoparticles have been the subject of much research interest even before the 1950s. Because of the surge in the field of nanoscience and nanotechnology, studies on Nano magnetism emerged as a major area of research due to the potential applications of the Nano sized magnetic materials. Thus, investigation of magnetic nanoparticles is a subject of intensive research from the viewpoint of probing their magnetic behavior (size and surface effects) and applications.

 

Some of the major applications of magnetic nanoparticles are in information storage where single nanoparticle can act as an individual bit of information providing high density data storage, in magneto optical switches, sensors based on giant magnetoresistance (GMR), magnetically controllable single electron transistor devices, in magnetic resonance imaging (MRI), targeted drug delivery, detection of bimolecular interactions, detection of DNA hybridization, diluted magnetic semiconductors (DMS), etc. Modern scientific research is aiming at improving and enhancing capabilities of devices based on “spintronic” (spin + electronics). This is towards the development of smallest and cheapest microchips for use in computers, cell phones and other electronic components for military applications.

 

2. MAGNETIC DATA STORAGE

 

The most common forms of permanent data storage used in modern computers – hard disks and archival tapes – represent data as magnetized “bars” on the surface of a thin medium, where writing and reading of data requires mechanical movement of the medium. This type of data storage was used in the very first computers at the end of the 1940s. In the succeeding 55 years the technology has been improved to an extent that is just as remarkable as improvements to computer electronic circuitry over the same period.

 

Magnetic storage in the form of wire recording—audio recording on a wire—was publicized by Oberlin Smith in the Sept 8, 1888 issue of the Electrical World. Smith had previously filed a patent in September, 1878 but found no opportunity to pursue the idea as his business was machine tools. The first publicly demonstrated (Paris Exposition of 1900) magnetic recorder was invented by Valdemar Poulsen in 1898. Poulsen’s device recorded a signal on a wire wrapped around a drum. In 1928, Fritz Pfleumer developed the first magnetic tape recorder. Early magnetic storage devices were designed to record analog audio signals. Computers and now most audio and video magnetic storage devices record digital data.

 

In the late 1940s, when the designers of the first computers needed to provide fast storage/ retrieval of digital data they could draw on the technology for recording analog audio information using magnetism that had been developed over the preceding 50 years. The concept behind the technology was outlined by the American Oberlin Smith in 1878 and publicized more-widely 10 years later. However, it was not until 1898 that the first functioning magnetic recorder, the telegraphone, was demonstrated by the Danish inventor Valdemar Poulsen. The technology developed slowly during the first part of the 20th century for use in dictaphones, for telephone message recoding and for delayed radio broadcasting. The magnetic media were steel wires and steel tapes (though there were devices that used disks following the same format as gramophones.) Major breakthroughs were made in the 1930s with the use of coated plastic tapes – by the early 1940s the German tape recorders called magnetophones were being used to make high-quality orchestral recordings.

 

The earliest digital magnetic stores used induction to read the information stored. The medium is moved past a “read head” which is an electromagnet used “in reverse” (sometimes the same as the “write head”). A change in magnetization direction induces a current in the electromagnet’s wires. In modern devices the magnetization is sensed by other means but the tradition is maintained that the two states of raw bits are represented by a transition or no transition in the direction of magnetization.

 

Computers as proposed by von Neumann and Turing required memory that would contain both data and programs. The memory had to be fast (operate at electronic speeds) and random access (take the same time for all words of memory). Although the technology for processing data electronically was available from the mid-1940s there was no matching technology for computer main memory. This did not really become available until the development of magnetic core memory in the mid to late 1950s.

 

One possibility was to use magnetic data storage. This was of reasonable price and capacity but had the defect that it was slow and not truly random access – some data items took longer than others to access.

 

The first magnetic data stores used information stored on the surface of drums rather than disks. Drums were easier to design with the required tolerances and read/write heads could be provided for each track, staggered around the circumference. There was thus no head movement to control which made the maximum access time quite short – one revolution of the drum. With multiple heads, data could be transferred “word parallel” so data transfer rates could be quite high, compensating for the slow, non-random access.

 

The first drum development was started in the USA in 1946 with the founding of Engineering Research Associates operating out of St Paul, Minnesota. ERA performed research and development work for the US Navy (actually for cryptographic applications.) Two of the first working drums were delivered in 1948, but the first drum for a computer to use as main memory was for the Atlas computer delivered in 1950.

