There are two essential requirements for generating a magnetic field which are magnetic material and current. The magnetic field is a region in which the magnetic effect due to electric current or of a magnet is precipitated. When small magnet is brought in the vicinity then torque will be experienced on the test magnet up to this magnet becomes oriented in a particular direction. The magnitude of this torque is nothing but measurement of the strength of the magnetic field and the shown direction of orientation which is the direction of the field.
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Magnetism means it is a physical phenomenon involving magnetic fields and whose effects upon materials. The magnetic fields may be set by electric currents or by magnets. In the magnetic material, the individual atoms cause magnetic fields when whose electrons have a net magnetic moment due to their angular momentum. Due to angular momentum of charged particle a magnetic moment arises whose cooperative effect acquires the macroscopic magnetic field of a permanent magnet.
Hence only one kind of magnetism was known until 1821which was produced by iron magnets. It was proved that when an electric current flows in a wire then needle of compass moves nearby it. This new phenomenon was studied by Ampere, concluded that the nature of magnetism which was quite different from basically a force between electric currents. The two parallel currents which are in the same direction attract each other and on the other hand, the two parallel currents in opposite direction repel each other. According to the modern theory, magnetism in solids arises due to spins and orbital motion of electrons and also due to the spin of the nuclei of an atom.
Due to motion of electrons, the magnetic effects produced in magnetic materials. The magnetic moments associated with the atoms are due to three types of motions which are, one is the electron orbital motion, second is the change in orbital motion caused by an external magnetic field and third one is the spin of the electrons. In most of the atoms the electrons occur in pairs. When electron pair spins in opposite directions then they cancel each other means there is no net magnetic fields exist. In magnetic materials with some unpaired electrons show a net magnetic field and which react more to an external field. The major contribution is due to spin of unpaired valance electrons in magnetic field of the magnetic materials. These unpaired valence electrons produce permanent electronic magnetic moments. The nature of magnetization produced depends on presence of the number of unpaired valence electrons in the atoms of the solid and on the relative orientations of the neighboring magnetic moments.
For the generation of net non-zero magnetic moment, number of such magnetic dipoles may align parallel to each other, with or without the application of magnetic field.
As a magnetic material is kept in a magnetic field then it becomes magnetized. It means that the material itself becomes a magnet. Hence the intensity of the induced magnetism is called the magnetization. The magnetization is also called as the magnetic moment per unit volume of the material. The magnetic force anywhere in space of magnetic field is described by a vector field .It is also called the magnetic induction.
Let
H – Magnetic field
M – Intensity of magnetization
B – Magnetic induction
Then the magnetic induction B is given by,
B = µ0 (H+M)
Where µ0 – is the permeability of free space.
The magnetic field produces due to motion of an electric charge. The electrons in a bar magnet about atomic nuclei are in constant motion. The motion of charge creates a tiny current, hence produces a magnetic field that means every spinning electron is a tiny magnet. When two electrons which are spinning in the same direction creates a stronger magnetic field. When a pair of electrons is spinning in opposite direction then their net effect cancel each other, hence there is no magnetic field which occurs in substances such as rubber, wood and plastics etc. When magnetic field is applied then materials turn magnetic field, they acquire a nonzero magnetization. On the basis of magnetic property as per the application of an external field, magnetic materials are divided into temporary and permanent magnets. In case of temporary magnets after the removal of the applied field will lose whose all or most of their magnetic properties. On the other hand in permanent magnet magnetic properties will retain or keep for a very long time. These temporary magnets are made from the materials such as iron, nickel and cobalt. Hence, these materials are called as soft magnetic materials that mean outside a strong magnetic field they usually do not retain their magnetism.
The total energy of the crystal is determined by the distribution of cations in a given spinel oxide. This parameter depends on other factors such as the size of ions, the limited space between the repulsive forces , Coulomb interactions between charges of these ions, effects of polarisation and ordering of cations.
With the help of spontaneous magnetization Ferromagnets are characterized. In the absence of field, ferromagnets attain saturation magnetization in each of the domains. But the magnetization curve shows that an unmagnetized ferromagnetic sample displays no overall magnetization in zero fields. However, it requires an appreciable field to produce saturation magnetization value. Weiss domain hypothesis explained this discrepancy in between the theory and the observation. According to Weiss domain hypothesis, a ferromagnet of macroscopic size consists numerous regions is known as magnetic domains in the demagnetized state. In each domain all the atomic moments are aligned in same easy direction. In orders to minimize the magneto static energy, the direction of the spontaneous magnetization varies from domain to domain. The multi domains are formed, if the ferromagnetic grains are subdivided into many domains with walls between them. When the grain size which is in the order of the wall thickness, hence it is a single domain grain. The difference between multi domain and single domain grain is that the multi domain grains possesses a net zero magnetic moment due to different directions of the individual domain magnetization, but the single domain grains are always show the saturation intensity below its curie temperature. The single domain size range would be from less than 1OOOA° which is less than the lattice constant of the material and hence these grains are also called as fine particles. The hysteresis behavior of these particles shows reversible magnetization curves. In this case there is a zero value of the remenance and the coercive force. The multi domain particles require a much larger magnetic field than single domain particle and hence super paramagnetic particles acquire saturated magnetization. When the changes in appropriate temperature, the hysteresis loop of single domain and due to super paramagnetic particles are interchangeable. However the multi domain particles are independent of temperature. In magnetism the major contribution comes from the spin of unpaired valance electrons which produces permanent electronic magnetic moments.
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