Magnetism and Magnetic Materials PDF

Magnetism and Magnetic Materials CHAPTER – 1 Introduction to Magnetism and Magnetic Materials Callister: Ch-20 Endale Abebe 1  Introduction Recall to the basics What is magnet/magnetism?...

Magnetism and Magnetic Materials CHAPTER – 1 Introduction to Magnetism and Magnetic Materials Callister: Ch-20 Endale Abebe 1  Introduction Recall to the basics What is magnet/magnetism? How many poles can a magnet can have? Can you find each of the poles alone? Can you compare magnetic poles with the sign of electric charges? Is there any relationship between a magnetic and an electric field? Can you mention devices/instruments that use magnet? 2 E.A.G.  Magnetic Dipoles Magnetic forces are generated by moving electrically charged particles; these magnetic forces are in addition to any electrostatic forces that may prevail. Many times it is convenient to think of magnetic forces in terms of fields. Imaginary lines of force may be drawn to indicate the direction of the force at positions in the vicinity of the field source. Magnetic dipoles (analogous to electric dipoles) may be thought of as small bar magnets composed of north and south poles instead of positive and negative electric charges. Magnetic Field Vectors 3 E.A.G.  Magnetic Dipoles Magnetic dipoles are influenced by magnetic fields in a manner similar to the way in which electric dipoles are affected by electric fields. Within a magnetic field, the force of the field itself exerts a torque that tends to orient the dipoles with the field. This is the reason for a magnetic compass needle lines up with the earth’s magnetic field. 4 E.A.G.  Magnetic Field Vectors The externally applied magnetic field, sometimes called the magnetic field strength, is designated by H. If the magnetic field is generated by means of a cylindrical coil (or solenoid) consisting of N closely spaced turns, having a length l, and carrying a current of magnitude I, then 5 E.A.G.  Magnetic Field Vectors The magnetic field that is generated by the current loop and the bar magnet is an H field. The units of H are ampere-turns per meter, or just amperes per meter. Iron, some steels, and the naturally occurring mineral lodestone are well-known examples of materials that exhibit magnetic properties. The magnetic induction, or magnetic flux density, denoted by B, represents the magnitude of the internal field strength within a substance that is subjected to an H field. The units for B are teslas [or webers per square meter (Wb/m2)]. Both B and H are field vectors, being characterized not only by magnitude, but also by direction in space. E.A.G.  Magnetic Field Vectors The magnetic flux density in the presence of a vacuum is where is μ0 the permeability of a vacuum, 4π×10-7 H/m. The magnetic flux density within a solid material is where μ is the permeability of the solid material. The ratio of the permeability in a material to the permeability in a vacuum is called the relative permeability, which is E.A.G.  Origins of Magnetic Moments The macroscopic magnetic properties of materials are a consequence of magnetic moments associated with individual electrons. Each electron in an atom has magnetic moments that originate from two sources. One is related to its orbital motion around the nucleus; being a moving charge, an electron may be considered to be a small current loop, generating a very small magnetic field, and having a magnetic moment along its axis of rotation 8 E.A.G.  Origins of Magnetic Moments Each electron may also be thought of as spinning around an axis; the other magnetic moment originates from this electron spin, which is directed along the spin axis. Spin magnetic moments may be only in an “up” direction or in an antiparallel “down” direction. Thus each electron in an atom may be thought of as being a small magnet having permanent orbital and spin magnetic moments. The most fundamental magnetic moment is the Bohr magneton, μB = 9.27×10-24 Am2. 9 E.A.G.  Origins of Magnetic Moments For each electron in an atom the spin magnetic moment is ±μB (plus for spin up, minus for spin down). Furthermore, the orbital magnetic moment contribution is equal to mlμB, ml being the magnetic quantum number of the electron. In each individual atom, orbital moments of some electron pairs cancel each other; this also holds for the spin moments. For example, the spin moment of an electron with spin up will cancel that of one with spin down. 10 E.A.G.  Origins of Magnetic Moments The net magnetic moment, then, for an atom is just the sum of the magnetic moments of each of the constituent electrons, including both orbital and spin contributions, and taking into account moment cancellation. For an atom having completely filled electron shells or subshells, when all electrons are considered, there is total cancellation of both orbital and spin moments. Thus materials composed of atoms having completely filled electron shells are not capable of being permanently magnetized. This category includes the inert gases (He, Ne, Ar, etc.) as well as some ionic materials. 11 E.A.G.  