Fri, Apr 17, 2015, 1:00 pm to 2:00 pm
Antiferromagnets are characterized by interactions between spins that favor configurations with nearest neighbors pointing anti-parallel. In some systems, the topology of the spins produces a situation where spins cannot find an orientation that fully satisfies all of the interaction constraints. This is known as geometrical frustration. Such frustrated systems exhibit macroscopically large ground state degeneracy. As a result of this large degeneracy, a perfectly frustrated system does not settle into any particular configuration and hence does not form an ordered state at any finite temperature. However, the ground state degeneracy is very fragile, and can be broken by effects such as quenched disorder and thermal and quantum fluctuations. These effects can relieve the frustration, allowing the formation of ordered states, a phenomenon known as “order by disorder.” We studied the effects of disorder in the archetypal frustrated magnet Gadolinium Gallium Garnet ($Gd_3$ $Ga_5$ $O_1$ $_2$)by introducing controlled amounts of a magnetic dopant. Measurements of the ac magnetic susceptibility in the low-field linear-response regime found an onset of a short-range ordered state at T~90 mK, which is suppressed to below 30 mK with the introduction of a 1% dopant concentration. Doping actually seems to reduce the effective disorder and increase the degree of frustration. Nonlinear susceptibility measurements excite and probe coherent clusters of hundreds of spins. We find similarly that the introduction of dopants acts to reduce the effective degree of disorder of these spin clusters.