  Since the discovery of spin glasses in dilute magnetic systems, their study
has been largely focused on understanding randomness and defects as the driving
mechanism. The same paradigm has also been applied to explain glassy states
found in dense frustrated systems. Recently, however, it has been theoretically
suggested that different mechanisms, such as quantum fluctuations and
topological features, may induce glassy states in defect-free spin systems, far
from the conventional dilute limit. Here we report experimental evidence for
the existence of a glassy state, that we call a spin jam, in the vicinity of
the clean limit of a frustrated magnet, which is insensitive to a low
concentration of defects. We have studied the effect of impurities on
SrCr9pGa12-9pO19 (SCGO(p)), a highly frustrated magnet, in which the magnetic
Cr3+ (s=3/2) ions form a quasi-two-dimensional triangular system of
bi-pyramids. Our experimental data shows that as the nonmagnetic Ga3+ impurity
concentration is changed, there are two distinct phases of glassiness: a
distinct exotic glassy state, which we call a "spin jam", for high magnetic
concentration region (p>0.8) and a cluster spin glass for lower magnetic
concentration, (p<0.8). This observation indicates that a spin jam is a unique
vantage point from which the class of glassy states in dense frustrated magnets
can be understood.
