Why can't we use an iron bar as magnet

[AlonsoMcLaren]

A really basic question: since room temperature (300K) is much lower than the Curie point of iron (1043K), the spins in an iron bar at room temperature should be lined up even when the external magnetic field B=0

Then why can't we use any iron bar as magnet? Instead we have to rub the iron bar against a magnet to make the iron bar into a magnet?

 

[jerromyjon]

Then why can't we use any iron bar as magnet

Because when it is made it settles into domains which cancel the overall field. When you force many to tend one direction is when it becomes a magnet, until it gets back up to the curie temp and it releases all magnetic moments to cancel again.

 

[Vanadium 50]

Then why can't we use any iron bar as magnet?

You can. Most pieces of iron are magnetized at a very low level. (Before asking why something is true, it is important to ask if it is true). The reason the level is low is because of what jerromyjon said.

 

[M Quack]

Even permanent magnets that you buy such as Nd2Fe14B have random domains when they are made and have to be magnetized first.
A good "hard" permanent magnet must hold this magnetization very well. This usually means that you need a very high magnetic field to magnetize it. "Soft" magnets depolarize easily. These are useful as cores in transformers, electric motors and electromagnets.