Magnetic Properties of Matter
Magnetism is a property of matter and it occurs in different forms and degrees in various conductors and insulators. For example, at low temperatures, metallic systems exhibit either superconducting or magnetic order. The degree of magnetism of a substance is due to the intrinsic magnetic dipole moment of its electrons. The degree of magnetism is also called magnetization and it is defined as the net magnetic dipole moment of the substance per unit volume. The magnitude of the magnetic dipole moment of an electron is given by the Bohr magneton, m = 9.27 x10-24 Am2 . Magnetization is further defined by measuring a quantity called the magnetic susceptibility.
In the nineteenth century, Michael Faraday was the first to start classifying substances according to their magnetic properties. Faraday classified them as either diamagnetic or paramagnetic and he based his classification on the force exerted on the materials when placed in an inhomogeneous magnetic field.
Diamagnetic substances have a negative magnetic susceptibility, (i.e., they are materials in which the magnetization and magnetic field are opposite). The electrons in the atoms of diamagnetic materials are all paired and there is no intrinsic magnetic moment. When a material is placed into a magnetic field, its atoms acquire an induced magnetic moment pointing in a direction opposite to that of the external field, and the material becomes magnetic. The diamagnetic field produced by the material opposes the external field, although this diamagnetic field is very weak (except in superconductors). If the atoms of a material have no magnetic moment of their own, then diamagnetism is the only magnetic property of the material and the material is called diamagnetic. Copper exhibits such diamagnetism.
Paramagnetic substances have a weak positive magnetic susceptibility and their atoms usually have unpaired electrons of the same spin. Some metals, rare earth, and actinides are paramagnetic. All the magnetic moments of the electrons in their atoms do not completely cancel out, and each atom has a magnetic moment. Such materials thus have a permanent magnetic moment and they can interact with a magnetic field. An external magnetic field tends to align the magnetic moments in the direction of the applied field, but thermal motion tends to randomize the directions. If only a relatively small fraction of the atoms are aligned with the field, then the magnetization obeys Curie's law. Curie's law states that if the applied magnetic field is increased, the magnetization of the material also increases. This is because a stronger magnetic field will align a greater quantity of dipoles. Curie's law also states that the magnetization decreases with increasing temperature. The magnetic field produced by the aligned magnetic moments of paramagnetic materials strengthens the external field, but at standard temperatures it averages no more than 10 times stronger than a diamagnetic field and is, therefore, still very weak.
Ferromagnetic materials have the highest magnetic susceptibilities. In these materials, the spins of neighboring atoms do align even in the absence of an externally applied field through a quantum effect known as exchange coupling. Besides iron, examples of ferromagnetic materials are nickel, cobalt, and alnico, an aluminum-nickel-cobalt alloy. In these materials, all metals, the electrons give rise to permanent dipole moments that can align with those of their neighbors, creating magnetic domains that produce a magnetic field. Above a certain temperature, called the Curie temperature, a ferromagnetic material ceases to be ferromagnetic because the addition of thermal energy increases the motion of the atoms, thus destroying the alignment of the dipole moments. The material then becomes paramagnetic with weak magnetic susceptibility. The magnetic domains of ferromagnetic materials allow them to be turned into permanent magnets. If a ferromagnetic material is placed in a strong magnetic field, its magnetic domains converge into large domains aligned with the externally applied field. Upon removal of the external field, the electrons maintain the alignment and the magnetism remains.
The magnetic properties of matter are used in a wide variety of applications. For example, magnetization is the acting principle of magnetic memory and it is used to make audio and video tapes, as well as magnetic disk storage devices for computers. In such applications, the recording head of a tape recorder, or the write head of a disk drive, applies a field that magnetizes a small segment of the tape or disk, which remains magnetized until another magnetic field changes it. A magnetic tape reader can then take the information from the magnetic tape and convert it into an electrical signal that, in the case of a cassette player, is sent to a speaker and converted into sound. When a magnetic tape is passed under the reading device, a magnetic field is induced through the plane of the wire loops. Different orientations of the atoms in the tape material means that the magnetic field is changing, which subsequently induces a current. This electrical signal is then passed on to another device for further processing.
Diamagnets also exhibit useful properties in the presence of externally applied magnetic fields. In these materials, eddy currents consisting of moving electrons are induced and the magnetic effects cancel part of the applied external field. An example of a device incorporating a diamagnet is the metal detector. In this instrument, the magnetic field is generated by an electromagnet, which then forms eddy currents. The magnetic fields from the induced currents are in turn picked up by the metal detector in the form of small currents.
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