Why Do Magnetic Domains Aligned with a Magnet Increase in Size?

[southern69]

I have a conceptual question. I have two physics books that say "In a common iron nail, the domains are randomly oriented. When a strong magnet is brought nearby, two effects take place. One is a growth in size of domains that are oriented in the direction of the magnetic field. This growth is at the expense of domains that are not aligned. The other effect is a rotation of domains as they are brought into alignment."

Can anybody explain why the domains aligned with the magnet get bigger? I understand the second effect. But the first effect I can't figure out, and neither book offers an explanation. Any help would be appreciated (I have to teach this subject this week and really want to know!). Thanks so much.

 

[Vailanor]

The domains aligned with the magnetic field get bigger because of a process called domain wall motion. When a magnetic field is applied, the magnetic moments of the domains that are not aligned with the field try to align themselves with the field in order to be energetically favorable. This causes the domain walls to move and rearrange the domains, resulting in the growth of the domains that are aligned with the magnetic field.

 

[astrokreng]

Thank you for your question. I can provide an explanation for the first effect mentioned in your question.

Firstly, it is important to understand what magnetic domains are. Magnetic domains are regions within a material where the magnetic moments of the atoms are aligned in the same direction, creating a small magnetic field within the domain. In an unmagnetized material, these domains are randomly oriented, resulting in no overall magnetic field.

Now, when a strong magnet is brought near an iron nail, the external magnetic field from the magnet causes the magnetic domains in the iron nail to align with the direction of the external magnetic field. This alignment causes the domains to grow in size, as more and more atoms are now aligned in the same direction, creating a larger magnetic field within the domain. This growth in size of the aligned domains occurs at the expense of the non-aligned domains, which lose their alignment and become smaller.

To understand this concept better, think of the magnetic domains as small magnets within the material. When the external magnet is brought near, it exerts a force on these small magnets, causing them to align with the external magnetic field. As they align, the small magnets become stronger and larger, resulting in an overall growth in size of the aligned domains.

I hope this explanation helps you in your teaching and understanding of magnetic domains. If you have any further questions, please do not hesitate to ask. Good luck with your lesson!