  Phases of matter are conventionally characterized by order parameters
describing the type and degree of order in a system. For example, crystals
consist of spatially ordered arrays of atoms, an order that is lost as the
crystal melts. Like- wise in ferromagnets, the magnetic moments of the
constituent particles align only below the Curie temperature, TC. These two
examples reflect two classes of phase transitions: the melting of a crystal is
a first-order phase transition (the crystalline order vanishes abruptly) and
the onset of magnetism is a second- order phase transition (the magnetization
increases continuously from zero as the temperature falls below TC). Such
magnetism is robust in systems with localized magnetic particles, and yet rare
in model itinerant systems where the particles are free to move about. Here for
the first time, we explore the itinerant magnetic phases present in a spin-1
spin-orbit coupled atomic Bose gas; in this system, itinerant ferromagnetic
order is stabilized by the spin-orbit coupling, vanishing in its absence. We
first located a second-order phase transition that continuously stiffens until,
at a tricritical point, it transforms into a first- order transition (with
observed width as small as h x 4 Hz). We then studied the long-lived metastable
states associated with the first-order transition. These measurements are all
in agreement with theory.
