A magnetic field can be generated By moving charged practical such as electrons. The magnetic force FB depends on depends on velocity v and the magnetic field B.The magnitude of F is given by:F=qvBsin (N) on the presence of both electric field E and magnetic field B , the Lorentz force is :F = q(E+vB)• The SI unit of electric field E is (Newton / Coulomb )• The SI unit of magnetic field is the tesla (T) , 1 Tesla= 1 N/A.mIf current passes through a coil of length L and number of turns n, then a magnetic field will be generated. The magnetic field strength, H is given by :H =nI/L• H is Measured in (A/m).Magnetism is created in the material due to Nuclear spin and two types of motion of electrons in atoms. The movement of electrons in specific orbits around the nucleus and the spherical rotation of the electrons around its axes. The orbital and integral movements lead individually to the formation of a magnetic torque on each electron that behaves like a fine magnet. The momentum of electrons are equal. In each orbit, there are a number of pairs of electrons, each of which has two opposite directions. Thus, the final result of the magnetism is zero. However, there are some substances that contain odd numbers of electrons such as iron, in these material the total magnetism has a certain value these materials called materials MagnetismIt can be said that a magnetic material is magnetized when subjected to an external magnetic fieldThe electrons behave like tiny magnets which are aligned in same direction of the applied magnetic field. magnetic materials vary depends on their behaviour to external magnetic fields.For that , materials are classified into the following five major groups depending on the magnetic behaviour : 1.DIAMAGNETIC SUBSTANCES 2.PARAMAGNETIC SUBSTANCES 3.FERRO MAGNETIC SUBSTANCES 4.FERRI MAGNETIC SUBSTANCES 5.ANTI FERRO MAGNETIC SUBSTANCESThe relationship between the magnetic flux density (induction) B and the magnetic strength H can be defined by: B = U0(H + M) (1)• ?0 is a constant – it’s called the permeability of free space and has a value (in SI) 4?.10–7• M is the magnetic moment per unit volume. Measured in N·m/T the magnetic properties of a material are often defined by how they change with an applied magnetic field. So, the ratio of M to H is called the magnetic susceptibility XX= M/H Diamagnetism . In diamagnetic materials such as the gold and the Copper , atoms have paired electrons thus the net magnetic moment of the atom of is zero. when external magnetic field is applied, these materials are magnetized opposite to the field direction which means a negative magnetization and a negative susceptibility.The susceptibility X of this type of materials is independent of temperature.as an example of this kind of materials Inert gases; many metals e.g, , Hg; non-metallic elements e.g. B, Si. http://www.themagnetguide.com/ www.irm.umn.edParamagnetism In paramagnetic materials such as Aluminium, each atom has Atoms have randomlyoriented magnetic moments therfore no interaction between these atomic magnets. In the presence of external magnetic field there will be a partial alignment of these atomic magnetic moments in the direction of the magnetic field which means a positive magnetization and a positive susceptibility. www.irm.umn.edBoth dia- and para- magnetic materials are considered as non-magnetic because they show magnetization only in presence of an external field.Ferromagnetism Ferromagnetism occurs in some substrates such as iron, cobalt, nickel. In this kind of materials, Atoms have parallelaligned magnetic moments resulting in a strong permanent magnets. These atoms are grouped together in a small subunit called a domain. A domain can aligns in the same or random direction. The total magnetic fields for the randomly aligned domain will be zero.That means in the absence of a magnetic field, the domain arrangement minimizes the external energy and the bulk material appears unmagnetized.. When an external magnetic field is applied the atoms take a position as the direction of the magnetic field , so the material becomes magnetized. In “hard” ferromagnetic material it is hard to shift the domains, so a significant fraction of the magnetization is retained when the external field is removed. This is how permanent magnets are made.In “soft” ferromagnetic material the domains more closely follow the external field, and not much net magnetization remains when the external field is removed. A good application of this is an electromagnet, which has a strong magnetic field when a current is turned on and very little field when the current is removed. http://elec-trical.blogspot.co.ukIn ferromagnetic materials, thermal energy conquers electrons exchange forces and produces a randomizing effect. This can be achieved at a particular temperature called the Curie temperature (TC). above the Curie temperature, the ferromagnet is disordered and becomes paramagnetic while below it is, ordered. At the Curie temperature The saturation magnetization goes to zero. In addition to the Curie temperature , when applied external field is removed, ferromagnets tend to retain the magnetism in phenomenon so-called , hysteresis. The graph between the variation of magnetization with appled magnetic field is called a hysteresis curve. http://chemical-biological.tpub.comAs it can be seen from figure , the relationship between B and H is non-linear. The magnetic field strength depends on the magnetisation current, I .If current is increased in a positive direction to some value the magnetic field strength H increases linearly with i then the magnetic field density B will also increase until it reach saturation point.Now if the applied magnetic field strength, H is reduced to zero (by reduced the current to zero) , the magnetic field also reduces to zero. When the magnetic field strength, H reaches zero the magnetic field will not reach zero due to the present residual magnetism. To reduce the magnetic field density at point b to zero we need to reverse the current therefore H will has negative value . The magnetizing force which must be applied to null the residual flux density is called a “Coercive Force”. An increase in this reverse current causes resulting in magnetization in the opposite direction and increasing this magnetisation current further will make the magnatec field to reach saturation point but in the opposite direction. Again reversing the magnetising current flowing through the coil this time into a positive direction will cause the magnetic flux to reach zero, as before increasing the magnetisation