Magnetic Materials PDF

MAGNETIC MATERIALS MCP, PH121 Ridhima Gahrotra 1  INTRODUCTION The phenomenon of magnetism is the one by which a material exerts either attractive or repulsive force on another. The fundamental source of magnetism is the rotation of electrically charged particles....

MAGNETIC MATERIALS MCP, PH121 Ridhima Gahrotra 1  INTRODUCTION The phenomenon of magnetism is the one by which a material exerts either attractive or repulsive force on another. The fundamental source of magnetism is the rotation of electrically charged particles. Thus magnetic behavior of a material can be drawn from the structure of atoms. The electrons in atoms rotate around the nucleus in circular orbits. This orbital motion and its own spin cause magnetic moments on the atoms, which contribute to the magnetic behavior of materials. Thus every material can respond to a magnetic field. However, the manner in which a material responds depends much upon its atomic structure, and determines whether the material will have strong or weak magnetic properties. MCP, PH121 Ridhima Gahrotra 2  Iron, some steels, and the naturally occurring mineral lodestone are well known examples of materials that exhibit magnetic properties. MCP, PH121 Ridhima Gahrotra 3  Magnetic Dipoles Magnetic dipoles are found to exist in magnetic materials, like the electric dipoles. A magnetic dipole is a small magnet composed of north and south poles instead of positive and negative charges. 4 MCP, PH121 Ridhima Gahrotra  Magnetic Field Strength The magnetic field strength is the externally applied magnetic field denoted by H. The magnetic field generated by means of a cylindrical coil (or solenoid) consisting of N closely spaced turns, having a length l and carrying a current i is given by. The units of H are ampere-turns per meter, or just amperes per meter. Intensity of Magnetisation (I) It is defined as the magnetic moment per unit volume of the magnetized substance Magnetic Susceptibility (χm) It is the ratio of the magnetic moment per unit volume (I) to the magnetic field strength (H) of the magnetizing field. It is positive for a paramagnetic material and negative for a diamagnetics. MCP, PH121 Ridhima Gahrotra 5  Relative Permeability (µr) It is the ratio of the magnetic permeability (µ) of the substance to thepermeability of the free space (µ0). Magnetic Flux Density 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 tesla or weber per square meter. Both B and H are magnetic field vectors. The relation between magnetic field strength and flux density is given by B = H, Where is the permeability of a material which is a measure of the degree to which the material can be magnetized, or the ease with which a magnetic field (B) can be induced in the presence of an external field H. The magneticflux density due to magnetization in material can be written as below B= MCP, PH121 Ridhima Gahrotra 6 MCP, PH121 Ridhima Gahrotra 7  Classification of Magnetism Magnetic materials can be classified mainly into three categories namely diamagnetic, paramagnetic and ferromagnetic. Ferromagnetic MCP, PH121 Ridhima Gahrotra 8  Paramagnetic Materials Diamagnetic Materials Ferromagnetic Materials These materials have Very small These materials have Very small These materials have positive but positive magnetic but negative susceptibility and large magnetic susceptibility susceptibility (~10-6) (~ 10-6) (~106) The relative permeability is µr is slightly less than unity The µr for a ferromagnetic slightly more than unity (µr˃1) (µr˂1)(-ve) material is of the order of few thousands The magnetic susceptibility The magnetic susceptibility of The magnetic susceptibility depends strongly on temperature diamagnetic materials is almost decreases with increase in and varies inversely with independent of temperature temperature temperature When a bar of a paramagnetic When a bar of these materials is When a bar of these materials is material is suspended between suspended between the poles of suspended between the poles of the poles of a magnet, it stays a magnet, it stays perpendicular a magnet, it behaves like a parallel to the lines of force. to the magnetic field paramagnetic material If these materials are placed in a If these materials are placed in a These materials behave like non-uniform field, they are non-uniform field, they are Paramagnetic substances, if attracted towards the stronger attracted towards the weaker placed in a non uniform field field field 9 MCP, PH121 Ridhima Gahrotra  CLASSICAL THEORY OF FERROMAGNETISM Ferromagnetic materials show spontaneous magnetization due to internal field arising as a result of mutual interactions between magnetic domains. When placed in external magnetic field they acquire very large and permanent magnetization in the direction of applied field. Each atom of ferromagnetic material has a permanent magnetic moment like the paramagnetic substances. In general, a specimen of a ferromagnetic substance contains a number of small regions called domains. These domains are typically very small (about 50 μm) or less and contain a large number of atoms, nearly 1017 to 1022, and have the dimensions of about 10-6 cm3 to 10-2cm3. Each domain consists of magnetic moments that are aligned, giving rise to a permanent net magnetic moment per domain. Each of these domains is separated from the rest