How do magnetic cores (ferrites) guide magnetic field? As far as I know, magnetic materials have magnetic dipole moments which align when they are under influence of the outside magnetic field.
Basically they increase the magnetic field strength. But people also say that they guide magnetic field. Does the magnetic field strength decrease around them compared to the state before they were introduced near a coil? Do they really guide magnetic field, or just increase the magnetic field inside them so that magnetic field around them seems small, even though it hasn't decreased?
 A: Ferromagnetic materials add their own magnetic field to the external field that causes an alignment of their atomic dipoles. If the external field is from a fixed current in a solenoid, the induced field has no effect on the external external field. If the external field is from another ferro-magnet, the induced field may modify the magnetization in the external magnet. If an external solenoid is part of an AC circuit, then the induced field will change the inductance of the solenoid and that will change the current and the external field.
A: A ferrous metal will attract the magnetic field lines around it to pass through it. Thus, it will decrease the density of magnetic field lines outside the magnet and increase the same inside itself. 
So, yes the magnetic field strength decreases in area where a ferrous metal is introduced.
Ferrous metals attract the magnetic lines and cause the ones near it to pass through it. this is what some people refer to as guiding magnetic field.
Overall the magnetic field remains the same, but it is in the precise environment of the ferrous metal introduced, that one can see the changes.
The behavior will depend on permeability of the ferrous metal and the intensity of the external magnetic field.
