  Magnetic materials have found wide application ranging from electronics and
memories to medicine. Essential to these advances is the control of the
magnetic order. To date, most room-temperature applications have a fixed
magnetic moment whose orientation is manipulated for functionality. Here we
demonstrate an iron-oxide and graphene oxide nanocomposite based device that
acts as a tunable ferromagnet at room temperature. Not only can we tune its
transition temperature in a wide range of temperatures around room temperature,
but the magnetization can also be tuned from zero to 0.011 A.m$^2$/kg through
an initialization process with two readily accessible knobs (magnetic field and
electric current), after which the system retains its magnetic properties
semi-permanently until the next initialization process. We construct a
theoretical model to illustrate that this tunability originates from an
indirect exchange interaction mediated by spin-imbalanced electrons inside the
nanocomposite.
