材料的磁性能
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Prof. Xiaoqin Yan
xqyan@mater.ustb.edu.cn
2012.08
Leabharlann Baidu
Contents
Optical Properties Electrical Properties Thermal Properties Magnetic Properties
Other Properties of Materials (For Self-Study)
(磁致伸缩、磁电阻)
Optical Properties:
Electrooptic, Photoelectric and Magnetooptic Property
(电光、光电、磁光性能)
Acoustic Properties:
Propagation, Absorption and Electroacoustic Property (声音的传播、吸收、电声性能)
Atomic dipole configuration for a paramagnetic material
Schematic representation of B - H for diamagnetic and paramagnetic materials
4.3 FERROMAGNETISM
Elasticity:
Anelasticity and Internal Friction (滞弹性与内耗)
4. Magnetic Properties
4.1 BASIC CONCEPTS
Magnetic Dipoles (磁偶极子): small bar magnets composed of north and south. Magnetic forces (fields) are generated by moving electrically charged particles.
Electrical Properties：
Thermoelectric, Pyroelectric and Magnetoelectric Property (热电、热释电、磁电性能)
Magnetic Properties： Magnetostriction and Magnetoresistance
Magnetic field lines of force around a current loop and a bar magnet
The magnetic moment as designated by an arrow
Magnetic field strength (磁场强度) H: the externally applied magnetic field. H=NI / l (4.1) A cylindrical coil (or solenoid): N closely spaced turns, length l, current of magnitude I. Unit of H: amperes per meter. Magnetic field strength within a coil—dependence on number of turns, applied current, and coil length. Magnetic induction (磁感应强度) / magnetic flux density (磁通密度) B: the magnitude of the internal field strength within a substance that is subjected to an H field. B = μH (4.2) Permeability (磁导率) μ: The property of the specific medium through which the H field passes and in which B is measured. The permeability has dimensions of webers per ampere-meter (Wb/A m) or henries per meter (H/m). Both B and H are field vectors. Magnetic flux density in a material— dependence on permeability and magnetic field strength.
When placed between the poles
of a strong electromagnet, diamagnetic materials are attracted toward regions where the field is weak. This form of magnetism is of no practical importance.
A coil and the H, B
Magnetization (磁化强度) M: B = μ0 H + μ0 M (4.5) Magnetic susceptibility (磁化率) χm,(unitless): M = χmH (4.6) Magnetization of a material — dependence on susceptibility and magnetic field strength. Relationship between magnetic susceptibility and relative permeability: χm = μr - 1 (4.7) Origins of Magnetic Moments Each electron in an atom has magnetic moments that originate from two sources: orbital moments and spin moments. In each individual atom, orbital moments of some electron pairs cancel each other; this also holds for the spin moments. The net magnetic moment for an atom is 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, there is total cancellation of both orbital and spin moments, and is not capable of being permanently magnetized (He, Ne, Ar, etc.).
In a vacuum:
B0 = μ0H (4.3) Permeability of a vacuum (真空磁导率) μ0: a universal constant, which has a value of 4π x 10-7 (1.257 x 10-6) H/m. B0: the flux density within a vacuum. μ r = μ / μ0 (4.4) Relative permeability (相对磁导率) μr: a measure of the degree to which the material can be magnetized, or the ease with which a B field can be induced in the presence of an external H field.
Atomic dipole configuration for a diamagnetic material
Paramagnetism (顺磁性) 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. Atomic dipoles are free to rotate, and acted on individually with no mutual interaction between adjacent dipoles. μr is greater than unity, and to a relatively small but positive magnetic susceptibility. χm range from about 10-5 to 10-2. Both diamagnetic and paramagnetic materials are considered nonmagnetic.
Ferromagnetism(铁磁性): A permanent magnetic moment in the absence of an external field, and manifest very large and permanent magnetizations. Fe (as α-ferrite), Co, Ni, and some of the rare earth metals such as Gd. χm as high as 106, and H << M: B ≈ μ0 M (4.8) Permanent magnetic moments result from atomic magnetic moments due to uncancelled electron spins and a smaller orbital magnetic moment. Coupling interactions cause net spin magnetic moments of adjacent atoms to align with one another, even in the absence of an external field. Domain (磁畴): Mutual spin alignment exists over relatively large-volume regions of the crystal. Saturation magnetization (饱和磁化强度) Ms: The maximum possible magnetization. There is also a corresponding saturation flux density (饱和磁通密 度) Bs.
