Interplay of disorder and geometrical frustration in doped gadolinium gallium garnet

The geometrically frustrated triangular antiferromagnet Gadolinium Gallium Garnet (Gd_3Ga_5O_(12) or GGG) exhibits a rich mix of short-range order and isolated quantum states. We investigate the effects of up to 1% neodymium substitution for gallium on the ac magnetic response at temperatures below 1 K in both the linear and nonlinear regimes. Substitutional disorder actually drives the system toward a more perfectly frustrated state, apparently compensating for the effects of imperfect gadolinium/gallium stoichiometry, while at the same time more closely demarcating the boundaries of isolated, coherent clusters composed of hundreds of spins. Optical measurements of the local Nd environment substantiate the picture of an increased frustration index with doping