1. Magnetic Properties and Superconductivity
Lecture 8
Magnetic Properties and Superconductivity
Md Arafat Hossain
Outlines

2. Soft and Hard Magnetic Materials
Based on their B-H behavior, engineering materials are typically classified into soft and hard magnetic materials
Soft magnetic materials are easy to magnetize and demagnetize and hence require relatively low magnetic field intensities. Put differently, their B-H loops are narrow (Fig. green).
Significance and applications
The hysteresis loop has a small area, so the hysteresis power loss per cycle is small.
Suitable for applications where repeated cycles of magnetization and demagnetization are involved (e.g. electric motors, transformers, and inductors).
These applications also require low hysteresis losses, or small hysteresis loop area.
Electromagnetic relays that have to be turned on and off require the relay iron to be magnetized and demagnetized and therefore need soft magnetic materials

3. Soft and Hard Magnetic Materials
Hard magnetic materials, are difficult to magnetize and demagnetize and hence require relatively large magnetic field intensities. Their B-H curves are broad and almost rectangular (Fig. pink).
Significance and applications
Relatively large coercivities (millions of times greater), which means that they need large applied fields to be demagnetized.
useful as permanent magnets in a variety of applications.
As the coercivity is strong, both the states +Br and -Br persist until a suitable (large) magnetic field intensity
switches the field from one direction to the other.
Can also be used in magnetic storage of digital data, where the states +Br and -Br can be made to represent 1 and 0 (or vice versa).

4. Initial and Maximum Permeability
Relative permeability is useful to compare various materials and even use it in various calculations.
Maximum permeability
Initial permeability
But it is not constant….
rather slope of the B-H curve that passes through origin O
Permeability is maximum when the line becomes a tangent to the B-H curve at P.
Maximum relative permeability….this is also know as ‘knee’ point of the B-H curve
Represents the initial slope of the initial
B-H curve as the material is first magnetized from an unmagnetized state.
This definition is useful for soft magnetic materials that are employed at very low magnetic fields {e.g., small signals in electronics and communications engineering). In practice, weak applied magnetic fields where \xri is useful are typically less than 10"4 T. In contrast, Mr.max is useful when the magnetic field in the material is not far removed from saturation. Initial relative permeability of a magnetically soft material can vary by orders ofmagnitude. For example, /xr/ for iron is 150, whereas for supemumetal-200, a commercial alloy of nickel and iron, it is about 2 x 105.

5. Soft Magnetic Materials: Examples and Uses

6. Hard Magnetic Materials: Examples and Uses

7. Hard Magnetic Materials: Chaterteristics
Very large coercivity (Hc) and remanent magnetic field (Br).
Since they are used as permanent magnets, the energy stored per unit volume in the external magnetic field should be as large as possible since this is the energy available to do work.
Maximum value of the product BH in the second quadrant of the B-H characteristics and is denoted as (BH)max

8. When the size of a ferromagnetic sample falls below a certain critical dimension, of the order of 0.1 μm for Co, the whole sample becomes a single domain – single domain fine particles.
Source of hard magnetism
High magnetocrystalline anisotropy energy
Shape anisotropy
A single domain elongated particle.
Due to shape anisotropy, magnetization prefers to be along the long axis as in (a). Work has to be done to change M from (a) to (b) to (c).