  A Ce-based metallic compound CeCoSi with a tetragonal structure exhibits
successive phase transitions: the one whose order parameter is unidentified at
T_0 = 12 K and the antiferromagnetic one at T_N = 9.4 K. We performed specific
heat, magnetoresistivity (MR), and magnetization measurements for a single
crystal CeCoSi at low temperatures in magnetic fields B of up to 14 T and
constructed detailed magnetic field--temperature phase diagrams for both B ||
[100] and [001]. The longitudinal MR measured for B || [100] shows a sign
change from negative to positive across T_0 = 13 K updated in the present
sample, indicating a clear change in an electronic state. In addition, the
constructed magnetic phase diagrams for both the field directions have a
B-induced region in each ordered state. The presence of the newly found regions
would be attributed to a change in the symmetry of the order parameter or
domain alignment by applying B.
