In the 1600's Willam Gilbert concluded that the earth itself is a giant magnet with north and south poles.
Also in 1820 Andre Marie a french physicist worked out the mathematical relationship between the current of electricity and the strength of th magnetic field. He also had a theory that the electric current in atoms had a magnetic field.
In 1830 Micheal Faraday an english scientist and Joseph henery an american ohysicist discovered that a changing magnetic field produced a current in a coil of wire.
In the early 1900's a lot of physicists began exploring the behaivor of electrons and other particles in a magnet which became known as quantum mechanics. Also the understanding of magnetic properties of atoms and molecues.
In the 1900's Edward M Purcell and flix Bloch both american physicists developed a way to measure the magnetic field of the nuclei. This discovery led to MRI also known as Magnetic Renosance Imaging.
In the 1960's and 1970's scientists developed superconducting materials. Only when the magnet is cooled to absolute zero these magnets can denerate magnetic fields up to 200,000 gauss. Superconducting magnets are used in maglev trains and in nuclear research.