Understanding Magnetic Domain Walls and Hysteresis Behavior: Can You Help?

[Zahid Iftikhar]

Please help in the explanation of BH curve (See attached file).
At the starting point, more current is required to bring the material to its magnetic saturation point( in fig form origin to pt.a shown by purple line ) as compared to when the material is fully demagnetized and then brought to same saturation point (in fig from point d to point a, shown by green line). At the origin and point d of the graph the material is totally demagnetized with B=0, then different current is required to reach the saturation point?
Regards
Zahid

 

[ZapperZ]

What exactly do you mean by explaining the HB curve? Are you asking for the cause or origin of the hysteresis?

http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/hyst.html

Zz.

 

[Zahid Iftikhar]

What exactly do you mean by explaining the HB curve? Are you asking for the cause or origin of the hysteresis?

http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/hyst.html

Zz.

Thanks Sir for the reply.
I can't understand why there are two different values of current to produce the same magnetism. At the start there is higher current needed to cause saturation (path shown as dotted purple in the figure, starting from origin and reaching point a) and afterwards lower current needed to reach the same saturation point (shown by path d to a in green). In both the case material is fully demagnetized at the start and becomes fully magnetized (saturation point) at the end. I hope I have clarified my question.
Regards

 

[ZapperZ]

Did you read the link? Do you know what magnetic domains are?

Zz.

 

[Zahid Iftikhar]

Did you read the link? Do you know what magnetic domains are?

Zz.

Yes I do. I know domains get fully aligned at saturation point and the material retains it magnetism because the domains don't fully go to complete disorderliness when the current becomes zero. I wonder why my question is not being understood.

 

[ZapperZ]

Yes I do. I know domains get fully aligned at saturation point and the material retains it magnetism because the domains don't fully go to complete disorderliness when the current becomes zero. I wonder why my question is not being understood.

Yes, you should wonder why your question is not being understood.

Zz.

 

[berkeman]

I can't understand why there are two different values of current to produce the same magnetism.

Your Profile page says that you have a Masters degree in Physics, so presumably you understand the basics of this phenomena. Do you want a more detailed explanation at the solid state physics level? If so, we could move this thread to that forum.

Or, if you want a simpler answer, there are always just the energy considerations, no?

 

[Zahid Iftikhar]

Yes, you should wonder why your question is not being understood.

Zz.

Your Profile page says that you have a Masters degree in Physics, so presumably you understand the basics of this phenomena. Do you want a more detailed explanation at the solid state physics level? If so, we could move this thread to that forum.

Or, if you want a simpler answer, there are always just the energy considerations, no?

Thanks for the reply.
Yes please explain it with respect to energy consideration.

 

[sophiecentaur]

I can't understand why there are two different values of current to produce the same magnetism.

Could the answer to your question just be that you have to look at the Arrows on the diagram? The green line shows the current / applied field to achieve a value of B. When the H is reduced, the B just 'hangs on' until H is lower than on the way up. That's just a description of what the graph is telling us, of course. An 'explanation of why this occurs will be that an increasing H will be doing work on altering the domains and, when H is decreased, work is done by taking Energy from the domains as B reduces.
There is a direct analogy with a spring laying on a friction surface. Energy is dissipated as the spring is stretched and when it returns to its unstressed length.

 

[Merlin3189]

It seems to me that the origin point is different to point d.
If we were generating this curve, say with a current in a coil round the iron sample, and we reached point d, then, instead of continuing to increase the current, we reduced the current to 0. I don't think the iron would move to the origin. I'm not sure that it would return to point e, but I think it would return to a point near to e.

That seems to imply to me, that the material is in a different state at d than at the origin.

I don't think the OP statement, "At the origin and point d of the graph the material is totally demagnetized with B=0" is correct.
B=0 says flux is 0. But flux is made of two contributions, the externally applied H field and the internal H field produced by the magnetised domains. Since these fields are opposite, there needs to be external H field opposing the internal remnance field to bring the flux to zero.

Caveat: I'm not a physicist and know v.little about this subject. Just my interpretation of a phenomenon I find as difficult as OP.

 

[sophiecentaur]

That seems to imply to me, that the material is in a different state at d than at the origin.

I think this is a matter of interpreting the graph right. You always have to follow the curve - you can't take another route.The H axis (the line) contains all the ways that B can be zero. The path around that curve is determined by the 'history' of the applied H. Point C and point d show the same value of B (=zero). If the sample had been heated and annealed, it would also have B=0 but, once you apply an H, it will follow the dotted path and B will be non-zero, even if you put H to zero. You can stop at any value of H and return B to zero but only by applying some negative H (following a curve that lies inside the outer curves in the diagram.
My analogy of the spring on a friction surface applies. once the end of the spring has been moved, you have to pull it 'the other way' to return it to the start point. If you just remove the force, the spring will have a non-zero extension.

