domain theory hysteresis loop and magnetioresistance.pptx

Domain theory and hysteresis loop

1. MATERIAL SCIENCE SPECIAL MATERIALS DOMAIN THEORY HYSTERESIS LOOP APPLICATIONS SOFT AND HARD MAGNETIC MATERIALS MAGNETO RESISTANCE AND GIANT MAGNETORESISTANCE KIRUTHIKA M, I M.Sc., CHEMISTRY 2023-2025

2. DOMAIN THEORY OF FERRO MAGNETISM: This theory was proposed by Weiss in 1907. It explains the hysteresis and the properties of ferromagnetic materials. Postulates of Domain theory : A ferromagnetic material - divided into a large number of small region called Domains (0.1 to 1 of area) In each domain the magnetic moments are in same direction. Magnetic moment varies from domain to domain Net magnetization is zero. In the absence external magnetic field all the magnetic moments are in different direction. When a magnetic field is applied there are two process takes place By the motion of domain walls By the rotation of domains.

3. By the motion of domain wall: when a small amount of magnetic field applied- the dipoles aligned parallel - increases domain area by the motion of domain walls. By the rotation of Domains On further increased, the domains are rotated parallel to the field direction by the rotation of domains.

4. Origin of Domain theory of Ferromagnetism: Total internal energy of the domain structure in a ferromagnets Exchange energy (or) Magnetic field energy Crystalline energy (or) Anisotropy energy Domain wall energy (or) Bloch wall energy Magnetostriction energy 1. Exchange energy (or) Magnetic Field energy “The interaction energy which makes the adjacent dipoles align themselves” Assembling atomic magnets into a single domain. The volume of the domain varies range :10^–2 to 10^–6cm

5. 2. Anisotropy energy The excess energy is required to magnetize a specimen along the hard direction . In ferromagnetic materials - two directions of magnetization, Easy direction - weak field can apply Hard directions – strong field can apply Crystalline anisotropy energy is energy of magnetization - function of crystal orientation. 3. Domain wall energy or Bloch wall energy A thin boundary or region that separates adjacent domains in different directions . Size of the Bloch walls 200 to 300 lattice constant thickness. The energy of domain wall is due to both exchange energy and anisotropic energy. Based on the spin alignments Thick wall: Spins at the boundary are misaligned and if the direction of the spin changes gradually - leads to a thick Bloch wall.

6. Thin wall: When the spins at the boundaries changes abruptly resulting magnetization in the domain wall

7. 4. Magetostriction energy When a material is magnetized, it is found that it suffers a change in dimensions – Magnetostrictive . work done by the magnetic field against the elastic restoring forces is called magneto-elastic energy or Magnetostrictive energy.

8. Domain Theory and Hysteresis Loop: Magnitude of the M field for the entire solid is the vector sum of the magnetizations of all the domains – Each domain contribution is by its volume fraction. Flux density and Field density H, is not proportional for ferro & ferri -magnets. Curve description: Point U: No net B (or M ) field. Moments of the constituent domains are randomly oriented Point V – X: As the external field is applied, the domains that are oriented in unfavorable directions are nearly aligning themselves with the applied field. P oint Y : with increasing field strength, the macroscopic specimen becomes a single domain, which is nearly aligned with the field. Point Z : Saturation is achieved when this domain, by means of rotation, becomes oriented with the H field

9. Point U to Z is equivalent to point O to S in the curve. Point S : As the H field is reduced by reversal of field direction, hysteresis effect is produced. Point R : At H=0 there exists a residual B field called the remanence, or remanent flux density, B r , point C : To reduce the B field within the specimen to zero, As H field of magnitude − Hc must be applied in a direction opposite to that of the original field; Hc is called the coercivity, or sometimes the coercive force. Point S’: Upon continuation of the applied field in this reverse direction, saturation is ultimately achieved in the opposite sense. A second reversal of the field to the point of the initial saturation (point S ) completes the symmetrical hysteresis loop and also yields both a negative remanence (− B r ) and a positive coercivity (+ H c ).

