weiss molecular theory of ferromagnetism

Weiss' Theory (Domain theory of ferromag : According to weiss, a feromagnetic substance. contains atoms with permanent magnetic. moments, as in a paramagnetic substance, but due to special form of interaction.

1. ASSIGNMENT ON Weiss molecular theory & ferromagnetism Submitted To: Dr . Manju tiwari Submitted by:- Shraddha Sahu Prachi M anhar Rishita M arkande Santosh Khute Bhupendra kumar Latia SVCAET & RS RAIPUR

2. Content:- Introduction Ferromagnetism & Ferromagnetic material Weiss Molecular Theory Ferromagetism Domains Theory Internal molecular field Application of Ferromagnetism Conclusion

3. Introduction- Diamagnetic and paramagnetic material get magnetised in external magnetic field. Their magnetisation to the strength of external magnetic field. When external magnetic field is removed diamagnetic and paramagnetic materials gets unmagnetised . That means diamagnetic and paramagnetic materials can’t be used to make permanent magnets.

4. Ferromagnetism & Ferromagnetic material Substance which are used to make permanent magnets called ferromagnetic materials. Ferromagnetic materials have + ve value of susceptiblity The elemants fe , Ni & co and many theis alloys are typical ferromagnetic materials. Ferromagnetism is a property of many materials like Fe . Ferromagnetism is strongest type of magnetism

5. Weiss molecular theory In 1907 Weiss postulated domain theroy to explain ferromagnetism This theory consist of 2 postulates (1) Ferromagnetic Domains (2) Internal molecular field

6. DOMAIN THEORY Ferromagnetic Domains A ferromagnetic material is divided into a large number of small region is called domains. Each domain is spontaneously magnetized. The direction of magnetization various from domain to domain and the net magnetization is zero , in the absence external mag. Field. The two domain separates by domain wall or Black wall.

7. When the magnetic field is applied to the Ferromagnetic material , the magnetization produced by two ways By the motion of Domain walls The weak magnetic field is applied , the domains having dipoles parallel to the applied magnetic field increases in area by the motion of domain walls. 2. By the rotation of Domains if the strong magnetic field applied, the domains are parallel to the field direction .

8. Internal Molecular Field Theory Spontaneous magnetigation is because of interaction between dipoles called EXCHANGE COUPLING magnetization in ferromagnetic substance even in absence of external magnetic field Acording to 2 nd postulate Spontaneous magnetism of each domains the absence of external magnetic field Due to the presence of internal moleculer field called exchange field on weiss field

9. The intenal molecules field tends to arrange the atoms in such a way that their magnetic dipole moments are parallel to each other producing strong spontaneous magnetism in each domain. Spontaneous magnetism ( M s ) is the magnetic moment per unit volume or mass. In orderd materials , the exchange interact promotes the magnetic moments of the atoms to be parallel, and a net saturation magnetigation appears spontaneously within a domain.

10. Introducing the ideas of domains Weiss assume the relat ion The spontaneass magnetisation of each domains is due to the presence of an exchange field . Which tends to produce a parallel allignment of the atomic Dipoles the field is assumed to be prapostianal to the magnetisation M of each domains H M M H M = λM ………….(1) = Coefficient called the weiss field coefficent and . Independent of temperature = molecular field as the weiss field H M λ H M H M

11. In the presence of external magnetic field the resultant field acting on any atom is In ferromagnetism resultant magnetic field is more then applied magnetic field. So material shows strong magnetic effect. If molecular field coefficient or weiss field coefficent λ Is positive then the possibilit of self magnetisation of these domains even in the absence of external magnetic field. B eff = B + B E . . . B E = µ ᵒ H M B eff = B + µ ᵒ H M B eff = B + µ ᵒ λ M ………. (2)

12. Drawbacks of weiss theory It could not explain why & how internal fields between the molecules of ferromagnetic materials possess such large value. Why the linear relationship expressed by curie weiss law linear by breaks wear curie point.

13. Application of Ferromagnetism: - . Ferromagnetic materials are used for making permanent magnets becauses it’s magnetization lasts longer. . Ferrormagnetic materical are used for coating magnetic tapes in a cassette player and for building a memory store in a modern computer. .Ferromagnetic material uesd for making transformer core and choke because ferromagnetic Material have a large value of magnetic induction.

14. Conclusion - Ferromagnetic materials are used to make permanent magnet. Domain – The small region in which atoms of ferromagnetic material grouped together In absence of external the magnetisation is zero. In presence of external the magnetic dipoles align along the direction of Spontanenus magnetism of each domain in the absence of external is due to exchange field. Spontaneous magnetism is magnetic moment per unit volume. In postulate given by wiess

15. Weiss could not explain about the large value of exchange field. Why the linear relationship expressed by curie weiss law breaks near curie point. Ferromagnetic material are used for coating magnetic tape and for making transformer core . DRAWBACK APPLICATION

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