 

[Lord Jestocost]

It seems to me that the origin point is different to point d.

The dashed purple line in the figure represents the virgin magnetization curve. The origin point and point d differ in the sense that the material exhibts different microscopic domain structures. Have a look at the graph on page 2 of
Chapter 7 Micromagnetism, domains and hysteresis

 

[Zahid Iftikhar]

Could the answer to your question just be that you have to look at the Arrows on the diagram? The green line shows the current / applied field to achieve a value of B. When the H is reduced, the B just 'hangs on' until H is lower than on the way up. That's just a description of what the graph is telling us, of course. An 'explanation of why this occurs will be that an increasing H will be doing work on altering the domains and, when H is decreased, work is done by taking Energy from the domains as B reduces.
There is a direct analogy with a spring laying on a friction surface. Energy is dissipated as the spring is stretched and when it returns to its unstressed length.

Thanks dear Sir for your time and answer. I d like to argue on this part of your answer.
"An 'explanation of why this occurs will be that an increasing H will be doing work on altering the domains and, when H is decreased, work is done by taking Energy from the domains as B reduces."
I have no problem in understanding what happens when H decreases. I have issues with a) when H increases and B reaches to saturation at the very beginning. It is shown by dotted path in the figure 1 I have just added. It talks about higher H needed to get to saturation. b) when the cycle of H completes and completes and we have to apply coercive H to bring the material to zero B (point d). You see from the graph this time less current or H makes the material saturated.

 

[Zahid Iftikhar]

It seems to me that the origin point is different to point d.
If we were generating this curve, say with a current in a coil round the iron sample, and we reached point d, then, instead of continuing to increase the current, we reduced the current to 0. I don't think the iron would move to the origin. I'm not sure that it would return to point e, but I think it would return to a point near to e.

That seems to imply to me, that the material is in a different state at d than at the origin.

I don't think the OP statement, "At the origin and point d of the graph the material is totally demagnetized with B=0" is correct.
B=0 says flux is 0. But flux is made of two contributions, the externally applied H field and the internal H field produced by the magnetised domains. Since these fields are opposite, there needs to be external H field opposing the internal remnance field to bring the flux to zero.

Caveat: I'm not a physicist and know v.little about this subject. Just my interpretation of a phenomenon I find as difficult as OP.

Thank God, my question is picked. So far I was struggling to clarify what I am asking for.

 

[ZapperZ]

Thank God, my question is picked. So far I was struggling to clarify what I am asking for.

But this is what has been addressed already! It is why I am puzzled why you continue to ask about it, as if we missed it!

The fact that there is still net magnetization is exactly why we have permanent magnets! It is also why this effect is present in ferromagnets but not paramagnets!

In magnetism, one cannot simply look at individual magnetic moments. Rather, this is a many-body problem and requires that you look at not only how many other magnetic moments are in the vicinity, but also how they are arranged! It is why this is a topic not in classical E&M, but rather in solid-state physics/condensed matter physics.

I pointed out way in the beginning about the magnetic domain walls, and you indicated that you know what they are, but your questions do not indicate so. The initial formation of the domain walls is CRUCIAL in understanding why the hysteresis occurs in the first place! These domain walls do not migrate or disperse immediately once the external magnetic field is switched off.

Zz.

 

[sophiecentaur]

It talks about higher H needed to get to saturation

etc.
I see it this way:
Reaching the saturation point requires the same H whenever. That's what the graph says; it's not ΔH but H that's involved. There are two options on the existing graph - either the H max takes B from 0 to the saturation point or from the negative retention value. When you release the B from that value and take it to the saturation point, a different amount of Energy is needed. But H and Energy are two different things when there is Hysteresis present. Go back to my spring analogy again and see that Energy is put into the spring and actually stored until released by unlocking the friction that's sticking the spring in a partly deformed state.

 

[Zahid Iftikhar]

etc.
I see it this way:
Reaching the saturation point requires the same H whenever. That's what the graph says; it's not ΔH but H that's involved. There are two options on the existing graph - either the H max takes B from 0 to the saturation point or from the negative retention value. When you release the B from that value and take it to the saturation point, a different amount of Energy is needed. But H and Energy are two different things when there is Hysteresis present. Go back to my spring analogy again and see that Energy is put into the spring and actually stored until released by unlocking the friction that's sticking the spring in a partly deformed state.