10. Hysteresis behavior and permanent magnetization - explained by the motion of domain walls. Upon reversal of the field direction from saturation the process, Rotation of the single domain with the reversed field. Domains having magnetic moments aligned with the new field form Grow at the expense of the former domains. Critical to this explanation is the resistance to movement of domain walls that occurs in response to the increase of the magnetic field in the opposite direction. A hysteresis curve at less than saturation (curve NP ) within the saturation loop for a ferromagnetic material. The B – H behavior for field reversal at other than saturation is indicated by curve LM .

11. Why PARAMAGNETIC AND DIAMAGNETIC MATERIALS are considered as non-magnets.

12. The graph provides comparison of B -versus- H behaviors for ferromagnetic/ferrimagnetic and diamagnetic/paramagnetic materials.

13. A pplication of hysteresis loop: Magnetic recording: In magnetic recording devices like hard drives and magnetic tapes 2) Magnetic Sensors: Used in automotive sensors, position sensors and compasses 3) Electric Transformers and Inductors: Designing efficient transformers and inductors. 4) Magnetic Materials Characterization: Material scientists and Engineers use hysteresis loops to characterize magnetic materials. 5) Magnetic Memory devices: Design of magnetic memory devices like Magnetic Random Access Memory (MRAM) for non-volatile data storage. 6) Microwave Devices: In microwave devices, it affect the performance of magnetic materials used in components such as circulators and isolators.

14. Both ferro- and ferrimagnetic materials are classified as either soft or hard on the basis of their hysteresis characteristics.

15. SOFT MAGNETIC MATERIALS: Soft magnetic materials are those materials that are easily magnetized and demagnetized ie ., domain walls can easily migrate which makes the ferromagnetic materials to magnetise easily at low magnetic field. This type of ferromagnetic materials is called soft magnetic materials. Relative area of hysteresis loop must be small; thin and narrow Having a high initial permeability μ initial 10^2 -10^5 maximum permeability μ max 10^3-10^5 A low coercivity. < 1000 A/m Possess low hysteresis energy losses (magnetized and demagnetized by relatively low applied field) Possess high electrical resistivity in order to reduce the eddy current loss. Possess small number of defects such as crystal grain.

16. APPLICATION OF SOFT MAGNETIC MATERIALS:

17. Energy loss Hysteresis loss - it is related with the area inside the hysteresis loop. Eddy current loss – generation of electric currents in the magnetic material and the related resistive losses. Anomalous loss – movement of domain walls inside the material.

18. HARD MAGNETIC MATERIALS : (permanent magnets)/(HARD FERROMAGNETIC MATERIALS) Hard magnetic materials are those materials that retain their magnetism after being magnetised. In other words, these types of ferromagnetic materials are difficult to magnetise, but once magnetised, it is difficult to demagnetise. Domain walls are difficult to migrate. i.e., magnetization of the ferromagnetic materials occurs only when high magnetic filed is applied. high retentivity/ remenance high saturation flux density, low initial permeability high hysteresis energy losses. High magnetocrystalline anisotropy High coercivity Magnetism is permanent.

19. Coercivity- Energy product ( BH )max ( BH )max is larger harder the material Hard magnetic materials fall within two main categories. conventional hard magnets high energy hard magnets Conventional hard magnets: The conventional materials have ( BH )max values that range between about 2 and 80 kJ/m^3 These include ferromagnetic materials—magnet steels, Cunife (Cu–Ni–Fe) alloys, and Alnico (Al–Ni–Co) alloys—as well as the hexagonal ferrites ( BaO –6Fe2O3).

20. High energy permanent magnets: ( BH )max in excess of about 80 kJ/m3 These are recently developed intermetallic compounds that have a variety of compositions. T wo commercial exploitation SmCo5 and Nd 2 Fe 14 B Samarium–Cobalt Magnets Samarium–cobalt, SmCo5, is a member of a group of alloys that are combinations of cobalt or iron and a light rare earth element. It exhibit high-energy, hard magnetic behavior. The energy product ( BH )max, of these SmCo5 materials [typically between 120 and 240 kJ/m3 considerably higher than those of the conventional hard magnetic materials. Relatively large coercivities. Powder metallurgical techniques are used to fabricate SmCo5 magnets.

