Question

Why does magnetic field increases the resistivity of all metals except iron , cobalt and nickel ?

Answer 1

In metals there are two types of electrons: bound electrons and free electrons. The free electrons are free to move between atoms, and are the cause of conductivity in metals. The bound electrons are stuck to the individual atoms.

Each electron, in addition to having charge, also has a “magnetic moment” which is a fancy way of saying that it’s a tiny bar magnet. Generally the bound electrons will be paired off in opposite spin pairs. This is like putting a North-South magnet next to a South-North magnet. They almost completely cancel each other out. However, sometimes (in iron, nickel, and cobalt for example) you’ll have one or more un-paired electrons. The magnetic fields of these electrons aren’t canceled out by another, oppositely-oriented, electron. As such they lend an overall magnetic field to the atom they inhabit.
Thus these unpaired electrons can move around freely even in the presence of magnetic field, the resistivity of such metals are lower than others

Hope this helps
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