FERROMAGNETIC AND ANTIFERROMAGNETIC MATERIALS

3. Ferromagnetic Materials • Ferromagnetism • Ferromagnetism  comes from the term ‘ferrous’ meaning iron, the first type of metal discovered to exhibit attraction to magnetic fields. • Ferromagnetism is the basic method in which a compound forms a permanent magnet or is attracted to a magnetic field.  • ferromagnetism, physical phenomenon in which certain electrically uncharged materials strongly attract others.  • Two materials found in nature, lodestone (or magnetite, an oxide of iron, Fe3O4) and iron, have the ability to acquire such attractive powers, and they are often called natural ferromagnets.  • They were discovered more than 2,000 years ago, and all early scientific studies of magnetism were conducted on these materials. • Ferromagnetic materials have large, positive susceptibility to an external magnetic field. • Ferromagnetic materials exhibit parallel alignment of moments resulting in large net magnetization even in the absence of a magnetic field. • In ferromagnetic materials, when a magnetizing force is applied, the magnetic domains become aligned to produce a strong magnetic field within the material. • Example: iron, nickel, cobalt etc. • Ferromagnetism is the basic mechanism by which certain materials form permanent magnets or they are attracted towards the magnets. • Today, ferromagnetic materials are used in a wide variety of devices essential to everyday life—e.g., Transformers, Electromagnets, Magnetic tape recording, Hard drives, Generators, Telephones, Loudspeakers, Electric motors, Hard disk and Magnetic Storage.

5. Continue… • Two distinct characteristics of ferromagnetic materials are their • (1) Spontaneous magnetization   • (2) Existence of magnetic ordering temperature • Spontaneous magnetization :- The spontaneous magnetization is the net magnetization that exists inside a uniformly magnetized microscopic volume in the absence of a field. The magnitude of this magnetization, at 0 K, is dependent on the spin magnetic moments of electrons. A related term is the saturation magnetization which we can measure in the laboratory. The saturation magnetization is the maximum induced magnetic moment that can be obtained in a magnetic field (Hsat); beyond this field no further increase in magnetization occurs.

6. Existence of magnetic ordering temperature:- Magnetic Ordering Temperature or Neel Temperature is the temperature above which an Anti-Ferro-magnetic or Ferri-magnetic material becomes Para-magnetic. Thermal Ferrites are materials that exhibit this phenomenon.

7. Ferromagnetic Ordering • Magnetic moments originate on an atomic scale from the orbital and spin motion of electrons. These effects are also influenced by the electronic configuration of the elements. • Electrons in matter have two properties- Spin and Orbit. • In materials, their magnetic properties are observed due to the spins of electrons rather than their orbital moments. • The orbital moment contribution is negligible in comparison to the spin of the electrons. • Ferromagnetic ordering of magnetic moments means they are aligned parallel to one another which is accomplished by a ferromagnetic exchange interaction. • The characteristic temperature of ferromagnets is the curie temperature at which the susceptibility diverges. • Ferromagnetism in magnetic material is a special case of paramagnetism in which the individual spin magnetic moment interacts I.e. the moments are coupled. • In crystals of a ferromagnetic materials the net magnetic moment observed due to the coupling of spins in a preferred orientation . • quantum mechanical in nature. • Ferromagnetic materials have strong ,positive magnetic susceptibility unlike paramagnets. • When the applied field is removed, ferromagnetic material retains a component of magnetization in the direction of the applied field- they are permanently magnetized and they have hysteresis property

8. ELECTROAGNETS Hysteresis loop: 1)Retentivity: When applied magnetized field is removed the magnetism B or I that remains in material is called retentivity or remenant magnetism. 2)Coercivity: or Coercive force: The Magnetism force or H applied in negative direction to make retentivity zero is called coercivity. Electromagnetic magnetics are made from materials with high retentivity and low coercivity. If the energy required to make that material is low (low hysteresis loop Area) then it is good electromagnet. Materials like alnico, Al, Fe, Ni, Co and carbon steel are used for making electromagnets. Soft Magnet/Temporary Magnet: Materials which show magnetic properties only in presence of external magnet. Hard Magnetic: Permanent Magnet.

9. EXCHANGE FIELD • If a paramagnet consisting of N ions of spin S. • Suppose, there is some local interaction which produces a magnetic field I.e., the exchange field or Weiss field-B. • According to mean-field approximation B=(lambda)*M M: Magnetization

10. Anti-Ferromagnetic Materials • Antiferromagnetic structures were first shown through neutron diffraction of transition metal oxides such as nickel, iron, and manganese oxides. The experiments, performed by Clifford Shull, gave the first results showing that magnetic dipoles could be oriented in an antiferromagnetic structure. • Antiferromagnetic materials occur commonly among transition metal compounds, especially oxides. Examples include hematite, metals such as chromium, alloys such as iron manganese (FeMn), and oxides such as nickel oxide (NiO). • There are also numerous examples among high nuclearity metal clusters. • Organic molecules can also exhibit antiferromagnetic coupling under rare circumstances, as seen in radicals such as 5-dehydro-m-xylylene. • Antiferromagnets can couple to ferromagnets, for instance, through a mechanism known as exchange bias, in which the ferromagnetic film is either grown upon the antiferromagnet or annealed in an aligning magnetic field, causing the surface atoms of the ferromagnet to align with the surface atoms of the antiferromagnet. • This provides the ability to "pin" the orientation of a ferromagnetic film, which provides one of the main uses in so-called spin valves, which are the basis of magnetic sensors including modern hard disk drive read heads. • The temperature at or above which an antiferromagnetic layer loses its ability to "pin" the magnetization direction of an adjacent ferromagnetic layer is called the blocking temperature of that layer and is usually lower than the Néel temperature.

11. Anti Ferromagnetic Ordering • In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usually related to the spins of electrons, align in a regular pattern with neighboring spins (on different sublattices) pointing in opposite directions. This is, like ferromagnetism and ferrimagnetism, a manifestation of ordered magnetism. • Generally, antiferromagnetic order may exist at sufficiently low temperatures, but vanishes at and above the Néel temperature – named after Louis Néel, who had first identified this type of magnetic ordering. Above the Néel temperature, the material is typically paramagnetic.