by domain boundaries called Bloch walls which are about 100 nm thick. Domains exist even in the absence of external field. In a material that has never been exposed to a magnetic field, the individual domains have a random orientation. This type of arrangement represents the lowest free energy. MCP, PH121 Ridhima Gahrotra 10  When the bulk material is unmagnetized (no external magnetic field is applied), the net magnetization of these domains is zero, because adjacent domains are orientated randomly in any direction, effectively canceling each other’s out figure (a). When a magnetic field is applied on the material, domains that are nearly lined up with the field (favourable domains) grow at the expense of unaligned domains figure (b). The Bloch walls move, the external field provides the force required for this movement and this process continues until only the most favorably oriented domains remains. When the domain growth is completed, a further increase in the magnetic field causes the domains to rotate and align parallel to the applied field figure (c). At this instant material reaches saturation magnetization and no further increase in magnetizationwill take place on increasing the strength of the external field. Under these conditions the permeability of these materials becomes quite small. The variation of magnetization with applied magnetic field H is shown is figure below. 11 MCP, PH121 Ridhima Gahrotra Materials with ferro-magnetism (e.g. Fe, Co, Ni, Gd) possess magnetic susceptibilities ~ 106. Above the Curie temperature, ferro-magnetic materials behave as para-magnetic materials and their susceptibility is given by the Curie-Weiss law, defined as Where C is the Curie constant, T being temperature and Tc is called Curie temperature. MCP, PH121 Ridhima Gahrotra 12  Antiferromagnetism: In anti ferromagnetic materials the magnetic moments of neighboring electrons point in opposite direction. Therefore, it has zero net magnetic moment. In these materials the alignment of magnetic movement of the atoms are combinations of both parallel and anti parallel. Ferrimagnetism: In ferrimagnetic materials, the opposing moments are unequal and a spontaneous magnetization remains. MCP, PH121 Ridhima Gahrotra 13 Variation of Susceptibility with temperature MCP, PH121 Ridhima Gahrotra 14  Magnetic Hysteresis The magnetization behaviour of the ferromagnetic materials is described by the B-H curve (hysteresis loop) as shown in figure. (i) Retentivity - It is the ability of a material' to retain a certain amount of residual magnetic field when the magnetizing force is removed after achieving saturation. (ii) Coercive force (field) - The amount of reverse magnetic field which mustbe applied to a magnetic material to make the magnetic flux inside the material to return tozero. (iii) Permeability - A property of a material that describes the ease withwhich a magnetic flux is established in the component. MCP, PH121 Ridhima Gahrotra 15  Energy Loss Due to Hysteresis During the process of magnetization and demagnetization, a loss of energy is always involved in aligning the domains (motion of domain walls and rotation of dipoles) in the direction of the applied magnetic field. When the direction of an external magnetic field is reversed, the absorbed energy is not completely recovered and rest energy in sample is lost in the form of heat. This loss of energy is called hysteresis loss. Calculation of Hysteresis Loss It can be proved that the energy lost per unit volume of the substance in a complete cycle ofmagnetization is equal to the area of the hysteresis loop. We consider a unit volume of the ferromagnetic substance, which has N magnetic domains. Let M be the magnetic moment of each magnetic domain which makes an angle θ with the direction of the magnetic field H. So, the total magnetic moment per unit volume in the direction of magnetizing field MCP, PH121 Ridhima Gahrotra 16 MCP, PH121 Ridhima Gahrotra 17 MCP, PH121 Ridhima Gahrotra 18 TYPES OF MAGNETIC MATERIALS MCP, PH121 Ridhima Gahrotra 19  INTRODUCTION TO NANOMATERIALS A nanometer is one millionth of a millimeter - approximately 100,000 times smaller than the diameter of a human hair. Nano dwarf Small objects MCP, PH121 Ridhima Gahrotra 20  SUPERPARAMAGNETISM The Magnetic Nano Particles (MNPs) might be considered to be composed of a single magnetic domain if its size decreses below a critical level. It might display a superparamagnetic behaviour as long as the temperature is above a particular temperature which is called Blocking Temperature. MNPs possess large magnetic moment that changes orientation continuously. At temperatures below, magnetic moments of the nanoparticles freeze in random orientations. Blocked and unblocked states in superparamagnetic materials MCP, PH121 Ridhima Gahrotra 21 The magnetization curve of superparamagnetic materials is not defined by a hysteresis curve as observed for the ferromagnetic materials. The curve is reversible with no remanent magnetization and no coercivity. B-H curve of superparamagnetic materials Comparison among M-H curves of paramagnetic, ferromagnetic and superparamagnetic materials MCP, PH121 Ridhima Gahrotra 22 MCP, PH121 Ridhima Gahrotra 23 Time for Assignment Questions (5 min) MCP, PH121 Ridhima Gahrotra 24 MCP, PH121 Ridhima Gahrotra 25 MCP, PH121 Ridhima Gahrotra 26