Phases of matter are characterized by order parameters describing the type and degree of order in a system. Here we experimentally explore the magnetic phases present in a near-zero temperature spin-1 spin-orbit-coupled atomic Bose gas and the quantum phase transitions between these phases. We observe ferromagnetic and unpolarized phases, which are stabilized by spin-orbit coupling s explicit locking between spin and motion. These phases are separated by a critical curve containing both first-and second-order transitions joined at a tricritical point. The first-order transition, with observed width as small as h x 4 Hz, gives rise to long-lived metastable states. These measurements are all in agreement with theory.