  Glassiness is ubiquitous and diverse in characteristics in nature.
Understanding their differences and classification remains a major scientific
challenge. Here, we show that scaling of magnetic memories with time can be
used to classify magnetic glassy materials into two distinct classes. The
systems studied are high-temperature superconductor-related materials,
spin-orbit Mott insulators, frustrated magnets, and dilute magnetic alloys. Our
bulk magnetization measurements reveal that most densely populated magnets
exhibit similar memory behavior characterized by a relaxation exponent of 1-n ~
0.6(1). This exponent is different from 1-n ~ 1/3 of dilute magnetic alloys
that was ascribed to their hierarchical and fractal energy landscape and is
also different from 1-n=1 of the conventional Debye relaxation expected for a
spin solid, a state with long range order. Furthermore, our systematic study on
dilute magnetic alloys with varying magnetic concentration exhibits crossovers
among the two glassy states and spin solid.