 

Magnetic drums were also researched at the University of Manchester in England and developed as a commercial product by Ferranti for their Mark 1 computer. The Manchester drums had a different use – rather than being the fastest memory they were used to supplement a real parallel memory and extend its capacity. They accessed data serially rather in parallel as it was more appropriate for this application.

 

3. THE FIRST HARD DISK

 

The history of magnetic disk storage for digital data has a very precise beginning. The first such storage was the result of an undertaking by IBM, from their new (1952) San Jose (California) laboratories to develop both a disk store, the IBM 350, and a computer system that was based around the use of on-line storage, the RAMAC (Random Access Method of Accounting and Control) in 1956. For the first 20 years of development of disks they were very much an IBM story. Nowadays there are others pushing the technology although IBM remains and important player.

 

The final IBM 350 disk storage had 50 24-inch diameter double sided disk platters on one spindle rotating at 1200 rpm. There was one dual-head mechanism that was inserted between disks to read the disk above or that below – the head had to be retracted and moved vertically to the appropriate gap when there was a change in the surface being read. One problem with large rotating disks is that they wobble (called run-out.) To get the head consistently near enough to the surface it was necessary to force it close using air under pressure pumped to the head. The head was stopped from contacting the disk surface by a jet of air from the head forming an “air bearing” that caused the head to float like a hydrocraft at 800 micro inches – 20 micro metres. This allowed the IBM 350 to store 100 bits per inch with tracks spaced at 20 per inch.

 

Dividing its capacity by its volume we can see that the first disk stored data at about the same density as the early drums, although of much greater capacity. Compared to a drum memory, which essentially has a 2 dimensional surface for storage, a disk device makes better use of space by stacking disks in 3 dimensions, thus disk storage was subject to more-intense development and improvement than drums, which has resulted in the close to a billion-fold improvement.

 

Electrons have spin as well as charge. This is of course the origin of ferromagnetism, and hence magnetic memories, but their miniaturization has been limited not by the ultimate size of a ferromagnetic domain but by the sensitivity of magnetic sensors. In other words, the main limitation has not been the ability to make very small storage cells, but the ability to detect very small magnetic fields.

 

The influence of spin on electron conductivity was invoked by Nevill Mott in 1936, but remained practically a least investigated and unexploited until the discovery of giant magnetoresistance (GMR) in 1988. The main present application of spintronics, is the development of ultrasensitive magnetic sensors for reading magnetic memories. Spin transistors, in which the barrier height is determined by controlling the nature of the electron spins moving across it, and devices in which logical states are represented by spin belong to the future. Giant magnetoresistance (GMR) is observed in thin (a few nanometres) alternating layers (super lattices) of ferromagnetic and nonmagnetic metals (e.g., iron and chromium). Depending on the width of the nonmagnetic spacer layer, there can be a ferromagnetic or antiferromagnetic interaction between the magnetic layers, and the antiferromagnetic state of the magnetic layers can be transformed into the ferromagnetic state by an external magnetic field. The spin-dependent scattering of the conduction electrons in the nonmagnetic layer is minimal, causing a small resistance of the material, when the magnetic moments of the neighbouring layers are aligned in parallel, whereas for the antiparallel alignment the resistance is high.

 

The technology is nowadays used for the read–write heads in computer hard drives. The discovery of GMR depended on the development of methods for making high-quality ultrathin films (such as molecular beam epitaxy). Magnetic sensor is based on the magnetic tunnel junction (MTJ), in which a very thin dielectric layer separates ferromagnetic (electrode) layers, and electrons tunnel through this non-conducting barrier under the influence of an applied voltage. The tunnel conductivity depends on the relative orientation of the electrode magnetizations and the tunnel magnetoresistance (TMR): it is low for parallel alignment of electrode magnetization and high in the opposite case. The magnetic field sensitivity is even greater than for GMR. MTJ devices also have high impedance, enabling large signal outputs. In contrast with GMR devices, the electrodes are magnetically independent and can have different critical fields for changing the magnetic moment orientation. The first laboratory samples of MTJ structures (NiFe–Al2O3–Co) were demonstrated in 1995.

 

4. PRIMARY STORAGE DEVICE:

 

Random Access Memory (RAM) and cache are both examples of a primary storage device. The image shows three different types of storage for computer data. Primary storage’s key differences from the others are that it is directly accessible by the CPU, it is volatile, and it is non-removable.

 

Fig: 2- Difference between secondary and primary storage devices.

