Magnetic Properties of Matter, Physics tutorial

Introduction:

Magnetism is the class of physical phenomena which are mediated through magnetic fields. Electric currents and the basic magnetic moments of elementary particles give mount to a magnetic field that acts on other currents and magnetic moments. All the materials are affected to some degree through a magnetic field. The most well-known consequence is on permanent magnets that contain persistent magnetic moments caused by means of ferromagnetism. Most of the materials don't have permanent moments. A few are attracted to a magnetic field (that is, paramagnetism); others are repulsed through a magnetic field (that is, diamagnetism); others encompass a much more complex relationship by an applied magnetic field (spin glass behavior and antiferromagnetism). The substances which are negligibly influenced by magnetic fields are termed as non-magnetic substances. They comprise copper, aluminum, gases and plastic. Pure oxygen shows magnetic properties whenever cooled to a liquid state.

The magnetic state (or stage) of a material based on temperature (and other variables like pressure and the applied magnetic field) in such a way that a material might represent more than one form of magnetism based on its temperature and so on.

Basic concepts of Magnetic Properties of Matter:

Magnetic Dipoles:

The magnetic dipole is the limit of either a closed loop of electric current or a pair of poles as the dimensions of the source are decreased to zero whereas keeping the magnetic moment constant. This is a magnetic analogue of the electric dipole; however the analogy is not complete. In specific, a magnetic monopole, the magnetic analogue of an electric charge, has never been noticed. Furthermore, one form of magnetic dipole moment is related by a basic quantum property-the spin of elementary particles.

Magnetic Field Vectors:

The externally applied magnetic field, at times termed as the magnetic field strength, is designated by H. When the magnetic field is produced by means of a cylindrical coil (or solenoid) comprising of 'N' closely spaced turns, containing a length 'l', and carrying a current of magnitude 'I', then:

H = NI/l

Magnetic induction:

The magnetic flux density or magnetic induction, represented by B, represents the magnitude of the internal field strength in a substance which is subjected to an H field. The units for B are teslas (wb/m2) Both B and H are field vectors, being characterized not just through magnitude, however as well through direction in space.

The magnetic field strength and flux density are associated according to:

B = μH

Magnetization and Susceptibility:

The Magnetic susceptibility is a measure of the capability of a material to be magnetized. The proportional constant links the magnetization to the applied magnetic field intensity.

The H-field within a long solenoid is nI. If there is a vacuum within the solenoid, the B-field is µoH = µo nI. If we put an iron rod of permeability 'µ' within the solenoid, this does not change H that remains nI. The B-field, though, is now B = µH. This is greater than µoH, and we can write;

B = µo (H + M)

The quantity 'M' is termed as the magnetization of the material. In SI units, it is represented in A m-1. We are familiar that there are two components to B that is, µoH = µo nI, which is the externally imposed field and the component µoM, originating as an outcome of somewhat that has happened in the material.

This might have happened to you that you would have preferred to state the magnetization from B = µoH + M, in such a way that the magnetization would be the surplus of B over µoH. The equation B = µoH + M, would be equivalent to the familiar D = εoE + P, and the magnetization would then be deduced in tesla instead of A m-1. This point of view does certainly have much to commend it, however so does B = µo (H + M). The latter is the suggested statement in the SI approach, and that is what we will use here.

The ratio of magnetization 'M' (the result) to 'H' (the cause) that is apparently a measure of how susceptible the material is to becoming magnetized, is termed as the magnetic susceptibility χm of the material:

 M = χm H

On merging this with the equation B = µo (H + M) and B = µH, we readily observe that the magnetic susceptibility (that is dimensionless) is associated to the relative permeability µr = µ/µo by:

 µr = 1 + χm

Diamagnetism:

Diamagnetism is an extremely weak form of magnetism which is nonpermanent and persists just while an external field is being applied. This is induced through a change in the orbital motion of electrons due to an applied magnetic field. The magnitude of induced magnetic moment is very small and in a direction opposite to that of the applied field. Therefore, the relative permeability is less than unity (though, merely very slightly) and the magnetic susceptibility is negative; that is, the magnitude of the 'B' field in a diamagnetic solid is less than that in the vacuum. The volume susceptibility for diamagnetic solid materials is on the order of whenever positioned among the poles of a strong electromagnet; diamagnetic materials are attracted toward areas where the field is weak.

