  Time and history dependent magnetization has been observed in a wide variety
of materials, which are collectively termed as the glassy magnetic systems.
However, such systems showing similar non-equilibrium magnetic response can be
microscopically very different and they can be distinguished by carefully
looking into the details of the observed metastable magnetic behavior.
Canonical spin glass is the most well studied member of this class and has been
extensively investigated both experimentally and theoretically over the last
five decades. In canonical spin glasses, the low temperature magnetic state
obtained by cooling across the spin glass transition temperature in presence of
an applied magnetic field is known as the field cooled (FC) state. This FC
state in canonical spin glass is widely believed as an equilibrium state
arising out of a thermodynamic second order phase transition. Here, we show
that the FC state in canonical spin glass is not really an equilibrium state of
the system. We report careful dc magnetization and ac susceptibility
measurements on two canonical spin glass systems, AuMn (1.8%) and AgMn (1.1%).
The dc magnetization in the FC state shows clear temperature dependence. In
addition, the magnetization shows a distinct thermal hysteresis in the
temperature regime below the spin glass transition temperature. On the other
hand, the temperature dependence of ac susceptibility has clear frequency
dispersion below spin glass transition in the FC state prepared by cooling the
sample in the presence of a dc-bias field. We further distinguish the
metastable response of the FC state of canonical spin glass from the metastable
response the FC state in an entirely different class of glassy magnetic system
namely magnetic glass, where the non-equilibrium behavior is associated with
the kinetic-arrest of a first order magnetic phase transition.
