Dipole sequences in the 143Sm nucleus have been investigated via the 124Sn (24Mg, 5n) reaction at Elab = 107 MeV using the Indian National Gamma Array (INGA). The spin-parity of the associated levels have been firmly established from the spectroscopic measurement. Level lifetimes of several levels in the dipole bands have been measured using the Doppler Shift Attenuation Method. The decreasing trend of the measured B(M1) and B(E2) transition strengths in one of the sequence (DB I) spells out its origin as Magnetic Rotation (MR). Though, the trends of B(M1) and B(E2) in DB I are reproduced well in the theoretical calculations using the Shears mechanism with the Principal Axis Cranking (SPAC) model, the calculations fail to reproduce the sharp rise in the B(M1)/B(E2) ratio at the highest spins in DB I. Such rise in the B(M1)/B(E2) values is in well agreement with the theoretical calculations within the shears mechanism as prescribed by the Clark and Macchiavelli considering the smooth decrease of the core rotation along the sequence. The experimental observations along with the theoretical calculations for the second dipole band (DB II), indicate that the core rotation, rather than the shears mechanism, is being favored for angular momentum generation. This represents a unique observation of forking of the shears band DB I from an abrupt phase change of the core from spherical into the deformed one. Keywords: Magnetic rotation, Lifetime measurement, Shears mechanism with the principal axis cranking model, 143Sm
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