Why Does a Steel Core Remain Magnetized After Current is Turned Off?

[poojarao]

When the current in solenoid is turned off a steel core stays magnetized. The tiny atomic magnets remain lined up, even when the external field is removed. Why is that so?

 

[f95toli]

It has nothing do to with the atoms; they always have the same magnetic moment. It is essentially a macroscopic effect.
In a magnetic material the moments will try to orient themselves in such a way that the total energy is minimized and this leads to the creation of so called magnetic domains. These domains differ in size but they can be quite big. Now, all moments in a given domain point in the same direction but since the system is trying to minimize the energy of the WHOLE sample neighboring domains are NOT neccesarily oriented in the same way.
There are many domain configurations which are local minima for the energy and the system can't move from one to the other without extra energy. Which configuration the system ends up in when it is created is (in the absence of an external field) essentially just random and in most cases the net magnetization will be zero.

What is happening when you magnetize a material is that you are adding energy to the sample (via the external magnetic field which has to do some work in order to flip domains) and you are also giving the system a "prefered direction" meaning all domains will re-aling to point along the external field. As long as the domains find a new configuration that is ALSO an energy minima the system will stay in that state even if the external field is removed.

 

[Valikie22]

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[poojarao]

thnx a lot!

 

[phys_student1]

really smart answer , but I think this is the case with ferromagnetic and paramagnetic materials only , right ?