What happens if a compass' needle is held fixed in the presence of an external B field? If the B field is strong enough, the microscopic magnetic dipoles forming the magnet (Fig. 1) will reorient themselves within the material so that they get aligned parallel to B (with the needle's orientation remaining the same). It is this functionality that allows ferromagnetic materials to work as memory bits in a computer hard drive. A ferromagnet with M pointing up can represent bit "1", while M pointing down can represent bit "0". Writing "0" or "1" to a magnetic memory requires the application of a B field that is stronger than the magnetic anisotropy of the material. In today's computers this is achieved by running a large electric current to the hard drive write head [See "Spin- based memory devices", by Arora et al. in this issue].
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