Thanks indeed for the prompt reply. I need to go through some attachments which are recommended. Hopefully I will be getting the answer with the courtesy of all scholars of PF.
High Regards

 

[sophiecentaur]

Thanks indeed for the prompt reply. I need to go through some attachments which are recommended. Hopefully I will be getting the answer with the courtesy of all scholars of PF.
High Regards

You only need to look at the graph and see its true message. Forget the Physics details, your annotations on the graph are showing that you are not reading it properly.

 

[SydneyBird]

Hi everyone!

We studied about hysteresis last week and our instructor did a good job at explaining it. I thought that I should highlight some of the points here to help the OP. The spring analogy is really helpful. So, I'm going to Google this thread for the OP because it looks like he doesn't intend to read the material carefully! :)

Did you read the link? Do you know what magnetic domains are?

Zz.

That was the answer! If someone has a little bit of understanding of magnetic domains, it becomes easier.

Your Profile page says that you have a Masters degree in Physics, so presumably you understand the basics of this phenomena. Do you want a more detailed explanation at the solid state physics level? If so, we could move this thread to that forum.

Or, if you want a simpler answer, there are always just the energy considerations, no?

One doesn't need a BS to understand this phenomenon, let alone a Masters! I believe that solid state physics treatment of this phenomenon would only be helpful if you'd would like to understand the manufacturing or use of ferromagnetic materials for practical purposes.

There is a direct analogy with a spring laying on a friction surface. Energy is dissipated as the spring is stretched and when it returns to its unstressed length.

My analogy of the spring on a friction surface applies. once the end of the spring has been moved, you have to pull it 'the other way' to return it to the start point. If you just remove the force, the spring will have a non-zero extension.

Thank you for the analogy. It gave me another perspective.

That seems to imply to me, that the material is in a different state at d than at the origin.

Can't agree more and that's what the attachment shows which was extracted from @Lord Jestocost's provided reference.

If the sample had been heated and annealed, it would also have B=0 but, once you apply an H, it will follow the dotted path and B will be non-zero, even if you put H to zero. You can stop at any value of H and return B to zero but only by applying some negative H (following a curve that lies inside the outer curves in the diagram.

Have a look at the graph on page 2 of
Chapter 7 Micromagnetism, domains and hysteresis

Thank you for the reference. I found it really helpful.

Thank God, my question is picked. So far I was struggling to clarify what I am asking for.

On the contrary, it's the other way around. Your question was 'picked' at the very beginning but for some reason you couldn't 'pick' the answers.

These domain walls do not migrate or disperse immediately once the external magnetic field is switched off.

Thanks for the linked reference.

I need to go through some attachments which are recommended. Hopefully I will be getting the answer with the courtesy of all scholars of PF.

Seriously! As you have already been told that you don't need to go through anything else for the required answer. Please look at the attachment. You can see that B=0 for both circled regions, yet the arrangements of domains look different. In the left region, domains are not that much disorganized or random as they are in the right region, i.e. at the origin.

Thanks!

 

[sophiecentaur]

@Zahid Iftikhar Hysteresis is a general principle and it can be identified in many (if not most) classical phenomena to some extent or another. Choosing to address it in the context of Magnetism is one of the hardest ways to get to know it. Once you have appreciated what 'those graphs' are actually telling you and once you realize that they describe a Journey, rather than a fixed one - to - one relationship between variables, you can get a lot out of them. Remember, the area in the middle of a Hysteresis loop graph represents loss (or transfer) of Energy. It's found in Heat Engines, Springs, Magnetism and many mechanisms with friction.

 

[Zahid Iftikhar]

@Zahid Iftikhar Hysteresis is a general principle and it can be identified in many (if not most) classical phenomena to some extent or another. Choosing to address it in the context of Magnetism is one of the hardest ways to get to know it. Once you have appreciated what 'those graphs' are actually telling you and once you realize that they describe a Journey, rather than a fixed one - to - one relationship between variables, you can get a lot out of them. Remember, the area in the middle of a Hysteresis loop graph represents loss (or transfer) of Energy. It's found in Heat Engines, Springs, Magnetism and many mechanisms with friction.

Thanks indeed.
I always love to have generic view of laws, applicable to most of the situations. The dimensions you have mentioned, have really opened up my mind to apply this phenomenon in other areas of Physics.

 

[Zahid Iftikhar]

Hi everyone!

