  The intermetallic compound SmMn$_2$Ge$_2$, displaying multiple magnetic phase
transitions, is being investigated in detail for its magnetization behavior
near the 145 K first order ferromagnetic to antiferromagnetic transition
occuring on cooling, in particular for thermomagnetic history effects in the
magnetization data. The most unusual finding is that the thermomagnetic
irreversibility, [= M$^{FCW}$(T)-M$^{ZFC}$(T)] at 135 K is higher in
intermediate magnetic field strengths. By studying the response of the sample
(i.e., thermomagnetic irreversibility and thermal hysteresis) to different
histories of application of magnetic field and temperature, we demonstrate how
the supercooling and superheating of the metastable magnetic phases across the
first order transition at 145 K contribute to overall thermomagnetic
irreversibility.