current further in a positive direction will cause to reach saturation at point.the B-H curve follows figure as the magnetising current flowing between a positive and negative value such as the cycle of an AC voltage. This path is called a Hysteresis Loop.it is related to the existence of magnetic domains in the material. Once the magnetic domains are reoriented, it takes some energy to turn them back again. This property of ferrromagnetic materials is useful as a magnetic “memory”. Some compositions of ferromagnetic materials will retain an imposed magnetization indefinitely and are useful as “permanent magnets”. The magnetic memory aspects of iron and chromium oxides make them useful in audio tape recording and for the magnetic storage of data on computer disksThe energy dissipated (as heat) by a ferromagnet as it is taken around a circuitof its hysteresis loop is proportional to the area of that loop. If the area issmall, the material is said to be magnetically soft. If the area is large, thematerial is said to be magnetically hard. Domain walls traverse a sample as thefield is cycled and we can distinguish these two broad classes of ferromagneticmaterials based on how easy it is for domains to move through a sample.(1) Soft magnetic materials are easy to magnetize. Soft magnets are usedin transformer coils, generators and motors. In these applications themagnetization must be reversed many times a second and it is importantthat the energy dissipated in each cycle is minimized. Soft materials havebroad domain walls (small anisotropy energy K) which are thus easyto move. This leads to small coercive fields. Low magnetostriction isoften desirable so that internal strains will not induce a local anisotropyenergy. An example is permalloy (a commercial Ni/Fe alloy with anadditional ingredient) which has a coercive field Bc ~ 2 x 10-7 T.(2) Hard magnetic materials are difficult to magnetize and thus difficultto demagnetize. Hard magnets are used as permanent magnets (e.g.in the back of loudspeakers, in motors, and of course on the front ofyour refrigerator!) and in magnetic tape recording (in powder form).In these applications, the magnetization needs to be preserved for aslong as possible. The energy dissipated in a hysteresis loop cycle is aslarge as possible so that the magnetization will not occur spontaneously.Hard magnets have large hysteresis and narrow domain walls (large K)so that it is easy to have domain wall pinning. Large ion momentsand large crystal fields are helpful for hard magnetic properties andsuitable materials often involve rare earths, e.g. Nd2Fe14B which hasTc = 585 K and coercive field Bc = 1.2TThe amount of magnetism remaining in the ferromagnetic sample, when the applied field is zero, is called retentivity. The reverse magnetic field that should be applied on the ferromagnetic sample to reduce the saturation magnetization to zero is called coercivity. Hysteresis parameters like retentivity and coercivity etc, are not purely intrinsic properties. They depend on the grain size, domain state, stress, temperature. That means these hysteresis parameters are useful in magnetic grain sizing of natural samples.When a ferromagnetic material is magnetized in one direction, it will not relax back to zero magnetization when the imposed magnetizing field is removed. It must be driven back to zero by a field in the opposite direction. If an alternating magnetic field is applied to the material, its magnetization will trace out a loop called a hysteresis loop. The lack of retraceability of the magnetization curve is the property called hysteresis and it is related to the existence of magnetic domains in the material. Once the magnetic domains are reoriented, it takes some energy to turn them back again. This property of ferrromagnetic materials is useful as a magnetic “memory”. Some compositions of ferromagnetic materials will retain an imposed magnetization indefinitely and are useful as “permanent magnets”. The magnetic memory aspects of iron and chromium oxides make them useful in audio tape recording and for the magnetic storage of data on computer disks http://hyperphysics.phy-astr.gsu.edu http://hyperphysics.phy-astr.gsu.eduuseful way to visualize the difference between conductors, insulatorsand semiconductors is. Instead of having discrete energies as in the case of free atoms, the available energy states form bands. Crucial to the conduction process is whether or not there are electrons in the conduction band. In insulators the electrons in the valence band are separated by a large gap from the conduction band, in conductors like metals the valence band overlaps the conduction band, and in semiconductors there is a small enough gap between the valence and conduction bands that thermal or other excitations can bridge the gap. With such a small gap, the presence of a small percentage of a dopingmaterial can increase conductivity dramatically.An important parameter in the band theory is the Fermi level, the top of the available electron energy levels at low temperatures. The position of the Fermi level with the relation to the conduction band is a crucial factor in determining electrical properties.The main characteristics of the hysteresis loop are the saturation magnetization Ms (saturation is attained when allthe magnetic moments are aligned along some common direction resulting in the largest value of the magnetization),the remanent magnetization Mr (leftover magnetization, that is magnetization when the field H = 0) and thecoercive field Hc (at which M = 0) and the anisotropy field HK.). The characteristic is non-linear and the outputM is delayed with respect to input H. The input-output delay is proportional to the width of the loop. The ratioMr/Ms called squareness is close to 1 when the applied magnetic field is close to some orientation defined as the easyaxis (EA) and the hysteresis loop is closest to a square shape. Once the EA is determined, the angle the magneticfield makes with the EA (say ?) is varied and the hysteresis loop is graphed for different angles (see fig. 2). Whenthe angle ? is increased the opening of the hysteresis loop is reduced; it is largest when the magnetic field is mostparallel to the EA and smallest when the magnetic field is most parallel to the so called hard axis (in simple systemsthe hard axis is perpendicular to the EA). As shown in fig. 1, most characteristics of the hysteresis loop are depictedand for a given temperature and frequency of the applied field H, quantities such as the remanent magnetizationMr, and the coercive field Hc depend on the angle ?.