Tài liệu Chapter XII Magnetic materials pdf

4/1/2008 1 GENERAL PHYSICS II Electromagnetism & Thermal Physics 4/1/2008 2 Chapter XII Magnetic materials §1. Atomic magnetic moment - Bohr magneton §2. Magnetization, paramagnetism and diamagnetism §3. Ferromagnetism 4/1/2008 3  Investigation of the magnetic properties of materials is very important, because magnetic phenomena have various scientific and technical applications.  The macroscopic properties of matter are a manifestation of the microscopic properties of the atoms of which it is composed.  The magnetic properties of materials may be very different for types of material, depending on their nature and structure. 4/1/2008 4 §1. Atomic magnetic moment – Bohr magneton: In order to understand the magnetic properties of matter we must know the magnetic properties of atoms. 1.1 The magnetic moment of an orbiting charge :  Moving electrons, protons, neutrons create currents they have magnetic dipole moments.  The motions of these particles can be decomposed into orbital motion and spinning motion The current created by the orbiting particle is 4/1/2008 5 • Its magnetic moment is The vector form: where L 0 is angular momentum of the orbital motion. It is known that for an electron in the ground (non-excited) state of the hydrogen atom the angular momentum equals to 1.05 x 10 -34 J.s (we will learn later in quantum physics), so we have for the orbital magnetic moment of electron: This quantity is the fundamental unit of magnetic moment, it is called Bohr magneton   = 9.22 x 10 -24 A.m 2 . 4/1/2008 6 2.3 The magnetic moment of a spinning charge: Consider a spinning charge: A ring dq of the spinning charge creates the current: The corresponding magnetic moment of the ring is Summing over all the rings: Assume that the charge is distributed in the same way as the mass, we can write I= L The spin magnetic moment vector 4/1/2008 7 The particular case of an electron: An electron is known to have spin angular momentum of 0.527 x 10 -34 J.s. So, its spin magnetic moment is However, experiments give the result of twice bigger. Why? The problem lies with the assumption about the charge distribution that we have used. To correct this mistake one introduces the factor g called “gyromagnetic ratio“, and writes For electrons g = 2. 4/1/2008 8 §2. Magnetization, paramagnetism and diamagnetisme: 2.1 Some general view on the magnetic properties of materials:  The material which has the most striking magnetic properties is iron. Similar magnetic properties are shared also by nickel, cobalt,…. That kind of magnetic properties is called ferromagnetism.  All other ordinary substances do show some magnetic effects, but very small ones – a thousand → million times less than the effects in ferromagnetic materials. This small magnetism is of two kinds. In other words, there are two signs to the magnetic effects: paramagnetism and diamagnetisme. Strong magnetic effects Weak magnetic effects in ferromagnetic materials in paramagnetic materials in diamagnetic materials 4/1/2008 9 Two signs to the magnetic effect: • If the small cylinder is of bismuth → it is repelled by the sharp pole • If the small cylinder is of aliminium → it is attracted by the sharp pole 4/1/2008 10 a) b) Applying the formula of magnetic field for a ring current on its axis, we have for each atomic current: 2.2 Magnetization: First we consider a piece of paramagnetic material. When no external magnetic field is present, the atomic magnetic dipoles are randomly aligned (pic. a). The total magnetic field due to all the dipoles cancels to zero. If we apply an external magnetic field B 0 (pic. b), the dipoles tend to align with the applied field, and the vector sum of all atomic magnetic moments becomes non-zero vector, then we denote it by B’ : [...]... Strong magnetization of ferromagnetic materials:  The third type of materials is called ferromagnetic materials, which includes iron, nickel, cobalt, … These materials manifest strong magnetic effects  The magnetic field inside them is much larger than the applied external field, the relative permeability Km is of the order of 1.000 to 10.000 The properties of ferromagnetic materials are explained by... tend to align with the external magnetic field, then B > B0 and M > 0 However, by experiments one observed that this is true not for all materials, but only for most common materials Such materials are called paramagnetic For these materials M > 0 but fairly small 4/1/2008 12 2.3 Permeability and susceptibility: We have said that for paramagnetic materials the total magnetic field inside the material... of the magnetic field in materials relates to that in vacuum by the replacement  by   Km  =  which is called the permeability of the material 4/1/2008 13  Since for paramagnetic materials the value of Km is a small deviation from unity, it is convenient to introduce the quantity χ = Km – 1 m which is called the magnetic susceptibility 4/1/2008 14 2.4 Diamagnetism: There are a type of materials. .. microscopic structure: In these materials the atomic magnetic moments are extremely easy to align together, due to strong interactions between them Inside the material there exist regions called magnetic domains, even when no external field is applied In each domain the atomic magnetic moments are parallel to each other In the absence of external magnetic field the magnetic moments 4/1/2008 of different... domain magnetic moments tend to orient themselves paralell to the field that leads to the shift of domain boundaries: domains that have magnetic moments parallel to the external field will grow, other domains will shrink The magnetic moment of each domain have the order of thousands of Bohr magnetons, the torques that tend to align the domains with the external field are much stronger than in paramagnetic... external magnetic field, that is M < 0 Why? We can explain this phenomenon in a simplified version as follows:  In the absence of external fields the electrons move randomly (pic a)  When the external magetic field is applied (pic b), the electrons begin move in circular orbits This orbiting electrons create a field which is opposite to the external field This type of materials is called diamagnetic... obtain the total magnetic field is The quantity M is called the magnetization of the material 4/1/2008 11  Units:  The units of magnetic moment are (current) x (area), that is A.m 2  The units of magnetization M is (A / m2) / m3 = A / m  The units of  M is the same as the units of B (as it must be):  ( T.m / A )( A / m ) = T  It is natural to think that the sum of the atomic magnetic moments... magnetic moment of each domain have the order of thousands of Bohr magnetons, the torques that tend to align the domains with the external field are much stronger than in paramagnetic materials After rearrangment of domain magnetic moments the magnitude of the magnetization vector of the material is much larger than the external field Two important features of ferromagnetism are • the saturation of magnetization... of external magnetic field Increase B 0 from zero → the magnitude of magetization increases M Msat B0 When B0 reachs to some enough large value, further increase of the external field causes no increase in magnetization This phenomenon is called the saturation of magnetization The saturation of magnetization is explained as follows: When the external field is enough large, all the domain magnetic moments... some magnetization remains when B0 = 0 The material becomes then a permanent magnet It has own magnetic moment when the external field is removed B0 To reduce the magnetization to zero We must apply an external field in the inverse dirction The variation of the magnetization with the change of the applied magnetic field is described by the hysteresis loop 4/1/2008 Hysteresis loop 19 . Strong magnetic effects Weak magnetic effects in ferromagnetic materials in paramagnetic materials in diamagnetic materials 4/1/2008 9 Two signs to the magnetic. magnetization of ferromagnetic materials:  The third type of materials is called ferromagnetic materials, which includes iron, nickel, cobalt, … These materials manifest