Every substance has some magnetic properties associated with it. The origin of these properties lies in the magnetic moments associated with the electrons. The magnetic moment of electron originates from two types of motions (i) its orbital motion around the nucleus and (ii) its spin around its own axis. Thus electrons behave like tiny magnets. Thus, each electron has a permanent spin and an orbital magnetic moment associated with it.
The magnitude of this magnetic moment is very small and is measured in the unit called Bohr magneton μB. It is equal to 9.27 × 10–24 A m2.
The magnitude of magnetic moment due to spin is ± μB and is directed along the axis of the spin. The magnitude of orbital motion is equal to mlμB and is directed along the axis of rotation. Where ml is the spin quantum number of the electron.
Types of Magnetic Material:
On the basis of their magnetic properties, substances can be classified into five categories: (i) paramagnetic (ii) diamagnetic (iii) ferromagnetic (iv) antiferromagnetic and (v) ferrimagnetic.
Diamagnetic substances are weakly repelled by a magnetic field. The magnetism exhibited by such substance is called diamagnetism.
Examples: H2O, TiO2, V2O5, NaCl and C6H6
They are weakly magnetized in a magnetic field in opposite direction.
Diamagnetism is shown by those substances in which all the electrons are paired and there are no unpaired electrons. The pairing of electrons cancels their magnetic moments and they lose their magnetic character.
Paramagnetic substances are weakly attracted by a magnetic field.
Examples: O2, Cu2+, Fe3+, Cr3+, TiO, Ti2O3,VO, VO2, and CuO
They are magnetized in a magnetic field in the same direction.
They lose their magnetism in the absence of magnetic field. They are temporary magnets.
Paramagnetism is due to the presence of one or more unpaired electrons which are attracted by the magnetic field.
The substances which are attracted very strongly by a magnetic field are called ferromagnetic substances.
Examples: iron, cobalt, nickel, gadolinium, and CrO2
Besides strong attractions, these substances can be permanently magnetised.
In the solid state, the metal ions of ferromagnetic substances are grouped together into small regions called domains. Thus, each domain acts as a tiny magnet. In an unmagnetized piece of a ferromagnetic substance, the domains are randomly oriented and their magnetic moments get canceled. When the substance is placed in a magnetic field all the domains get oriented in the direction of the magnetic field and a strong magnetic effect is produced. The alignments of domains persist even when the magnetic field is removed and the ferromagnetic substance becomes a permanent magnet.
Substances like MnO showing antiferromagnetism have domain structure similar to ferromagnetic substance, but their domains are oppositely oriented and cancel out each other’s magnetic moment.
Ferrimagnetism is observed when the magnetic moments of the domains in the substance are aligned in parallel and anti-parallel directions in unequal numbers.
They are weakly attracted by a magnetic field as compared to ferromagnetic substances.
Examples: Fe3O4 (magnetite) and ferrites like MgFe2O4 and ZnFe2O4.
These substances also lose ferrimagnetism on heating and become paramagnetic.
Iron is strongly ferromagnetic:
Iron shows the strong magnetic properties. It is ferromagnetic substance and it can be magnetized permanently.
The atomic number of iron is 28. Its electronic configuration is [Ar] 3d6 4s2.
The box diagram of its electronic configuration is
There are four unpaired electrons. i.e. their spins are not neutralized. Hence they show strong magnetic properties. Hence iron is strongly magnetic.
Guoy’s Method of Studying Magnetic Properties of Solids:
The method consists of weighing the substances in an out of the magnetic field.
If the substance is diamagnetic it weighs less in the magnetic field due to opposite direction of the magnetic field set in the diamagnetic substance (repulsion).
If the substance is paramagnetic it weighs more in the magnetic field due to the same direction of magnetic field set in paramagnetic substance (attraction).
If the substance is ferromagnetic, then the effect is same as that in case of paramagnetic substance but the extent of pull is more than that in case of paramagnetic substance.
Dielectric Properties of Solids
In insulators, electrons in the individual atom or ion are bound to corresponding nuclei. Hence they are not able to migrate. Thus they are localized. Due to this localization, no electrons are available for conduction and insulators do not conduct electricity to them. If the external electric field is applied these atoms or ions undergo polarization due to the formation of dipoles.
Now the dipoles produced can behave in two ways. They may align themselves in such a way that there is net dipole moment of the crystal or they may align themselves in such a manner that they cancel each other’s dipole moment and the net dipole moment of the crystal is zero.
Due to polarization, the solid shows some interesting properties as discussed below.
The crystal in which the individual dipoles formed due to polarization align themselves in such a way that there is net dipole moment of the crystal shows piezoelectricity.
When mechanical stress (pressure) is applied to such crystal, the atoms or ions in the crystal are displaced and the crystal produces electricity. Conversely, when an electric field is applied to such crystal, there is displacement 0f the ions or atoms (anti piezoelectricity). Due to these properties, they are used as mechanical-electrical transducers.
They are used in a pickup of record players, pressure sensors, engine knock sensors, etc.
Some piezoelectric crystals on heating produce electricity. Electricity produced by this method is called pyroelectricity. The phenomenon is called pyroelectric effect.
They are used in passive infrared (PIR) sensors. They are a common type of motion detector thermal sensors, which can detect the movement of human beings, animals, objects, etc., They are used in infrared non-contact thermometers.
In some piezoelectric crystals, the dipoles are permanently aligned even in the absence of the electric field. In such crystals, the direction of polarization can be shifted by applying external electrical field. This phenomenon is known as ferroelectricity.
They are used in capacitors as a dielectric, in non-volatile memory, in ultrasound imaging and actuators, for making thermistors, oscillators, filters, light deflectors, modulators and displays.
Examples of such materials are Barium Titanate (BaTiO3), Lead Titanate (PbTiO3), sodium potassium tartarate (Rochelle salt), Potassium dihydrogen phosphate (KH2PO4), etc.
When the dipole in alternate polyhedra point up and down, the net dipole moment of the crystal is zero. Such a crystal is called antiferroelectric.