  We report on the first experimental observation of a current-driven
instability developing in a quasi-neutral matter-antimatter beam. Strong
magnetic fields ($\geq$ 1 T) are measured, via means of a proton radiography
technique, after the propagation of a neutral electron-positron beam through a
background electron-ion plasma.The experimentally determined equipartition
parameter of $\epsilon_B \approx 10^{-3}$, is typical of values inferred from
models of astrophysical gamma-ray bursts, in which the relativistic flows are
also expected to be pair dominated. The data, supported by Particle-In-Cell
simulations and simple analytical estimates, indicate that these magnetic
fields persist in the background plasma for thousands of inverse plasma
frequencies. The existence of such long-lived magnetic fields can be related to
analog astrophysical systems, such as those prevalent in lepton-dominated jets.