Paramagnetism:

Diamagnetism forms itself obvious in atoms and molecules which have no permanent magnetic moment. A few atoms or molecules, though, do encompass a permanent magnetic moment and such materials are paramagnetic. They should still be diamagnetic, however often the paramagnetism will outweigh the diamagnetism. The magnetic moment of an atom of the molecule is usually whenever order of a Bohr magneton. The presence of a permanent magnetic moment is frequently the outcome of unpaired electron spins. An illustration frequently quoted is the oxygen molecule O2. Liquid oxygen certainly is paramagnetic. If a paramagnetic material is positioned in a magnetic field, the magnetic moments experience a torque and they tend to orient themselves in the direction of the magnetic field, therefore augmenting, instead of diminishing, 'B'. Unsurprisingly the effect is greatest at low temperatures, where the arbitrary motion of atoms and molecules is low. At liquid helium temperatures of order 1 K, susceptibilities can be of order +10-3 or +10-2, therefore greatly exceeding the small negative susceptibility. At room temperature, paramagnetic susceptibilities are much less - typically around +10-5, barely beyond the diamagnetic susceptibility.

Ferromagnetism:

A ferromagnet, similar to a paramagnetic substance, comprise of unpaired electrons. Though, in addition to the electron's intrinsic magnetic moment's tendency to be parallel to the applied field, there is as well in these materials a tendency for such magnetic moments to orient parallel to one other to maintain a lowered-energy state. Therefore, even in the absence of the applied field, the magnetic moments of the electrons in the material suddenly line up parallel to one other.

Each and every ferromagnetic substance consists of its own individual temperature, termed as the Curie temperature, or Curie point, above which it loses the ferromagnetic properties. This is due to the reason that the thermal tendency to disorder overwhelms the energy-lowering due to the ferromagnetic order.

Ferromagnetism only takes place in some substances; the general ones are iron, cobalt, nickel and their alloys, and several alloys of rare earth metals.

Antiferromagnetism:

Antiferromagnetism is whenever the electrons in a material coalesce, making a chain of oppositely charged particles, even although the material as a whole doesn't appear to encompass any magnetic quality. Antiferromagnetism is the opposite of ferromagnetism, where particles line up themselves and takes place in materials like manganese oxide. The phenomenon usually reduces as the temperature increases, the electrons scattering arbitrarily and no longer forming chains. The temperature at which this takes place is termed to as the Neel temperature.

Antiferromagnetism signifies that the electrons in the material don't align themselves having the similar magnetic polarity. Even in the specific domains, the material doesn't display any magnetic qualities. As an outcome of the electrons not aligning themselves in the similar polarity, they cancel one other out by the specific chain of electrons. This is dissimilar from ferromagnetism, as in ferromagnetic materials, the chains comprise of electrons having matching polarity; however the different chains cancel one other out.

Choice of Magnetic Materials for Different Purposes:

The B-H loop lets us to judge the appropriateness of a material for use in some electrical devices.

1) Transformer and generator cores:

The cores are taken via 50 cycles of magnetization per second. They should, thus, encompass narrow hysteresis loops, high permeability for low values of H and low coercivity. The materials appropriate for this aim are silicon irons and mumetal (76% nickel, 17% iron and small percentages of aluminum and copper).

2) Permanent Magnets:

As the materials used for permanent magnets have never to be taken through cycles of magnetization, hysteresis loss is not of any significance. Their remanence and coercivity, though, should be big.

Materials appropriate for the purpose is cobalt steel (that is, alloy of cobalt, tungsten and carbon) and tincal (having cobalt, titanium, nickel and aluminum).

3) Electromagnets:

The major needs are big values of magnetization, 'M' for a specific field 'H' and low coercivity. The general material employed is silicon steel (having 4 percent of silicon).

4) Ferrites:

These are the alloys of iron oxide having other materials. One form of ferrites consists of high permeability, low hysteresis and high resistivity. It is employed for receiver aerial in the transistor receivers.

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