  The conduction electrons in a metal experience competing interactions with
each other and the atomic nuclei. This competition can lead to many types of
magnetic order in metals. For example, in chromium the electrons order to form
a spin-density-wave (SDW) antiferromagnetic state. A magnetic field may be used
to perturb or tune materials with delicately balanced electronic interactions.
Here we show that the application of a magnetic field can induce SDW magnetic
order in a metal, where none exists in the absence of the field. We use
magnetic neutron scattering to show that the application of a large (~8T)
magnetic field to the metamagnetic perovskite metal Sr3Ru2O7 can be used to
tune the material through two magnetically-ordered SDW states. The ordered
states exist over relatively small ranges in field (<0.4T) suggesting that
their origin is due to a new mechanism related to the electronic fine structure
near the Fermi energy, possibly combined with the stabilising effect of
magnetic fluctuations. The magnetic field direction is shown to control the SDW
domain populations which naturally explains the strong resistivity anisotropy
or electronic nematic behaviour observed in this material.