 

 

5. MEMORY STORING MECHANISM:

 

Each head has a tiny electromagnet, which consists of an iron core wrapped with wire. The electromagnet applies a magnetic flux to the oxide on the media, and the oxide permanently “remembers” the flux it sees. During writing, the data signal is sent through the coil of wire to create a magnetic field in the core. At the gap, the magnetic flux forms a fringe pattern. This pattern bridges the gap, and the flux magnetizes the oxide on the media. When the data is read by the drive, the read head pulls a varying magnetic field across the gap, creating a varying magnetic field in the core and therefore a signal in the coil. This signal is then sent to the computer as binary data.

Fig: 3 – Sliding of read head over memory storing (magnetic domains) of hard disk.

 

6.  VARIOUS TYPES OF SECONDARY STORAGE DEVICES ARE AS FOLLOWINGS: –

 

  1. Winchester Disk: Another term for hard disk drive. The term Winchester comes from an early type of disk drive developed by IBM that had 30MB of fixed storage and 30MB of removable storage; so its inventors called it a Winchester in honor of its 30/30 rifle. Although modern disk drives are faster and hold more data, the basic technology is the same, so Winchester has become synonymous with hard.
  2.  Magnetic Drum: A direct-access, or random-access, storage device. A magnetic drum, also referred to as drum, is a metal cylinder coated with magnetic iron-oxide material on which data and programs can be stored. Magnetic drums were once used as a primary storage device but have since been implemented as auxiliary storage devices. The tracks on a magnetic drum are assigned to channels located around the circumference of the drum, forming adjacent circular bands that wind around the drum. A single drum can have up to 200 tracks. As the drum rotates at a speed of up to 3,000 rpm, the device’s read/write heads deposit magnetized spots on the drum during the write operation and sense these spots during a read operation. This action is similar to that of a magnetic tape or disk drive.
  3. Magnetic Tapes: The Magnetic Tapes is the Type of Secondary Storage Device and this Device is used for taking back up of data and this Tape contains some magnetic fields and the Magnetic Tapes are used Accessing the data into the Sequential Form and the Tape Also Contains a Ribbon which is coated on the Single Side of the Tape and also contains a head which reads the data which is Recorded on to the Tape.
  4. Optical Disks: The Optical Disks are also called as the CD-ROM’s means Compact Disk Read Only Memory which is also used for Storing the data into the Disk and this is called as the Optical Disk because the CD-ROM ‘s are made up of the Golden or Aluminum Material and the data is Stored on the Disk in the Form of the Tracks and Sectors. The Whole Disk is Divided into the Number of Tracks and the Single Track is Divided into the Number of Sectors and the Data is Stored into the Sectors.Fig: 4 – Magnetic Disk Drive and Thin Film. (Bharat Bhushan, Springer, Handbook of Nanotechnology)
  5. Magnetic Disks: – This is also called as the hard disk and this is made from the thin metal platter which is coated on the both sides of the magnetic Disks. Platters into a single Hard Disk are made from the magnetic materials. Disks are rotated from the 700 to 3600 rpm means. The Hard Disk also contains a head which is used for both Reading and Writing the Data.

 

The Disk is first divided into the Number of Tracks and the Tracks are further divided into the sectors and the Number of Tracks Makes a Cylinder. All the data is Stored into the disk by using Some Sectors and each sectors belongs to a Tracks. The Data is accessed from the Disk by using the heads, all the heads have Some Arm those are used for Reading the Data from the Particular Tracks and sector. For reading the data from the Disk the arm touches with the particular Track and read the data from that location.

 

7.  ADDRESSES, SEEK TIME, LATENCY TIME, DATA TRANSFER TIME:

 

  1. Address means the Number of Cylinder, Number of Track and Number of Sectors from which user wants to read the data. With the help of these Read and Write heads we can also Read the Data from the Disk and we can also store some data onto the Disk.
  2. Seek Time: – The Total Time which is Taken to Move on the Desired track is known as the seek Time. And time is always measured by using the Milliseconds.
  3. Latency Time: – The time required to Bring the Particular Track to the Desired Location Means the Total Time to bring the Correct the Sector for Reading or for the read and Write head. This is also called as the Average Time.
  4. Data Transfer Time: The Total Time which is required for Reading and Writing the data into the Disk is known as the Data transfer Time.

 

8.  ANALOG RECORDING & DIGITAL RECORDING:

 

Analog recording is based on the fact that remnant magnetization of a given material depends on the magnitude of the applied field.

