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Magnetic Properties of Materials: Detailed Explanation

According to Ampere’s law, when current flows through a wire or solenoid then magnetic field is produced around it. The magnetism produced by an atom can be due to 2 types of motion of electrons; spin and orbital motion. The net magnetic field created by the electrons in an atom is due to the combined effect of orbital and spin motions. Either these 2 will reinforce each other or will cancel each other. Magnets attract a variety of elements, which are collectively referred to as magnetic materials. These are the same materials that can be magnetized to make permanent magnets. Magnetic materials can be classified as either magnetically hard or magnetically soft. Magnetic properties of materials are discussed here.

What are soft magnetic materials?

Magnetically soft surfaces can be magnetized quickly, but the induced magnetism is typically temporary. When you rub a permanent magnet over a nail, the nail becomes temporarily magnetized and emits its own weak magnetic field. This is due to the fact that a large number of their iron atoms are aligned in the same direction at the same time. But such materials are magnetized for very short term, as you will remove external field, they will return back to their de-magnetized state.

Magnetic Properties of Materials: Detailed Explanation

What are hard magnetic materials?

Hard magnetic materials can be magnetized by a strong external magnetic field, such as those generated by an electromagnet. The difference being that magnetically hard materials will remain magnetized indefinitely, unless they are demagnetized by an opposing magnetic field, raised above their curie temperature. Magnetically hard materials are used to create permanent magnets. Permanent magnets are difficult to magnetize as unlike magnetically soft materials their atomic magnetic domains aren’t easily aligned but once they are aligned, they remain so indefinitely.

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Classification of magnetic materials

Although the term “magnetic materials” is usually reserved for ferromagnetic materials, but actually there are 5 types of magnetic materials based on their properties.

  • Ferromagnetic materials
  • Paramagnetic materials
  • Diamagnetic materials
  • Ferromagnetic materials
  • Anti-ferromagnetic materials

Magnetic Properties of Materials

What are ferromagnetic materials?

Magnetic Properties of Materials: Detailed Explanation

Ferromagnetism is the basic mechanism by which materials like iron become permanent magnets. This means the compound shows permanent magnetic properties rather than exhibiting them only in the presence of a magnetic field. These are the materials which are strongly attracted by a magnetic field or magnet. In a ferromagnetic element, electrons of atoms are grouped into domains in which each domain has the same charge. In the presence of a magnetic field, these domains line up so that charges are parallel throughout the entire compound. They are excellent conductors of electricity as they have free available electrons.

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Ferromagnetic materials have some unpaired electrons in their atoms and therefore generate a net magnetic field, although very weak. This is because the individual atoms or groups of atoms, known as magnetic domains, are randomly aligned thus cancelling each other out. When an external magnetic field is applied to the ferromagnetic material, the individual domains are forced into alignment which they maintain, once the external field is removed therefore maintaining their magnetism, known as remanence. Iron, nickel and cobalt are all ferromagnetic materials.

Magnetic Properties of Materials: Detailed Explanation

The permeability of these materials is very high. The opposite magnetic effects of electron orbital motion and electron spin do not eliminate each other in an atom of such a material.

Soft ferromagnetic materials

They have a high relative permeability, a low persuasive power, are easily magnetized and demagnetized and have a low hysteresis loss. Iron and its various alloys of nickel, cobalt, tungsten and aluminum are soft ferromagnetic materials. Soft ferromagnetic materials have a large magnetic moment at room temperature, making them ideal for magnetic circuits.

Magnetic Properties of Materials: Detailed Explanation

Hard ferromagnetic materials

They have a low permeability but a powerful coercive force. It’s difficult to magnetize and de-magnetize these. Cobalt steel and various ferromagnetic alloys of cobalt, aluminum and nickel are examples of hard ferromagnetic materials. They have a high hysteresis loss and maintain a high percentage of their magnetization. They are ideal for use as permanent magnets in microphones, measuring instruments, and other applications.

What are ferrites?

Ferrites are a type of ferromagnetic material that falls somewhere between ferromagnetic and non-ferromagnetic. They are made up of extremely fine particles of a high-permeability. Ferrites contain enough magnetization to be commercially valuable, but their magnetic saturation is not as high as ferromagnetic materials. Ferrites also come in soft and hard varieties.

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What are paramagnetic materials?

Magnetic Properties of Materials: Detailed Explanation

Paramagnetic is a term that is used to describe a type of magnetic materials which cannot retain its magnetic properties until the external magnetic field is removed, unlike ferromagnetic materials. Although most elements are paramagnetic, their attractive force is thousands of times weaker than that of ferromagnetic material, so they are classified as “non-magnetic.” When external magnetic field is applied, they align their domains in the direction of applied field and are demagnetized as external field is removed.

What is Curie temperature?

It is the value of temperature at which a material losses its alignment of domains and thus it loses its ferromagnetic behavior, above this temperature material is paramagnetic.

What are diamagnetic materials?

Magnetic Properties of Materials: Detailed Explanation

An externally applied magnetic field repels diamagnetic materials. When stimulated by a magnetic field, their magnetic domains oppose the external magnetic field. Since all of the electrons in diamagnetic materials’ atoms are paired, they do not produce their own net magnetic field. There are no unpaired electrons in diamagnetic compounds. The electron spins are directed in opposite direction to the external field. As a result, the magnetic fields of the electrons cancel out, leaving no net magnetic moment and preventing the atom from being drawn to a magnetic field.

What are ferri-magnetic materials?

A magnetic material with magnetic moments of unequal magnitude and opposite direction is ferri-magnetic. The magnetic moments of ferromagnetic materials are aligned antiparallel to each other thus the net magnetic moment is non-zero. They show spontaneous magnetization to the external field.

Magnetic Properties of Materials: Detailed Explanation

What are anti-ferromagnetic materials?

Anti-ferromagnetic materials are the magnetic materials that behave as tiny magnets spontaneously as they can align their domains at low temperatures in the opposite direction to the external field. In these materials, net magnetic moment is cancelled out between atoms aligned in one direction and the atoms that are aligned in the reverse direction. Their antiparallel coupling is disturbed by heating and above Neel’s temperature, it completely disappears. At low temperatures, these materials don’t respond to the external magnetic field as the antiparallel ordering is maintained rigidly. However at higher temperatures, atoms can break bonds and become free thus they show a weak net magnetic moment. They have maximum net magnetic moment at Neel’s temperature. Above this value of temperature, thermal agitation starts and it prevents the alignment of atoms with respect to magnetic field. Hence alignment of atoms decreases by the increase in temperature.

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