Question

Describe the working of magnetic hard disc based on GMR sensor.

Mention its advantages and disadvantages.​

Answer 1

Answer:

Giant magnetoresistance (GMR) is a quantum mechanical magnetoresistance effect observed in multilayers composed of alternating ferromagnetic and non-magnetic conductive layers. The 2007 Nobel Prize in Physics was awarded to Albert Fert and Peter Grünberg for the discovery of GMR.

The effect is observed as a significant change in the electrical resistance depending on whether the magnetization of adjacent ferromagnetic layers are in a parallel or an antiparallel alignment. The overall resistance is relatively low for parallel alignment and relatively high for antiparallel alignment. The magnetization direction can be controlled, for example, by applying an external magnetic field. The effect is based on the dependence of electron scattering on spin orientation.

The main application of GMR is in magnetic field sensors, which are used to read data in hard disk drives, biosensors, microelectromechanical systems (MEMS) and other devices.[1] GMR multilayer structures are also used in magnetoresistive random-access memory (MRAM) as cells that store one bit of information.

In literature, the term giant magnetoresistance is sometimes confused with colossal magnetoresistance of ferromagnetic and antiferromagnetic semiconductors, which is not related to a multilayer structure.[2][3]



The founding results of Albert Fert and Peter Grünberg (1988): change in the resistance of Fe/Cr superlattices at 4.2 K in external magnetic field H. The current and magnetic field were parallel to the [110] axis. The arrow to the right shows maximum resistance change. Hs is saturation field.[note 

Answer 2

Working of Magnetic hard disc based on GMR sensor:

The giant magneto resistive phenomenon, discovered in 1988, is an effect found in metallic thin films consisting of magnetic layers a few nanometer thick separated by equally thin nonmagnetic layers. Researchers observed a large decrease in the resistance with a magnetic field applied to the films. This effect is based partly on the increasing resistivity of conductors as their thickness decreases to a few atomic layers. In bulk material form, conduction electrons in these materials can travel along distance before “scattering,” or changing direction, due to a collision with another atomic particle. The average length that the electron travels before being scattered is called the mean free path length. However, in materials that are very thin, an electron cannot travel the maximum mean free path length; it is more likely that the electron will reach the boundary of the material and scatter there, rather than scatter off another atomic particle. This results in a lower mean free path length for very thin materials. It is therefore more difficult for conduction electrons to travel in this material, and the result is higher electrical resistivity.

Advantages:

• GMR sensors can operate at significantly higher areal densities than MR sensors.
• Their percent change in resistance is greater, making them more sensitive to magnetic fields from the disc.

Disadvantages:

• Non-linearity
• Hysteresis
• Offset
• Temperature dependent output that can reduce measurement accuracy
• In addition, the output of some of GMR sensors is unipolar, which limits its application in AC measurements

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