Magnetism

William Gilbert was the first to investigate the phenomenon of magnetism .He also discovered that the earth is itself a weak magnet Hans Christian Oerstad (1777-1851) first suggested a link between electricity and magnetism .One day He was performing a experiment and noticed that when an electric current is passed through a conducting wire ,a magnetic field is produced around it . The presence of magnetic field at a point around a current carrying wore can be detected with the help of a compass needle . Ancient humans discovered the property of magnetism from lodestone.A lodestone is a naturally magnetised piece of the mineral magnetite .They are naturally occurring magnets,which can attract iron . MAGNETISM Depending on the existence and alignment of magnetic moments with or without application of magnetic field ,three types of magnetism can be defined . Diamagnetism Paramagnetism Ferromagnetism (Magnetic moments are like dipole moment or simply we can say that they are dipoles) DIAMAGNETISM It is weak form of magnetisation which arises only when an external field is applied . Diamagnetic materials are composed of atoms which have no net magnetic moments i.e all the orbital shells are filled and there are no unpaired electrons .There is no magnetic dipoles in the absence of a magnetic field and when a magnetic field is applied the dipole moments are aligned opposite to the field direction .examples Cu,Au,Si,Zn . PARAMAGNETISM It is a form of magnetism whereby certain materials are weakly attracted by an externally applied magnetic field and form internal induced magnetic field in the direction of the applied magnetic field. Whereas , diamagnetic materials are repelled paramagnetism is due to the presence of unpaired electrons in the material ,so all atoms with incompletely filled atomic orbitals are paramagnetic .When a field is applied all the dipole moment are aligned in the direction of the field ,causing a net attraction . Paramagnetic materials include aluminium ,oxygen ,titanium and iron oxide . PARAMAGNETISM do not retain any magnetism in the absence of an externally applied magnetic Field because thermal motion randomised the spin orientations .Even in the presence of the field there is only a small induced magnetisation because only a small fraction. Of spin will be oriented by the field .exmaples are Al,Cr,Mo,Ti. There magnetism disappears when the field is removed . FERROMAGNETISM Few substances like iron ,cobalt,Mikel,gadolinium are strongly attracted by a magnetic field .These substances can be permanently magnetised .The metal ions of ferromagnetic substances are grouped together into small regions called domains .So that each domain acts as a tiny magnet .The domains are aligned randomly when no field is applied a.The domains of unmagnetised piece of a magnetic moments get cancelled out. When such substances are placed in a magnetic field ,all the domains get oriented in the direction of the magnetic field .This ordering of domains remains same even when the magnetic field is removed and become a permanent magnet . Why paramagnetic and diamagnetic materials doesn't show domains like ferromagnetic materials ? The individual magnetic moments of the atoms are aligned with one another and point in same direction .Dipoles align in response to an external field but in case of para and dia magnetic ,dipoles do not have magnetic domains . Diamagnetic materials have zero net magnetic moment and paramagnetic material have very small net magnetic moment Further there are antiferromagnetic materials and ferromagnetic materials . ANTIFERROMAGNETISM Substances have domain structure similar to ferromagnetic substance .The domains of antiferromagnetic substance are oppositely oriented and cancel out each other's magnetic moment .example MnO There is no net magnetic moment Generally antiferromagnetic order may exist at sufficiently low temperatures ,but vanishes at and above the Neel temperature .The Neel temperature is the temperature above which antiferromagnetic materials becomes paramagnetic FERROMAGNETISM When the magnetic moments of the domains in the substance are aligned in parallel and antiparallel direction in unequal numbers . Ferromagnetic substances are weakly attracted by magnetic field as compared to ferromagnetic substances . examples Fe3O4 ,MgFe2O4 and ZnFe2O4. On heating these substances lose their ferromagnetism and become paramagnetic . Uses of magnets 1. In computers ,floppy disks and hard disks record data on thin magnetic coating . 2. Credit ,debit and ATM cards : all of these cards have magnetic strip on back and strip contains identity information which connect with their accounts . 3. Electric motors and generators 4. In MRI (magnetic resonance imaging) is used to spot problems in a patient organs without invasive surgery 5. Compasses A compass is magnetised pointer free to align itself with a magnetic field ,most commonly earth's magnetic field .