For diamagnetic materials, when an external magnetic field is applied, the electrons having orbital magnetic moment in opposite directions spped up. how does this happen? The direction of magnetic momwnt is given by right hand thum rule. If there is an electron revolving in anticlockwise direction, magnetic moment is upwards and magnetic field is upwards. So when external magnetic field is applied in downward direction, the magnetic field through the orbit decreases because the two magnetic fields cancel each other. So the electron has to revolve faster to conserve the initial magnetic moment. Is this the reason?