Magnetization In the presence of an H field, the magnetic moments within a material tend to become aligned with the field and to reinforce it by virtue of their magnetic fields. The reinforcing field is called the magnetization (M) of the material, which is defined as The stronger the H field the larger will be the magnetization, hence their proportionality constant is called magnetic susceptibility, χm: The magnetic susceptibility and the relative permeability are related as follows: Hence what must be the unit of χm? What about that of M? 12 E.A.G.  The types of magnetism 13 E.A.G.  The types of magnetism: Diamagnetism Diamagnetism is a very weak form of magnetism that is nonpermanent and persists only while an external field is being applied. It is induced by a change in the orbital motion of electrons due to an applied magnetic field. The magnitude of the induced magnetic moment is extremely small, and in a direction opposite to that of the applied field. Thus, the relative permeability is less than unity (very slightly), and the magnetic susceptibility is negative; that is, the magnitude of the B field within a diamagnetic solid is less than that in a vacuum. When placed between the poles of a strong electromagnet, diamagnetic materials are attracted toward regions where the field is weak. 14 E.A.G.  The types of magnetism: Paramagnetism Paramagnetism results when the magnetic moments preferentially align, by rotation, with an external field. These magnetic dipoles are acted on individually with no mutual interaction between adjacent dipoles. Inasmuch as the dipoles align with the external field, they enhance it, giving rise to a relative permeability that is greater than unity, and to a relatively small but positive magnetic susceptibility. 15 E.A.G.  The types of magnetism: Ferromagnetism For some solid materials, each atom possesses a permanent dipole moment by virtue of incomplete cancellation of electron spin and/or orbital magnetic moments. In the absence of an external magnetic field, the orientations of these atomic magnetic moments are random, such that a piece of material possesses no net macroscopic magnetization. Certain metallic materials possess a permanent magnetic moment in the absence of an external field, and manifest very large and permanent magnetizations. These are the characteristics of ferromagnetism, and they are displayed by the transition metals iron (as BCC α ferrite), cobalt, nickel, and some of the rare earth metals such as gadolinium (Gd). They have high magnetic susceptibilities. Consequently, H≪M, and 16 E.A.G.  The types of magnetism: Ferromagnetism Permanent magnetic moments in ferromagnetic materials result from atomic magnetic moments due to electron spin—uncancelled electron spins as a consequence of the electron structure. There is also an orbital magnetic moment contribution that is small in comparison to the spin moment. Furthermore, in a ferromagnetic material, coupling interactions cause net spin magnetic moments of adjacent atoms to align with one another, even in the absence of an external field. This mutual spin alignment exists over relatively large volume regions of the crystal called domains, 17 E.A.G.  The types of magnetism: Ferromagnetism The maximum possible magnetization, or saturation magnetization, Ms, of a ferromagnetic material represents the magnetization that results when all the magnetic dipoles in a solid piece are mutually aligned with the external field; there is a corresponding saturation flux density, Bs. The saturation magnetization is equal to the product of the net magnetic moment for each atom and the number of atoms present. For each of iron, cobalt, and nickel, the net magnetic moments per atom are 2.22, 1.72, and 0.60 Bohr magnetons, respectively. 18 E.A.G.  Antiferromagnetism The alignment of the spin moments of neighboring atoms or ions in exactly opposite directions is termed antiferromagnetism. Manganese oxide (MnO) is one ceramic material that displays this behavior. No net magnetic moment possessed by O, but Mn, atom. Obviously, the opposing magnetic moments cancel one another, and, as a consequence, the solid as a whole possesses no net magnetic moment. 19 E.A.G.  Ferrimagnetism Some ceramics also exhibit a permanent magnetization, termed ferrimagnetism. The macroscopic magnetic characteristics of ferromagnets and ferrimagnets are similar; the distinction lies in the source of the net magnetic moments. For example Fe3O4 can be written as Fe2+O2--(Fe3+)2(O2-)3 in which the Fe ions exist in both 2 and 3 valence states in the ratio of 1:2. 20 E.A.G.  Ferrimagnetism A net spin magnetic moment exists for each Fe2+ and Fe3+ ion, which corresponds to 4 and 5 Bohr magnetons, respectively, for the two ion types. Furthermore, the O2- ions are magnetically neutral. There are antiparallel spin-coupling interactions between the Fe ions, similar in character to antiferromagnetism. However, the net ferromagnetic moment arises from the incomplete cancellation of spin moments. Antiferromagnetism and ferrimagnetism are considered to be subclasses of. ferromagnetism 21 E.A.G.  Temperature and Magnetic Behavior 22 E.A.G.  