Magnetic Hard Disk Drive [HDD] with GMR Sensor

MAGNETIC HARD DISK DRIVE [HDD] WITH GMR SENSOR

Introduction

Hard disk drives [HDD] made up of magnetic garnet materials are called magnetic hard disk drives. It is a mass data storage device recently used for storing data to a very high level in terms of Tera bytes.

Now-a-days GMR [Giant Magneto Resistive] sensors, which has a very high magnetic sensitivity are used to read the data at greater speed.

Principle

In Hard disk drives, the binary data in terms of zero's (0) and one's (1) are stored by inducing magnetic moment in a thin magnetic layer and GMR effect is used as the principle to read the data in HDD.

Here zero (0) represents missing transition and one (1) represents transition in the medium.

Construction

The HDD consists of recording medium madeup of thin layer of magnetic garnets grown over the substrate The GMR sensor, which is made up of ferrites and anti ferromagnetic materials is used as reading element. The writing element is made up of inductive magnetic transducer.

The writing element and the GMR sensor shall be made to slide over the recording media in the longitudinal direction as shown in Fig. 3.40. Hence this method is also called as longitudinal recording.

The flow of current through the GMR sensor and writing element shall be adjusted and inturn the magnetization is sensed (or) controlled in the recording media.

Working

Writing / Storing

1. Initially the current is passed through the writing element and a magnetic field is induced in between the gap of the inductive magnetic transducer.

2. During writing, the amplitude of current is kept constant, and the direction of current is reversed.

3. Due to reversal of current, the magnetization orientation is reversed in the recording medium i.e., from South → North as shown in Fig. 3.40.

4. When the induced magnetic field is greater than the coercivity of the recording media, then data is recorded in the form of 1.

5. Thus one (1) is stored as data in the recording medium as a magnetic transition.

6. When there is no magnetic transition, then it is referred as zero (0).

7. In this way the zero's (0's) and one's (1's) are stored in the recording medium.

Reading/Retrieving

8. Giant magnetoresistive (GMR) effect is the principle used to read/retrieve the data from the recording medium.

9. When the GMR sensor is made to move near the recorded medium, then, the resistance of the GMR sensor varies, with respect to the orientation of the magnetic moments as follows.

10. When the layers are magnetised in parallel manner, then the resistance in the GMR sensor is minimum and therefore maximum current flows through the sensor, which represents the data as one (1), as shown in Fig. 3.41.

11. When the layers are magnetised in antiparallel manner, then the resistance in the GMR sensor will be maximum and therefore minimum (or) no current will flow through the sensor, which represents the data as zero (0), as shown in Fig. 3.41.

12. Therefore, with the help of the reading current, the zero's (0's) and one's (1's) can be retrieved from the magnetic hard disk drive.

Note: Recently spin valve sensors, which consists of GMR trilayer (Two ferromagnetic layers, separated by one non-metallic inter layer), adjacent to an antiferromagntic layer are used as reading element for superior magnetic sensitivity and better performance.

Advantages

1. HDD's can store data interms of Terabytes and in future it can store data interms of Petabytes and Exabytes.

Note: 1 Terabyte = 1024 Gigabytes

1 Petabyte = 1024 Tetrabytes

1 Exabytes = 1024 Petabytes

2. It is has very large storage capacity.

3. It is compact in size and can be easily transferred from one place to another.

4. The size of the recording medium in HDD shall be reduced upto few nano-metres using nanotechnology.

5. GMR sensors are non diffusive and are very sensitive in reading.

Disadvantages

1. HDD is slower than SSD [solid state drives].

2. They consume more power and will damage, when dropped even at a smaller distance.

3. Sometimes the data in HDD may be corrupted, due to thermal radiation.

4. HDD has bulkier form factor.

5. The GMR noise ratio is high for the nano size recording media as it is temperature dependent.

Applications

1. HDD's are used as storage devices in cloud applications.

2. They are used in coding and signal processing units.

3. It is used in many Engineering fields such control systems, nanoelectronics, etc.

4. The GMR and spin valve sensors are used in modern HDD by IBM.