  Recent research dealing with magnetocaloric effect (MCE) study of
antiferromagnetic (AFM) low dimensional spin systems have revealed a number of
fascinating ground-state crossover characteristics upon application of external
magnetic field. Herein, through MCE investigation we have explored
field-induced quantum level-crossing characteristics of one such spin system:
NH4CuPO4.H2O (NCP), an AFM spin 1/2 dimer. Experimental magnetization and
specific heat data are presented and the data have been employed to evaluate
entropy, magnetic energy and magnetocaloric properties. We witness a sign
change in magnetic Grueneisen parameter across the level-crossing field B_C. An
adiabatic cooling is observed at low temperature by tracing the isentropic
curves in temperature-magnetic field plane. Energy-level crossover
characteristics in NCP interpreted through MCE analysis are well consistent
with the observations made from magnetization and specific heat data.