4.2 DIAMAGNETISM AND PARAMAGNETISM
Diamagnetism (抗磁性/逆磁性/反磁性) Very weak form of magnetism that is nonpermanent, and persists only while an external field is being applied. 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. μr is less than unity (only very slightly), and χm is negative; B is less than that in a vacuum, χm is on the order of -10-5.
xqyan@mater.ustb.edu.cn
2012.08
Leabharlann Baidu
Contents
Optical Properties Electrical Properties Thermal Properties Magnetic Properties
Other Properties of Materials (For Self-Study)
(磁致伸缩、磁电阻)
Optical Properties:
Electrooptic, Photoelectric and Magnetooptic Property
(电光、光电、磁光性能)
Acoustic Properties:
Propagation, Absorption and Electroacoustic Property (声音的传播、吸收、电声性能)
Atomic dipole configuration for a paramagnetic material
Schematic representation of B - H for diamagnetic and paramagnetic materials
4.3 FERROMAGNETISM
Elasticity:
Anelasticity and Internal Friction (滞弹性与内耗)
4. Magnetic Properties
4.1 BASIC CONCEPTS
Magnetic Dipoles (磁偶极子): small bar magnets composed of north and south. Magnetic forces (fields) are generated by moving electrically charged particles.
Electrical Properties：
Thermoelectric, Pyroelectric and Magnetoelectric Property (热电、热释电、磁电性能)
Magnetic Properties： Magnetostriction and Magnetoresistance
Magnetic field lines of force around a current loop and a bar magnet
The magnetic moment as designated by an arrow
Magnetic field strength (磁场强度) H: the externally applied magnetic field. H=NI / l (4.1) A cylindrical coil (or solenoid): N closely spaced turns, length l, current of magnitude I. Unit of H: amperes per meter. Magnetic field strength within a coil—dependence on number of turns, applied current, and coil length. Magnetic induction (磁感应强度) / magnetic flux density (磁通密度) B: the magnitude of the internal field strength within a substance that is subjected to an H field. B = μH (4.2) Permeability (磁导率) μ: The property of the specific medium through which the H field passes and in which B is measured. The permeability has dimensions of webers per ampere-meter (Wb/A m) or henries per meter (H/m). Both B and H are field vectors. Magnetic flux density in a material— dependence on permeability and magnetic field strength.
When placed between the poles
of a strong electromagnet, diamagnetic materials are attracted toward regions where the field is weak. This form of magnetism is of no practical importance.
A coil and the H, B
Magnetization (磁化强度) M: B = μ0 H + μ0 M (4.5) Magnetic susceptibility (磁化率) χm,(unitless): M = χmH (4.6) Magnetization of a material — dependence on susceptibility and magnetic field strength. Relationship between magnetic susceptibility and relative permeability: χm = μr - 1 (4.7) Origins of Magnetic Moments Each electron in an atom has magnetic moments that originate from two sources: orbital moments and spin moments. In each individual atom, orbital moments of some electron pairs cancel each other; this also holds for the spin moments. The net magnetic moment for an atom is 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, there is total cancellation of both orbital and spin moments, and is not capable of being permanently magnetized (He, Ne, Ar, etc.).
In a vacuum:
B0 = μ0H (4.3) Permeability of a vacuum (真空磁导率) μ0: a universal constant, which has a value of 4π x 10-7 (1.257 x 10-6) H/m. B0: the flux density within a vacuum. μ r = μ / μ0 (4.4) Relative permeability (相对磁导率) μr: a measure of the degree to which the material can be magnetized, or the ease with which a B field can be induced in the presence of an external H field.
Atomic dipole configuration for a diamagnetic material
Paramagnetism (顺磁性) 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. Atomic dipoles are free to rotate, and acted on individually with no mutual interaction between adjacent dipoles. μr is greater than unity, and to a relatively small but positive magnetic susceptibility. χm range from about 10-5 to 10-2. Both diamagnetic and paramagnetic materials are considered nonmagnetic.
Ferromagnetism(铁磁性): A permanent magnetic moment in the absence of an external field, and manifest very large and permanent magnetizations. Fe (as α-ferrite), Co, Ni, and some of the rare earth metals such as Gd. χm as high as 106, and H << M: B ≈ μ0 M (4.8) Permanent magnetic moments result from atomic magnetic moments due to uncancelled electron spins and a smaller orbital magnetic moment. Coupling interactions cause net spin magnetic moments of adjacent atoms to align with one another, even in the absence of an external field. Domain (磁畴): Mutual spin alignment exists over relatively large-volume regions of the crystal. Saturation magnetization (饱和磁化强度) Ms: The maximum possible magnetization. There is also a corresponding saturation flux density (饱和磁通密 度) Bs.
4.2 DIAMAGNETISM AND PARAMAGNETISM
Diamagnetism (抗磁性/逆磁性/反磁性) Very weak form of magnetism that is nonpermanent, and persists only while an external field is being applied. 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. μr is less than unity (only very slightly), and χm is negative; B is less than that in a vacuum, χm is on the order of -10-5.