 

Instead of creating a magnetization distribution in analog recording, digital recording only needs two stable magnetic states, which are the +Ms and -Ms on the hysteresis loop. Examples of digital recording are floppy disks and hard disk drives (HDDs). Digital recording has also been carried out on tapes.

 

9. MAGNETIC TAPE SOUND RECORDING:

 

The magnetic material is normally in the form of tape, with the tape in its blank form being initially demagnetized. When recording, the tape runs at a constant speed. The writing head magnetizes the tape with current proportional to the signal. A magnetization distribution is achieved along the magnetic tape.

 

Finally, the distribution of the magnetization can be read out, reproducing the original signal. The commonly used magnetic particles are Iron oxide particles or Chromium oxide and metal particles with size of 0.5 micrometers. Analog recording was the most popular method of audio and video recording.

 

10. MAGNETO-OPTICAL RECORDING:

 

Magneto-optical recording writes/reads optically. When writing, the magnetic medium is heated locally by a laser, which induces a rapid decrease of coercive field. Then, a small magnetic field can be used to switch the magnetization. The reading process is based on magneto-optical Kerr effect. The magnetic medium is typically amorphous R-FeCo thin film (R being a rare earth element). Magneto-optical recording is not very popular.

 

11. DOMAIN PROPAGATION MEMORY:

 

Domain propagation memory is also called bubble memory. The basic idea is to control domain wall motion in a magnetic medium that is free of microstructure. Bubble refers to a stable cylindrical domain. Data is then recorded by the presence/absence of a bubble domain. Domain propagation memory has high insensitivity to shock and vibration, so its application is usually in space and aeronautics.

Fig: 5 – The behavior of a multidomain material in an increasing magnetic field (top), the associated changes in the magnetization (bottom left) and a typical magnetic hysteresis loop (bottom right).

 

In a crystalline material, the magnetic properties depend up on the direction of magnetization. This is known as magnetocrystalline anisotropy. Magnetocrystalline anisotropy originates from the spin orbit interactions of the electrons. The magnetic moments are directed towards certain directions in the crystal lattice, called the easy directions and it is easy to attain saturation magnetization when the magnetic field is applied along these directions of a crystal. Along the hard directions, very large fields are required for saturation. Very large magnetic fields are required to rotate the moments away from the easy axis, and therefore, higher negative fields are required to demagnetize the material.

 

Therefore, high magnetocrystalline anisotropies give rise to larger coercivities.

 

When a ferromagnet is cooled below its TC, in the absence of an applied field, there develops a magnetization by the exchange coupling and this is called spontaneous magnetization.

 

In a domain, all groups of spins point in the same direction and act cooperatively. In larger particles, it is the energetic considerations which favor the formation of the magnetic domains. Such particles are called multi-domain particles. Individual domains are separated by regions called domain walls with a characteristic width. The directions of magnetization of the various domains are such that the specimen as a whole has no net magnetization.

 

In the case of the multi-domain ferromagnetic particles, magnetization occurs through the nucleation and motion of these walls. The role of an applied magnetic field is to convert a multi-domain specimen to a single domain state, magnetized in the direction of the applied field.

 

In a typical M-H curve of a ferro- or ferrimagnet, magnetization at low fields increases due to the movement of the domain wall.

 

As a result, domains which are favorably oriented to the direction of the applied field, grow at the expense of others. At still higher fields, changes in the magnetization occur by domain rotation, leading to saturation of magnetization. When the temperature increases, the saturation magnetization of a ferromagnetic substance decreases and at TC the domain structure is collapsed, so that the material will act as a paramagnetic substance above TC.

 

Magnetism in solids arises from the magnetic ions or atoms distributed throughout a regular crystalline lattice on equivalent sites. The cooperative magnetism in solids, arising from the interaction (coupling) between the atomic moments, is more complex than that of the isolated atoms. Because of the alignment of the magnetic moments across many unit cells in a solid, ferromagnetic, ferrimagnetic and antiferromagnetic materials are said to exhibit “long-range ordering” of the moments in the lattice. For ferro- and ferrimagnetic substances, the field dependence of magnetization is non– linear and at large values of H, the magnetization M becomes constant at its saturation value Ms. But once saturated, a decrease in H to zero does not reduce M to zero. Hence, it possesses some magnetization, called remnant magnetization (Mr). In order to demagnetise the substance after saturation, a reverse field is required. The magnitude of this field is called coercivity (Hc). The M-H curve in the case of Ferro and ferrimagnets is called the magnetic hysteresis loop.

