The ancient Greeks were aware of the phenomenon of magnetic order in lodestone, a type of rock containing the ferromagnet magnetiteFe3O4.Magnetic moments in a ferromagnet tend to align and thereby sum to an easily observed macroscopic magnetic moment. The absence of such a moment even in ordered antiferromagnets is the reason their discovery is comparatively recent. It had to await the development of Louis Ne´el's microscopic theory of spin interactions in the 1930s and the neutron diffraction measurement of MnO in 1949 by Clifford Shull and Stuart Smart.