I know that magnetism is due to the alignment of domains of a magnetic substance. Due to this reason, an iron bar may be magnetized by stroking it with a permanent magnet such that its domains are aligned.
Similarly, when a wire is coiled around a soft iron core and current flows through it, the iron core starts behaving as a magnet as it's domains are aligned.
My question is that why does the iron core lose its magnetic properties once the current flowing through the coil is stopped. Shouldn't the domains that were aligned remain aligned?
There might be a flaw in my reasoning too, so please tell me where I'm going wrong with this concept.
Best Answer
The reason the domains don't remain aligned is the same reason the material splits into domains to start with: to minimize the magneto-static energy. One of the differences between the "soft" magnetic materials (like this described in the question) and the "hard" materials (these that remain magnetized after the external field is removed consists in how easy is for the domain walls to move.
In soft materials the walls can move easily (low energy required) and the minimization of the magneto-static energy is the main factor. If the motion of the walls requires too much energy the material maintain a considerable magnetization even though this state is not a minimum of magneto-static energy. Things that can impede the motion of the domain walls may be impurities, defects in the crystal lattice, the boundaries between grains.