  Neutron diffraction measurements, performed in presence of an external
magnetic field, have been used to show structural evidence for the kinetic
arrest of the first-order phase transition from the high temperature austenite
phase to the low temperature martensite phase in the magnetic shape memory
alloy Ni37Co11Mn42.5Sn9.5 and the formation of a glass-like arrested state
(GLAS). The CHUF (cooling and heating under unequal fields) protocol has been
used to establish phase coexistence of metastable and equilibrium states of
GLAS in the neutron diffraction patterns. We also explore the field-temperature
(H,T) phase diagram for this composition which depicts the kinetic arrest line
TK(H). TK is seen to increase as H increases.