Temperature and Magnetic Behavior Raising the temperature of a solid results in an increase in the magnitude of the thermal vibrations of atoms. The atomic magnetic moments are free to rotate; hence, with rising temperature, the increased thermal motion of the atoms tends to randomize the directions of any moments that may be aligned. For ferromagnetic, antiferromagnetic, and ferrimagnetic materials, the atomic thermal motions counteract the coupling forces between the adjacent atomic dipole moments, causing some dipole misalignment, regardless of whether an external field is present. This results in a decrease in the saturation magnetization for both ferro and ferrimagnets. 23 E.A.G.  Temperature and Magnetic Behavior The saturation magnetization is a maximum at 0 K, at which temperature the thermal vibrations are a minimum. With increasing temperature, the saturation magnetization diminishes gradually and then abruptly drops to zero at what is called the Curie temperature, Tc. Above Tc both ferromagnetic and ferrimagnetic materials are paramagnetic. Antiferromagnetism is also affected by temperature; this behavior vanishes at what is called the Néel temperature, above which, antiferromagnetic materials also become paramagnetic. 24 E.A.G.  Domains and Hysteresis in a macroscopic piece of material, there will be a large number of domains, and all may have different magnetization orientations. Each domain is magnetized to its saturation magnetization. The magnitude of the M field for the entire solid is the vector sum of the magnetizations of all the domains, each domain contribution being weighted by its volume fraction. For an unmagnetized specimen, the appropriately weighted vector sum of the magnetizations of all the domains is zero. Flux density B and field intensity H are not proportional for ferromagnets and ferrimagnets. If the material is initially unmagnetized, then B varies as a function of H. 25 E.A.G.  Domains and Hysteresis Section 20.7 is your reading H.W. 26 E.A.G.  Hysteresis and the different magnetism 27 E.A.G.  Soft and Hard Magnetic Materials One method of demagnetizing a ferromagnet or ferrimagnet is to repeatedly cycle it in an H field that alternates direction and decreases in magnitude. The area within a loop represents a magnetic energy loss per unit volume of material per magnetization–demagnetization cycle; this energy loss is manifested as heat that is generated within the magnetic specimen and is capable of raising its temperature. Soft magnetic materials are used in devices that are subjected to alternating magnetic fields and in which energy losses must be low; one familiar example consists of transformer cores. 28 E.A.G.  Soft and Hard Magnetic Materials For this reason the relative area within the hysteresis loop must be small; it is characteristically thin and narrow. Consequently, a soft magnetic material must have a high initial permeability and a low coercivity. A material possessing these properties may reach its saturation magnetization with a relatively low applied field (i.e., is easily magnetized and demagnetized) and still has low hysteresis energy losses. 29 E.A.G.  Soft and Hard Magnetic Materials Hard magnetic materials are utilized in permanent magnets, which must have a high resistance to demagnetization. In terms of hysteresis behavior, a hard magnetic material has a high remanence, coercivity, and saturation flux density, as well as a low initial permeability, and high hysteresis energy losses. 30 E.A.G.  Superconductivity As most high-purity metals are cooled down to temperatures nearing 0 K, the electrical resistivity decreases gradually, approaching some small yet finite value that is characteristic of the particular metal. There are a few materials, however, for which the resistivity, at a very low temperature, abruptly plunges from a finite value to one that is virtually zero and remains there upon further cooling. Materials that display this latter behavior are called superconductors, and the temperature at which they attain superconductivity is called the critical temperature, Tc. 31 E.A.G.  Superconductivity At temperatures below Tc, the superconducting state will cease upon application of a sufficiently large magnetic field, termed the critical field which depends on temperature and decreases with increasing temperature. 32 E.A.G.  Magnetic response of superconducting materials Type I materials, while in the superconducting state, are completely diamagnetic; i.e., all of an applied magnetic field will be excluded from the body of material, a phenomenon known as the Meissner effect. As H is increased, the material remains diamagnetic until the critical magnetic field is reached. At this point, conduction becomes normal, and complete magnetic flux penetration takes place 33 E.A.G.  Magnetic response of superconducting materials Type II superconductors are completely diamagnetic at low applied fields, and field exclusion is total. However, the transition from the superconducting state to the normal state is gradual and occurs between lower critical (Hc1) and upper critical (Hc2) fields. The magnetic flux lines begin to penetrate into the body of material at Hc1 and with increasing applied magnetic field, this penetration continues; at field Hc2 penetration is complete. 34 E.A.G.  35 E.A.G.  36 E.A.G.