Spin-Trimer Antiferromagnetism in La$_4$Cu$_3$MoO$_{12}$

La$_4$Cu$_3$MoO$_{12}$ is a cluster antiferromagnet where copper spin-1/2 form a network of st rongly coupled spin-trimers. The magnetic properties of this material have bee n examined using magnetic neutron scattering. At low temperatures, excitations from the ground state are observed at 7.5(3) meV and 132.5(5) meV. An additio nal peak in the neutron scattering spectrum, which appears at 125.0(5) meV on heating is ascribed to a transition between excited states. The wave-vector an d temperature-dependence of the inelastic magnetic scattering cross section is consistent with intra-trimer transitions. Magnetic neutron diffraction reveal s antiferromagnetic order below T$_N$=2.6 K that doubles the unit cell along t he {\bf a} direction. The ordered magnetic structure is described as inter-tri mer order where spin correlations within trimers are controlled by the strong intra-trimer interactions. Combining the information derived from elastic and inelastic magnetic neutron scattering with group theoretical analysis, a consi stent set of intra-trimer interactions and ordered magnetic structures are der ived. The experiment provides a simple worked example of magnetism associated with inter-atomic composite degrees of freedom in the extreme quantum limit.