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.
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