32-11 Ferromagnetism Ferromagnetic material having strong permanent magnetism Diamagnetic or paramagnetic material having weak

Transcription

1 32-11 Ferromagnetism Ferromagnetic material having strong permanent magnetism Diamagnetic or paramagnetic material having weak temporary magnetism. Iron, cobalt, nickel, gadolinium, dysprosium exhibit ferromagnetism due to a quantum physical effect exchange coupling the electron spins of one atom interact with those of neighboring atoms. Alignment of of the atoms is the result of that interaction gives ferromagnetic materials their permanent magnetism. When the temperature of ferromagnetic materials raised above a certain critical value Curie temperature 1043 K (= 770 C) the exchange coupling stops most materials become simply paramagnetic The magnetization of a ferromagnetic material 232

2 iron can be studied with an arrangement Rowland ring Fig The material is formed into a thin toroidal core 233

3 of circular cross section. Primary coil P having n turns per unit length wrapped around the core and carries current i p Iron core not present the magnitude of the magnetic field inside the coil would be B M the magnitude of the magnetic field contributed by the iron core due to exchange coupling and to the applied magnetic field B o Contribution B M proportional to the magnetic dipole moment per unit volume M of the iron. Secondary coil S used to determine B M by measured B compute B o with Eq Figure shows a magnetization curve for a ferromagnetic material in a Rowland ring: 234

4 The ratio B M, max the maximum possible value of B M, corresponding to saturation plotted versus B o. The curve like Fig the magnetization curve for a paramagnetic substance: 235

5 Both curves show the extent to an applied magnetic field can align the at atomic dipole moments of a material. Fig the ferromagnetic core yielding the alignment of the dipole moments is about 70% complete for B o = 1 X 10-3 T. If B o increased to 1 T the alignment would be almost complete. Complete saturation B o = 1 T is quite difficult to obtain. Magnetic Domains Exchange coupling produces strong alignment of adjacent atomic dipoles in a ferromagnetic material at a temperature below Curie temperature. Why Isn't the material naturally at saturation 236

6 even when there is no applied B o? Isn't every piece of iron such as an iron nail naturally strong magnet? Understanding this Consider a specimen of a ferromagnetic material iron in the form of a single crystal. The arrangement to make it in its normal state be made up of a number of magnetic domains regions of the crystal throughout the alignment of the atomic dipoles is essentially perfect. 237

7 Figure a magnified photograph of such an assembly of domains in a single crystal of nickel. Exchange coupling and domain shifting give us the following result Mural Paintings Record Earth's Magnetic Field The red pigments used in many mural paintings as the ones shown in this chapter's opening photograph contain grains of the iron oxide hematite. Each grain consists of a single domain having a particular magnetic dipole moment. Artists' pigments are a suspension of various solids in a liquid carrier. 238

8 When a pigment applied to a wall as a mural being created each grain rotates in the liquid until its dipole moment aligns with Earth's magnetic field. When the paint dries the moments locked into place and record the direction of Earth's magnetic field at the time of the painting. Figure suggests the alignment of the moments in a mural painted in 1740 when geomagnetic north in the direction indicated by N

9 Evidence from mural paintings and many other sources reveal that the direction of geomagnetic north has varied gradually but continuously over recorded history. التخلفية نزعة المادة الممغنطة إلي البقاء في حالة مغناطيسية Hysteresis Magnetization curves for ferromagnetic materials not retraced as we increase and decrease the external magnetic field B o. Figure a plot of B M versus B o during the following operations with a Rowland ring: 240

10 1. Starting with the iron unmagnetized point a increase the current in the toroid until B o = o in has the value corresponding to point b 2. Reduce the current in the toroid winding thus B o back to zero point c 3. Reverse the toroid current and increase it in magnitude until B o has the value corresponding to point d 4. Reduce the current to zero again point e. 5. Reverse the current once more until point b reached again Fig shows curve bcdeb loop of hysteresis Familiar phenomenon of permanent magnetism at points c e iron core is magnetized even though there is no current in the toroid windings Hysteresis understood through the concept of 241

11 magnetic domains the motions of the domain boundaries and the reorientations of the domain directions are not totally reversible. When the applied magnetic field B o increased and decreased back to its initial value he domains do not return completely to their original configuration but retain some "memory" of their alignment after the initial increase. This memory of magnetic materials is the essential for the magnetic storage of information as on magnetic tapes disks. This memory of the alignment of domains also occur naturally. When lightning sends currents along multiple tortuous paths through the ground the currents produce intense magnetic fields suddenly magnetize any ferromagnetic material in nearby rock. Because of hysteresis rock material retains some 242

12 of that magnetization after the lightning strike the currents disappear. Pieces of the rock later exposed broken and loosened by weathering are then lodestones a natural magnet 243

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