Soft and hard magnetic materials, Physics

Magnetic materials are divided in terms of their magnetic properties and their applications. If a material is effortlessly magnetized and demagnetized, then it is known to as a soft magnetic material whereas if it is hard to demagnetize then it is known to be hard (or permanent) magnetic materials. Materials in between hard and soft are almost exclusively used as recording media and have no other general term to describe them. Other classifications for types of magnetic materials are subsets of soft or hard materials, such as magnetostrictive materials.

SOFT MAGNETIC MATERIALS: Soft magnetic materials are simple to magnetize and demagnetize. Hence they require relatively low magnetic field intensities. The hysteresis loops are highly narrow i.e. has little area. So hysteresis power loss per cycle is small. Soft magnetic materials are typically suitable for applications where repeated cycles of magnetization and demagnetization are involved such as in electric motors, transformers and inductors where the magnetic field varies cyclically. Properties of soft magnetic materials:

(1)         High permeability and low coercive force,

(2)         Easy magnetization and demagnetization.

(3)         Low residual magnetism.

(4)         Small area of hysteresis loop and

(5)         High resistivity.

APPLICATIONS: They are used for construction of core plates of transformers, electromagnets, electric machines, reactors, etc. Examples are pure iron, iron silicon alloys, nickel iron alloys, soft ferrites (these are non-metallic compounds consisting of ferric oxide and one or two bivalent metal oxides such as, gunmetal (this is alloy of iron, Nickel, Copper and Chromium).

HARD MAGNETIC MATERIALS: Hard magnetic materials are hard to magnetize and demagnetize. Hence they require large magnetic field intensities. Their B_H curves (Hysteresis loop) are board and almost rectangular. They posses' relatively large coercivities which mean that they need large applied fields to get demagnetized. The coercive field for hard magnetic materials can be a millions of times greater than those for soft magnetic materials. It is obvious that hard magnetic materials can also be utilized in magnetic storage of digital data.

Properties of hard magnetic materials:

(1)         Gradually rising magnetization curve,

(2)         Large area of hysteresis loop and hence large losses,

(3)         High value of BH product,

(4)         High receptivity,

(5)         High coercively,

(6)         Low permeability, and

(7)         High magnetization force is required to attain magnetic saturation

APPLICATIONS:

They are useful in making permanent magnets. They also find application in miniature electronic devices such as printers, electron beam focussing assemblies, magnetic bearings, loudspeakers, microwave devices, travelling wave tubes, magnetron tubes, etc.

EXAMPLES: Rare earth cobalt, tungsten steel, cobalt steel, Alnico (it is a family of alloys contain metals Fe, Co, and Ni plus small amount of Al and Cu), hard ferrites, etc.

Posted Date: 7/7/2012 8:24:52 AM | Location : United States







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