Question

a. Explain why magnetic materials that are to be used in information storage should have either square or rectangular-shaped hysteresis loops.

b. Why cannot pure iron metal be used in transformer cores?

Answer 1

Solution:

a. Magnetic materials that are to be used in information storage should have either square or rectangular-shaped hysteresis loops because,

1. Magnetic materials are used for data storage. Memory is stored by magnetizing the material in a certain direction. For example, if the north pole is up, the bit of information stored is 1. If the north pole is down, then a0 is stored. For this application the materials with a square hysteresis loop, a low remanance, alow saturation magnetization, and a low coercive field are preferable.

2. Hard ferrites based on Ba, CrO₂, acicular iron particles, and γ-Fe₂O₃ satisfy these requirements. The stripe on credit cards and bank machine cards are made using γ-Fe₂O₃ or Fe₃O₄ particles. The square loop ensures that a bit of information placed in the material by a field remains stored; a steep and abrupt change in magnetization is required to remove the information from storage in the ferromagnet. Furthermore, the magnetization produced by small external fields keeps the coercive field (Hc), saturation magnetization, and remanance (Br) low.

3. Many new alloys based on Co-Pt-Ta-Cr have been developed for the manufacture of hard disks. Computer hard disks are made using sputtered thin films of these materials. Also many different alloys, such as those based on nanostructured Fe-Pt and Fe-Pd, are being developed for data storage applications. More recently, a technology known as spintronics(spin-based electronics) has evolved. In spintronics, the main idea is to make use of the spin of electrons as a way of affecting the flow of electrical current (known as spin-polarized current) to make devices such as field effect transistors(FET).

4. The spin of the electrons (up or down) is also being considered as a way of storing information. A very successful example of a real-world spintronic-based device is a giant magneto resistance (GMR) sensor that is used for reading information from computer hard disks.

b. Pure iron metal cannot be used in transformer cores because,

1. A material with higher permeability and less conductivity is chosen for the transformer core. High permeability so that it can easily allow the magnetic field through it and low conductivity so that the strength of Eddy current is less and hence losses are less. Such materials are iron, steel (steel has better permeability than iron) , it is ferromagnetic material and have high permeability, but it also have good conductivity hence to decrease the conductivity 4- 5 % of silicon is added to it , hence the material is called silicon steel.

2. Transformers feature primary, secondary and tertiary windings, whose working depends on the flux that is produced between these windings. Magnetic cores in the transformers serve as the pathway of the flux. These cores are magnetic materials with high magnetic permeability used to guide magnetic fields in transformers and other electrical or magnetic devices. They are made of ferromagnetic metals or compounds.

3. Pure iron metal have more conductivity and low permeability due to its pure characteristics. Such material is not effective in transformer because it cannot reduces current intensity with increase in conductivity of the current. Due to this such transformers are not useful in any applications.

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