 

12. SOLIDS CLASSIFIED INTO TWO MAIN CATEGORIES:

 

  1. Crystalline materials with complete long-range order,
  2. Amorphous materials with short-range order.

 

Amongst the nanometer-sized crystalline materials there are long-range order and short-range order which can also coexist. These materials are polycrystalline, in which the size of the individual crystallite or particle is in the order of 1–100 nm.

 

13.  NANOSTRUCTURED MATERIALS AND ITS TWO COMPONENTS:

 

  1. A crystalline component, formed by all atoms located inside the crystallites or particles and an interfacial component comprising all the atoms, which are situated in the boundaries between the crystallites or in the interface (surface).
  2. The interfacial component is proposed to represent solid-state structures without long- or short-range order.

 

Nanostructured magnetic materials exhibit interesting and unexpected properties due to their very high surface area to volume ratio. It is also important to mention that the percentage of atoms at the surface of the material becomes significant. The morphology of the nanoparticles can be varied with different specialized synthetic routes. Similarly, the magnetic particles can also be made in to different shapes, such as Nano spheres, Nano rods, nanocubes, nanotubes, nanowires, nanoribbons or nanoplates.

 

14. MAGNETIC NANOSTRUCTURES AND THEIR PROPERTIES AND MORPHOLOGIES:

 

Fig : 6 – Morphologies and properties of magnetic nanostructures.

 

The morpholigies are as follows:-

 

1.      2- dimensional crystalline lattice of magnetic ions containing large number of unit cells.

2.        Smaller crystalline or nanoparticles containing fewer unit cells.

3.      A magnetic nanoparticles consisting of a finite number of atoms or ions.

4.        Ideal non- interacting magnetic nanoparticles.

5.      Magnetic nanoparticles surrounded by non-magnetic shells.

6.      Surfactant coated magnetic nanoparticles.

7.      Patterned array of magnetic nanoparticles.

8.      Magnetic nanocomposites in a nonmagnetic matrix.

9.      Magnetic nanoclusters in crystalline matrix

 

 

(a). Non-magnetic atom/ion.

 

(b). Magnetic atom/ion.

 

 

15.  SUMMARY

 

  1. Magnetic data storage has become more efficient with the advent of time and advancements in technologies.
  2. The first magnetic data stores used information stored on the surface of drums rather than disks. Drums were easier to design with the required tolerances and read/write heads could be provided for each track, staggered around the circumference.
  3. The first such storage was the result of an undertaking by IBM, from their new (1952) San Jose (California) laboratories to develop both a disk store, the IBM 350, and a computer system that was based around the use of on-line storage, the RAMAC (Random Access Method of Accounting and Control) in 1956.
  4. The various types of Secondary Storage Devices are Winchester Disk, Magnetic Drum, Magnetic Tapes, Optical Disks, Magnetic Disks etc.
  5. Some important parameters for data storage are Addresses, Seek Time, Latency Time, Data Transfer Time.
  6. The magnetic material is normally in the form of tape, with the tape in its blank form being initially demagnetized.
  7. Digital recording: Instead of creating a magnetization distribution in analog recording, digital recording only needs two stable magnetic states, which are the +Ms and -Ms on the hysteresis loop.
  8. Magneto-optical recording writes/reads optically. When writing, the magnetic medium is heated locally by a laser, which induces a rapid decrease of coercive field.
  9. Domain propagation memory is also called bubble memory.
  10. A high magnetocrystalline anisotropies give rise to larger coercivities.
  11. Solids may be classified into two main categories:

 

1.  Crystalline materials with complete long-range order,

 

2.    Amorphous materials with short-range order.

 

12.  Nanostructured magnetic materials exhibit interesting and unexpected properties due to their very high surface area to volume ratio.

13. Magnetic nanostructures can be divided into different groups based on their properties and morphologies as

  1. 2- dimensional crystalline lattice of magnetic ions cantaining large number of unit cells.
  2. Smaller crystalline or nanoparticles containing fewer unit cells.
  3.  magnetic nanoparticles consisting of a finite number of atoms or ions.
  4. Ideal non- interacting magnetic nanoparticles.
  5. Magnetic nanoparticles surrounded by non-magnetic shells.
  6. Surfactant coated magnetic nanoparticles.
  7. Patterned array of magnetic nanoparticles.
  8. Magnetic nanocomposites in a nonmagnetic matrix.
  9. Magnetic nanoclusters in crystalline matrix

 

(a). Non-magnetic atom/ion.

 

(b). Magnetic atom/ion.

 

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