21. Neodymium–Iron–Boron Magnets Neodymium–Iron–boron, Nd 2 Fe 14 B, alloys wide diversity of applications. Coercivities - energy products ( BH )max, 255 kJ/m 3 . are relatively high. The magnetization–demagnetization behavior of these materials is a function of domain wall mobility, Two different processing techniques are available for the fabrication of Nd2Fe14B magnets: powder metallurgy (sintering) and rapid solidification (melt spinning). Application of H ard magnetic materials: Familiar motor applications - cordless drills and screwdrivers. In automobiles (starters; window winders, wipers, and washers; fan motors). In audio and video recorders; clocks; speakers in audio systems, lightweight earphones, and hearing aids; and computer peripherals.

22. SI.NO PROPERTIES SOFT MAGNETIC MATERIALS HARD MAGNETIC MATERIALS 1 When external field is removed Magnetisation disappears Magnetisation persists 2 Area of the loop small Large 3 retentivity low High 4 Coercivity low High 5 Susceptibility and magnetic permeability high Low 6 Hysteresis loss less More 7 Movement of domain wall Domain walls of soft magnetic materials can move easily Domain walls of hard magnetic materials do not move easily 8 uses Solenoid core, transformer core and electromagnets Permanent magnets 9 examples Soft iron, Mumetal , Stalloy etc Steel, Alnico, Lodestone, etc. SUMMARY: DIFFERENCE BETWEEN SOFT AND HARD MAGNETIC MATERIALS

23. MAGNETORESISTANCE: The resistance of some of the metal and the semiconductor material varies in the presence of the magnetic field, this effect is called the magnetoresistance. The element which has these effects is known as the magnetoresistor . The magnetoresistor is used for determining the presence of a magnetic field their strength and the direction of the force . It is made of the indium antimonide or indium arsenide semiconductor material. The resistance of the magneto resistor is directly proportional to the magnetic field. The magnetoresistor operates without physical contacts which is their major advantage. A pplication : hard disk of the computer An electronic compass

24. Working Principle of Magnetoresistor : It works on the principle of electrodynamics , which states that the force acting on the current placed in the magnetic field changes their direction. In the unavailability of the magnetic field, the charge carriers of the magneto resistor move in the straight path . In the presence of the magnetic field, the direction of the current becomes changes, and it flows in the different direction. The indirect path of the current increases the mobility of their charge carrier which causes the collision. The collision increases the loss of energy in the form of heat. This heat increases the resistance of the magneto resistor. The current of very small magnitude flows in the magnetoresistor because of few free electrons. The deflection of the magnetoresistor electrons depends on their mobility. It is more in the semiconductor material as compared to the metals. The mobility of the indium arsenides or indium antimonides is approximately 2.4m2 /Vs.

25. The Effect of Magnetic Fields on Resistance Resistance is caused by collisions between charge carriers. An electron moving through a perfect crystal of metal at a temperature of absolute zero will experience no collisions, so the crystal is in zero resistance. Imperfections and temperatures above absolute zero cause the atoms to vibrate out of their lattice locations . causes collisions, increasing the resistance of the crystal. Applying a magnetic field can also increase the resistance of a material, since the magnetic force on the moving charges will tend to increase the number of collisions between charges.

26. Characteristic of Magnetoresistor The sensitivity of the magnetoresistor depends on the strength of the magnetic field. In the absence of the magnetic field, the magnetization of the element becomes zero. When the magnetic field slightly increases the resistance of the material reaches near to b. The magnetoresistor element moves by an angle of 45º because of the presence of a magnetic field. The further increases in the magnetic field make the curve saturates, point C. The magnetoresistive element either operates at O or at b.

27. Types of Magneto resistor The magnetoresistor is classified into three categories. Giant Magnetoresistance (GMR) Extraordinary Magnetoresistance Tunnel Magneto Resistor Giant Magnetoresistance (GMR) In this effect, The resistance of the magneto resistor - small - ferromagnetic layers are parallel to each other. The resistance - very high - antiparallel alignment of the layer.

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29. REFERENCES: Material Science and Engineering, by “William D Callister, Jr., David G Rathwish ”. Lecture Material from Rohini College of Engineering 303. L17 material Epg pathshala Images Wikipedia William D Callister - Material Science and Engineering book Research Gate V ideos Y outube Channel : Physics Videos by Eugene Khutoryansky – Hysteresis Loop

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