It was believed that the magnetic field to be generated deep down in the Earth's core. But the truth is the flow of liquid iron generates electric currents, which produces magnetic fields.
But the truth is no one really knows for certain so the only thing we know is that the spin of the earth is the cause of its magnetic field
Our planet's magnetic field is believed to be generated deep down in the Earth's core. ... The Coriolis force, resulting from the Earth's spin, also causes swirling whirlpools. This flow of liquid iron generates electric currents, which in turn produce magnetic fields.
