ORDERING DUE TO DISORDER IN AN ANTIFERROMAGNET WITH CONTINUOUSLY DEGENERATE NEEL GROUND-STATE - A COMBINED NEUTRON-DIFFRACTION AND MAGNETIZATION STUDY OF (FEXGA1-X)(2) CA3GE3O(12)

In general, fluctuations suppress order in ''conventional'' many body systems. Here we present results of an experimental study intended to test recent ideas about how quantum, thermal and quenched noise may in duce ordering in highly frustrated magnets. Our model system, the garn et Fe2Ca3Ge3O12, is an antiferromagnet with a continuously degenerate Neel ground state. We report combined magnetization- and neutron diffr action studies on the dilution series (FexGa1-x)(2)Ca3Ge3O12 (0 less t han or equal to x less than or equal to 1). We propose a complex magne tic phase diagram with a spin glass phase around x = 0.5. Within a wid e concentration range, we observe a micro-domain state, where true lon g range order is never established. The correlation lengths lie betwee n some 300 Angstrom for x = 0.4 and greater than or equal to 900 Angst rom for x greater than or equal to 0.8. We relate our findings to rece nt theories about how fluctuations select a definite spin structure in highly frustrated antiferromagnets. This leads us to propose two diff erent spin structures for the pure and the diluted samples: our observ ations are in accordance with a stabilization of a collinear structure via quantum and thermal fluctuations, while quenched stoichiometric d isorder lifts the ground state degeneracy but favours a canted spin ar rangement.