We studied about hysteresis last week and our instructor did a good job at explaining it. I thought that I should highlight some of the points here to help the OP. The spring analogy is really helpful. So, I'm going to Google this thread for the OP because it looks like he doesn't intend to read the material carefully! :)
That was the answer! If someone has a little bit of understanding of magnetic domains, it becomes easier.
One doesn't need a BS to understand this phenomenon, let alone a Masters! I believe that solid state physics treatment of this phenomenon would only be helpful if you'd would like to understand the manufacturing or use of ferromagnetic materials for practical purposes.Thank you for the analogy. It gave me another perspective.
Can't agree more and that's what the attachment shows which was extracted from @Lord Jestocost's provided reference.Thank you for the reference. I found it really helpful.
On the contrary, it's the other way around. Your question was 'picked' at the very beginning but for some reason you couldn't 'pick' the answers.
Thanks for the linked reference.
Seriously! As you have already been told that you don't need to go through anything else for the required answer. Please look at the attachment. You can see that B=0 for both circled regions, yet the arrangements of domains look different. In the left region, domains are not that much disorganized or random as they are in the right region, i.e. at the origin.

Thanks!

Thanks Sir for the reply. Of course B=0 despite the fact domains have different organization. I can guess there is some reason behind two different values of H at the two positions. My really question was to figure out the reasons why domains are not alike. If we take the material out of the external magnetic field one by one at these two points, what the difference will be in the value of B. May we say at one point, domains are less disorganized than at the other, even then B=0. I am sorry for my trivial knowledge.

 

[Zahid Iftikhar]

Thanks Sir for the reply. May I know why this happens.

 

[Zahid Iftikhar]

So, I'm going to Google this thread for the OP because it looks like he doesn't intend to read the material carefully! :)

I tried to but actually the link provided by @Lord Jestocost is too advanced for me. I don't have deep knowledge of this topic.

 

[SydneyBird]

Hi!

Just found out today that you're still looking for an answer.

Did you read the link? Do you know what magnetic domains are?

Zz.

Yes I do. I know domains get fully aligned at saturation point and the material retains it magnetism because the domains don't fully go to complete disorderliness when the current becomes zero. I wonder why my question is not being understood.

I pointed out way in the beginning about the magnetic domain walls, and you indicated that you know what they are, but your questions do not indicate so. The initial formation of the domain walls is CRUCIAL in understanding why the hysteresis occurs in the first place! These domain walls do not migrate or disperse immediately once the external magnetic field is switched off.

I'm going to link some PDFs which you might find helpful. Yes, it does involve math but you could simply try mentally picture it looking at the given diagrams. Please also Google something like 'magnetic domain walls'. No problem, I could do it for you!

http://www.ifmpan.poznan.pl/~urbaniak/Wyklady2012/urbifmpan2012lect6_04.pdf
http://www.ifmpan.poznan.pl/~urbaniak/Wyklady2012/urbifmpan2012lect7_04powy.pdf
https://www.birmingham.ac.uk/Docume...d/Magnetic-Materials-Background-6-Domains.pdf
https://www.google.com.au/search?q=magnetic+domain+walls+hysteresis

Just a side note. Anyone who tries to shun math in favor of 'wordy' explanations of a scientific phenomenon is doomed to fail in the long run. You were rightly asked if you knew about magnetic domains because the answer lies there. But your knowledge of magnetic domains is more like of quantum physics in this scenario. Question: "Do you know about quantum physics" Answer: Yes, I do. It's a branch of physics which involves quantum! :)

I'd suggest that you think of domains as those springs on a friction surface in the analogy. Some springs encounter more friction, some less, some are more aligned in the same direction... That's the best intuitive explanation of magnetic domains.

I had a look at your profile and it says you're an instructor and your background is Masters in physics. However, the most probable scenario is that you did your Masters a long time ago and started job as an instructor just recently and probably just trying to refresh the material. In my opinion, some refresher courses could be more helpful. I'll try to forward you few introductory physics ebooks which could help you to revise the subject.

My really question was to figure out the reasons why domains are not alike.

You nowhere mentioned that your question was about the domains. Nonetheless, you were given the right directions where you were asked about your knowledge of the domains. Asking one question and later after some discussion switching and saying that your real question was something else and implying that you already understand the material being discussed is a little offensive. I once was helping someone with 'why work done is a vector quantity'. I had to explain the difference between scalars and vectors, scalar product, vector product, etc. but after all the discussion that someone said "Well, I already understood all these things and my question was simply about calculating work done from force and displacement!". That someone just switched to a new question and believe me that someone didn't even understand the difference between vectors and scalars well.

Best of luck!