Magnetic Properties of Material Classification of Magnetic Materials On the basis of their magnetic properties different materials are classified as: Diamagnetic substance Paramagnetic substance Ferromagnetic substance Diamagnetic Substance
The materials are weakly magnetized in a direction opposite to the direction of applied magnetic field. Eg. Gold, silver, Zinc, lead, bismuth, mercury, glass, water, air, helium, hydrogen, hydrochloride salt. Michael Faraday discovered that a specimen of bismuth was repelled by a strong magnet. Diamagnetism occurs in all materials. These materials are those in which individual atoms do not possess any net magnetic moment. [Their orbital and spin magnetic moment add vectorially to become zero]. The atoms of such material however acquire an induced dipole moments when they are placed in an external magnetic field. The diamagnetic materials are Type 1 superconductors as they exhibit perfect conductivity and perfect diamagnetization when cooled to very low temperature. The superconductor repels a magnet and in turn is repelled. Such perfect diamagnetism in superconductors exhibiting the above phenomena is called Meissner effect.
Some important properties are: When suspended in a uniform magnetic field they set their longest axis at right angles to the field as shown 2) In a non-uniform magnetic material, these substances move from stronger parts of the field to the weaker parts. For e.g.,. when diamagnetic liquid is put in a watch glass placed on the two pole pieces of an electromagnet and current is switched on the liquid accumulates on the sides. 3) A diamagnetic liquid in a U shaped tube is depressed, when subjected to a magnetic field. 4) The lines of force do not prefer to pass through the specimen, since the ability of a material to permit the passage of magnetic lines of force through it is less. 5) The permeability of the substance, that is, r < 1. 6) The substance loses its magnetization as soon as the magnetizing field is removed. 7) Such specimen cannot be easily magnetized and so their susceptibility is negative. Example: Bismuth, antimony, copper, gold, quartz, mercury, water, alcohol, air, hydrogen etc.
Paramagnetic Substance Paramagnetic substance are attracted by a magnet very feebly. Which are weakly magnetized in the direction of applied mag. Field. Eg. Aluminium, Chromium, manganese, platinum, antimony, sodium, copper, Salt solution of iron and nickel, liquid oxygen ….
In a sample of a paramagnetic material, the atomic dipole moments initially are randomly oriented in space. When an external field is applied, the dipoles rotate into alignment with field as shown The vector sum of the individual dipole moments is no longer zero. Some important properties are: The paramagnetic substance develops a weak magnetization in the direction of the field. When a paramagnetic rod is suspended freely in a uniform magnetic field, it aligns itself in the direction of magnetic field. The lines of force prefer to pass through the material rather than air that is r > 1 that is their permeability is greater than one. As soon as the magnetizing field is removed the paramagnetics lose their magnetization. In a non-uniform magnetic, the specimen move from weaker parts of the field to the stronger parts (that is it accumulates in the middle). A paramagnetic liquid in U tube placed between two poles of a magnet is elevated. The magnetization of paramagnetism decreases with increase in temperature. This is because the thermal motion of the atoms tend to disturb the alignment of the dipoles.
Ferromagnetic Substance
The material which are strongly magnetized in the direction of the applied magnetic field. Eg, steel, iron, nickel, cobalt, and alloys like alnico(Al, Ni, Cobalt). Ferromagnetism, like paramagnetism, occurs in materials in which atoms have permanent magnetic dipole moments. The strong interaction between neighboring atomic dipole moments keeps them aligned even when the external magnetic field is removed. Some important properties are: These substances get strongly magnetized in the direction of field. The lines of force prefer to pass through the material rather than air that is r>1 that is their permeability is greater than one. In a non-uniform magnetic, the specimen move from weaker parts of the field to the stronger parts (that is it accumulates in the middle). A paramagnetic liquid in U tube placed between two poles of a magnet is elevated. For ferromagnetic materials r is very large and so its susceptibility i.e., Xm is positive. Ferromagnetic substances retain their magnetism even after the magnetizing field is removed. The effectiveness of coupling between the neighboring atoms that causes ferromagnetism decreases by increasing the temperature of the substance. The temperature at which a ferromagnetic material becomes paramagnetic is called its curie temperature. For example the curie temperature of iron is 1418oF, which means above this temperature, iron is paramagnetic. Example: Iron, cobalt, nickel and number of alloys.
Curie Law in Magnetism Pierre Curie experimentally discovered that intensity of magnetization I is directly proportional to Bo (flux density in Vacuum) and inversely proportional to the absolute temperature. T of the material That is I Bo/T or I=c Bo/T Here, C is Curies constant. m C is the Curie constant. Curie temperature for iron is abt 1000K, for cobalt it is abt 1400K and for Nickel it is abt 600K. The saturation region explains that at a particular stage, all atomic dipoles present in the specimen align in the direction of the field. Domain Theory With ferromagnetic substance, there are regions roughly 0.01 mm in size in which coupling of dipoles is perfect. Such regions are called domains. In each domain, however, the dipoles point in different directions and so add up vectorially to give zero in an unmagnetized ferromagnet as shown below. (a) On placing ferromagnets in an external magnetic field, the domains having magnetic moments in the direction of magnetic field start growing in size at the cost of other domains.
(b) Thus, the number of magnetic moments pointing in the direction of the magnetic field increases and for a strong field, the material gets strongly magnetized. Hysteresis Consider an iron being magnetized slowly by a changing magnetizing field (H). The intensity of magnetization is found to increase along OA. On decreasing H slowly, I also decreases but does not follow AO. When H = 0, I has a non-zero valve equal to OB. This implies that some magnetism is left in the specimen. This value of I which is non-zero when H = 0 that is OB is called retentivity or residual magnetism. When the field is applied in the reverse direction, the I decreases along BC till its zero at C. The valve of H which has to be applied to the magnetic material in reverse direction so as to reduce its residual magnetism to zero, is called its coercivity. On increasing H further, I increases along CD till it acquires a saturation at D. On changing the field, I follows a path DEFA. This closed loop is called hysteresis loop and represents cycles of magnetization a specimen has undergone. The hysteresis therefore refers to lagging behind. Here I lags behind H. The shape and size of hysteresis loop is characteristic of each material, because of their difference in retentivity, coercivity etc.
The Earth's Magnetism A magnetic compass was used to help the sailors for navigational purpose. But recently it has been discovered that some migrant birds have magnetic sensors in their heads, which help to guide them using the Earth's magnetic field. William Gilbert suggested that Earth itself is a huge magnet from various observations he had made: On disturbing a freely suspended magnet it returns quickly to N-S direction. The north pole of this huge magnet must be towards geographic south as to attract South Pole of the suspended magnet. Soft iron pieces buried under surface of Earth are found to acquire magnetic properties. On mapping magnetic field lines due to bar magnet, we come across neutral points. These points are those where magnetic field of the bar magnet cancel with that of Earth's field. But for the latter, we cannot obtain neutral points. The exact cause of magnetic field of Earth is not yet known but some important postulates are: Magnetic field of Earth may be due to molten charged metallic fluid in core. This rotating fluid results in currents thus magnetising the Earth. Since every substance is made up of charged particles, these substances rotating about an axis is equivalent to a circulating current and hence is responsible for the Earth's magnetisation. As the earth rotates, strong electric currents are set up due to movement of charged iron (due to showers of cosmic ray). These moving ions magnetise the Earth. Features of Earth's Magnetic Field The earth's magnetic field has an axis which is inclined 20o west of the axis of rotation of earth. The point where this huge earth's magnet cuts the earth's surface are the magnetic poles. A freely suspended magnet has its north pole pointing towards geographic north; we therefore designate the earth's magnetic pole close to geographic north as magnetic south. The same argument follows for the south pole of the freely suspended magnet. The magnetic equator divides the earth's surface into two. The lines of force enter geographic north and come out of the geographic south.
Magnetic Elements The strength of the earth's magnetic field is about 10-4 tesla or 1 gauss. To describe the magnetic field of earth at any place three quantities or elements are required. They are: Magnetic declination () Magnetic inclination () Horizontal component (BH) Magnetic declination () Magnetic declination is the angle between magnetic axis and the geographic axis. Magnetic field around the earth
Magnetic dip or Inclination The angle between the direction of total intensity of Earth's field with the horizontal line in magnetic meridian. It is represented as . At poles, the angle of dip = 90o and at the equator, the angle of dip = 0o. The dip at a place can be determined by an apparatus known as dipcircle as shown below. The needle rotates freely in the vertical plane of scale. The pointed ends move over the graduations on the scale, which are marked 0-0 in the horizontal and 90-90 in the vertical directions. Simple Dip Needle Horizontal component Tangent Law B= BH Tanθ Horizontal component is the component of the total intensity of Earth's magnetic field in the horizontal direction in magnetic meridian.
When a small Magnet suspended in two uniform field B and BH which are at right angle to each other, The magnet comes to rest at an angle θ w.r.t BH such that B= BH Tanθ Global and Temporal Variation in Earth's Magnetic Field The dipole pattern of earth's magnetic field is disturbed due to solar winds. Solar winds are a stream of charged particles coming from the sun. These particles ionise the atmosphere above these poles which display a light high up in the atmosphere. This phenomenon occurring in the arctic region is called aurora borealis or northern lights and in south it is called aurora australis. The earth's magnetic field is found to change with time. The magnetic poles of earth keep shifting their position which is short term change. Detailed charts are maintained and revised periodically. The changes occurring over long term come from the evidence of basalt. The basalt from volcano cools and solidifies and provides the picture of earth's magnetic field. As the basalt can be dated back, a clear picture of the earth's magnetic field has emerged. The currents in the earth's core slow down, stop and pick up in the opposite direction.